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Wikipedia describes ground effect as "the increased lift (force) and decreased aerodynamic drag that an aircraft's wings generate when they are close to a fixed surface."

That's all fair and good, but when they state "close to a fixed surface" they pretty much assume the surface would be the ground or another large surface under the wing (hence the name).

What I'm wondering is, in the unlikely event that a plane were to fly near a ceiling (so the fixed surface would be over the plane), what would be the net effect? Will there be more, or less lift? Drag? Is there something else that could be affected which I'm not considering?

I can't think of any realistic scenarios where this could happen besides someone flying an RC toy plane in a warehouse (that's probably why it's much less documented than ground effect) but I was thinking about this and was curious.

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  • $\begingroup$ You can experience this with an indoor helicopter: It needs much less power to fly once it is close to the ceiling. Since the rotor is on top, the effect is even more pronounced than the "conventional" ground effect. $\endgroup$ – Peter Kämpf 2 days ago
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A plane flying just below a ceiling would experience a beneficial effect very similar to a ground effect. It would have greater lift and smaller drag than if there were no ceiling.

This can be seen most easily by considering a typical 3-D wing to be operating in an adverse flow field caused by counter rotating vortices located at the wingtips (normally called a downwash flow).

Adding a planar surface either below or above the wing can be modeled using reflections of the original tip vortices. The reflected vortices have signs opposite to the original ones, so they create an upwash flow that partially offsets the original adverse downwash flow. I repeat, this effect is the same whether the reflection plane is above or below the wing.

This sketch shows the wing with its tip vortices above a ground plane: enter image description here

And this very similar sketch shows the wing & its tip vortices below a "ceiling plane":

enter image description here

A realistic scenario where the "ceiling effect" can happen is a hydrofoil below the surface of the water. In linearized flow models, there are two conditions where the water's surface can be considered planar. In the low-speed limit (small Froude Number), the reflected vortices have opposite sign to the originals, just like in your "ceiling-effect" case, and the effect is beneficial. At the high speed limit however (large Froude Number), they have the same sign, so the effect is detrimental.

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