The XB-70's wing tips folded down in flight to:
- improve directional stability at supersonic speeds
- reduce shifting of aerodynamic center at supersonic speeds
- reduce drag at high supersonic speeds
With thanks to Peter Kämpf's answer. Actually, this question was prompted by that answer, and quite a bit of it is 'borrowed' from there.
Wings in flight:
Landing with wings straight:
My question is why did they fold, instead of being fixed in the down position?
My assumption is that the plane already sat very high off the ground (probably to give clearance for take off/landing rotation), and lengthening the landing gear enough to allow the wing droop would have been a greater engineering issue than making the wing folding mechanism. Is there validity in that assumption, or, were the benefits of the wing droop at supersonic speeds detrimental at subsonic speed?
Some perspective of just how tall the XB70 was (click to zoom - that's a tall bird!):