Would a reverse wing sweep be better for speeds above Mach 2

[Joseph Wood]

I am curious about a reverse wing sweep because it could be used on an aircraft for turning at high speeds at higher than Mach two.

 

[FactChecker]

There are a lot of trade-offs with a forward swept wing. One that comes to mind in modern fighters is that the yaw stability is reduced. If stealth is desired, the vertical tail is minimized and there is already a danger of yaw instability. But that is just one of many considerations.

 

[Borek]

One that comes to mind in modern fighters is that the yaw stability is reduced.

I thought stability is the last thing you want in a modern fighter? I mean - aren't they designed to be unstable, as it allows fast maneuvering?

 

[FactChecker]

I thought stability is the last thing you want in a modern fighter? I mean - aren't they designed to be unstable, as it allows fast maneuvering?

You are right that you don't want too much stability because the plane will be slow to react. But that is mostly in the roll and pitch axes that are critical for maneuvering. I don't think that there is any benefit to having a low yaw stability margin (this is pushing the limits of my knowledge of the subject). Of course the plane must not be so unstable in any axis that the flight controls can not handle it. The Air Force will specify that the plane must have a certain minimal closed loop stability phase and gain margins. The margin allows for some safety, even with deterioration in the plane and controls, future modifications, and new weapons loaded.

 

[gleem]

Grumman X-29 tested this concept
https://en.wikipedia.org/wiki/Grumman_X-29

 

[Noisy Rhysling]

Grumman X-29 tested this concept
https://en.wikipedia.org/wiki/Grumman_X-29

Just the bird I was thinking of.

 

[FactChecker]

Some additional interesting information on the X-29 is at https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-008-DFRC.html

 

[Noisy Rhysling]

Earlier concept.

 

[gleem]

Earlier concept.

Considering that Grumman need a computer to control the X-29 I wonder what the results of the test flight of the German prototype were.

 

[Noisy Rhysling]

Considering that Grumman need a computer to control the X-29 I wonder what the results of the test flight of the German prototype were.

Never flew.

 

[Jonathan Scott]

A big problem with reverse wing sweep is that it is liable to cause nasty instability and flutter problems (described as "aeroelastic divergent twisting" in the Wikipedia article on the X-29) when the wing flexes. A small amount of flexing obviously tends to twist the end in a direction which will tend to amplify the effect. For the X-29 they did some clever engineering tricks to overcome that such that bending the wing one way caused the end to twist in the opposite direction.