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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:

XB70 in flight
Source: interceptor.com

Landing with wings straight:

XB70 landingSource: 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!):

XB70 perspective
Source: cybermodeler.com

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    $\begingroup$ Look at how far the wingtips extend downwards past the engines in the in-flight picture, and then look at how close the engines are to the runway in the next picture. $\endgroup$ – fooot Nov 6 '15 at 17:22
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    $\begingroup$ A confirmation of my assumption, @fooot? aeroalias' answer indicates there are aerodynamic issues, as well. $\endgroup$ – FreeMan Nov 6 '15 at 17:26
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    $\begingroup$ Yes, just pointing out that landing gear would have to be even longer than is apparent when it is level. $\endgroup$ – fooot Nov 6 '15 at 17:36
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    $\begingroup$ FWIW, the XB-70 wingtips folded down to exploit a phenomenon called "compression lift" $\endgroup$ – Anthony X Nov 8 '15 at 18:10
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    $\begingroup$ Thanks, @AnthonyX, as mentioned in the question, I understood the value of the drooped wingtips, the question was why did they fold instead of being permanently drooped. $\endgroup$ – FreeMan Nov 9 '15 at 14:22
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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.

XB-70A wing testing

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.

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    $\begingroup$ Thanks for finding that shot of the wing fold check. I worked at North American 1960 -1962, and coded the Fortran software that analyzed the static-solution loads off the wind tunnel data for the six hydraulic hinge actuators in each wing that moved the wingtips. $\endgroup$ – Terry Nov 6 '15 at 18:01
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    $\begingroup$ @Terry, seriously, will you be my grandpa so I can sit on your knee and you can tell me all your stories? I come complete with a wife and kids who will soon be producing great-great-grandkids for you! I've rummaged around your website a bit - it would be awesome if you would just start documenting all these stories there. $\endgroup$ – FreeMan Nov 9 '15 at 14:25
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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.

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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".

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  • $\begingroup$ Thanks for that. As noted in the question, I understood the purposes of the droop. The question, bolded in the OP, was why the wings were movable instead of being fixed in a droop. While adding a couple of extra benefits of the droop, unfortunately, your answer doesn't address the question at all. $\endgroup$ – FreeMan Sep 4 '18 at 11:15
  • $\begingroup$ Having the wingtips fixed at a 65 degree angle might have made takeoff and landing hazardous I think. Take another look at the photos above, and you may see what I mean... $\endgroup$ – J. Southworth Sep 4 '18 at 18:01
  • $\begingroup$ Having the wingtips fixed at 65 degrees down would have necessitated extremely long main and nosewheel gears, which would have made landing, which was already quite demanding due to the height differential between the nosewheel and the cockpit, even more difficult. So it made more sense to design the aircraft with folding wingtips. $\endgroup$ – J. Southworth Sep 4 '18 at 18:34

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