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Biplanes have lower minimum power and material strength demands. However these slow planes have worse glide ratios and thus distance-based fuel consumption. Today with high performance engines and materials these planes are very niche and monoplanes are the norm.

But these traits seem best for maximizing human-powered flight time. Humans are very weak engines, and the aircraft must have airliner-sized wingspans with an under 100kg weight. This presents an extreme material demand which would be lessened in a biplane design. One could argue that time in the air is more important than speed or distance for the purposes of "having a fun toy to play with". Given these demands, why are human powered biplanes not more common?

Kevin Kostlan
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  • From your quoted question: But the main reason is that they're inefficient, and don't really do much that a monoplane can't. – Jim Oct 03 '22 at 20:52
  • @Jim: It's not doing much. A pedal biplane would fly maybe <10 mph and the pilot could be beaten on foot. It's a toy and a teaching tool, not much more. But the power usage in flight would be lower so more people could fly it and for longer. – Kevin Kostlan Oct 03 '22 at 20:56
  • Still need to look at the drag from two wings vs from one. – Jim Oct 03 '22 at 20:57
  • @Jim: The L/D ratio is worse. But the speed is so low (as described in the linked question) that the minimum power is lower even though the thrust must be higher (at constant aircraft weight). At least that's the hope! And presumably the aircraft could be (even) lighter due to the structural advantage. – Kevin Kostlan Oct 03 '22 at 21:00
  • Question could be improved to better specify exactly what you are seeking to optimize. Perhaps, "maximum possible endurance till pilot is exhausted, for 20-year-old male in average physical condition"? Etc you get the idea. Based on existing HPA history, I doubt the biplane will ever come out the winner, but we can at least get a better idea of exactly what problem we are trying to solve--. But I don't think that changing the goal from range to endurance (duration) will decisively tip the scales toward the biplane's favor. – quiet flyer Oct 03 '22 at 21:03
  • @quietflyer: Fixed. – Kevin Kostlan Oct 03 '22 at 21:08
  • Biplanes originally existed because there was not the material science to make wings long enough to provide enough lift. Due to the juxtaposition of the wings, there is more drag than a single longer wing. – WPNSGuy Oct 03 '22 at 22:13
  • "A pedal biplane would fly maybe <10 mph..." This is a disconnect. Heavier than air craft, no matter how light weight or efficient, need to generate lift. Lift comes from forward speed and <10 mph will barely move a kite, let alone something carrying >100 lbs of meat. – Michael Hall Oct 04 '22 at 15:07
  • Why not a box-wing aka PrandtlPlane aka Best Wing System? That should be even better than a biplane – sophit Oct 04 '22 at 18:28
  • @sophit the box-wing gets the fanfare but is it really better? I see it as an attempt to magick away those evil tip vortexes which can't be done for any finite wingspan. What matters is minimum power given material constraints (i.e. longer wings weigh more). – Kevin Kostlan Oct 04 '22 at 19:21
  • I don't think it is easy to say a priori what is lighter between biplane and box-wing. For sure PrandtlPlane is aerodynamically more efficient, so I'd give a try in the pre-design phase. – sophit Oct 04 '22 at 19:28
  • @MichaelHall Paragliders stall at around 12mph (20kph) with a comparable wingspan. Pushing this a little lower with a biplane design and airfoil shape isn't out of the question. – Kevin Kostlan Oct 04 '22 at 19:29
  • @KevinKostlan, can they maintain powered level flight at 12mph, or is that in a descent? If powered, does the motor put out power comparable to what a person could achieve? If so, perhaps that's a better solution than a biplane? – Michael Hall Oct 04 '22 at 19:37
  • @MichaelHall I am not sure. No human powered plane uses a para-motor design that I am aware of. I am going by our first planes that flew, back when engines and materials were weaker, which were biplanes not para-motors. – Kevin Kostlan Oct 04 '22 at 19:47
  • If I don't remember wrong the very first human powered airplanes had a speed of some 10mph – sophit Oct 04 '22 at 19:53
  • It's counterintuitive to me that you could better a paraglider's stall speed with a rigid wing of any sort. Especially one that needs to support the significant weight of "a very weak engine". Windspeed of <10 mph on the Beaufort scale is merely leaves rustling, and instinctually I cannot fathom flying at that speed. (let alone enjoying it if I'm huffing and puffing to supply the power...) Hopefully someone with some math chops can verify my hunch. Anyway, good luck! – Michael Hall Oct 04 '22 at 20:04
  • @KevinKostlan - Just a thought about paragliders. Ripstop nylon and paracord didn’t exist back then either nor did the idea of the ram-air chute. – Jim Oct 05 '22 at 01:03
  • @jim all materials improved not just fabric and rope. And the pioneers didn't care much about safety features such as ripstops. Although your point stands about the concept and design of the canopy. – Kevin Kostlan Oct 06 '22 at 02:32
  • I actually flew a human powered biplane called Chrysalis in the late 1970's while a student at MIT. As I remember it, the biplane configuration was driven mostly by the limits of hanger space rather than aerodynamics. https://en.wikipedia.org/wiki/MIT_Chrysalis – Eric S Jun 23 '23 at 17:09

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