[nf05papsjfVbc]

Are you suggesting that if we have two planes of the same size and shape, the heavier one displaces more air?

I can understand that objects floating on water follow that principle but that's because they are floating. Once fully suspended in water, I think the amount of water they displace is just their volume. Shouldn't it be likewise for aircrafts in the atmosphere?

[mcguire]

To maintain level flight, a plane has to generate lift equal to its weight. It generates lift by forcing a mass of air down. More mass means more air disturbed.

[nf05papsjfVbc]

Interesting. I never thought of it this way. Thanks for helping me see the missing part of the picture.

[mcguire]

When people explain flight through Bournoulli or whatever, they tend to forget Newton's basic laws, which really are the simplest way to understand flight. It won't make you an aerospace engineer, though.

[brian_cloutier]

Ah, you're right, thanks for clarifying. My comment now looks pretty dumb, wish it didn't get so many upvotes.

[mzs]

You are describing ground effect not lift.

[mcguire]

Note: I am not an aerospace engineer.

Not quite. Outside of ground effect, a downward force is applied to the air mass around the aircraft, resulting in lift on the aircraft. IIUC, this results in compressed, high pressure air below the plane and low pressure air above it. The result of that is a pair of vortices roughly centered on the plane's wingtips. These vortices themselves are Somehow Important to the whole thing. Anyway, you can see the deal when a plane flies close to a cloud top.[1]

In ground effect (i.e. the aircraft is flying within roughly one wingspan of the ground), the proximity of the ground blocks the formation of the wingtip vortices (?) and greatly enhances the efficiency of lift production (by some form of magic, AFAIK).

[1] https://s-media-cache-ak0.pinimg.com/564x/1c/33/e2/1c33e2f2f...

[mzs]

You need to sum craft weight and lift when applying Newton's laws in figuring downward pressure, more lift -> less ground pressure across all craft weight.