Airchair Questions & Answers

Last Revision: October 10, 2008 (new #66)


This is a listing of selected questions that I have been asked about the Bug and Goat gliders, along with my answers. I do appreciate comments, new questions, and corrections, especially from people who have read all the Website pages, which you probably have if you have read this far. My e-mail address is "m--sandlin" followed by " " (I give my address in two parts to confound harvesting by advertisers). The newest questions will be found at the bottom of the list. 

For  more, go to page2.


Subject Index

Bug4 vs. Goat .... Motor for self launch.... Heavy pilot.. .. Easiest to build..... Performance

Movable flaps..... Airspeeds.... Cost.... Other airchairs..... Building time....... My liability stance

Spray paint....... Rolling takeoff......Stronger aluminum alloys.... Control stick forces

Covering fabric..... Building instructions..... Pitch balance..... CAD software

Cable braced wings..... Seeing an airchair.... Learning to fly..... Two axis control

Goat1 vs. Goat2..... Goat2 good ideas..... Push start..... Drogue chute..... Bill of materials

Airfoil optimization....... My drawings offered elsewhere..... G3 airfoil & stalls

Undercamber fabric attachment.......Motor floater........Load test.........Weight limit

U.S. Aviation Superfloater........Visiting my shop.........Hillhopper drawings (none)

Adequate workspace........Finding Tubes, not in U.S.A, ........Rib Stitching.......Bending Loads

Fabric Covering Website......Cable tangs, Bolt orientation.........Goat4 Stalls & Spins

Emergency Parachute........Primary Gliders, PlanA vs.PlanB


1. "I was wondering if you had put down any rough ideas or notes on the key differences between the Bug4 and the Goat."

Answer: The Goat flies more quietly than the Bug4 and with higher performance. It does not roll as fast in flight, but seems to have more static roll authority (when stopped but ready to roll launch, keeping the wings level in the start position is easier in the Goat). There is an awareness of greater wing tip ground tip clearance, but no real advantage has yet been demonstrated.

The wing panel footprint of the Goat when collapsed for transport is much larger than that of the Bug panel and will probably be harder for one person to load onto a vehicle in windy conditions. In gusty wind out in the open I could disassemble the Bug4 but sometimes had to wait for help to get the panels up onto the truck rack because I could not hold control them reliably . An uncontrolled panel can blow away and damage itself or damage an adjacent vehicle in a parking area (my Bug wing once blew off my rack and landed on a motorcycle). I expect this to be more of a problem for the Goat than for the Bug.

The elevator push rod and sliding tube have been moved and are now aligned vertically rather than horizontally. They are still inside the vertical stabilizer but are located just in front of the rudder post and can now drive the elevator from below. This allows the direct connection of the push rod to the elevator, eliminating the two short folding arms which were needed on the Bug, thus simplifying elevator hookup and preflight inspection. This is an example of a case where the old Bug design would work just as well mechanically but is more complex to build and assemble, and not as easy to inspect.

I'm using the next larger size Spectron 12 line now for the control system (1/8" instead of 7/64") because it is stiffer and stronger and will inspire pilot confidence (i.e., it looks good). (The bulkier knots required to connect this larger line are not a problem for the Goat but might be restrictive on the Bug4.)

In those places where I clamped a marine pulley in place with a bolt I am now using a more conventional, "stand up" spring swiveling pulley, a readily available "off the shelf " commercial item. This will relieve the pulley of side loads, a condition in which it does not achieve full strength.

I'm sitting in a light folding boat seat, another convenient "off the shelf "commercial item, but this is fixed in position and does not readily allow for different size pilots, so it might be changed (see No.3 below).


2. "...We were wondering if you had given any thought to having a very small engine to self launch put on this design with two wheels with a brake as well? Any thoughts?"

Answer: I'm not doing airplanes right now, since I think they require about twice the effort and expense of glider building and flying. (The Lady Bug,

a Canadian Bug-like airchair, may soon have an engine because it's over the Canadian 100 lb. glider weight limit, but it can meet the much greater airplane weight limit, so it needs at least a token motor for regulatory reasons!) I have no special ideas to offer when it comes to a Goat motor. In general I expect airplanes to be simple and tough, whereas gliders are light and efficient, so a glider may not make a good airplane.

The Bug2-2, in Germany, has two wheels, side by side but close together, which reportedly acts sort of like one very wide wheel. For the Goat-like airplane, I presume you want wheel brakes for steering while in taxi mode, but why taxi at all? On the ground, why not just walk the plane out to the runway? When it comes to takeoff and landing, if you can roll launch with one wheel and no brakes, I don't see why that same setup wouldn't work for airplane operation, so why not just skip the extra wheel and brakes. This is all just my speculation, since I am not an ultralight airplane pilot, but it seems to me that considerable complexity and burden might be avoided by deleting the taxi capability from a very light airplane that can be pushed around on its wheel.


3. "I am a tall and heavy guy, and would like to know if you have a design in mind that would be simple to build, using hand tools, and can carry a 260 pound person. I saw a picture of a biplane type glider, and was told it was your design, called a "lady bug" I think. is there any way to modify it to carry the xtra weight?"

Answer: Airchair gliders can provide cockpit space for any size person if the controls are placed to fit, but redesign may be necessary to get the heavy pilot far enough back for good balance, and to strengthen the structure if the heavy pilot wants to fly at the same faster speeds and maneuver at the same g levels as average weight pilots. A larger wing area will also be needed if the heavy pilot wants to slow down and turn as tight as a normal or light pilot. If a two person airchair comes along, that might be just what you need, but I myself have no intention right now of designing gliders like that. A glider for a tall or heavy pilot could be a specialty design, an enlarged version of a Bug or Goat, but I'm dreaming up new stuff and I don't want to stop and develop old designs. You may need to find a big, heavy designer to do this job.

"What is the pilot weight range?"

I haven't established any formal weight range. A pilot weighing 145 lbs. was checked for static margin (with the existing fixed seat position and without the 6.5 lb. emergency parachute) and was balanced on the wheel, therefore too tail heavy, and he had to add some ballast forward. I weigh 155-160 lbs. and don't use any special ballast. A heavy pilot has flown the Goat (over 200 lbs. I think) and he didn't mention having any special problems.


4. "I would just like to ask which is easiest to soar Bug or Goat? And when building Goat did it take less time being a "monoplane"?  "

Answer: In general, the Goat is better than the Bug as a soaring glider because it has a lower sink rate and has a better glide ratio for moving around from thermal to thermal. This is a result of a greater wing span, fewer cables across the wind, and fewer wing tips. When soaring, better usually means easier. The Bug, however, may roll faster than the Goat, and might do better in some conditions.

A Goat-like glider (monoplane) would probably take less time to build than a Bug-like glider (biplane) since they use the same technology and the lower wing construction is time consuming.


5. "I have not been able to find any mention of the sink rate or the l/D. If you have not posted these any where will you venture a speculative figure? "

Answer: I have no measured performance figures for the Goat, but I seem to soar with the weekend mix of hang gliders, perhaps with similar low speed efficiency. Several hang glider pilots I have flown with have commented that they don't see any relative performance deficiency or superiority. I think my stability and handling are much better than a hang glider, especially in turbulent air.


6. "Have you considered make the flaps moveable so they could be used for landing."

Answer: Yes, this comes up a lot. A flap could be useful to lower the stall speed for a rolling takeoff, and for glide path control (added drag) when landing. I doubt that flaps would much enhance the soaring or gliding of the Goat. The flaps are already down, I think, and more deflection would just add drag that would not improve the thermalling climb rate. Adding glide speed by raising the flap panels might not accomplish much, considering the large amounts of parasite drag from the exposed pilot and structures.

These would be interesting things to try, and I suppose some mechanical changes could be made to allow the flaps to be adjusted in the air, but I'm not going to do it. Adding another control system is a considerable increase in complexity, an additional burden on the pilot, and I would rather move toward simpler aircraft.

Right now I use a drogue chute to add drag for landings rather than lowering flaps.


7. "What is the typical airspeed of the Goat?"

Answer: Floyd Fronius has flown the Goat with his airspeed indicator and reports an indicated slow flight speed of 21 mph. and a fast speed of 40 mph.

My impression is that my airspeeds are like a hang glider, stalling around 23 mph. or so, soaring maybe in the mid or low 20's, but I don't fly with an airspeed indicator. The weight of the pilot will make a difference.


8. "Do you have a ... cost, using new materials?"

Answer: I do not have an exact cost estimate for the Bug or Goat. It was reported that a Bug4-like glider was built in Utah for just under $3k, and materials for a Goat-like glider should cost about the same.

The choice of emergency parachute system is the major cost variable for an airchair. It might cost from $300 to $3,000 or more. I could use a used chute, a new one, a new high tech design or an older one, a rocket deployed system or hand thrown, etc. I actually use an ordinary (but old fashioned) hang glider hand deployed parachute, 22 gore round canopy with a standard length bridle, from the low end of the cost scale.


9. "Has anyone else built one? (Or working on one.) If so, do they have a website..."

Answer: There are certainly Super Floaters flying, and maybe some other airchairs, and there is at least one new Bug-like glider flying. See the Yahoo Airchair discussion group website for remarks by builders. Some of them do have websites, and there is a link to a builder's list.


10. "... how long it might take one (me!) who is a first-time aircraft builder but handy with tools to build one?"

Answer: I can build a glider in five or six months. It can no doubt be done faster or slower.


11. "By the looks of your drawings, I'd say you're in the drafting or aircraft business, or something related. Care to reveal your day job?"

Answer: For liability reasons, I don't wish to present myself as any kind of expert, and so I generally don't comment on my professional or aviation experience levels. This policy sometimes breaks down, but I am going to try to maintain it as long as it contributes to allowing me to design, build, and describe my experimental aircraft.

My current liability stance is:

1. My words and technical drawings are descriptions of what I have done, they are not plans and are not advice.

2. My designs and procedures are experimental, often unconventional, untested to any recognized standard, and have no extensive history of flight safety.

3. I am not a technical expert in the design of aircraft structures or control systems.

4. I am an amateur, I have nothing for sale and no commercial involvement in general aviation.

It would be a favor to me if other builders would give unique names to their aircraft and acknowledge their design as original and as their own.


12. "Getting ready to cover my BUG4. Please tell me what you add to Polybrush in order to get color for trim."

Answer: For color I use spray paint from a can, just like the local taggers. If you want color that is chemically compatible with Polybrush, use Aerothane, the finish coat of the Polyfiber (Stitts) process (it's a certified covering system).


"Regards rolling T/O: What wind speed for Go/NoGo for a rolling T/o on say a 15 degree slope?"

Answer: For a rolling takeoff, the wind required depends on the length of the takeoff slope. You just have to reach flying airspeed before you're in the bushes, and the headwind is the airspeed you start with. In theory, 18 feet of vertical descent is adequate for a no wind launch, but this does not account for drag losses or rolling friction, so don't depend on it. (This distance "18 feet" is derived just by dropping a mass and asking how far it would have to free fall to reach 23 mph. or so). For moderate winds, the "Rule of 18" seems to provide a rough guideline: "To get flying speed, the vertical descent in feet plus the headwind in miles per hour will equal 18". Don't confuse the "vertical descent" with the rolling distance, which will be a lot longer, of course. In practice, you'll use extra rolling distance anyway, just to get some extra margin of airspeed. Nobody will want to take off as short as I have described unless they have to hop over a snake or something.


13. "...we only have a few trikes and one Bailey Moyes Dragonfly to aero tow behind here in Norway and there is only a few places where it is possible to launch the Goat by rolling on the wheel as you usually do...."

Answer: Remember that you can launch by vehicle towing with a scooter and a piece of rope. This is not an easy or efficient way to go soaring, but it works out on the flats, can be fun, and you will get to do a lot of takeoffs and landings.

"Have you considered using the European 7075 alloy tubes to reduce the weight of the Goat wings and frame?"

Answer: Stronger aluminum might be used, as long as it is not used for the formed (bent) parts, because it may lose strength when sharply bent. (7075 is widely used for hang glider ribs, however, which involves only large radius bending.) Not much is gained by using stronger aluminum for the main spars, because they are mostly in compression and for that only the stiffness of the material really matters, not so much the strength, and strong aluminum alloys do not vary much in stiffness. Generally, however, stronger aluminum is by no means a problem. The big advantages of European tubing are: 1. 7075 alloy, 2. metric sizing, and 3. broad availability of sizes and thicknesses. Someone should design a metric airchair!

14. "How is the control and stick forces on the Goat. Is it comparable to an old glider like the Ka8 or similar gliders?"

Answer: The control forces generally seem light to me, compared to most sailplanes. The Goat control system can be greatly improved, no doubt, since it is now just a crude prototype. Silicone spray helps a lot.


15. "I thought you might like to know that your plans are being offered for sale on eBay ..."

Answer: Thanks for the information. I have no involvement with this. I have heard about these activities before and my response is: no comment.

I try to get people not to use the word "plans" for my "technical drawings", which are freely available in the public domain, and which are a description of what I have done, not instructions or advice to others.


16. "Interested to know how long the original BUG4 covering lasted, was it always stored inside and when on the truck rack did it have a cover over it?"

Answer: The polyester fabric (Dacron) which I use to cover airchair structures is the same material as that used for boat sails, hang glider wings, and paragliders, so I follow the same practices as they do. My gliders, which use unprotected fabric covering, are always stored under cover to minimize solar exposure, which would cause cumulative weakening of the exposed, unprotected fabric over time. (Another result of radiation exposure is that it appears to cause unprotected Polybrush to turn brown, which looks bad on white fabric).

I've considered using conventional protective coatings of aluminized paint, which would allow me to tie my glider down outside for as long as I wanted to. All I have to do to protect my fabric against radiation is to complete more of the Polyfiber process by using the protective coat, Polyspray. It's just additional work, and a little additional weight (I wish somebody would do it and tell me how much weight is added, a pound or two, maybe?). The flexible sails mentioned above cannot effectively use these protective aluminized coatings because they must be rolled up when put away, but of course my airchair fabric remains tensioned on the frame at all times, like that of airplanes and sailplanes, so I can take advantage of this stuff if I want to.

The reason I don't use the protective coating is that my gliders are prototypes, which are sometimes torn to shreds after a single flight test for massive rebuilding and modification. I just want to do a fast and simple fabric job, not something of lasting quality. The original fabric is still (in pieces) on the Bug4 and Goat., but Bug2 was recovered when it was modified to the Bug4 dimensions.

There has been discussion of boxes and covers for transport, but none have been used that I know of. My personal philosophy remains unchanged: don't use any special padding or covers, just make the wing tougher. For car top transport, it might be possible to designate one surface of one wing as the "on top" surface, and give that surface an especially thorough ultraviolet protective coating. (See also #58)


17. Is the wing rib just a French curve smoothing through the high point using the Aluminum tube L.E. for the nose radius and the bottom of the rib chord is flat and tangent to the leading edge tube? Or do you use an airfoil the has "plotable" points and has a set of polars?

Answer: A garden hose, not a french curve (an archaic drafting device), but mostly that's about right. This is an airfoil of convenience that I made up, not plotable, and without analytical calculations or empirical performance data (polars). The front upper curve of the nominal airfoil is a quarter ellipse, approximately vertical and tangent to the leading edge tube at the front, and horizontal at the high point.

18. Where's "horse canyon"?

Answer: The Horse Canyon flying site is east of San Diego on Interstate 8, just east of the Buckman Springs Rest Stop. For more info., contact the local hang gliding shop or club. I fly there on some weekends (in season), usually on Sunday, but I don't schedule anything much in advance (the Website is the show).


19. "I can not fine the part Strut Junction Channel (W11) 1.25" x 1.25 x .125" Chan., 3" in drawing no. G1W11. Can you conform it the part in this drawing."

Answer: The part is there, but it needs a label and reference (I'll update the drawing). The strut junction (or hinge) part is that thing on the outboard end of the strut.


20. "I was wondering if you please could send me all the possible information about how to build one. Something like a manual with steps."

Answer: I regret that here is no complete recipe for building an airchair that I know of. This kind of aircraft building is too new, too small in scale, and changing too fast for good popular presentation. I have only the drawings to offer at this time.

There are some additional sources of information: The fabric covering process does have a manual available (Polyfiber manual, from them or a vendor) which is a step by step guide. Other builders have websites with information, and specific questions can also be posted to the Airchair discussion group. I wish someone would establish a website for collecting methods of construction for airchairs, then I could just link to it.

There should be a manual with steps, for operations (flying, maintenance, transport, etc.) as well as construction, for the Goat as well as other gliders, and I have considered writing one, but this is unlikely to happen because it's too big a job for me while I am still working on new designs.

21. "Seated and found the rudder pedals canted too far forward..."

Answer: This question is about getting ready to fly and then finding the rudder lines had stretched, presumably due to a previous hard landing (or thermal expansion, or something). The Spectron 12 line is well known for taking a permanent elongation after a big load (it is sometimes sold "pre-stretched"). This does not involve any loss of strength. On gliders I have had no trouble with it. I have had to adjust some lines to take slack out, on occasion, but this goes away after a while. The steel cable is the same, to a lesser extent. All lines on my gliders, steel and Spectron, are rigged to allow slack to be taken out periodically, although most lines on existing gliders have not shown significant slackening and therefore have not been adjusted.


22. "...I'm not an experienced hang glider pilot I'm not familiar with the use of the "quick Assembly pins" as a primary connecting device...From the plans it looks like the rapid assembly pins are shoved in place then held in place by a rubber band. It looks like they are used for the actual pinning of the structural pieces during flight and not just a temporary alignment pin to be used then pulled out and a bolt with nut inserted. Correct?"

Answer: (First, it's "drawings", not "plans".)

My quick assembly pins are not found on hang gliders, they are my own invention. The design goals: fast assembly, easy to check during the preflight, and no detachment from the airframe. The current version (a very recent update to the Goat drawings) consists of an aircraft bolt tapered at the insertion end, with a handle on the other end, to assist in parts alignment during assembly (as a drift pin) and then to remain in place as a structural fastener in shear. The (Goat2) pin is retained in place by two robust elastic loops (two for redundancy in case one fails). These devices are still developing but I am satisfied with my progress up to now.

23. "1. Did you stop making the hang gliders because they were not as sucsesful as you hoped or prehaps you felt it was better to develope the design by enclosing the pilot in a protective cockpit ? 2. I want to build a foot launch biplane similar to your Bugs and i wondered why for simplicity of construction you did not use the same chord of wing for both top and bottom wings ? was it just for improved visability ?"

Answer: This question is mainly about my experimental hang gliders (see "X-Gliders" page). I don't hang glide anymore because (true confession) I got older and was less bold and less athletic, weaker and less able or willing to tolerate physical stress and injury. Also, I developed new interests in other ultralight soaring possibilities, and crash protection remains among my major pursuits. The great majority of ultralight soaring injuries and fatalities can be eliminated by the design and general acceptance of aircraft that protect the pilot from impact. Ultimately I think we could create a new branch of aviation which would be entirely out of the high risk category, flying with hazards comparable to those of riding bicycles.

My Bug biplanes are really sesquiplanes (one and a half plane wings) for structural simplicity and compactness. I can think of no compelling reason why biplane upper and lower wings should have the same chord, airfoil, or angle of attack (and they often don't). A good reason for not matching the upper and lower wings is so one of them will stall before the other, which results in a milder stall than when they both stall together. Staggering the wings and setting the forward wing to a slightly higher angle of attack is one way of trying to establish this forgiving stall characteristic, although it seems to me that wing stagger is more often set by considerations of visibility or cockpit access rather than aerodynamics.

24. "- if you strap yourself into the BUG cockpit and for example have the tail on the ground; can you lift the tail without aid? That is, just by balancing, without a helper or a breeze that will lift the stablizer? With the BUG4 beak prototype I have noticed the same effect, but also with regard to rolling the wings: once the movement starts it is hard to stop it because the pilot's weight will shift to the wrong side of the wheel and aggravate the effect....."

Answer: If the tail goes down and there is no wind, you are in fact, stuck unless a firm breeze returns or you get out or get assistance to push the nose down. This is not a problem in normal operation, since the only time you let the tail go down is for a stylish landing, and then you are going to get out anyway.

Statically, the airchairs are indeed unstable in roll and they do tend to put a wing down when they are not moving (I presume the "movement" mentioned in the question is sideways tipping, not forward rolling). In any light breeze the ailerons will immediately control this, or you can keep your feet on the ground to keep the wings level until the breeze cycles up. Once you start rolling (moving forward) and get some airspeed, these minor static problems go away and you confront the real problem: the need for smooth, simultaneous coordination of a three axis control system.

It mighy be useful for training to have side by side wheels to keep the wings level until actual flight speed is reached, so the novice does not have to deal with the roll control until good airspeed is attained. Is this what that LAK primary is doing? I haven't heard about or tried anything like this yet.

25. "... which cad software you used ... and what dimension scale you used. Also I would like to know what length of tow rope you use when truck towing..."

Answer: My computer assisted design (CAD) software is Autodesk Autosketch 2.1, circa 1996, buggy and obsolescent, but I like it more than the later Autosketch versions or any of the current programs that I have tried. It allows me to draw lines quickly and simply, and is otherwise adequate. In general I avoid using any scale other than full scale, but scale can always be determined just by measuring the length of a specified distance (i.e., a part specified as one inch long, which, when measured on the drawing, is one half inch long, has been drawn at half scale).

For airchair static line towing I have used lines that are hundreds of feet long, but I don't know their actual length. I think I have reached tow angles of about 45 degrees up or a little more, so I have achieved altitudes of about 3/4 of the length of the line. It takes continuous coordination between the driver and pilot to keep the line tension comfortable on a simple rope tow. An underpowered tow will not allow the glider to climb (airspeed is low and any attempt to pull up to a higher angle will slow the glider down too much, into the semi-stalled wallowing mode), whereas an over pulling line will force the pilot to drop the nose to lower the excessive airspeed (a temporary measure, just slackening the line to buy a little time for the tow vehicle to slow down). If the pull is not quickly reduced with the nose down the airspeed will still be excessive and the pilot will have to release. There are line tension measuring instruments and active "constant tension" equipment which can be used to hold a proper and comfortable line tension through the varying conditions of the ground tow. Most of my ground tows have been by payout winch, which is a good constant tension system when it is working right.

I suspect that the simplest thing to do for easy rope towing is to limit the motor power. Something like a motor scooter using full power on every tow might be about right. The Bug has been launched by a stationary scooter (scooter winch) with good results.

For more on ultralight towing, see the book "Towing Aloft" by Dennis Pagen, Sport Aviation Publications, 1998.

26. "...about the "6061 T6" aluminum, Could I use aluminum "6063 T5" to build the Goat? ... second question is: Can I build the wing ribs in other material like plywood, or policarbonato sheets? what material do I need exactly to cover the wing? Dacron, polyester? is it the same material that is use in the Wind Surf wings? "

Answer: Comments on material substitutions will not be made by me, since I will only describe what I have done. I will make general statements, such as to point out that if weaker materials are used, a greater weight will be required to achieve the same strength, if it can be achieved at all.

The fabric covering material is specified on the Goat Materials and Parts List. It is polyester fabric, a commercial name for which is Dacron (see questions16 & 20 above), and it is used for sail cloth when manufactured with a flexible resin applied that reduces permeability.



Can you be more specific about what you want? Have you looked at all the pages and drawings? I have posted nearly all the information I have, so be sure to have a complete look. If performance measurements are what you are looking for, they have mostly not been made, but a lot of comments and comparisons can be found in the text.

 28. " 1) Can you or give another world for "cabane"? 2) Is hardware (screws) stainless? 3) Talking about bolts, what is meaning letters as AN, A, B....and numbers as 3, 4, 42.....? 4) En G1A10, is fabric really heatshrunck before recovered with polybrush ? 5) I never used "polyfiber ". On web, I found product catalogue. How can I get "polyfiber process manual" ? 6) En G1W1, to nullify gap between the tubes, they are wrapped with vinyl tape ; is it only at ends or all along tube ? 7) For more facility, which order would you recommend for Goat construction? 8) You write me "I am told there is an article in French about Bug4 in a French soaring magazine: Vol a Voile, July August issue (No. 117) . This is the only French source I know of. "I have looked for this article and never found it, so I think there is a mistake. Do you have others informations ?"


The cabane is a strut structure that holds the wing to the fuselage in some designs. I use "cabane" to refer to the inboard folding assembly of struts connecting the wing to the nose section, the "seat back" for the Bug4.

American aircraft bolts are specified as AN or MS bolts. The bolts I use are strong steel, cadmium plated, not stainless. I do not know how to translate this into a European equivalent, which would be metric, I assume. Use what the hang gliders use.

At the website, the page "Information and Samples" has the process manual, a video, and a sample kit, the works! And, at the page "Technical Questions", there are specific discussions about covering ultralights, exactly relevant to airchairs.

Yes, most of the fabric shrinking is done before sealing, but some shrinking can also be done after sealing, since the fabric shrinks in response to heat rather than chemicals.

Sleeves should be tape padded at least at the ends and middle, and at additional points for long sleeves (perhaps at every five or six diameters). This is a good question, I will include some of this in the Goat2 drawings, which I am working on now (Jan. '05).

Most homebuilders, myself included, start with the rudder, it's not too critical.

The french publication reference came to me second hand, so I cannot verify it. Other than the publications mentioned on my website, I have no further sources of information to offer at this time.

29. "...perhaps you could comment ...on the design issues going from a strutt-braced wing to a cable braced wings. " (Jan. '05)

Answer: Cable braced ultralights are rugged and light, but usually are assumed to pay a drag penalty relative to designs with strut braced wings. Cables are unfaired, and those cables on top of the wing are causing flow problems in the worst possible place, but experience with cable braced designs has shown them to be a practical alternative when speed is not the issue.

Struts below the wing must sustain downward (negative) loads in compression (positive loads, in tension, are usually not critical for tubes), so the open span lengths must be minimized for strength. Thus on Goat1 the nose section (base tube) is wide, the dihedral angle is low, and the outer cantilever portion of the span is large, all as a result of keeping the struts short. For Goat2, with its cables, the nose section can be narrow (for easier transport), the dihedral can be greater (better ground clearance and stability, yaw/roll coupling, etc.), and the cantilever outboard span can be short (the internal sleeve tubes can be much shorter and lighter).

Overall the weight of the heaviest "pick up" part, the wing half, has been reduced by a half dozen pounds or so, which is quite important when dealing with the extreme of what a tired pilot can pick up and put on a car in adverse conditions.

The reduced wing bulk resulting from cable rigging (no folding struts in the stack up) may allow two Goats to be piled on the same vehicle, so airchair pilots can fly cross country with shared resources, as hang glider pilots do. Also, it's a great relief in ground handling not to have to worry so much about where some helper may grab hold and push, since the cables are not nearly as vulnerable to mishandling damage as the tube struts and booms were (especially the long lower tail boom, which is now replaced by a cable).

30. "I am a big guy 6'4" 220lbs....asside from possibly moving the pilot rearward a bit do you think the airframe would be strong enough in its standard configuration? Would the goat1 be a better choice since it has the added strength of the struts? Or possibly, would the heavier pilot be better in the goat2 since it is lighter and will offset some of the added pilot weight and maybe keep the wingloading down?" (Jan. '05)

Answer: My ideas about heavy pilots have not changed much (see Question 3 above). You need a designer who cares enough to develop a special machine.


31. "I am planning a fast trip ... my idea is to try to see theGoat (1 or 2) in action...Please let me know if there is a chance to visit you..." " But I'm realy tinking in build one,I just want to have a chance to see it closely and esc you some questions about your design....I can drive to you,just let me now your aveabelety." (Aug. '06)

Answer: As much as I approve of and wish to encourage your interest and enthusiasm, I have to duck these visits. It's too time consuming to try to do any demonstrations, and the weather cannot be predicted in advance. Unfortunately there is not enough regular airchair flying right now in San Diego to have a good place to go to see them, but this may soon change if a couple of gliders get based at Torrey Pines or start coming out to Horse Canyon. When some regular activity starts to appear I'll certainly point it out on my Update Page and elsewhere.

Visits to my shop I must also discourage, since it would take up the time I need for new designs and it's not a good way to communicate. I want questions sent to me by email so I can post them and answer them for everybody, which I think is far more productive than me talking to people one at a time and often repeating things. The big show is the Experimental Soaring Society Western Workshop, where you can see one (on the ground), in Tehachapi , California, on Labor Day weekend.

32. "In your aircraft, how is it that you determined the required size of the horizontal stab?

...Did you find a value for this coefficient (airfoil's moment) for your airfoil?"

Answer: My horizontal tail plane sizing (stabilizer plus elevator) is primarily a matter of continuing to use the same size tail planes repeatedly because I have been pleased with them. Placement and size comparisons with other aircraft are performed by comparisons of calculated "horizontal tail volume", which is the tail plane area (as a fraction of the total wing area) multiplied by the tail moment arm (the distance between the main wing quarter chord and the tail plane quarter chord, expressed as a multiple of the mean wing chord). This number represents a basic measure of tail power for any ordinary cruciform aircraft layout. (This is called a "volume" because it is basically just a fancy version of the tail area times the moment arm, which is in units of inches cubed, analogous to volume.)[See "Aerodynamics, Aeronautics, and Flight Mechanics" by Barnes McCormick.]

I have no information or calculations on the moment coefficients of my (rib) airfoil at any flow condition or angle of attack. My assumption is that moments will be small and overcome by elevator deflection and pendulum effect (low center of mass), and of course my initial flight tests are done at low altitude, skimming down a training hill, in case there is a problem.

I try to place the tail plane at zero angle of attack at best glide. The 3.5 degree downward set of the tail plane leading edge (relative to airframe level reference, see drawings) is intended to account for both glide angle and main wing down wash , assuming a best glide of about ten to one with the airframe level.

33. "What are seen as major hurdles of conversion from 3 axis to 2 axis control in airchair. Is pilot strength/endurance big obstacle..."

Answer: A 2 axis control system would be a great thing for an airchair, and the foot launch gliders have already used them. For example, the original Super Floater was a 2 axis machine (elevator & rudder). For wheel launching, two side by side wheels would probably be a good setup, like the Sky Pup airplane. I think that two side by side wheels would hold the wing level until flight speed could be reached. I assume this is how the Sky Pup launches.

Trying to wheel launch a 2 axis glider with just one wheel doesn't work, I tried it with Bug3. The problem is you have to steer down the hill with the rudder, and if the wing starts to tilt while the wheel is still on the ground, there isn't much you can do about it (you can't go swerving across the hillside trying to keep the wing level), the wing just rolls until the tip is in the brush , causing a groundloop.

I can't think of any reason why pilot strength or endurance would matter at all.

34. "I have consulted your drawings of Goat 2 and all your update pages. I plane to make an airchair. I waver between Goat 1 or 2. To help my choise, can you answer theese questions: What are the differences between Goat 1 and 2 ?

- about stability (easiness of control)

- about maneuverability

- about maxi and mini horizontal speed, minimum failing speed and glide ratio

- about stall and easiness for short landing

what do you think about addind flaps controled by thepilot ?

In short, do you advise to build Goat 1 or Goat 2 ?

Answers: (Sept. 05) Thank you for telling me up front that you have read all the available materials. This makes answering much easier, since I know you already know what is on the website.

Goat1 or Goat2? Only Goat2 for me because the Goat1 wing/strut panel is so heavy it wears me out to lift it (a lighter wing panel, for easier transport, was the basic idea behind Goat2). Just a few pounds less makes all the difference when you are at your limit.

The only difference in stability would come from the increased dihedral angle of the Goat2 wing, presumably an improvement.

In the air, I don't feel any great difference between Goat1 and Goat2, but Goat1 probably glides better. Speeds and maneuvering seem the same to me, also performance, but the Goat1 is quieter, indicating less cable drag and probably a higher glide ratio. Did you notice that Goat2 has a sharper main wing trailing edge than Goat1 (smaller tube)? This is good for performance but the cable drag probably dominates the issue.

See Question 6 above for flap answer (no change).

Short field landings are the same, both gliders have good skid braking, can be slipped, and can use drogue chutes. If you really need to stop short during a ground roll, either Goat1 ot Goat2 could jam its skid into the ground and rock up on the nose, or even turn over upside down with little damage (this has been done!). I don't know of any other aircraft that can recommend this procedure.

The Goat2 cables are robust and will not be harmed by bad ground handling, whereas the struts on Goat1 may be vulnerable (not an in-flight problem, and we have not really seen the problem yet, after three seasons of flight).

Assembly and transport are similar, although Goat2 is less bulky. The Goat2 assembly is a little more complex and fussy due to all the cables, but the wing structure is probably stronger.

If I wanted to put a sailplane airfoil on the wing, the cable braced wing would allow it but the folding struts require a flat wing bottom.

The differences between these two gliders are small and favoring one over the other will have to be a matter of personal preference.

35. "I was wondering why you didn't design the goat with a fuselage structure like Bill's [Bill Spencer, Compact 110]. The Superfloater is another one that comes to mind. It would be just as simple to build and it would have eliminated the fore and aft wires that keep everything together. I realise of course, this would require mounting the glider on a trailer such as a small modified boat trailer with a box tray, rather than carrying it on roof racks. Is a modification possible for this suggestion or not? Have you considered this option? It seems to me that rigging and de-rigging would be easier and quicker if the fuselage was in one piece."

Answer: (Oct.. 05) Basically you are right about all that. The choice in favor of the sweep cables as opposed to more tail structure comes from the Goat design priorities, which favor car top transport and pilot crash protection. In an impact situation or very rough landing, the forward seeep cables keep the nose rigid and protect the pilot from ground obstacles (very useful for bush top landings, of which there have already been a couple).

Some fliers would do well with a trailer, and light parts might indeed allow a fast setup. It would make sense then to have the conventional sailplane setup, a monolithic fuselage from the nose to the rudder. I don't want to enclose the pilot, but a faired fuselage with at least the sides left open might be a good option. I would like to see something like a ULF but with the pilot completely under the unswept cantilever wing, faired by a full bodied aft fuselage (so a large vertical stabilizer won't be needed). The main issue of ultralight sailplane design is how to get the pilot in the right place, and I think the answer is to put him completely under an unswept wing.

36. “...I have never piloted an aircraft ...I plan to learn slow and just hill hop around until I get this something even novices can learn to do on their own? And where you ever licensed before starting to fly or did you gradually learn on your own? ..any...advice?” (Dec. 05)

Answer: The Wright brothers and some of the early hang glider pilots were self taught by careful skimming down small hills in good wind conditions, and I suppose you could do the same if you wanted to. It would be fun, and a Goat-like airchair would provide good crash protection at slow flying speeds when things went wrong. There are alternatives, however, which would probably result in quicker learning and less risk of injury or damage to the glider. Consider:

1. Find a pilot, get him to go out to the hill and fly or at least advise. (A demonstration flight will give you confidence that the glider is okay.)

2. Take some flying lessons, with emphasis on stick and rudder control.

3. Take some hang gliding or paragliding lessons to get the sensation of flight at airchair speeds, learn the importance of wind, ground handling, preflight inspection, etc. Hang gliding is really the best preparation for airchair flying, and when you soar you will be flying with hang glider pilots, so if you think the risks are acceptable this is a good approach.

4. Find a hang glider or sailplane pilot who flies cross country and offer to drive for him or her. This will be an immense education at no expense.

You could have fun with your friends skimming down hillsides for as long as you want. In the early years of hang gliding there were many of us who did not fly high, we just wanted to get off the ground and for us hillside flying was safe and satisfying. We had a magazine called “Ground Skimmer” and the title reflected the idea that high flying was not yet safe enough to be considered a sport. I was an experienced sailplane pilot when I taught myself to hang glide, but even then I wiped out a lot of aluminum tubing and became quite skilled at absorbing impact. Today standardized hang gliding lessons are available and those would be better.

37. “We are up to the point of rigging our glider (Goat2-like) but I haven’t been able to find any information regard control surface deflections. Could you ... tell where I may be missing it in the plans. Particularly I’m interested in the up and down elevator deflections? Additionally can you provide up with some direction on what a good CG location for the first flight should be relative to the wheel?" (Jan. 06)

Answer: There are some elevator deflection numbers given on the bottom of drawing G2T8 (-39 deg., + 45 deg.), but these are just nominal. These are about what I would look for from any control surface. I probably would have accepted 30 degrees for the downward elevator or downward aileron deflections. There are some issues in getting a good downward elevator deflection from a folding horizopntal tail, and some re-design is in order, eventually.

The center of gravity for every flight is determined by the specification "slightly nose heavy when level", balancing on the wheel when fully loaded on the ground prior to flight. This is a safety measure intended to asist in spin recovery, which might be inhibited by a center of gravity too far aft. (I don't know the exact center of gravity location for any of the flights that have been made in any glider.) On a low test flight this won't make much difference (in fact, aircraft notoriously fly more efficiently when the c.g. is too far aft, because the tail is contributing lift instead of just drag).

38. From Airchair Group Website: what items on Goat2 (cable braced monoplane wing) might have application on a Glider like Goat1 (strut braced monoplane wing)? (Jan. '06)

Answer: As to which new Goat2 (cable braced wing) items might well be applied to airchairs similar to Goat1 (strut braced wing), the following may be of interest.

The ideas listed here were first used on Goat2 (a monoplane airchair with a cable braced wing) but I think they would have worked well had they been adapted for use by Goat1, and I intend to use them for Goat3, which will be a strut braced airchair similar to Goat1.

1. Monolithic nose and tail tube center fixtures

On G2W5 note the “wing center fixture”, which combines the function of attaching the compression struts to the spars with the assembly of the nose and upper tail tubes to the wing. One Goat2 part replaces two Goat1 parts, and is stiffer and stronger.

2. Small snaphook/eyebolt tail attachments

Goat2 uses small swivel snaphooks to attach the tailplane to three eyebolts (see G2T14, G2T17). This is quick and working well so far.

3. Simplified Quick Pins and tensioners

See G2A5 for the new quick pin retainers that don’t use the former system of locking with an “O” ring (G1A5). The “O” ring deteriorated quickly in service and was deemed unnecessary when the elastic loops were made stronger.

4. Smaller main wheel

The Goat2 main wheel/tire (see G2N3) is 14 in. in diameter, as opposed to the 16 in. diameter of Goat1 (i.e., the new setup uses the next size down wheelbarrow wheel, or an ultralight wheel). Runway operation allows use of the small tire, whereas bushwhacking or novice training favors the large tire. I have destroyed several wheels by landing in rough spots, but the ultralight wheel I am now using is tough and the smaller size is my choice for the moment. A small tire might need a little more wind or a little more roll distance on a rolling launch from a rough slope.

5. Sharper trailing edge for main wing

For a lifting body, a sharp trailing edge is the most efficient. Note on G2S2 & G2S3 that the wing panel trailing edge tubes are 1/4 in. diameter, as opposed to 3/8 in. diameter for Goat1.

6. Stiffer rudder pedals

The composite rudder pedals of G2N7 are stiffer than the old version and give better control feel when using the rudder.

7. Upgrades to fabric covering method

Spiral wrapping of selected tubes with fabric tape allows strong fabric attachment, and on the top of the main wing, extra chordwise slack prior to fabric shrinking reduces the finished chordwise fabric tension, reducing bowing between the ribs (see G2S14 & G2N15).

8. Simplified nose truss structure

Goat2 uses fewer and larger nose structural tubes to simplify the truss structure (G2N3).

9. Simplified leading edge shell for main wing

Goat2 uses a shallower nose shell, one block of Styrofoam deep instead of two, just for better return on fabrication effort (G2S9).

10. New rudder shape & structure

The rounded planform of the Goat2 rudder (G2T1) makes it simpler to shape and more robust, resistant to warping from fabric tension or ground handling accidents.

11. New control line standards

As per G2A9 the control line standards have been simplified. Steel thimbles are no longer used for non-steel lines, and marine deadeyes (specialized marine hardware) are no longer used.

12. Wing tip wheel for ground handling

For one person moving the glider on the ground from the tail, a wing tip wheel allows the glider to be moved easily (G2S9). On Goat3 I may use either a wheel or a rounded skid/handle.

13. Airfoil with shallower ramp

The Goat2 airfoil (G2S8) is still an airfoil of convenience built around a ladder frame, but the forward ramp profile has been lowered to be more in accordance with well known, aerodynamically developed airfoils.

14. New tow release plate

A new tow release plate (see G2N12) replaces the figure 8 tang used before in order to allow a hole to be drilled which can be used to retain the elastic loop wound around the plate (used to retain the tow release pin when there is no line tension). When there was no hole to use to retain the elastic loop it would sometimes come off and be lost.

15. Simlper, lighter seat belts

The Goat2 uses a punch button automotive seat belt (with added shoulder straps), standard modern car "after market" equipment, which is a good airchair setup.

Afternote: After three years of operation, there has been no reason to modify or revise anything on Goat1, with the exception of proposals for dealing with a small bend in the base tube caused by forward jolts from the lower tail boom tube. This bend is small and no action has yet been taken, but substituting a full length sleeve in place of the base tube end sleeves has been discussed (see G1N3). The real problem is that the diagonal braces are not anchored to anything stiff, and are not adequate to support the base tube against loads from the tail tube attachment eyebolt (“centerline eyebolt” of G1N3). The proposed sleeving might be enough to fix the problem, but a more elegant fix would be to extend each brace tube farther forward and anchor it to a vertex of the nose truss (near the outboard end of the forward seat tube) instead of just being attached to the middle of an element. The Goat3 nose, under construction, proposes to attach the lower tail boom to a rear extension of the nose truss, a new structural hardpoint at the extreme rear of the nose structure, which will eliminate the bending load on the base tube and should have the additional benefit of shortening the length of the lower tail boom.

39. "Have you ever had anyone assist in a hill or cliff launch by pushing somewhere on the structure to get you to flying speed with a shorter ground run?" (Jan. '06)

Answer: Yes, there was a lot of this early on, but it didnn't seem that pulling or pushing added much energy, and it interfered with the pilot's control of the glider, so in the end it was better just to roll freely ratther than to wear the helpers out or injure them from tumbling down the hill. Waiting at launch and then rolling into the air usually requires no assistance, and it's better not to use any help. Sometimes, while waiting in in a shifty cross wind, in a monoplane, it may be good to have a helper standing by to re-level the wing if it gets to the ground, but this is not vital.


40. "(I) would like to know if you knew one of your product having been bought in France? And has an entreprise recently shown interest in commercailising your Bug, Goat, Hillhopper?....will it be possible to perform a test flight in oder to write a test flight article for (a specific French freeflight magazine)?" (Feb. '06)


Answer: I would love to hear about an airchair in France. I have visited some French hang gliding sites, they would be wonderful places to fly, but I don't know of any actual airchairs there.

I don't have any product, I'm just a home builder, and there is no confirmed, current commercial airchair product anywhere that I know of, neither aircraft nor kits. There has been interest in airchairs in Europe, but no activity that I know of except a few homebuilts in progress (see Yahoo Airchair discussion site). I favor a commercially available glider so pilots can buy who don't want to build, and also to provide a well engineered, well tested baseline design, but I have no personal interest in commercial design. I think I can be most effective just by flying and designing homebuilts, then providing the descriptive drawings.

The big photo images in my gallery are provided so magazines (and anyone else) can use them, so please do! I regret that there is no flight test data, just the subjective impressions provided on the Webpages.


41. "Where does the (drogue) chute attach on the Goat2?" (Feb. '06)

Answer: On all the Bug and Goat gliders the drogue chute is mounted on the flying cables (or main struts) on the left side, about 27 inches off the aircraft centerline. It is secured so that the attachment point cannot slide farther inboard or outboard, as this might cause problems. I can feel it bumping around in flight, but this is not a major consideration, since the drogue chute is the only elegant means of landing on a considerably down sloping landing area. The video clip taken from the pilot seat of my landing in Goat2 (see Video Page) was made with the drogue chute deployed.


42. "Do you have a "condensed" list for ordering the tubes? Not as each single piece needed, but how many full lenght tubes from each size to order, to later here cut them out to make all the parts for the glider... " (Feb. '06)

Answer: Sorry, but there is no "Bill of Materials" from which the needed quantities of tubing or any other needed item can be directly ordered. The Materials Parts List shows everything needed, and lists sources, but there is no summary or grouping into commercially available quantities. This is mostly just an accounting job, and anybody who wanted to do it could make up a spread sheet for this (and I wish they would!). I have not done it because it is boring, and by the time I finish a materials list I am well into next year's design, and anxious to get on to new things.


43. "I have analysed the goat 2 wing, using CFD methods used in the industry and the university wind tunnel and have obtained performance results, such as CL, CD, CM, glide ratio, etc.. .By evaluating these results it might be possible to optimize the wing design..any furthur information (?). (March '06)

Answer: A serious analysis and optimization of airchair airfoils is certainly an effort I wish to support. I’ll try to make some of the comments you asked for.

My airfoils are “airfoils of convenience”, designed for simple construction around a tubular ladder frame with a folding trailing edge panel. Generally they are 12% thick at about 26% chord, the high point being far forward just to fit the shape onto the frame. I try to place my wheel at the center of lift position, which I usually assume to be at 29% chord. All the wing fabric surfaces are flat except the top of the main wing panel, which is shaped by the ribs. The leading and trailing edges are circular (tubes). The ramp (on top of the wing, in front of the high point up to the leading edge tube) used to be a quarter ellipse (Goat1), but now I use an approximate average between that and a circular curve horizontal and tangent at the high point and tangent to the upper front of the leading edge tube. Behind the high point the shape of the top surface is circular back about half way to the rear spar, then straight.

The trailing edge panels, ailerons and the inboard panels (which fly in a fixed position), are intended to be aligned (ailerons in neutral position) so as to create no change in the slope of the mean camber line where it joins the main wing panel. The things that are easy to change are the shape of the rib and the neutral position of the rear panels.

An airchair is for slow, easy flying, like an airborne garden chair. I don’t have much interest in the high speed glide, but I would like to see high lift coupled with a low sink rate at the low speed end. With an inherently high drag from having the pilot out in the open, we must slow down and turn tight in order to stay up in light conditions (I call this “Plan B” soaring, as practiced by paragliders). The wing loading has already been minimized (the Goat already has the largest transportable wing panel built as light as readily possible). A broader soaring window (staying up in even lighter conditions) could best be achieved by getting more lift out of an efficient airfoil using the same wing planform.

The airfoil pattern provided is for the rib airfoils, which have fabric dips between them on the top main surface, so the average airfoil is a little inside the nominal profile in that area. I also observe a slight undercamber in flight on the flat undersurfaces of the wing (see the Gallery Page photos). The maximum lift angle of attack of the airfoil might be a useful number. It might be desirable to set the tailskid height to limit the rotation of the glider on the ground to the maximum lift angle. This would lower the skid from its current position, and might serve to prevent a nervous pilot from over rotating on takeoff.


44. "Was there anything that led you to use a "modified" FX63-120 foil on your G3?...Are you not concerned that the change of the le radius incorporated and difficulty in building such a thin cambered te could significantly change the foils characteristics especially the stall?" (April '06)

Answer: I chose the Wortmann 63 because it has been extensively used and tested with good results. Many recreational soaring aircraft use airfoils from this family, including the ULF and Schweizer 1-34. The 12% thickness (-120) was selected to be thick enough for good performance but thin enough for wing stacking on top of a car. I always have concerns that this or any other airfoil may have some adverse stall characteristics, so I will proceed carefully during prototype testing.

So far, none of the airchairs, Bug or Goat, have demonstrated a very dramatic response to an unaccelerated stall. If you slowly bring the stick to the full back position and hold it there, the glider will just wallow, rumble, and sink, but will not do much of a nose drop (depending on the weight of the pilot), and it will still respond to aileron and rudder control. Since all of the airchair control surfaces are sized to operate while sitting on the ground in a strong breeze, it is a pleasant surprise that the stabilizer/elevator is aparently not big enough to induce unaccelerated stall problems at these low wing loadings.

45. "It appears that the G3 airfoil has a concave lower surface. How will you keep the fabric from separating from the lower surface of the ribs?" (April '06)

Answer: Rib lacing is the conventional method of attaching the fabric to the ribs, both for inward and outward curvature, and that is what I will use in the undercambered areas (see Polyfiber website for online information, instruction manual, fabric training videos, sample fabric practice kit, etc.). My initial fabric attachment will employ spanwise tension with a lot of chordwise slack (as mentioned on the appropriate G2 drawing), so the fabric after shrinking will have more tension spanwise than chordwise, enhancing conformity to the rib contours. Once again, this fancy airfoil is generating a lot of extra work, but those thin, curving trailing edges should look elegant when finished.


46. " the tail structure on goat 1 and goat 3 going to be the same?? if they are then i can continue to get things together to start construction of the tail...." Another writer: " there anything largely different on the goat 3 that you feel is different enough (from Goat1) that I should wait on starting to build to see how the goat 3 turns out?" (April '06)

Answer: You can wait all you want, but I'm not going to encourage it. Goat3 details are not available because it has not flown, is not a confirmed success, and so the final configuration has not been determined. I'm gratified that Goat3 is percieved as an object of desire, but it will be many months before drawings are available, if ever.


 47. "I hope that you'll approve my membership on your yahoo group soon so I can see more. I know that you answered someone about putting an engine on one of your gliders and you said you aren't necessarily interested in flying a motorized craft, but I'm curious if you've ever known anyone to try. I was actually thinking about trying to build one with a very small electric motor just to help self-launch since I'm in Florida and don't have very many hills..." (May '06)

Answer: Does Florida need hills? It has airparks, like Wallaby Ranch, Quest, etc.

(I am not the moderator of the Yahoo Airchair group, so I can't help you, but I suppose he/she will approve you as long as you don't seem to be an advertiser.)

Only the Lady Bug project has been planning an engine on an airchair, and this has not flown as far as I know.

Mini-trikes and powered hang glider harness setups are essentially the same as a powered airchair, since they have the same wing loading and minimal engine power. The ethic of those who fly these craft is to take off from a local field (slope or level), putter to the nearest thermal, and climb with the engine off, using the least fuel of any type of flatland recreational soaring launch. I like this idea a lot. It can be done almost anywhere, it minimizes fuel use, and reduces driving distances. I would like to fly what I call a "motor floater", a very light and lightly loaded soarable airplane with the pilot seated out in the open, essentially a simplified airchair with a small engine. This would probably fly a lot like a mini-trike, but it might differ in having greater stability, lighter control forces, and superior crash safety. It might also be home built with a reasonable amount of satisfaction.

My airframe ideas for a motor floater center around the Hillhopper prototype. If I build something like this I will probably start by building it as a glider, to get it flying early on.



48. " Have you destructively tested your wing (i.e. turned it upside down on a rig and loaded it with sand bags until failure)? ... I was just going to build a wing with no ribs or covering, put a piece of plywood on top and symmetrically load the wing. Since the wing strength is not dictated by ribs or covering I do not see a need to add them. Do you concur with this idea or do you think I should build a complete wing with ribs, covering and all for a valid strength test?" (Aug. '06)

Answer: Really, there have been no tests and no data has been withheld. Your idea of a load test is a good one, though, and I wish you or someone else would do it.

You will have to choose a "standard" design and let that represent all the rest (every home built airchair may have a slightly different structure). Your plan is okay, just shovel a known weight of sand onto the upside down wing with a realistic distribution (spanwise: Schenk, chord wise: center at 25-30% chord ). A big load of sand on each wing/strut (or even just one, the setup does not have to be symmetrical if you have a strong test stand) representing a big positive load will lend confidence to the pilots, even though each may have a different idea of what it represents. The real prize would be a dramatic, unexpected failure of some critical component, indicating an unrecognized weak point that should be corrected.

The sweep cables contribute to the strength of the wing, so if you leave them out an early break will be discounted but a high load break will be all the more encouraging. Bringing the fuselage and pilot's seat into the picture would also add realism. A truck test would add realism because it would allow for load reduction due to wing twisting at high speeds and angles of attack, but like a static test it still lacks realistic reaction forces so it is not perfect.

Suppose each wing reached a load of 850 lbs. during a simple static test. Cables stretched and holes elongated, but that's okay as long as the basic structure held together so as to allow marginal control for a safe landing (i.e., ignore yielding). Using a glider weight of 140 lbs. with a 200 lb. pilot, the gross weight is 340 lbs., but I would discount half of the weight of the aircraft as self supporting wing weight which does not contribute stress to the main load path, so I will use an adjusted gross weight of 340-70 = 270 lbs. My load level is now 1700/270 = 6.3 g's ultimate, which is an acceptable ultimate load to me because it is over 6 g's.


49. "I know FAR Part 103 limits un-powered ultralights to 155lbs, but do you include safety equipment (Reserve Parachute) in that weight? ... Where are oxygen bottles/gps receives/etc. accounted for? If it's strapped on, is it included?" (Aug. '06)

Answer: I don't know the answer to this question, but my guess is that the unpowered ultralight is like the powered one, so the nonessential equipment is not included in the weight limit. I think that in effect this weight limit has served mainly as a guideline, since in the almost 23 years these rules have been in effect I have not heard of a single instance of a glider being weighed as part of a requested and thus required inspection.

I haven't had to deal with the weight limit because my empty weights have been well short of it. I insist on my airchairs having about the same wing loading as a hang glider, and the wing must be small enough to go on the top of my little truck, so the squeeze between those two requirements effectively limits the weight to about 140 lbs. or less, even with the emergency parachute. Another limit is the weight of the heaviest part, which I think should be held down to 40 lbs. for one man assembly. Heavier gliders have to break down into more parts to keep the parts light, and that adds to the operational burden, which argues in favor of keeping the glider as light as possible.

I like and support the weight limit, I think it is a good thing for open air flying and helps prevent the reinvention of the sailplane, which would lead away from the airchair experience. I hope that airchair development will lead toward lighter wing loadings, not heavier ones.

50. "are the drwings for the HillHopper available?...Are your intimations of an engine powered armchair any closer to fruition?" (Sept. '06)

 Answer: ...There are no HillHopper drawings so don't look for these. A motorfloater may be upcoming.

51. " familiar are you with the Super Floater and what are the pros/cons of that design compared to your BUGs/GOATs?"

 Answer: The commercial U.S. Aviation Superfloater (not the original home built, which was quite different) is an airchair, a lot like a Goat, and seems to a well appreciated glider. It uses replaceable (if you can find them) sailcloth wing fabric panels, not the shrink-on cloth. I'm told he assembly is not quick, usually requires two people, and it does not go on top of a car, (uses a trailer). It has a side stick rather than a center stick. The pilot sits low and close to the ground, not having the deep Goat-type nose section (for protection from ground impact and objects). The wheel is small, so rolling take offs from rough ground might be difficult , and again, ground impact protection provided by the tire might not be as good as that of the Goat . (I don't know if any SF has ever made a rolling take off.). These planes are no longer in production but can sometimes be bought used.

The Superfloater fliers are well represented in the Airchair Yahoo discussion group and you can ask them about how they like it. I have not flown one. There is a lot of information on the web about this glider, including video clips.

52. "....concidering building a Goat3. I am limited in space and wanted to ask you what you think would be a mimimum work area for construction."  (Jan. '07)

 Answer: My work shed is 25' by 18', and has no heat or installed electricity. It has always suited my needs for workspace and material storage, as well as accomodationg a finished Bug or Goat. I suppose a minimum would be room for a workbench and to layout the entire wing half horizontally on saw horses, maybe eight by twenty feet.


53. ".... Just a thought that if a BRS is ever contemplated, and the pilot is going to depend on that BRS saving him as well as the plane, that The BRS needs to have a good connection to the pilots harness" (Feb. '07)

 Answer: Yes, the emergency chute is always attached directly to the pilot's harness in ultralight gliders, and the airframe just comes along for the ride. My chutes are all set up this way. My use of the parachute has nothing to do with "saving the plane", the airframe provides additional drag and crash protection. I don't see why "BRS" matters (this is a company name), or do you really mean a rocket deployed chute? I don't see why this would be any different from a hand deployed system as far as attachment to the pilot is concerned.


54. "...I could not find 6061-T6, instead I found 6013. Is it ok to use 6013 is I can find the appropriate sizes? ...Another question: Is it ok to convert the ribs to blue foam+epoxy with carbon and Kevlar uni reinforcement on top and bottom of the ribs..." (March '07)

 Answer: I will not comment on other people's material selections, I'll only continue to describe what I have done for my own purposes. See #13 above for some furthur comments on aluminum tubing.


55. "Did you rib stitch the wing covering, glue it to the ribs, or attach to the ribs in any other way." (April '07)

 Answer: On Goat4 I don't do any ordinary rib stitching or special attachments, except for some attachment of the bottom surface wing fabric to the compression struts (there are no lower surface ribs at all) to reduce the size of the unsupported fabric panels. (This results in a slight undercamber.) I spiral wrap the compression struts with fabric tape, and when the main fabric has been put on over that, I stitch the wing fabric to the strut tape using an ordinary household needle. The final tape job hides the thread in the usual way.

The conventional aircraft rib thread is rather bulky, I don't use it, I use doubled light polyester thread. Upper surface rib stitching has been done in the past, I used a hook needle to thread around the tubular rib. I can't think of any objections to rib stitching, but I don't find it worthwhile on the Goat for the moment.


56. "After I looked for a supplier of 6061-T6 aluminum tube in Brazil, I found only one that is producing this material. So, buying material and building a GOAT here is much more expensive than in USA.." (April '07)

 Answer: I can't really respond to the problems of getting aluminum tubing in various areas of the world, I just don't know how to do it. Contacting the domestic manufacturers of hang gliders and motorized ultra lights might be one way to get an expanded view of the situation, since they use the same or similar materials. Also, addressing these questions to the Airchair forum on the Web might stir up some useful ideas. No.13 above deals with stronger aluminum alloys.

I have a tube supplier here in San Diego, but they say that the tubes I use often come from Canada or Europe, so perhaps ordering from there is not impractical. Some of the bar stock on Goat4 is marked "Made in Brazil".


57. "... I have done some quick simplified hand calculations on your GOAT 4 wing but I conclude that the leading edge spar would yield in bending at around 4.5g mid-way between the root and the flying cable attachment and fail in bening at around 5 g, i.e. well before your indicated 9g ultimate load capability. This assumes rectangular wing loading as opposed to elliptical...." (May '07)

 Answer: Our results are different but for now I have no way to resolve them. I do size for a calculated 9g (based on a 200 lb. pilot and not using the mass of the wings), but I do this in order to be somewhat confident of actually getting 6g, so this is the load I am really counting on.

I presume that the critical load on the inboard spars is a combined load of long column compression (Euler method) and bending in the vertical plane. This critical combined load will be reached long before the load required for breakage in pure bending, so our bending calculations are not in agreement.

For beam bending calculations I use "Beamboy", a freeware program. As a next step, perhaps the values of the writer could be put into that to see what happens.

A sand bag load test is what we really need, but this is a big a job, and I'd rather go flying or build something new.

For span wise air loads I use a (simplified) Schrenk approximation, which is an exact average between an even loading over the area (very close to rectangular, in this case) and a pure elliptical loading. This will give me less overall bending load than you are seeing, but more inboard lift.

My confidence in my airframe comes mainly from comparison to previous similar structures which flew well and established a firm history of safety. The Fledgling hang glider, in particular, flew at a wing loading close to that of an airchair and with a similar tube and cable structure. This glider flew for many years as the favorite of rigid wing hang glider pilots, and was accepted as a standard of strength, especially for aerobatics.

I don't care much about "yield", this is just a matter of the stressed metal being permanently bent, not a loss of strength. I assume I can still control the glider enough with yielded parts to make a safe landing, and that is what matters. I don't care about the possibility of bent parts because presumably this high load will occur only once in a hundred glider lifetimes, anyway, so this bending is an unlikely penalty.

What I truly desire is a program for critical loads of thin wall tubing in a combined loading of long column compression and bending. Right now I'm using the "mix ratio" method for combining the compression and bending loads, but this is not really right because the failure mechanisms of these two loadings are quite different. I suspect that the correct approach will involve combining deflections and comparing to a critical deflection value, rather than looking at stress, but this is beyond me for the moment.


 58. "1. Can the aircraft fly at low altitude ie 50 to 100 feet......2. Is there some website from where i can obtain detailed info on how to cover the wing and what kind of fabrics to use......3. What kind of engineering and mathematical concepts did you use to make your designs." (June 2007)

Answer: For the fabric covering methods, see the Goat Materials Page for fabric specification, then use a search engine to find "Polyfiber aircraft fabric (or something similar)". This website is big and tells all about aircraft fabric covering, offers a manual, sample kit, videos, etc.

For design concepts see the Goat Design page, although there is not much there yet. Regrettably, I would have to write a book to describe my design methods, and I have not had the time. (See also #57)

Now comes the interesting question. Can an airchair fly low? Yes, otherwise how could it fly at all (airdrop)? What does this question mean? You do fly close to terrain when soaring in ridge lift, within 50 feet, sometimes. On a training hill you may not get above half a wing span. Novice training on slopes is problematic and tough on the glider because of the three axis controls, which are difficult to learn all at once, but you can fly low forever and have plenty of fun if you want.


59. "In ... drawings, the cables, at some point terminate with a "Cable Tang" bolted to some part of the airframe. Are these an AN-parts or fabricated from stainless steel? ... with certified airplanes, most bolts are installed with the nuts either on the bottom or toward the aft so they can't fall out in the unlikely event the nut comes off in flight. I noticed several instances, such as the aft end of the control stick assembly, the nuts are on top. Can you comment on this or could the boltsbe inverted?" (July 2007)

Answer: Cable tangs are stainless steel parts used mostly to connect cables to bolts. Tangs are found on both ends of nearly all ultralight and hang glider cables, where they provide a cable attachment strong in tension but without applying any bending loads to the cable (as opposed to a clamped cable end, which can weaken the outer cable strands by fatigue, especially under conditions of vibration, i.e., if you have an engine). Certificated aircraft often use ball swages on cable ends, a fancier way of not bending the cable under load.

Tangs are ordered as ultralight parts, direct from the catalogue, not fabricated by the builder.

The bolt orientation convention of "threads down, inboard, or to the rear" is followed unless there is reason to do otherwise. For instance, the particular nuts you mention are on top to put the bolt heads on the bottom, to maximize clearance between the fasteners and the tire. More often bolt orientations are determined by: "load the heads, not the threads".


60. "I did read your description of the stall characteristics, wallow, rumble, and sink. I equate this with mushy control forces, buffeting, and loss of altitude in light single-engine high-wing aircraft.

You do mention accelerated stalling but not the effect or result. I assume this would also be similar to a whip stall.

Have you ever intentionally or not, encountered an accelerated stall or spin and what was the result in loss of altitude?

In the stalled configuration, is it basically just mushing its way down, and what is the sink rate? If you kick the rudder to initiate a spin, I assume spin recovery is the same, controls neutral, apply opposite rudder to stop rotation, elevator forward to break the stall.

Have you ever ridden your emergency parachute down? If so, what was the rate of sink and the result?" (December 2007)

Answer: Generally, the unaccelerated stall characteristics of an airchair at a wing loading near that of a hang glider ( 1.73 lbs. per square foot or less) are very mild and do not involve extreme loss of control, or sudden drops of the nose or wing tip. There will be "mushy control forces, buffeting, and loss of altitude" with the stick held full back, and often a bounce when you reach the ground (I have often landed in full mush mode while trying to skim as far as possible). Heavier pilots, flying at higher wing loadings, report more "normal" stall characteristics, including a slight nose drop as the result of an unaccellerated stall. My experiments with slightly accelerated stalls yield similar results. There have been no reports or indications of any wing tip stall or sudden drop. I have not tried spin entries or very many banked stalls.

I do not know what a "whip stall" is, and I have no sink rate data. I doubt that rudder use would guarantee a spin entry. There are plenty of aircraft that won't spin at all, especially when nose heavy, and the Goat4 might be one of them. My immediate response to any critical stall or spin indication would be conventional: "stick forward, apply opposite rudder" (i.e., if turning while stalled or spinning, apply rudder opposite to the turn), but I have never had to do this in an airchair.

I have not used my emergency parachute. Note that my parachute specs are on one of the nose section drawings, and this system is intended to be a conventionally sized, ordinary modern hang glider emergency parachute (22 gore, pull down apex, 20' bridle with swivel, hand thrown deployment bag, etc.). Presumably my sink rate using this parachute would be similar to that of a hang glider of comparable gross weight, and therefore acceptable, especially considereing that I am firmly attached to the airframe and hopefully will still have some extended structure to absorb crash energy and isolate me from contact with ground objects.


61. "I am sure you have seen the SG-38, Northrup, Cessna, Grasshopper, Australian, and Ron Sands replica, Primary gliders. These aircraft all have the same lineage of the German SG-38 in their appearance....." (Jan. '08)

Response: Primary gliders have much in common with the Goat and Bug, but at their original (heavy) wing loadings but they may not be satisfying to soar except in the best of conditions, so my interest in the originals is limited.

I see two successful soaring methods currently in use:

Plan A is sailplanes, with low drag and able to slow down moderately (for a low sink rate and tight turning in thermals), and with a good high speed glide (for search range).

Plan B is paragliders, which have a lot of drag but are able to slow way down to achieve a low sink rate and to turn in very small thermals. Looking at site records, we see that paragliders fly about 38% as far as hang gliders on cross country flights. This is pretty good for a type of glider with half the glide performance and a much lower gliding speed than the flex wings and rigid wings.

Hang gliders manage to span the two methods.

The problem with the traditional primary gliders is that they can’t soar well using either method. They have too much drag for Plan A (they have little search range) and are too fast for Plan B (they have a high sink rate and can’t slow down to turn tightly in small thermals). Even the more lightly loaded of the old primaries, like the Cessna primary, have wing loadings heavier than airchairs and hang gliders.


62. "When making radial ends on parts what tools do you use to shape them , I have hard time making them perfectly round can you help with advice ? i have found no concave files on the internet" (Jan. '08)

Answer: I hesitate to correct those who accuse me of doing something perfecrtly, but there are no perfect curves on my gliders, only on the drawings. I round things off with a flat hand file, kind of round, anyway, and that's as far as I go.

That part about the "concave files on the internet" really threw me (this is new, how do I download a concave file?). After a week or two I realized that I was the confused one, it refers to a tool, not a cybernaetic construct. I havn't seen one.


63."Hi Mike how tall are you? I'm 6'2 I want to know how far to move rudder pedals, enough headroom?"

Answer: I'm average height, but taller people have flown my gliders by bending their legs more. There's plenty of toe room, so I suppose the pedals could be moved wherever they're wanted. Headroom has not been a problem yet.

64. "The local sites all seem to be restricted to members of various local clubs requiring [USHPA] membership and hang glider rating. I would be interested in any comments on what folks actually do....any guidance / comment on the issues of access to those who are not hang glider pilots? Sure, I suppose one could go out to the desert... Actually, auto tow sounds like fun if one could find long clear roads to fly while on tow to get some experience in the glider - staying fairly low but using the tow as power while getting used to the controls.

For tow launch, I was wondering how the nose tow connection worked as it seemed that it would interfere with substantial pitch up. Whatever the connection, it looks like the forces during a ground tow launch might be substantial.

Is that a problem with the BUG and GOAT gliders?"

Answer: The club or private rating requirements for non-hang glider or non-paraglider pilots at hang glider sites is a problem for airchairs, since ultralight sailplanes do not have ratings comparable to those of the USHPA hang glider / paraglider system.. One reason for building a motorfloater, like a Pig with an engine for self launch, would be to get away from the hang glider sites and have more places to fly, especially out in the flats. If ultralight sailplanes ever become popular enough to make it worth while to establish a rating system or to promote acceptance for using the existing FAA sailplane ratings, then mountain launches and airparks might become more accessible.

My tow release is closely related to those in common use by paragliders and hang gliders, which are well suited to releasing from high or low angle tows. The Bug and Goat tow release has always worked well. Read the drawings, and you will note that the weak link breaking strength is intended to be no more than about 120% of gross weight.

A Schweizer-type tow hook, which will automatically release if pulled to the rear, might be a good alternative, but there is some question of binding in a strong side pull unless the hook is very stiffly made or allowed to rotate to the side (see LK-10 tow hook).

65. " I am in Canada and so I would like to get it to 99.99 pounds to get within our limits. The Goat has a foam upper surface sheeting from the [wing] leading edge partway back. ... is [there] any structural purpose to the foam surfacing ..." (Oct. '08)

Answer: That Canadian 100 lb. weight limit will require a lot of concessions with regard to performance and durability, if a practical and satisfying airchair can be built that light at all. I suspect that even many hang gliders are over this limit when the harness weight is included as part of the essential flying weight. 

On the Goat, the  leading edge Styrofoam is indeed non-structural with regard to primary air loads, and weighs about one pound per wing (with epoxy sealant). A smooth upper front ramp on the wing is generally considered worth some weight and effort  for aerodynamic reasons. There are some almost modern hang gliders that have a fabric (no stiffener) front ramp, but this is rare.

66    "...could you suggest any kind of software (free or share) to evaluate aerodynamical characteristics of a wing profile & to draw polar curves with a reasonable level of reliability? 2)    Could you put a number for cross-wind ability of your goat gliders? I mean the value of real 90° component." (Oct. 08)

Answer: I don't use any aerodynamic software, but the airchair  forum members could probably recommend something. X-plane is popular.

The cross-wind ability of the Goat has not been pushed much except on landings, where you can land in a big slip and roll for a ways, but as you get slow you will "weather vane" nose into the wind if you try to keep going. On a rolling takeoff  I stick to the cautious end of the hang glider range, for sure no more than a 45 degree cross wind. 

I don't think the limit could be based on any fixed 90 degree component, what you will see in the field is wind strength and cross wind angle. 

I suppose the cross wind limit on ground or aero tow is whatever you can get away with. If you can't contol the tow, you release, and try not to groud loop.