6 ms·

Something doesn't quite add up. Cessna 172 and Robinson R44 use similarly powered engines for similar payload, same on the the heavy end of transport planes vs

Something doesn't quite add up. Cessna 172 and Robinson R44 use similarly powered engines for similar payload, same on the the heavy end of transport planes vs transport choppers.

zackmorris 8y ago
You bring up a good point. There are a few things going on here: https://en.wikipedia.org/wiki/Aspect_ratio_(aeronautics) https://en.wikipedia.org/wiki/Aspect_ratio_(aeronautics) https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Theory https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Theory https://en.wikipedia.org/wiki/Helicopter_rotor#Overview https://en.wikipedia.org/wiki/Helicopter_rotor#Overview In general, the lift to drag ratio (L/D) of an airfoil is proportional to the square root of the aspect ratio (AR). For airplane wings, this hits a limit of about 60 simply due to the difficulty of reinforcing a long wing while keeping weight down. A helicopter rotor spins, so has different requirements (like greater strength near the hub) so can have a higher aspect ratio. Unfortunately the inner third of the rotor turns too slowly to contribute much lift, and the tips shed large toroidal vortices that diminish efficiency. The tips also encounter turbulence near the speed of sound which diminishes efficiency. So rotors (even on wind turbines) can't really get higher than 60% efficiency: https://en.wikipedia.org/wiki/Betz%27s_law#Betz's_law_and_coefficient_of_performance https://en.wikipedia.org/wiki/Betz%27s_law#Betz's_law_and_co... There has probably been more research in helicopter rotors over the years than increasing the L/D ratio of light aircraft, which explains how they reduced some of the discrepancy between the two. When I was doing research for the Sikorsky human powered helicopter competition (unfortunately someone already won it haha), I did a lot of napkin calculations for this sort of thing: https://en.wikipedia.org/wiki/Igor_I._Sikorsky_Human_Powered_Helicopter_Competition https://en.wikipedia.org/wiki/Igor_I._Sikorsky_Human_Powered... Success came down to having the fewest, longest, thinnest, lightest rotor blades possible: http://www.pitt.edu/~jdnorton/Goodies/TE-antiTE/ http://www.pitt.edu/~jdnorton/Goodies/TE-antiTE/ There are many ways of increasing lift using winglets, vortex generators, EHD devices, etc (a few other keywords to look for are boundary layer and Reynolds number): https://en.wikipedia.org/wiki/Wingtip_device https://en.wikipedia.org/wiki/Wingtip_device https://en.wikipedia.org/wiki/Vortex_generator https://en.wikipedia.org/wiki/Vortex_generator https://sbir.gsfc.nasa.gov/content/aerodynamic-efficiency-drag-reduction-technology-0 https://sbir.gsfc.nasa.gov/content/aerodynamic-efficiency-dr... Personally I don't think we've even scratched the surface of what's possible in L/D ratios. With modern materials science and computer simulation at our disposal, I'd like to see someone break 100. Probably by using an array of airfoils and some EHD effects. I don't think anyone is seriously looking at it though other than maybe Bigelow Aerospace (JP Aerospace) and their defunct airships-to-orbit project: https://rimstar.org/space/jp_aerospace_balloon_airship_to_orbit.htm https://rimstar.org/space/jp_aerospace_balloon_airship_to_or... https://en.wikipedia.org/wiki/JP_Aerospace https://en.wikipedia.org/wiki/JP_Aerospace https://en.wikipedia.org/wiki/Bigelow_Aerospace https://en.wikipedia.org/wiki/Bigelow_Aerospace I think this idea has merit and is one of the only ones that can compete on price with a space elevator. But it's not sexy enough, and people are still stuck on the idea of using helium instead of hydrogen, which sabotages it from ever succeeding (due to the limited supply of helium). I would love to work on revolutionary ideas like this, but like most of you, I'm building SPA CRUD apps on the web to pay the mortgage. Sorry there's a lot of info here, but hey it's black friday on HN. What else are we gonna do.

Something doesn't quite add up. Cessna 172 and Robinson R44 use similarly powered engines for similar payload, same on the the heavy end of transport planes vs transport choppers.

zackmorris 8y ago

You bring up a good point. There are a few things going on here: https://en.wikipedia.org/wiki/Aspect_ratio_(aeronautics) https://en.wikipedia.org/wiki/Aspect_ratio_(aeronautics) https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Theory https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Theory https://en.wikipedia.org/wiki/Helicopter_rotor#Overview https://en.wikipedia.org/wiki/Helicopter_rotor#Overview In general, the lift to drag ratio (L/D) of an airfoil is proportional to the square root of the aspect ratio (AR). For airplane wings, this hits a limit of about 60 simply due to the difficulty of reinforcing a long wing while keeping weight down. A helicopter rotor spins, so has different requirements (like greater strength near the hub) so can have a higher aspect ratio. Unfortunately the inner third of the rotor turns too slowly to contribute much lift, and the tips shed large toroidal vortices that diminish efficiency. The tips also encounter turbulence near the speed of sound which diminishes efficiency. So rotors (even on wind turbines) can't really get higher than 60% efficiency: https://en.wikipedia.org/wiki/Betz%27s_law#Betz's_law_and_coefficient_of_performance https://en.wikipedia.org/wiki/Betz%27s_law#Betz's_law_and_co... There has probably been more research in helicopter rotors over the years than increasing the L/D ratio of light aircraft, which explains how they reduced some of the discrepancy between the two. When I was doing research for the Sikorsky human powered helicopter competition (unfortunately someone already won it haha), I did a lot of napkin calculations for this sort of thing: https://en.wikipedia.org/wiki/Igor_I._Sikorsky_Human_Powered_Helicopter_Competition https://en.wikipedia.org/wiki/Igor_I._Sikorsky_Human_Powered... Success came down to having the fewest, longest, thinnest, lightest rotor blades possible: http://www.pitt.edu/~jdnorton/Goodies/TE-antiTE/ http://www.pitt.edu/~jdnorton/Goodies/TE-antiTE/ There are many ways of increasing lift using winglets, vortex generators, EHD devices, etc (a few other keywords to look for are boundary layer and Reynolds number): https://en.wikipedia.org/wiki/Wingtip_device https://en.wikipedia.org/wiki/Wingtip_device https://en.wikipedia.org/wiki/Vortex_generator https://en.wikipedia.org/wiki/Vortex_generator https://sbir.gsfc.nasa.gov/content/aerodynamic-efficiency-drag-reduction-technology-0 https://sbir.gsfc.nasa.gov/content/aerodynamic-efficiency-dr... Personally I don't think we've even scratched the surface of what's possible in L/D ratios. With modern materials science and computer simulation at our disposal, I'd like to see someone break 100. Probably by using an array of airfoils and some EHD effects. I don't think anyone is seriously looking at it though other than maybe Bigelow Aerospace (JP Aerospace) and their defunct airships-to-orbit project: https://rimstar.org/space/jp_aerospace_balloon_airship_to_orbit.htm https://rimstar.org/space/jp_aerospace_balloon_airship_to_or... https://en.wikipedia.org/wiki/JP_Aerospace https://en.wikipedia.org/wiki/JP_Aerospace https://en.wikipedia.org/wiki/Bigelow_Aerospace https://en.wikipedia.org/wiki/Bigelow_Aerospace I think this idea has merit and is one of the only ones that can compete on price with a space elevator. But it's not sexy enough, and people are still stuck on the idea of using helium instead of hydrogen, which sabotages it from ever succeeding (due to the limited supply of helium). I would love to work on revolutionary ideas like this, but like most of you, I'm building SPA CRUD apps on the web to pay the mortgage. Sorry there's a lot of info here, but hey it's black friday on HN. What else are we gonna do.