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 x-tra 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 thermaling 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. "... 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.
10. "Getting ready to cover my Plane. 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).
11. "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.
12. "...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.
13. "Have you considered using the European 7075 alloy tubes to reduce the weight of the Goat wings and frame?"
Answer: Stronger aluminum can 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 mainly in compression and for that only the stiffness of the material really matters, not so much the strength, and aluminum alloys do not vary much in stiffness. Generally, however, it's a good thing and some weight could be saved. 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. "Interested to know how long the original BUG4/Goat 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.
16. 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.
17. "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.
18. "...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.
19. "- 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 stabilizer? 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.
20. "... 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.
21. "...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 polycarbonate 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.
22. " 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 heat shrunk 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 information ?"
Answers:
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 polyfiber.com 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.
23. "...perhaps you could comment ...on the design issues going from a strut-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).
24. "I am a big guy 6'4" 220lbs....aside 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 wing loading 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.
25. "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.
Click to go to Plans page
26. "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.
27. " I plane to make an airchair. I waver between Goat 1 or 2. To help my choice, can you answer these 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 adding flaps controlled by the pilot ?
In short, do you advise to build Goat 1 or Goat 2 ?
Answers: 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 Goat2 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 or 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.
28. "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 realize 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 seep 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.
29. “...I have never piloted an aircraft ...I plan to learn slow and just hill hop around until I get better...is 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.
30. “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 horizontal 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 assist 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).
31. 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.
32. "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 didn'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 rather 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.
33. "Do you have a "condensed" list for ordering the tubes? Not as each single piece needed, but how many full length 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.
34. "I have analyzed 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. Aany further 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.
The airfoils you have used on G1&2 were well educated guestimates that seemed to work well. Was there anything that led you to use a "modified" FX63-120 foil on your G3?
35. "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 that 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 apparently not big enough to induce unaccelerated stall problems at these low wing loadings.
36. "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.
37. " 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?"
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.
38. "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?"
Answer: I don't know the answer to this question, but my guess is that the un-powered 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.
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