Does a v-tail offer true 3 axis control in all areas of a basic flight envelope?

Question

Does a v-tail offer true 3 axis control in all areas of a basic flight envelope?

** Based on the comments below, I've slightly changed the question from "Can a v-tail offer true 3 axis control?" to the above.

I understand in a classic configuration, the ailerons control roll, the elevator controls pitch, and the rudder controls yaw, but in a v-tail, the elevator and rudder are part of the same control surface.

So can a v-tail offer true 3 axis control in all areas of a basic flight envelope?

My gut feeling is .... No!!

I stand to be corrected but, I don't think you can use a v-tail to fly at maximum AoA during a forward slip with crossed controls). Eg. Full right rudder, full left aileron, full pitch up.

Is this correct?

Answer 1

Yes they do the job of both rudder and elevator at the same time. They're called "ruddervators".

To provide normal input/output function, you have to operate the ruddervator surfaces through a mixer unit, a mechanical summing device that operates the two surfaces one-up-one-down when providing a yaw force, that is, when moving in response to a rudder input, and both-up-both-down when providing a pitch force, that is, moving in response to an elevator input.

So when you move the stick back and forth, both ruddervator surfaces move up and down together, and when you move the rudder pedals, they both move from side to side together. Moving both controls at the same time results in a "blended" result that achieves the required pitch and yaw force in proportion to the control inputs.

So, for example, stick half way back, both surfaces half way up. Hold the stick at that position, and apply right rudder, the right ruddervator will move toward neutral, and the left one will move up MORE. This keeps the original pitch force in place, while also inducing a yaw force to move the nose right.

You have to allow for the full aft stick + full rudder case in the geometry of the linkage and the travel range of the surface itself, meaning the travel range of the surface must be more than normally required for each movement individually. So, for example, you might give the surfaces themselves 30 degrees of range up and down. Full up elevator might move the left and right surface to 15 degrees up. Full up elevator AND full right rudder may move the LEFT side to 30 degrees up, whereas the right surface might move down to neutral. Move the stick forward a bit while holding the rudder input, and the left surface might come down to say 25 degrees, and the right surface might go below neutral a bit. To accommodate all this, you may have to make them bigger, partly negating the drag benefit of only having two.

Answer 2

This is largely a matter of human conception, because of an arbitrary design choice about controls. But imagine it was another way: Pedals for ailerons, and a single stick for elevator/rudder.

Or, an intermediate linkage which has the same effect, allowing us to keep "normal" controls.

At the bottom of the stick, simply welded to it, is a circular swashplate. The swashplate corresponds to the vector of force you want from the tail. Wow, that's super straightforward!

Well now, you can have any number of fins (think fins on a rocket)... simply tie each fin to the corresponding point on the swashplate, phased** so it produces the correct net force to correspond to swashplate angle. You could have 3 fins at 120 degree angles, and simply attach their Bowden cables to 120 degree points on the swashplate. 5 fins, 7 fins, sure. Asymmetrical fins as airplanes have, not a problem.

The only thing you couldn't have is all fins in the same axis, because then you could only exert force in one axis and you'd be uncontrollable in the other.

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• Remember that if you want the "stick back for elevator up" paradigm, there will be a conceptual 90 degree rotation on the swashplate, so a horizontal elevator attaches to the top and/or bottom, with rudder on the left.

** However you would have to account for reversing/phase: to illustrate, suppose for redundancy's sake you attached left and right elevator separately to top and bottom of the swashplate. One would need a reverse action so they work together and not opposite.

Answer 3

Does a v-tail offer true 3 axis control in all areas of a basic flight envelope?

This updated question requires an updated answer: a v-tail does indeed provide full 3-axis flight control, until one of the ruddervators hits its travel stops.

When deflecting the pitch stick fully aft, the ruddervators deflect fully up. If we’re inputting pedal deflection on top of that, only 1 of the moving flight controls can deflect, the other one is in its travel stop already. This results in a flight envelope with more limited directional control. Same goes for pitch stick input with full pedal deflection of course.

The 3rd DoF mentioned in the question, roll, will add to the limited control credit per DoF, while not being very effective due to the limited moment arm compared to ailerons near the wing tips.

The 3 DoFs pitch, roll and yaw are mentioned in their order of importance to flight control:

• Pitch control is most critical for flight safety;
• Roll control most for manoeuvrability;
• Yaw control handy to have during sidewind landings with one engine out for instance.

Of course, the above is a generalisation, but does provide the design parameters for the control surfaces. Best to keep them independent - when combining them, an ability to trade in DoFs must be built in: make sure that it is always possible to input maximum pitch control.

Answer 4

Sure it can. A Pelican or V-tail affords pitch control via differential movement of the ruddervators and yaw control via synchronized movement of the ruddervators.