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As a spin-off to the recent success of the Boeing 777x maiden flight, the effects of folding the wing tips in flight were considered.

Folding them down in cruise, like the XB-70 should increase yaw stability and create a slight anhedralling effect, which reduces Dutch Roll tendency. More importantly, it would shift the Center of Pressure forward, eliminating the need for downforce from the tail. This creates a desirable, fuel saving, staticly nuetral configuration.

But the real safety improving benefit would be to fold them down on approach to landing, requiring downforce on the tail to balance the lifting forces.

Would this not create a safer, staticly stable configuration, where approach speed can be trimmed?

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    $\begingroup$ Can you think of a particular accident that this might have prevented? $\endgroup$ – Terran Swett Jan 26 at 15:03
  • $\begingroup$ @Terran Swett do you need an accident before improving safety? Folding the wing tips (for swept wings) moves the CP forward, eliminating the need for tail downforce. This is cruise configuration. The safety comes in converting back to tail downforce static stability on approach and landing. You can now trim for speed. This will be helpful on take-offs too. $\endgroup$ – Robert DiGiovanni Jan 26 at 22:58
  • $\begingroup$ @RobertDiGiovanni airliners are already longitudinally stable, do you want more stability? Why? Excessive stability is not necessarily a good thing. $\endgroup$ – AEhere supports Monica Jan 26 at 23:26
  • $\begingroup$ @AEhere supports Monica longitudinal stability and static stability are 2 different considerations. Static stability is undesirable in cruise, so "modern" aircraft set CG further back (but not so far back to be directionally unstable) to be staticly nuetral, which avoids drag penalty. The wing tip idea (for swept wings) is to move CP forward so tail downforce is not needed in cruise. On approach, wingtips are folded down and elevator downforce applied, creating a "staticly stable" configuration that can be trimmed for speed. $\endgroup$ – Robert DiGiovanni Jan 26 at 23:57
  • $\begingroup$ @RobertDiGiovanni " longitudinal stability and static stability are 2 different considerations" well, yes, in that each of those terms is independent of the other: you can have longitudinal static stability, for example. "Static stability is undesirable in cruise" are you sure of this? Or do you misunderstand the terminology? Because I'd say having an aircraft that does not try to pitch further into a dive is in fact, desirable. $\endgroup$ – AEhere supports Monica Feb 12 at 9:45
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A few issues make fold-up wing tips unfeasible:

Weight and complexity of a mechanism and control system for it.

Shifting CP forward makes the airplane less stable statically. In any case, if you want to reduce trim drag, just move the C of G aft to unload the tail to get the same result, and use computers to artificially induce static margins, which is already done.

The stall speed reduction from the extra wing area is quite small. A wing tip that added 10% to wing area when dropped would only drop the stall by a few knots. The stall speed reduction from adding flaps dwarfs the reduction you get from adding area.

At high altitudes, you want the extra span so you wouldn't want to retract the tips anyway. On an airliner that cruises sub-sonic at high altitudes, there's no benefit to reducing span. On sub-sonic airplanes, where you clip off the wing tips, it's to improve speeds at low altitudes (like the clip wing Spitfire, done to improve performance below 20000 ft - for the very high altitude versions used to take on JU-86s, they added span).

The only practical reason to have folding wing tips on an airliner is to reduce span on the ground, as an alternative to winglets (which are used to gain some of the benefit of added span without taking up space or adding to bending loads on the wing), but a winglet is a much cheaper solution.

Which, as always, is why you haven't seen anybody do folding wing tips in the last 60 years, whereas winglets are everywhere.

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  • $\begingroup$ Yes, but using computers is so old fashioned! And the 777 has a bit more power than the JU-86. I had worried about me mechanically moving the wing tips, then realized they could be unlocked and flown into position with aerodynamic force. Once in position they become .... winglets! $\endgroup$ – Robert DiGiovanni Jan 26 at 8:07

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