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In the rare event of the vertical stabilizers being ripped off and the rest of the plane surviving, could folding wingtips (given they are far enough behind the CoM to create a stabilizing moment) be used as a "replacement" for the vertical stabilizers?

enter image description here

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  • $\begingroup$ Not really folding wingtips but the Rutan VariEze uses that concept. $\endgroup$
    – sophit
    Nov 26, 2023 at 21:51
  • $\begingroup$ @sophit - As did the B-70. But those were foldable in flight, by design. aviation.stackexchange.com/questions/22745/… $\endgroup$
    – WPNSGuy
    Nov 26, 2023 at 22:14
  • $\begingroup$ @WPNSGuy: yes, very good example indeed 👍 Even if in the case of the Valkyrie there was more than just increasing lateral stability. $\endgroup$
    – sophit
    Nov 26, 2023 at 22:24

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If the vertical wing tips have aerodynamic centers that are aft of the center of mass, there will be some yaw stabilizing effect by the tips.

Problem is, the effect will be quite minor, the arm being so short, and without vertical stabilizers, the overall aerodynamic center of the entire body is going to be forward of the center of mass, because there is more fuselage area ahead of the center of mass than behind it.

The turned up tips having so little "leverage" they will likely just make the thing somewhat less unstable in yaw, but not positively stable. They are just too far forward to make more than a minor difference.

Remember, it's just a big weathervane pivoting about the center of mass. Look at a side profile without vertical fins, and imagine where the overall center of the finless profile presented is, relative to the center of mass (assume the center of mass is about 1/4 of the wing chord aft of the leading edge). If the total profile footprint is still forward of the center of mass, the weathervane, or airplane, will want to switch ends and point the other way. Then it will be stable in yaw, just going backwards.

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  • $\begingroup$ Agreed. unless the tips were as far back as the vertical stabilizers, then they would not provide as much stability as the vertical stabilizers for sure. $\endgroup$
    – Wyatt
    Nov 27, 2023 at 3:16
  • $\begingroup$ When you put floats on an airplane, they move the yaw axis neutral point (aerodynamic centre) forward, reducing yaw stability, b/c the floats have more surface area ahead of the CofG than aft. That's why you usually see little finlets or ventral fins added to the tail when floats are installed. On airplanes that DON'T include fins as part of the float kit,you will notice the decrease in yaw stability when floats are installed. Nose wanders in bumps, and adverse yaw with aileron increases. $\endgroup$
    – John K
    Nov 27, 2023 at 14:09
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Well, no. There is no control authority or code to tell the wingtips to act as a replacement for the vertical stab(s) which have been blown off.

Additionally, you can't fold the wingtips up in flight.

Might they act as sort of a yaw stabilizer? Maybe. But not with any steerable control from the cockpit.

If the system had been designed to do this, like the XB-70, then yes. Since it was not, this will not work. Especially if trying to compensate for having the existing vertical stabs "blown off".

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  • $\begingroup$ Yeah I was thinking more along the lines of yaw stabilizer. Just something to make you not immediately go into a spin. $\endgroup$
    – Wyatt
    Nov 25, 2023 at 22:50
  • $\begingroup$ @Wyatt - Whatever caused the RUD (Rapid Unplanned Disassembly) of the vertical stabs might have an influence on stability. $\endgroup$
    – WPNSGuy
    Nov 25, 2023 at 23:52
  • $\begingroup$ haha yeah you're probably right. Just was wondering if in a rare case if the wingtips could provide stability. $\endgroup$
    – Wyatt
    Nov 26, 2023 at 0:54
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    $\begingroup$ "you can't fold the wingtips up in flight" - Citation required. Why not? $\endgroup$
    – Richard
    Nov 26, 2023 at 10:37
  • $\begingroup$ @Richard - I believe it is controlled by a weight on wheels switch, much like munitions release. No weight on wheels, no fold. Also, only the C model has folding wings. The -A and -B do not. $\endgroup$
    – WPNSGuy
    Nov 26, 2023 at 13:47
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I agree with the diagram by Mins, however, creating a source of drag at the wingtips would create a yaw force in the correct (rearward) direction with ample 'arm'.

The MU-2 uses drag-brakes instead of ailerons to induce both yaw and bank, however, in general, it is frowned upon to induce drag as part of maneuvering as this creates unexpected stalls in low-speed turns. i.e. MU-2.

The wingtip design would not require the wingtips to pivot upright, just remain as-is while inducing drag, such as a split trailing edge as speed brake, just like the Space Shuttle vertical fin did.

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More or less what others already said, but with a picture...

Yaw control/stabilization is a rotation around the yaw axis, created by a force acting perpendicularly to its arm (left below).

enter image description here

In your suggestion the vertical tip would create a left/right force (right above), applied with an arm which is parallel (the line between the surface and the center of gravity. This results in:

  • The force is not amplified (arm parallel to the direction of the force).

  • The resulting moment tends to translate the aircraft left or right, it has no yaw action.

No mentioning the lack of crew control, the reduced lift of the wing, the strength of the unlocked tips, etc.

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  • $\begingroup$ ah, I thought that might be the case. I was thinking that sometimes swept wings will have tips aft of the CoM, creating a stabilization effect if the tips fold up. $\endgroup$
    – Wyatt
    Nov 26, 2023 at 20:13
  • $\begingroup$ @Wyatt: You're right on the principle, this is like a twin tail, however an inefficient one, the arm is short and the area of the tip is small, limiting the torque that can be created. The wingtip area is also subject to a vortex, limiting lift. $\endgroup$
    – mins
    Nov 26, 2023 at 21:56

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