Airchair Frequently Asked Questions

Last Update: October 15, 2009 ( No.9)

Here are some frequently questions about airchair gliders, along with my answers.

I do 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).

There is also a lot of good stuff to read on the older Q&A pages, starting at Q&A page 1.

1. What about building an airchair, making design changes and using different materials? Will you help me? (April 2009)

Answer: For 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, Voyager, etc.

2. Can I use different size tubing and other alloys and tempers? (April 2009)

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)? (April 2009)

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 them.
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 glider structure.
    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 lifted.
    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.

    Grasp the 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, though.

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 idea.
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 category.
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.

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