What produces lift during a knife-edge pass?

05 September 2013

Question

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 anything 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? It's just that very high angle of attack, isn't 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 they've 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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