Airchair Questions & Answers, Page 2

Last Revision: January 28, 2009 (No.10)


This is a listing of selected questions that I have been asked about the Bug, Goat, and Pig gliders, along with my answers. The newest questions will be found at the bottom of the list. This is page 2, there is a page1 with mostly older questions.

I do appreciate comments, new questions, and corrections, especially from people who have read all the Website pages. There is a lot of good stuff to read on the previous Q&A Page (Page1 above). My e-mail address is "m--sandlin" followed by " " (I give my address in two parts to confound harvesting by advertisers).

1. "...I have only been able to source 0.058 wall in this size [tubing].   Would this pose an issue with the intended design [Goat4-like] ..." (Oct. '08)

Answer: I can't  think of any place in any of my airchairs where I would have hesitated to use a thicker tube except for the added weight. Not all of the tube sizes I use are easy to find, but my sources are given on the Goat Materials Page. The tube specialty place had all the Goat sizes, as I recall. 

2. " What are the pilot weights that have successfully flown a GOAT?  Vary by model (1...4)?
  How many GOATs have you heard of flying?"  (Oct. '08)

Answer: several people over 200 lbs. have flown a Goat  like a 1, 2 or 4  (all the same wing, essentially). Whether these people could have  soared and been satisfied at these weights I don't know.  Wing loading is critical, and  anything over the hang glider range  (about 1.7 lbs./sqft.)  may restrict the pilot to high speeds so he can't slow down and can't turn tight, leading to soaring problems in light lift. 

Other than my gliders, only the Mojave Goat has flown that I know of.

3. "...there is a GOAT that is going to be built...I am going to finish my parts list / BOM... Also need to make the dye I was writing about for forming the wing ribs.
On the epoxy / glass fiber I am thinking of using carbon fiber cloth; have you ever used that? I think it would bring longevity to the structure but???
What is the longest service life of airframe components in the GOAT have you experienced?
I will send a photo of the bundle with a plot of the wing ribs I made on Pro-Engineer..." (Nov. '08)

Answer: There are lots of good things starting up here. First of all, send all this good information to the Airchair forum (Yahoo group) for sharing and comments, so we can all benefit. A copy of your Bill of Materials would be good (for which Goat-type?) as well as photos, software files, and software knowledge (Pro-Engineer?).

Even better, I urge you and all other airchair builders to post your materials to your personal  project website, one we all can link to, so all the information stays available to all interested parties. 

I don't see the point of carbon wet layups on small parts when the carbon composite material is already available as rod or flat stock. I use the rod, instead of just more layers of fiberglass tape, because using the rod is so easy. You just cut off a length and glue it into a groove in the foam.

Composite rib service life is not known to me in the normal sense because I have only built prototypes, which do not see normal soaring service. Sometimes, on the Pig,  a trailing edge rib will get soft due to overloads (local failure internally, due to  assembly on rough ground) , and I have to re-build it, making it stronger or using more ribs.

4. "I'm going to be in southern California the week of ....[is] there any way I could observe your flying or even just seeing one of your air chairs I would greatly appreciate it." (Nov. '08)

Answer: For the sake of using my time for other things, I discourage visits to see my shop or gliders. 

I don't know where to go to see an airchair flying, they are too rare and unpredictable (weather sensitive). When I see regular flying going on I will tell everybody.

5. "Thought you should know that someone... has yourultralight glider plans up for sale on Ebay" (Nov. '08)

Answer: I get a message like this fairly often. My comment is: no comment. 
I am not involved with this or any any commercial activities.

6. " I ... want to build the plane myself....and if I would not have a wife and two daughters, I would learn to fly by myself as well... I understand that in the USA rules permit you to build a plane (like the Goat)  under a certain weight limit, and roll it off a cliff or maintain without any proof of being capable to do so? .... I am investigating the french rules on your type of plane ... but the plan is eventually flying something familiar to a goat came to mind how great it would be to have a forward swept flying wing, like the Monarch from Jim Marske/Mat Redsell, built in the same "style" as your planes. would be very practical having even less parts, and shorter fuselage. Pitch control might be weightshift control further reducing the complexity in mechanical parts. ...t this would be in a sitting position like the Goat... some mix between a hangglider and a Goat.  It would be easier to pack because there is no stabilo, and shorter fuselage.... I am curious of your opinion about this type of configuration." (Nov. '08)

Answer: All of this could add up to the desire to build and fly an airchair. You might look at Ruppert's Archeopteryx web site, where he has a list of requirements, by country, to fly that glider, which is foot launchable but otherwise an airchair. The Swift is also flown in Europe, and is much like an airchair.
For Europe, maybe a light Goat  (80 kg. upper weight limit?) with nominal foot launch ability would fit the current rules.
To achieve lightness, the basic choice is: high tech structure(Archeopteryx or LightHawk style) or sacrifice performance (Goat style).

My opinion: having a tail makes a big difference, it makes the glider more comfortable and easier to fly (more stable).The atmosphere seems less turbulent when you have a tail. However, I have flown a lot without a tail, and I confirm that it is practical.

The big transport issues are: do you need a trailer, and can it be assembled by one person?

As far as I know, forward wing sweep is used only to position the pilot rearward relative to the center of lift, and is in all other ways it is a bad idea. If the pilot is put under the wing, then forward sweep should not be necessary.

Pitch control by weight shift as a mechanical simplification might be a good thing, as long as minimal pilot restraint for crash protection can be provided. Since I try to move toward design simplification, and the Pig is already flying without ailerons, maybe the elevator will be the next thing to go.

7. "What kinde of airfoil you ysed for goat 1 and where is its aerodynamic center per cent of chord? and if may, how much is the pitching moment coefficient of that airfoil?" (Nov. '08)

Answer: The Goat1 rib airfoil is on the drawing, of course. There is very little data and no analysis available for the airfoils I use or for the glider itself. I do not have these numbers. On the Goat Design Page I point out that the Goat rib airfoil is very similar to an ordinary flex wing hang glider rib airfoil.

8. "Mike, you once mentioned that handling the wings of the G1 was difficult because of the weight (i think the size too) How about a three piece wing? Smaller and lighter pieces." (Nov. '08)
Answer: I assume the "handling" referred to here is the loading onto the car, not control in the air. The size and weight of the Goat wing are indeed  limiting factors. The weight can't be more than about 40 lbs. maximum for any one part, since one person has to lift it  in wind (unlike a hang glider, which can be heavier because it is balanced and  compact). The size of the wing part, even with folding panels, is restricted to what can go on a car top.
A three piece wing could be used (like a Lighthawk or early Mitchell wing) , but I've already gone on to the four piece wing, the folding biplane wing I use for the Pig. I wanted a bigger wing and this seemed like the simplest way to do it.

9. ".. if  I  build the goat4, what would I need to do about flying.  Would I need wind at all, or could I just tow-launch it and it would stay in the air?  The videos I saw on YouTube made it appear as if there was vital wind acting on the launch, especially when you just rolled down that cliff.  I live in Ogden Utah... Is too much wind like that a problem?  When you talk about "weather permitting", does that mean you need wind or you don't want wind at all?" (Jan. '09)
Answer: Flying an airchair is like flying a hang glider, for soaring and wind and so forth. "Weather permitting" usuually means a moderate headwind at launch and no rain. Yes we want wind! Moderatly strong and from the right direction. A headwind is good for launching, zero wind is maybe okay if you have a long slope, and a tailwind or extreme cross wind is not a good idea. Too much wind can be a problem, but airchairs generally handle better than hang gliders while launching in strong winds. Go to Point of the Mountain down by Salt Lake City and look at the hang gliding there, and you'll see what can be done (good place for a Goat). The people there can tell you the difference between wind and soaring conditions.
 Hang gliding is a good start, so is sailplane flying. The Goat is a three axis glider that is going to require some sailplane flying experience. The Pig will be a two axis glider which can be flown directly by a hang glider or paraglider pilot, but it is still being developed.

10. "I am waiting ... to see your completed PIG Design...I have read your site} that you are leaving room for a small motor. Will your design handle this total aprox100lb (62"diam prop+mount+gear reduction + batteries +controller) type system that the uses ? "  (Jan. '09)
Answer: Weight is an issue, and I would use a light engine, if any, until the Pig does more flying. My weight calculations did include a 30 or 40 pound engine, but are they any good? No load test has been done.
A Pig-like motor floater could readily be built stronger than the prototype, if desired, for greater structural confidence at high speeds. It would remain to be seen what the flight characteristics are at the higher operating speeds required by flying with more weight.
I like the idea of an electric engine, especially since the option of locating the batteries as ballast means that the same pilot position can be used for both a pure glider and motor version.
I don't know if the smallest battery pack would get you into the first thermal, or nor, or how long the recharge period would be, but it would be fun to try it anyway.

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