Science Questions

What produces lift during a knife-edge pass?

Thu, 5th Sep 2013

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Question

Cameron Thompson asked:

When an airplane does a knife-edge pass (wings vertical), where is the vertical component of lift coming from to keep it in the air?

 

Thanks!

Cameron

 

 

 

Answer

Dave - So, the plane is basically flying on its side. The wings can't be doing Ercoupe_plane_1941anything because they're vertical. So, the lift must be coming from somewhere else. When you watch them doing it, they're actually quite a big angle. So, they're not flying horizontally. They're sort of flying at 20 or 30 degrees to the vertical. And so, you'll be getting some lift from the side of the body of the plane, from the side of the fuselage. Some from the tail plane itself will be giving you some lift and mostly, the kind of planes which do this have got very, very large propellers which can throw an awful lot of air backwards. So, because the plane is pointing upwards, that air is being thrown downwards, and so you get an equal opposite reaction and the plane gets pushed upwards.

Chris - Thanks, Dave. Similar to the sort of thing when planes fly upside down and people say, well, if the wings are generating lift in the right way up position, why should the plane be able to fly upside down? Itís just that very high angle of attack, isnít it?

Dave - Yeah, wings are optimised to fly the right way up normally unless you've got a really stunt plane. But if you fly them upside down, as long as theyíve got enough angle attack, and you push them through the air hard enough, you will get enough lift to stay up.

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During the knife-edge pass, the propeller/jets are angled slightly downwards, and this directs a flow of air downwards.
The body of the aeroplane is angled, and this directs a flow of air downwards.
This downwards force on the air provides an upward force on the aeroplane to keep it in the air. evan_au, Wed, 17th Jul 2013

I am not an expert at all, is it possible that the rudder also is used for this purpose? lightarrow, Wed, 17th Jul 2013

All of the above. A true 90 degree bank produces no upward lift from the wings, but every nonvertical surface is acting to produce lift. To sustain a knifedge at constant height you need to angle the thrustline upwards or tolerate a significant loss of speed throughout the manoeuver. alancalverd, Wed, 17th Jul 2013

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