Why weren't the XB-70's wingtips fixed in the down position?

Question

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:

^{Source: interceptor.com}

Landing with wings straight:

^{Source: Gawker Media}

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!)}:

^{Source: cybermodeler.com}

Answer 1

Well, for starters, the drooped wing is not producing any lift in subsonic speeds. So, the flat area (the fixed part) should be significantly larger for producing the lift. This increases weight, which leads to a vicious cycle.

Another thing is the wing would panels would not clear the ground while the aircraft was on its landing gear. For testing the wing fold in ground, the aircraft had to be put on tall jackstands.

Image from keypublishing.com

As you noted, lengthening the landing gear enough to allow the wing droop would have been a big issue not the least because the aircraft weight was already increasing alarmingly (the weight had shot up from 483,000 lb to 537,000 lb).

Also, you lose the outboard elevons; now for getting the same control response the inboard control surfaces have to be increased in size, which has to be come either from wing or increase the wing area- it is again a mess.

Answer 2

First, you need more wing area at low speeds in order to have a reasonable landing speed. Second, you want downward turned wingtips at supersonic speeds to create compression lift. At supersonic speeds the center of lift moves aft making the airplane pitch down. You have to trim this change away and that creates drag. In the case of Concord you pump some fuel to the back of the airplane to keep everything in balance but on the XB-70 you eliminate some wing area on the back of the airplane by turning the wingtips down. So at subsonic speeds you need the wingtips to create lift but not at supersonic speeds.

Answer 3

The purpose of the pivoting wingtips, which could be rotated downwards to an angle of 65 degrees from the horizontal during supersonic flight was to improve directional stability, while shifting the centre of lift forward to a more favourable position for high speed flight, and at the same time helping the compression lift effect by confining the shockwave generated by the sharp vertical central splitter plate of the air intake between the downturned wingtips, thereby creating a positive pressure area underneath the wing. This use of the compression lift principle made possible a 30% reduction in induced drag at high speeds. The XB70 in supersonic flight was said to "ride on it's own shockwave".