# Is the compressibility of air the reason high aspect ratio wings are more effecient

As stated many times here 1, 2, 3 wings generate lift by moving air downwards. This downward air creates an equal and opposite force pushing the aircraft up.

High aspect ratio wings have a greater lift/drag ratio because they are moving large amounts of air a little bit, vs lower aspect ratio wings which move a little bit of air a lot.

Now my question is why does this matter?

When using "solid" objects to generate "lift" this doesn't matter as long as the solid object is strong enough not to break. For example the image below shows 2 different sized poles (different aspect ratios) resisting the same amount of force.

Air is not a solid, and so when high pressure is exerted against it, air will compress absorbing some of that pressure and converting it into heat and potential energy.

There is also pockets of low pressure that will form which can try to "suck" the aircaft back into it creating drag. In some cases these low pressure pockets cause the water vapor in the air to even evaporate:

Is this the reason it's more effective to move a lot of air slowly, vs a little bit of air a lot? To prevent the energy from being lost to heat conversion/ storing potential energy that your aircraft will not beneficent from as you already flew out of the area of the release?

If that is true would the same apply to water which is a non compressible liquid? Is this why we don't often see boats with "high aspect ratio" displacements of water?

• Regarding the comparision to "high aspect ratio" boats, I submit that they do exist. Case in point: Rowing sculls, very long and narrow. (However, not for the same reasons.) Look up "hull speed" in Wikipedia for a better explanation than I am prepared to give. – Michael Hall Sep 4 '18 at 20:23
• Wouldn't a boat that's very long and narrow be comparable to a plane that has a low aspect ratio (short wings longer body) design? I'll look up hull speed as you suggested – YAHsaves Sep 4 '18 at 21:42
• Comparable, maybe... that's why I mentioned it. However, very different as well. Read up on hull speed, it is interesting if you are into this sort of thing. Cheers. – Michael Hall Sep 4 '18 at 21:46
• Correction: the water vapor condenses to form contrails or "clouds" around the aircraft (as in your picture), it doesn't evaporate. It's already evaporated, that's why it's water vapor in the first place. – FreeMan Sep 5 '18 at 13:32

The force to keep the object aloft is $$F=m_{object}g$$ The force generated by downwards momentum transfer is $$F=\dot{m}v$$ with $$\dot{m}$$ indicating mass flow (kilogram per second) of the air (not the mass of the object). The energy flow (power) required to impart this momentum on the airflow is $$P=\frac{1}{2}\dot{m}v^2$$ Here we can draw an important conclusion. The power requirement is arbitrarily small, by increasing the mass flow and decreasing the downwards velocity.
This is the main reason for high AR wings: by increasing the wingspan (so, for equivalent surface, we increase the AR), we increase the amount of mass flow $$\dot{m}$$ affected by the wing.