As many of you know, I've been toying around with designing a legal ultralight for a while. I've been noodling all sorts of ways to improve on the performance while still staying legal. Here are the pertinent thoughts that led me to a potential "eureka" moment.
- Most people do not like how UL's get blown around in even light winds, due to the typical wing loading.
- Most people don't like having such a limited top speed that most UL's have, due to high drag.
- The FAA allows a designer (though it is actually the responsibility of the pilot to prove) to choose to demonstrate max speed and/or stall speed using a published formula.
- If one was creative, they could use the formula for max speed to use a small wing with other drag inducing elements (eg struts) to keep the "calculated" speed below the threshhold.
- A UL with more complex high lift devices could possibly exchange the weight of such by reducing the overall size of the wing - and thus meet the "actual" stall speed requirement by actual demonstration (calculated with a 170 lb pilot with 5 gallons of fuel and no "removable equipment" on the aircraft (seat cushions, instrument panel, parachute, etc?)
SO... here is my "outside the box" idea for building a legal UL.
- .Have only an 80 square foot wing (wing loading would then be about 6-7psf, instead of 4-5).
- The pilot can be in a sleek and aerodynamic fuselage (formula calls such 'pilot drag factor' "totally enclosed, streamlined").
- With a smaller wing, the fuselage can be shorter, but if still fairly long, a much smaller tail group can be used.
- Overall, the airframe could be lighter than a typical size UL, while still using beefier structure.
- According to the max speed formula, a streamlined fuselage with an 80sf wing, using a 32hp engine, would require 10 struts (at least 4' long, 45-90 degrees into the air) to meet the calculated legal max speed - even if the aircraft can in fact actually go faster.
- Struts can be streamlined - and be included simply for inducing drag (do not have to have a structural purpose). Two struts on each wing, maybe four 'jury' struts, and or a number of struts on the tail group).
- Engine power must be based on the Manufacturer's label/designation.
- I want to use the Generac industrial engine, which is labelled as a 32hp engine, even though they show an 'alternate' power curve that shows the engine putting out 38hp (with 40-50 claimed on the Backyard Flyer).
- With a good airfoil (not laminar) and slotted flaps (maybe even double slotted) it should be possible to still get the aircraft to stall at the required speed.
THUS, if the 'actual' stall speed can be met, it would be possible to have a legal UL that can cruise faster (maybe 75 mph), while it looks and handles more like a "real" airplane.
- I envision a wing of about 27.5 feet long with a 3' chord (aspect ratio of over 9).
- Might possibly build a tapered wing, with a 30" tip chord and a 42" root chord.
- A heavier version could still be a motorglider.
- Wings would be two piece, with overlapping spars inside the cabin, typical of many sailplanes. It would have automatic control hook-ups. Each panel would be 15' long, and manageable for one person to easily install.
- Fuselage would be about 18' long, from tip of spinner to back edge of elevator.
- Engine would have a 1.9:1 redrive, allowing for 2100 rpm at max power. It would swing a 72-84" prop, and put out about 250-300 pounds of static thrust. That is almost a two-to-one ratio, and should allow the aircraft to have an incredible climb and very short takeoff.
- As mentioned, 'actual' top speed could be much higher, since it legally meets the requirements of the FAA.
- The airplane would have lots of removable equipment: canopy or doors (depending on low or high wing), plug and play instrument panel (full moving map, radio, etc), seat cushions, and anything else. Even the fuel tank might be easily removable (simply for the convenience of filling or swapping tanks).
Its styling will be inspired by the BMW 'Gina' (which is a "rag-and tube" automobile, pictured below).
NOW - what do you think?