Here are some questions about airchair gliders,
along with my answers.
appreciate comments, new questions, and corrections, but please read
the website first and ask questions that show me what you already know
(otherwise I'll just be referring you back to the website, which you
may have already seen). I know it's easier to send me a list of
questions than to read my (poorly organized) materials or to search
the web, but I can't allow e-mail to take up all my lazy time. My e-mail address is "m--sandlin"
followed by "@sbcglobal.net " (I give my address in two parts to
confound harvesting by advertisers).
14, 2017 "...the
super goat guys use foam ribs in the wing. It seems that foam would be
an easier material to build ribs from than the small aluminum and not to
mention cheaper. What is your take on foam ribs and would you suggest
them for the goat wing? I know the tail feathers already have a similar
you can see on my drawings, the main wing rib for the Bug, Goat, and
Bloop is just a formed tube that shapes the upper surface of the wing.
This is strong and simple, and has always worked well. I'm not sure what
people are trying to accomplish with the built-up ribs, I doubt that
trying to rigorously control the contour of the lower surface is worth
the effort or the weight. On the Bloop 4 I stitched an undercamber into
the lower surface to help tension the fabric, but that was quick and
easy work with a long needle and thread (see B4A10). Go to the photo
album "Bloop 1 & 2" and look at the wing frame on the lawn, it is
ready to cover, there are no built up ribs. The Goat wing is the same. I
would use this system again.
a Goat 1 from your dwgs on the web....Did you use the epoxy
reinforcing rod in the ribs or are they the same as the flaps no rod?"
Best wishes for your Goat project. The carbon rods are not called
for on the wing panels or ailerons of Goat 1 or Goat 4, but they
are probably a good idea for any plane that is going to be
transported on top of a car. The rods will stiffen and strengthen
the panels, which can get a lot of battering during transport.
Installing the rods is extra work but they don't add much weight.
September 8, 2015..."I love the goat
design but I want a higher Vne. If I build a goat out of 4130 chromoly
and tig weld and anneal all the joints will the glider be more likely
to go to say 65 mph as a Vne. I live in rural Missouri and locally we
have a rural airport with a 4000ft runway but no hills to launch from.
I want to build a goat1 with a 480cc tiny twin 16 hp briggs with a
redrive. I feel the structural limits of the aluminum goat is from
stress at the pin connections along with rivet points. I would just do
a high wing pusher ultralight but all the designs have a poor glide
ratio. I'm a vet, certified welder and I can bend all the tube as
needed. Thank you for posting plans for the public and please consider
a goat being used as a better ultralight for us that don't have
Response: The Bloop motorfloater should be
good for those who have rural airstrips and no hills, but I know
that biplanes are not for everybody. The Aviad MG 12 seems to be a
Goat with a small engine, and you can buy one, but that may not suit
the innovators who want to use the skills they already have
(welding, engine adaptation, etc.). When pilots ask for high speed
and high performance, I say that most light airplanes are already
trying to provide those things, so maybe you should look at
those. The airchairs and motorfloaters are intended to sacrifice
speed, range, climb and glide in order to get people's feet off the
ground, and pilots with performance requirements will probably not
be satisfied with them.
Structural innovation is fine as long as you keep the airframe
strong, but try not to add too much weight. Light wing loading is
something you can't see by looking at a plane, but it produces a
different kind of flying, the easy and safe kind. Light wing loading
produces low speed flying, which is bumpy in turbulence, but in
critical situations it is safer because there is more time to think
and react, and a typical crash will be slow instead of fast.
Ideally, after a crash you want to walk away from your totally
destroyed aircraft with a smile on your face, thinking "now I can
get a new one"!
I don't see any advantages to using steel for the larger structural
elements, its strength to weight ratio is inferior to aluminum.
Look at the structural
design drawings for my Goat,
you will see that I don't even mention bolts or rivets, my concerns
are mainly for the structural instabilities of the long columns in
compression, which are mainly a matter of spar geometry and material
stiffness. Steel and aluminum are about equivalent for stiffness
critical structures, since they have about the same stiffness
to density ratio, but with the aluminum you get a strength bonus for
no additional weight, so I'll go with that, as the aircraft industry
The best engines are the modern two stroke paramotor engines. If you
adapt an older engine and put your own propeller on it, how will you
know that you are getting all the thrust that can be provided? You
will have more confidence in a popular, integrated, fully developed
modern system. A custom engine and propeller is not the direct
approach to getting your feet off the ground, although of course I
can see the creative aspect of it.
October 11, 2014..."The plastic saddles as drawn in B4W7
and B4W10 for the joints from spar to compression strut on the upper wing
and lower spar to strut brackets: do you had any information on what type
of plastic you used? I assume you milled these to shape from dowel stock?
Secondly, in B4N1 and G4W3, there are spacers between the seat and nose
tubes; and between the compression strut and spar attach brackets on the
G4 drawing. Are these spacers sections of 6061 t6 tube cut to length, or
some type of AN hardware?"
No milling for me! Straight out of the catalog, (Aircraft Spruce, nylon
ultralight saddle). Spacers can be short lengths of small tubing.
April 25, 2014..." Where do you
buy the material for your control systems? i.e. "marine snaphooks",
"marine pulleys", and your control line" "Is the tow point called out in the
technical drawings suitable for auto tows?"
hardware, you may want to get the specification on the drawing and check
it out on a search engine. (I don't know which of a dozen aircraft you may
be referring to). The tow points on
the various airchairs, all on the front of the nose, are the ones I used
for all kinds of towing. I will not advise on what is suitable. I found
the tow point at the nose to be adequate, stable and easy to use.
March 30, 2014... "I have a private pilot's
license....and ... a few questions. First, can the Goat4 be built solely
from the technical drawings provided, or are there other plans out there?
Also, is there a complete build log I could view, so I know what I would
be getting into?"
Response: Do a
web search for "Goat Glider build blog", there are some out there. Also
look at the Airchair Yahoo Forum for alternate drawings and opinions
(better yet, join and ask specific questions). Generally, each homebuilt
airchair is different, but I haven't seen anything much better than my
Goat1, 2, or 4. My drawings are a description rather than a plan, but
they have been used as a basis for building (but they get changed, as
2014......... "I am an
experienced sailplane pilot / instructor with over 500 auto tow
launches. My planned launch method for the Goat 1 will be a similar
launch process with lighter line, 90% gross weight weak link, and much
slower launch airspeed. Without data, and only prior sailplane
experience to rely on, I estimate tow NEVER EXCEED SPEED to be 30 mph
airspeed. Do you have any experience with this launch method that might
define safe launch airspeed and / or other procedures that differ from
500 auto tows! Launching the Goat should not be much of a
challenge after those. I expect a slow and casual climb, so if the truck
speeds up so I can't climb without a lot of airspeed and back stick,
that's too fast, and I release. This may be different for you, depending
on your weight and whether you have moved the hookup point back from the
nose, as some people have. I favor the nose hookup for the initial
ground tows because it gives lots of feedback, a tow that is too fast
will pull your nose down, keeping down the line tension for the release.
I've flown the Goat with an airspeed indicator because somebody else put
it on, I don't use one for airspeed control, just for comparisons to
other aircraft. I don't recall reading the airspeed on ground tow. If the tow is too
slow, I can't climb or maintain normal airspeed, and again I release. I
agree that about a 30 mph airspeed on climb should work. The standard weak
link strength would be 100-120% gross weight, but I see no problem with
being a little weaker, as long as you are prep ared to respond quickly
to a line break.
December 2, 2013..... " I'm
living in Poland (Europe), I have 30 years old and I'm starting my
adventure with flying. I mean I would like to start. Now I'm starting with paragliding, for
this moment more theoretically but when spring will come, I would like to
make some practices.
On the internet and I've found your web
side and i'm very interesting the GOAT construction. Because glider can fly faster than
paragliders :) However I'm living on flat area and It
will be hard to find a cliff or mountain, so my first question is . Does GOAT construction can be with the
engine from paraglide for example? Does anybody was try this type of
My another question is , How many time is need to build this
Answer: You seem to be looking
for an adventurous and low burden way of getting off the ground.
Paragliding and hang gliding are
a good start, they get you up and teach the basics. They are an end in
themselves, and if you want to fly airplanes you can do that too. One drawback to a Goat glider or
airplane is the need for three axis control skills, which will require
instruction in an airplane or sailplane.
let you fly in flat areas without organizing a glider launch operation.
transition from hang gliding or paragliding to a slow, two axis airplane
will not require three axis skills. The Bloop, for instance, is a two
plane intended to provide simple, slow flying like a paramotor.
Paraglider pilots who want to motorize might want to fly a paramotor or
but it might be that the Bloop would be easier due to lack of canopy
The Aviad Zigolo is close to
a Goat and uses a paramotor engine. They do some soaring in it, they
sell kits, so have a look if you think this is a good mix. See the Goat Q&A website for
Goat building time estimates, but there is no fixed time, everybody does
November 4, 2013 "Goat...... find out about: Differential
aileron deflection. ?"
Answer: "Differential aileron deflection"
usually refers to rigging the controls so that the upward moving aileron
goes way up while the downward moving aileron only goes a little down.
This is intended to reduce the adverse yaw in turns, so you won't need
as much rudder deflection to get a coordinated turn. It's the normal
setup for conventional sailplanes and airplanes.
The Goat already has some differential aileron deflection, but it is
small because a big differential would reduce the rolling power of the
ailerons, which I don't consider a good trade off. The Goat ailerons are
sized for keeping the wings level in a moderate breeze while sitting on
the ground before takeoff, so you don't need a helper to run with the
wing tip and keep it level until a higher airspeed is reached.
July 2, 2013
"How do the controls fell on the goat 4 glider, de they have any
slack or play that is unconfortable, how do they do in time, do they
gain play because or the wearing of aluminum parts in the sistem?
prefer the cable braced wings than the strutted version, is it that much
draggier that everybody chouse the strutted version? the thicker
wall ..., i mean will it create any problems oather than weight? Being heavy it
means tall, and that means longer legs also, so the control stick is a
litle (4 inch) shorter fo me intend to make it longer, but that will
limit a bit of control surface moovment, my qestion is how much
(aproximatively in any measture) you think the ailerons and elevator
shoud move in each direction? The goat 1 flyes
for a wile (10 years o so) do you know what problems or maintenace dit
it need ( weared parts or so) exept fome fabric covering wich i know it
Answer: The Goat and Bug control lines can
develop slack due to stretching of the Spectron lines, which is minor.
The control lines can be re-tensioned anytime, or just use the new type
lines which are a Vectran/Dyneema mix and don't stretch (see G4A9). This
control line hardware is just sailboat running rigging, these problems
were encountered and worked out by sailboaters. There is additional slop
in the controls due to elasticity of mounting and structures, but I do
not find it "uncomfortable". Stiff controls are better, but if the plane
does what you tell it to do that is of greater importance.
of aluminum, steel, or control line has been detected, just busted up
landing gear and worn down skid plates. Maintenance might involve
control line tension adjustment, lubrication of moving parts (silicone
spray on control lines and pulleys works well), and air in the tire
(take a hand pump).
struts versus cables has been discussed elsewhere. I will not comment on
design changes such as control modifications.
1. What about building an airchair, making design changes and
using different materials? Will you help me? (April 2009)
liability reasons I will
not endorse specific design changes or material
substitutions. I may be willing to make some general comments as long as
the question has not already been asked and answered on my website.
The main issue with regard to design changes (or even without them!)
is pilot confidence when it comes time to fly. You won't get much
airtime if you have doubts about your glider, especially if you want
to soar and stay up in turbulent conditions. We airchair pilots are
aviation pioneers, and courage is required. Even without innovation,
when you soar an ultralight you will, at times, get thrashed and
trashed by the atmosphere, so get ready!
My confidence in my gliders (Bug, Goat, Pig) is based on using
structures well proven in hang gliders. I have not done any large scale
structural testing, but tubing ladder frames like mine, braced by
steel cables, have been used by rigid wing hang gliders for many years
with satisfaction. I fly at a hang glider wing loading, with about the
same weights and speeds as a hang
glider, so I feel that by
applying a small additional margin of design strength I can use similar
structures and take advantage of the history of safety that hang gliders
have provided. Well known hang gliders with structures similar to mine
might include the Fledglings, glider Quicksilvers, Icarus 2 & 5,
2. Can I use different size tubing and other alloys and tempers?
Answer: Generally, using thicker wall tubing will not weaken an airframe,
it will usually make it locally stronger but heavier.
Using a stronger alloy of aluminum should not be a
major problem, either. It will add some strength, although it may be more
apt to corrode and should not be used for parts formed by bending, since
stronger alloys are sometimes weak when bent or unsuitable for bending. I use mostly 6061 alloy aluminum (temper
T6 for tubing). It has a long history of safe use in hang gliders
and ultralights, and has good properties of strength, formability, and
corrosion resistance. Tubing and extrusions are readily available in this
alloy. 7075 alloy aluminum is the modern hang glider standard for ribs and
large tubes used under bending loads, but it's benefits are marginal. It
can be hard to get and is harder to work with than 6061. Remember, you can
accept a little extra weight when you don't have to pick the glider up and
run with it!
Aluminum tubing will bend or dent before it loses
strength, so the extent of possible damage due to some unfortunate
incident can usually be determined by visual examination (unlike many
composite tubes or structures). There will be hard landings and rough
handling, and decisions will have to be made in the field about whether
the glider is still airworthy.
3. How about using those fixed trailing edge "flap panels" on the
Goat as in-flight adjustable flaps (i.e., adding a flap control system)?
Answer: This could be done, but I doubt
that it would be worth the design, construction, operational
effort, or the increased burden on the pilot (an increase in control
complexity is always a burden on the pilot, even if there are other
benefits). Most simple sailplanes do not have flaps for exactly these
reasons. The high parasite drag of the Goat makes it essentially a one
speed glider, perhaps with little or no glide benefit to be gained from
raising the flaps to fly faster. Lowering the flap panels for soaring
might not benefit the Goat because it is already designed for slow speed
flight (i.e., the flaps are already down).
A flap might be useful to lower the stall
speed for a rolling takeoff, but the effect would be marginal and I
doubt that it would really make much practical difference.
In the landing pattern, using flaps for glide
path control might be a good thing, but I don't favor
First, at hang glider wing loadings there are no devices that will
allow any airchair to fly a fixed pattern in afternoon
turbulence without maneuvering for approach control, so why
install complex flaps which cannot serve as the primary method of
glide control anyway? Even if you did have landing flaps that were
powerful enough to overcome lift and sink at low wing loadings, they
would be too powerful for any low time pilot (or maybe any pilot) to
safely use. Only a few sailplanes use landing flaps as the primary
glide control because they are tricky and require somewhat involved
and critical procedures.
Secondly, even though the Goat is not
high performance even by hang glider standards and doesn't really need
special devices for getting down, there are already ways of adding
drag. Some like to slip (flying slightly sideways to add drag), but I
avoid anything requiring skill, so I use a drogue chute for almost
every Goat landing. Of course, in addition to slips or a drogue, you
will still have to maneuver to a landing under some lift and sink
conditions, but this is just what hang gliders and paragliders have
been doing all along.
The loads produced by my
use of a drogue chute attached outboard on the struts or flying cables
are not known to me, there have been no tests or structural
calculations. I expect 30 to 60 lbs. of force on the chute bridle,
and, as you see from my frequent use of it, I have decided that this
is acceptable. Concern over drogue chute loads might seem to favor
flaps, but the flap loads are also unknown and untested.
4. What are the trade offs on the Goat regarding the cable braced
wing versus the strut braced wing? (April 2009)
Answer: The Goat1 has a strut braced wing which is compact and quiet in
the air. The top of the wing is open and clean (look at the Gallery Page
for good photos of strut Goats and cable Goats). Presumably this is the
highest performance Goat, it has made a 100 km. flight. The struts fold up
flush with the wing, but perhaps could be made removable for transport.
Weight is the main draw back, both from the struts themselves and the long
internal sleeves inside the wings. The long sleeves provide a lot of
outboard cantilever wing, keeping the struts as short as possible to keep
them strong in compressive loads. The wide nose section of the Goat4 would
also contribute to keeping the struts short (the Goat4 nose section was
originally designed for Goat3, which had struts).
The Goat4 has a cable braced wing, a little draggier
and noisier, but light and easy to transport and store. This type of wing
has short internal sleeves and not as much outboard cantilever length,
because with cables there is no reason to minimize length (at least not
from a compressive load point of view). This creates a classic primary
These two gliders fly about the same. I do like the
quiet flight of the struts, but the weight of the struts plus wing panel
can be restrictive, especially for trying to load a wing on top of a car
in bad conditions. The total weight difference between the two schemes is
about 15 or 20 lbs., maybe 7 to 10 lbs. per disassembled wing panel, which
is a lot for one person already near the upper limits of what can be
If I were designing a Goat5 (which is always a design
concept but is not being seriously considered for construction at this
time) I would probably use struts, mainly to find a way of removing and
folding them for transport. I
would use some of my advanced or revived ideas which are found on the Pig,
such as prismatic rudder pedals and a boat seat, for instance. I might try
making the nose skid broader and less grabby, and perhaps the nose section
could be made stiffer in torsion by adding some shear bracing to the top
frame. The reason for attention to the nose and rudder pedal stiffness is
to try to duplicate the rudder feel of the Pig, which is excellent and
benefits from the very stiff box kite airframe. The rudder feel of the
Goat is a little spongy compared to the Pig.
concept: these gliders fly and soar just fine! The kind of improvements
that would really make a difference are not these little flying details
that people keep asking about (like what kind of fairings to use on the
tail struts). Instead, we should seek airchairs designs that are more
practical and attractive to own and use, involving easier transport,
quicker assembly, compact
storage, garage maintenance, better crash safety, etc.
In my vision of the sustainable future, airchairs are
commonly flown as a form of local aviation, along with the paragliders and
hang gliders, but at a much lower level of risk because they are more
stable, easier to fly, and protect the pilot better in ordinary
crashes. They will be commercially available, but kept severely simple and
made from readily available materials so they can
be maintained and
repaired at a garage or
hangar level (like the early hang gliders, they may never go back to the
manufacturer). Airchair flying will be accepted as recreational and
casual, mainly striving to stay up in light conditions, with cross country
flights seldom exceeding 100 kilometers. There will be a lack of emphasis
on serious contests, record setting, high performance, or increasing
market size, so that airchairs remain simple and can provide an entry
level to faster and fancier forms of aviation.
5. "I was wondering if there was a materials guide and a step-by-step
instruction on building this [Pig1]."
Answer: No, just the drawings
for now. Someone could make up a materials list from the drawings and post
it, which would be useful.
In the future I may add some pages onto the Pig Page
with some design and construction comments, but right now I'm still busy
working on the glider. I do plan to post a new zip file with revised
drawings sometime this summer, since some of the rigging and strut
details are changing.
6. "... I wanted to build a glider. My biggest concern is
pilot size, I am 6'1" and 215. Is the Goat4 a feasible flying
machine for me. Also. what areas in Southern California would I find
one of your designs flying, I live in Los Angeles and there is a hang
gliding hill right behind us, Sylmar." (August 2009).
Answer: Read more of this
website, and you'll see I've written a lot about this "heavy pilot" issue
down through the years. Size is not the problem, but a heavier wing
loading will change the flight characteristics of a Goat type airchair,
and I don't know if you'll like it or not. A pilot your size has flown one
of my gliders without any great problems, but it was not soared.
A Bug4 has been flying at Crestline (San Bernardino) in California, but it
may be down now for new fabric. Sylmar (a.k.a. Kagel) would be an
excellent place to fly an airchair (I flew it for years, it's a great
local site, and the launch is suitable for rolling take offs). You'll have
to get started getting a hang glider or paraglider rating to fly it,
7. "I am taller and heavier ...
than ...the goat was designed for, but I want to maintain the 1.7 lbs
per sq ft wing loading...would you recommend only increasing wing span
or chord or both to increase the wing area." (September 2009) Answer: For a
Goat airframe weight of 140 lbs., my weight at 160 lbs., the gross weight
is 300 lbs., the wing area is 174 sqft., so the wing loading is 300/174 =
1.72 lbs./sqft. I have had a lot of fun at this wing loading, which is in
the hang glider range, and when I tried a heavier wing loading (Goat
3) I was dissatisfied (could not slow down), so I think you have the right
The Mojave Goat reportedly added a foot of span to each wing, adding 10
sqft., allowing another 17 lbs. of weight, so a 187 lb. pilot can now fly
with full capability, minus whatever structural weight was added to get
the span increase (I have no details on this). They could have added the
same area by increasing the wing chord 3.5 inches, probably without tail
changes but maybe with adjustments to keep the wheel and pilot in the
right place relative to the center of lift. We're now up to a 200 lb.
pilot, if we increase both span and chord. Weight increases do
require more structural strength to get the same margins of safety, so
calculations must be made and structure added. Note that the Mojave Goat
folded wing is carried on a long trailer, whereas my Goat wing main
panel is sized for car top transport. You have 15 lbs. of airframe
weight that can be added (onto the current 140 lbs.) before you reach the
F.A.R. Part 103 limit of 155 lbs.
The simple answer is that this heavy pilot issue will keep coming back,
again and again, until a skilled and dedicated designer does the big job
and a new prototype gets built.
Large pilot size, as opposed to weight, has not been much of an issue in
this outdoor flying situation, but customization of the rudder pedal
location is possible.
8. "Regarding G2W10, is there a
sleeve for the Cabane lower part of the pipe?" (September 2009) Answer: In the
vertical center tube of the Goat cabane, there is no sleeve, but I
considered using them and saw no great penalty for using them there. I
decided not to use sleeves in areas where two bolts were holding two
plates sandwiched onto the tube, figuring that the bolted plates were
enough reenforcement without additional sleeves. This was a very marginal
situation, and I could have done it either way.
9. "...elevator gap-seal. Do you think
that (lack of this) will cause the elevator to be less
effective?...I'm contemplating using duct tape, because I already
painted the elevator... Any ideas?
...how did you arrive at your VNE (velocity not to be exceeded,
i.e.,maximum allowable airspeed) of 45mph? The mojave goat was pushed
to 50mph, and the croation goat to
55mph." (October 2009)
Answer: Yes, a hole in your wing will make it less effective
and add drag, so I use gap covers. It may be that the effect is small
enough to be neglected in many cases (as it often is neglected for reasons
of convenient maintenance and painting) but the effect is there, and I
don't choose to build beautiful wings and tails and then fly around
with holes in them. The kiss seal between the wing halves is in this same
I have used overlapping pieces of insignia cloth for gap
covers (and small fabric repairs) with good results. Insignia cloth
is a stick-on Dacron (polyester) fabric with the adhesive exposed by
a removing a paper skim from the back, used on sails for numbers, logos,
stripes, etc. It is durable and tenacious, and sticks to paint or dope. It
is sold off the roll by length from sailcloth outlets, sail makers, etc.
in several colors. Much better than duct tape.
I don't use insignia cloth on paint unless I'm sure the paint
is well attached to the wing. If a gap cover or other spanwise tape peels
up in flight, it can act like a little spoiler. I've had this happen, it
feels like a built in turn, so you have to land and check it out.
A basic maximum safe airspeed (for a fixed configuration) can
be calculated using the maximum angle of attack/load limiting method. If you stall (maximum angle of attack,
at 1 g) at 22.5 mph., then flying at twice that speed (45 mph.) will limit
your wing forces to four times the load (4 g) because the force on the
wing is proportional to the square of the airspeed. I can accept the idea
of a 4g load (even on my designs, which are untested), and for open air
flying I see little to be gained by flying faster.
"can you expain how to drill perpendicular and parallel bolt holes (like
two holes on the opposite ends of alumnium tube) on aluminum tubing? Do
i need a drill press and jig(s)?" (February 2010)
In response to this question I expanded section C3 of the Goat
Construction Notes. The new text is: "Holes are drilled with a
hand held power drill using a self centering drill bit. Every hole is
marked from the outside and drilled separately (see reference line note
.....). In tubing, matching holes are drilled through in pairs for
final finishing alignment after they are drilled from the outside, unless
there is an access problem. My holes tend to be sloppy and oversize, which
makes assembly easier."
idea of what Extrusions Floyd used to create the new struts on the now
red goat (and where he got the material)?
They look good and are flatter than the extrusions I have been
exploring for use (some made for Struts / jury struts on PMA
Taylorcraft parts etc). I can see how they will allow folding
for easy transport - and it looks like a simple and elegant solution
to the strut constructions." (February 2010) Answer:
Floyd Fronius used faired
cross section extrusions for the wing struts on the
Goat. This aluminum stock was originally produced for use as hang glider
"down tubes", in this case for Seedwings of Santa Barbara, California.
There are various versions of faired down tube stock for the various
hang glider manufacturers. I don't know if they want to sell it, or in
The extruded struts, with heavy internal tubular sleeving for strength,
added about ten pounds to the weight of the Red Goat relative to the
tubular struts I used originally. The sleeved tubular struts with
applied fairings, as shown on my Goat1 drawings, would still be my
favorites because they are lighter, and probably easier to repair or
replace if damaged.
12. "Why are the bug2 and bug4 wings
inclined at an angle, and the goats' wings are fixed without any
bending, but are straight all the way across?" (March 2010) Answer: On my gliders
the dihedral angle of the wings is
often related to ground
contact. The Bug biplane dihedral is set high mainly to provide ground
clearance for the lower wing tips while rolling on the single wheel
during launch or landing.
The strut braced Goats have a
small dihedral angle (the wing is not straight, see the drawings).
This allows short, and thus lighter and less bulky struts, as well as a
reduced sideways tipping angle when not flying, making it easier for the
pilot to sit down and strap in when the wing tip is on the ground.
Aerodynamically the wing when flying will be more efficient, less
stable, and the control might not feel as good or be as easy.
On the cable braced monoplane
Goat wings the dihedral
is intermediate, about 4 degrees, a good all around compromise. The
dihedral provides good yaw/roll coupling for easy flying, making the
rudder more effective in turns, probably reducing the need for precise
These dihedral effects are small and usually not noticed, at least not
by me. I once tried steering a cable braced Goat using just the rudder,
and it was okay but slow turning. I suspect that the Goats with less
wing dihedral will not steer as well using just the rudder.
CAN FIND THE PLANS TO BUILT THE SUNDOG, DO YOU HAVE THIS IN YOUR WEB ?"
Answer: Sorry, no hang glider
drawings, nor Hillhopper, nor some others. These drawings mostly don't
exist in any clear form.
would like to know your opinion on the use Goat as a basic glider training
for young pilots (11-15 year). What is the difference between different
versions of your glider Goat?"
Answer: Look at my webpage text and
photos, and you'll see that I have done basic training with the Goat and
Bug. It's fun, but difficult and hard on the glider because the student
can'y really stabilize in flight until after they have learned how to fly
with the three axis system. The Bug biplane is better than the Goat
monoplane for low tow or hillside training because the lower wing will
cause a shallow ground loop before a steep one can develop. Best of all is
the two axis Pig, which is ten times easier to fly and stays level on its
two wheels (see the video). As a teenage novice trainer, the Pig is by far
the most effective choice because it will provide immediate succesful
flights instead of bungled launches.
Goat project.... 1. With the rib stitching of
the undersurface, is the undersurface pulled tight
against the compression struts to create slight under
camber or is it stitched to keep it flat on the bottom but resist any blow down? 2. After shrinking the top
surface does the application of poly brush adhere
the fabric to the ribs? I
can rib stitch the top surface if I need to. 3. With your drawings you
mention one method of reducing scolloping between
ribs is to add span wise slack when covering the top surface. Are there any other techniques you
use? ie Do you shrink from the mid span
out, Edges first, one bay at a time?" (November 2010)
1. I did sometimes create undercamber by sewing up the lower surface to
the middle half of the strut with a curved needle. However, instead of
that, on most of my gliders I have a flat wing bottom. I
applied a balsa wood slat onto the strut to make the bottom of the wing
flat and flush to the leading and trailing edge spars. I don't see this
slat on the drawings (I wish I had shown it), but the flat wing bottom is
implied and works fine.
Airfoil variations have had no noticeable effect on flight, so I tend to
do what is simple and looks good.
2. Rib stitching with a curved needle around the rib tubes works well and
is easy to do. I have not done it in the past, but now I do it a little
bit, although I have not yet confirmed a need for it at these low
Fabric adhesion to metal can be strong if done right, but I don't count on
it for strength in flight (I depend on fabric to fabric bonding, creating
a continuous fabric envelope around the frame).
3. I'm happy with the method of initially attaching the top surface wing
fabric to the frame with spanwise tension and chordwise slack so
that the final fabric shrinkage will produce a smooth top surface with
minimal bowing down between the ribs. The sequence of shrinkage does not
seem to matter much. Wrinkles will form along the inboard and outboard
edges of the wing where the fabric attachment has to accomodate chordwise
slack, but these will mostly shrink out and then be tape covered, or can
be cut out and then tape covered if severe.
did Mike Sandlin use for the GOAT 4 drawings?" (January 2011)
current software is TurboCAD
Designer 14.2. TurboCAD is
akward but modern and functional, I chose it because it had a 2
dimensional version and was the most accesible to the most users. It came
on a disk in the mail that cost less than $10.
most drawings previous to the
Pig were done with the older Autosketch, which is no longer available in a
It takes time to learn, but CAD is a great design and documentation tool
and I encourage anyone with aircraft design interests use it. Anyone who
wants to take a serious look at my drawings will need it.
my thoughts are wandering around the concept of an electric powered Goat.
Perhaps a pusher unit just behind the pylon, between the top and bottom longerons? (May 2011)
Answer: There probably should be a small gas engine tried first, before
going to electric, to work out the design and procedures. No one has flown
much with a full size tail and a motor at a light wing loading.
Evaluations of electric battery ultralight propulsion systems seem to
agree that they provide good performance (like a gas motor) for a short
duration, followed by a long delay to re-charge. Even a pilot who
just wants a low soaring launch might only get only one or few launches
per day. Mechanically I favor the electric engines because they allow the
(battery) weight to be placed where desired, but I'll try a gas engine
first. There are persons who report that they are working on a Goat-like