This thread has been going on for awhile but hasn't converged yet. It seems to me that the obvious starting point for an inexpensive, simple design is a primary glider. Primary Glider Plans This vehicle only weighs 150 lbs with a 32' wingspan and 157 sq ft of wing area. It has a 17:1 glide ratio. It's obviously wood and fabric with kingpost and cable bracing. I like the idea of starting with a glider since any aircraft limited to ultralight specs (63 mph) isn't really that great for cross country anyway. If the weight stays low, you could probably use some type of RC engine to climb. With a longer wingspan and less drag, this vehicle might match an older sailplane like a Schweizer 1-26. And, even in my area in the Midwest, you can achieve bronze badges with a 1-26. I think an engine would be necessary because around me, they only have Pawnees for tugs and you can't tow an ultralight with one of these. Also, it would be nice to be able to fly even when there is no ultralight tug or an area of ridge lift. On grass, you could probably use a bungee and still be able to get airborne without a big engine. Let's try some math. A Schweizer 1-26 has a 40' wingspan and a sink rate of 175 ft/min with a wing loading of 4.4 lbs / sq ft. So, with 700 lbs and a sink rate of 175 ft/min: 175 / 60 = 2.92 ft/sec (700 lbs * 2.92 ft/sec) / 550 lb ft/sec = 3.7 HP for level flight Climbing at, say, 400 ft/min would require: (175 + 400) / 60 = 9.6 ft /sec (700 lbs * 9.6 ft/sec) / 550 lb ft/sec = 12.2 HP. Allowing 80% prop efficiency = 15.2 HP for a 1-26. Can we beat that? We could probably get a 40-44' wingspan if we used wing struts like on the Schweizer 2-33. That would also let us remove the kingpost and most of the cable bracing. Let's assume the same 175 ft/min sink rate and 400 ft/min climb rate. However, we should be able to keep the vehicle weight under 500 lbs (assuming 250 lb empty + pilot and fuel). (500 lbs / 700 lbs) * 15.2 HP = 10.9 HP. We might be able to get that lower if could get the sink rate down or if we accepted something smaller like a climb rate of 300 ft/min. A Turnigy Rotomax 100cc brushless outrunner can put out 10.1 HP so we are close to an electric solution. Unfortunately, the obvious place to mount it would be behind the pilot where the pilot's seat would be blocking most of the airstream. We would either have to use a tapered fairing and airscoops behind the pilot's seat or perhaps use two motors on the wings. If we went to two motors, it would be easier to size them and their propellers. Of course, folding props would be essential. Electric would also be attractive in terms of not having to pull start them while flying. A couple of Turnigy Rotomax 50cc outrunners could put out 7 HP each. The weight is 1080 grams each. I guess I need to figure the batteries to see if this is feasible. We need a 12s battery for a maximum of 44 volts. However, we'll figure this as 37 volts. We need 120 amps to get 6 HP out of each motor. Looks like 3 minutes of takeoff and climb to reach a reasonable altitude to soar. Okay, so we know that we'll need 6 amp hours each or 12,000 milliamp hours total. We also know that we need at least 20C since it is three minutes. Something like a Sky Lipo 4000mAh 40C 7.4 V is $25 and weighs 267 grams. 7.4 Volts is only 2S so we need 6 in series to reach 44 volts. This would give us 12S. Then we would need three in parallel to reach 12,000 mAh. 6 * 3 = 18. This would give us enough power to climb once. The weight would be 267 g * 6 * 3 = 4806 g. The motors are 1080 g apiece for 7 kg or 15.4 lbs total. That's about $450 for batteries and $385 for the two motors. These brushless motors require an Electronic Speed Controller to create 3-phase AC so another $105 for an ESC capable of 12S and 120 amps. And, then you need a servo tester to replace the receiver controller. That seems doable in terms of the power for climbing. One difficulty might be the propeller though. We can get a 30" x 8" propeller from JC Super Props (for about $76). A 32" would give us more thrust but I think 30" is about as much as we could handle in terms of tip speed. I also don't believe there is a folding hub available in this size. However, there are plans available for hinged autogyro hubs and these should be sufficiently robust. These brushless motors will run in either direction so we don't have to worry about the twist on the propeller although counter-rotating would be nice. Could we use a regular fuel Zenoah RC motor like the ZP 62cc? Maybe, but it wouldn't be easy. We can get one that puts out 6 HP. However, the good version requires a battery to drive the electronic ignition because the magneto version is hard to start and idles rough. Vibrations are bad so isolation mounts would be needed. And, if you plan to use a folding prop, you'll also need a flywheel and a clutch. The weight is now as high as the electric solution. The advantage though is that you can fly longer under power. However, endurance is not really that useful with a glider that only needs power to climb. I could see building a craft like this to go soaring. However, if someone was not interested in soaring then I can't see much advantage to the Part 103 ultralight spec. It would seem to be more logical to go all the way to Sport, shorten the wings, and pick up the cruise speed. You'd have something similar to the Starlite which was ultralight weight but faster. I guess I'll have to read through the whole thread now.