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I was looking at some aircraft as an inspiration for a project and found the X-02 Wyvern from the Ace Combat series and one feature that seemed interesting (let's ignore its Swiss Army knife wings) was the aft stabiliser's ability to change their angle.

Would there be any benefits to it if the airplane could change the angle dynamically, say from -35 to +35 degrees on the roll axis?

Aft stabilisers folded down

Stabilisers in their up position

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    $\begingroup$ Could you turn that round and consider how much it might cost to let the aft stabiliser's change their angle? Does that suggest the ability might be worth sqillions? If you doubt there'd be any benefits to a plane changing the angle, say from -X to +X degrees on any axis, what benefit do you suggest any given, fixed angle would offer? $\endgroup$ Jul 30 at 22:07
  • $\begingroup$ @RobbieGoodwin if there is a benefit, then the cost problem is, as they say, "just" engineering. $\endgroup$
    – fectin
    Jul 31 at 20:46

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An advantage of aircraft without vertical stabilizer is their lower radar signature.

A disadvantage is the poor stability on the yaw axis, which can be compensated by vector thrust or split elevons, both continuously adjusted by a flight computer.

Increasing the dihedral of the horizontal rear surfaces to make it a V-tail configuration would allow to gain yaw stability in situations where stealth is no longer important, and where yaw stability is most at risk, like maneuvering at low speed and altitude without any thrust to be vectored anywhere.

This variable-tail dihedral mechanism could also help when deeply stalled, maneuvering like Starship does when controlling its terminal velocity attitude.

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    $\begingroup$ could be really interesting if the plane was able to move then dynamically, could produce interesting maneouvring capabilities $\endgroup$ Jul 30 at 17:26
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While clearly a disadvantage during take-off and landing, an anhedral tail would be rather helpful at supersonic speed.

Vertical tails pointing up produce a rolling moment which runs counter to the desired effect when starting a turn. If they point down, their roll contribution becomes helpful. However, fighters use their ailerons for starting turns, not their rudders, so this effect has little relevance in real life.

Next, consider the graph below which shows that the ventral fins of the F-104 Starfighter contributed 30% of directional stability at Mach 2. Tails pointing up at that speed are much less effective because air density above the fuselage is much lower than below at supersonic speed.

F-104 ventral fin contribution to directional stability

This means that rotating the vertical tails around the roll axis allows to make them smaller because they are more effective when it counts. However, so far the downside of mechanical complexity and added mass seem to have prevented this.

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Yes, absolutely. They are called elevons and can be found on many delta designs.

Elevons can function oppositely as ailerons, or together as an elevator.

The benefit is that, at very high speeds, having them closer to the fuselage on a strong structure reduces torque stress on the wings, which can warp if the aileron force is too great.

If one wished to lower the vertical stabilizers to a horizontal position, this would result in a loss of directional stability on the yaw axis.

Raising or lowering an aft horizontal surface to vertical, as with the XB-70, can have beneficial effects by increasing yaw stability, and increasing lift coefficient at supersonic speeds when lowered.

Raising the canards would result in a loss of yaw stability. It may be better to bring them completely in line with the wings at higher speeds, as wing strakes, and to raise them up at lower speeds/higher Angle of Attack as stall inhibiting canards.

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  • $\begingroup$ The OP asked about rotation around the roll axis, elevons rotate around the pitch axis. $\endgroup$
    – Bianfable
    Jul 30 at 13:38
  • $\begingroup$ He's talking about their function as stabilizing surfaces, where the thing morphs by the introduction of dihedral from rudderless horizontal tail only (top picture), into a V tail (bottom picture). An elevon function involves the surface moving on its pitch axis. $\endgroup$
    – John K
    Jul 30 at 13:50
  • $\begingroup$ Please see edits. Perhaps loss of yaw stability may help for fighters, but be ready for ... "you're gonna need a bigger computer". The ability to raise or lower them would make for excellent spin recovery research. $\endgroup$ Jul 30 at 13:53
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Yes they have a benefit, as demonstrated by the F-16 all-flying-tail (the stabilators) for almost half a century.

enter image description hereCropped image from the wiki

With all of the horizontal tail deflecting there is a huge amount of pitching moment created, great for manoeuvrability and tight turns. There is also a varying amount of passive aerodynamic pitch stability, no problem if the airframe does not require that anyway because of the artificial stability generated by the fly-by-wire inputs.

So yes, very beneficial, with structural and controllability issues being introduced which can be solved by active control engineering.

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    $\begingroup$ I didn't know the F-16 stabilators could move around the roll axis. I thought they just rotate around the pitch axis. $\endgroup$
    – Bianfable
    Jul 31 at 10:14
  • $\begingroup$ Pitch rotation is normal, but if you look at the attached pics of the wyvern, you see that they are able to shift in the roll axis, that is what i am curious when it comes to it's potencial mobility characteristics $\endgroup$ Jul 31 at 13:25

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