I notice that, for example, human-powered flight operates at low altitudes. This might of course be due to safety but I wonder if in fact the delta in air pressure is greater at lower altitudes and this prevents low-powered aircraft from reaching higher altitudes?
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$\begingroup$ Size and shape of the wing? $\endgroup$– user207455Commented Jun 4, 2019 at 4:17
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$\begingroup$ @SolarMike: For a given wing shape, does it require the same power to maintain a lower altitude vs a high altitude. If the plane started at a very high point, like Denver, would it in fact not even be able to take off but at sea level the added air pressure would allow take off? $\endgroup$– releseabeCommented Jun 4, 2019 at 4:27
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$\begingroup$ what does your research show? $\endgroup$– user207455Commented Jun 4, 2019 at 4:28
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$\begingroup$ I have seen discussions of planes, which use air to burn fuel or produce thrust, needing longer runways at higher altitudes. I have never seen anything which discussed why specifically human-powered flight remains so close to the ground or even a record for altitude achieved (only speed and distance). $\endgroup$– releseabeCommented Jun 4, 2019 at 5:00
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$\begingroup$ human powered flight often makes use of thermals as do birds... $\endgroup$– user207455Commented Jun 4, 2019 at 5:03
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2 Answers
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Human-powered flight is only barely possible because humans cannot generate large amounts of power. Human-powered planes therefore fly almost exclusively in what is called ground effect, where the plane is no more than about one wingspan off the ground. When flying this close to the ground, the plane is partly supported by a "bubble" of air that is caught between the underside of the wing and the ground, which reduces the amount of drag experienced by the plane and enhances the lifting power of its wings. This in turn reduces the power requirement to keep the plane in the air.
Regarding small planes with low-powered (piston) engines, they are limited in how high they can fly because as they climb they move into air which is less dense and which therefore furnishes less oxygen for the engine to burn with its fuel. This causes the power output of the engine to decrease with increasing altitude, and at some point the engine's power output is so diminished that the plane can climb no further.
In small planes, this maximum altitude point can be increased by putting a bigger engine in the plane, reducing the plane's weight, or putting a compressor on the engine's intake to furnish it with denser air at high altitudes so it can deliver more power.
In large planes with jet engines, those engines are specifically designed to deliver high power and best economy at high altitudes.
answered Jun 4, 2019 at 5:16
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$\begingroup$ I think this ground effect thing makes sense. $\endgroup$ Commented Jun 4, 2019 at 7:32
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Here's what happens when airplanes go to higher altitudes.
First, there's less oxygen, so they have to have ways to get enough, like turbo-charging. Jet engines have built-in turbo-charging, and supersonic engines also use ram effect.
Second, since the air is less dense, they need to go faster to get the same lift. That's a good thing. It's why long-distance aircraft go as high as they do. But there's a problem. In colder air the speed of sound comes down, and if their airspeed approaches the speed of sound they can get into problems. This is called the "Q-corner" or "coffin-corner". To go faster than that they need to be designed for supersonic flight. So, for example, subsonic transports are limited to about 40k feet, while the supersonic Concorde cruised at 60k feet.
For human-powered flight, it's strictly a matter of having enough power to overcome drag. Niels is right about ground effect. When an aircraft is within one wing-length above the surface, drag is greatly reduced, so much less power is needed. If you take a flying lesson you learn to watch out for this, because you don't want your aircraft to "float along" when you're trying to get it on the ground. But it can help if you're taking off on a poor surface. You get into ground effect and stay there while you accelerate.
answered Jun 8, 2019 at 20:18