# Why are all-moving horizontal stabilizers common but all-moving rudders rare on fighters?

According to Wikipedia

Stabilators were developed to achieve adequate pitch control in supersonic flight, and are almost universal on modern military combat aircraft. All non-delta-winged supersonic aircraft use stabilators because with conventional control surfaces, shock waves can form past the elevator hinge, causing severe Mach tuck.

If this is the case with supersonic pitch control, why use an all-moving elevator but a conventional rudder? Doesn't yaw control have the same issue? Or is it because rudders are rarely used?

• The reason why you want a all moving vertical tails is you can use FBW to make it more effective hence smaller (lighter, lower drag). The reason why you don't want to do it is because it's expensive and hard to do right (e.g. reliably) and not as useful (there's no performance gain except smaller and lighter). – user3528438 Jan 31 '19 at 17:10
• @user3528438 what about control when flying supersonic? Does shockwave form past the rudder hinge? I get that mack tuck doesn't matter much for rudders because they are already at the rearmost part of the aircraft and tucking doesn't do much to yaw control. But it must've added quite a bit of drag in supersonic. I'm no pilot, but it seems you can turn by rolling with aileron and diff elevators without yaw input at all. – Meatball Princess Jan 31 '19 at 20:39
• My understanding is that Mach tuck is something else. It is caused by shift of centre of lift of the main wing from quarterchord to midchord as the flow over the wing separates as it locally exceeds the speed of sound (it is like stall, but the post-stall lift is still plenty to keep the plane flying at this speed, so it is only the difference in centre of lift that matters). – Jan Hudec Jan 31 '19 at 21:59
• … that of course does not mean shock wave from the bend between the fixed and movable surface is not a problem—it generates additional drag and reduces the effectiveness. – Jan Hudec Jan 31 '19 at 22:01
• Anyway, I suspect the answer is that the rudder is mostly needed for cross-wind take-off and landing—when at low speed changing camber with separate rudder is better than changing angle of attack—while at high speeds the passive yaw stabilization is enough—and if the rudder is not deflected, there won't be a shock wave at the hinge. – Jan Hudec Jan 31 '19 at 22:05

Basically, in fighter aircraft you need a lot more of elevator than rudder.

For most aircraft, the required size of the elevator is determined by one of the two corner cases:

• Sufficient control and balance at low speed, particularly on landing;
• Sufficient control for high-G manoeuvres.

For fighters and aerobatic aircraft, the latter is usually the main factor; for others, the former.

In contrast, rudder is usually sized for crosswind landing and, if applicable, for the asymmetric thrust (one engine out) case. It's easy to see that due to closely packed engines on most fighters and higher speeds, both these factors are relatively smaller compared to other aircraft (e.g. airliners).(1) Whereas the elevator demand gets worse. This is why.

There are two main problems that arise at supersonic speeds.

1. The centre of pressure of the wing (and the neutral point) moves back significantly. This obviously creates a strong nose-down moment (and makes the aircraft statically very stable in pitch for the same CG), requiring a lot of control to re-trim the aircraft and execute any manoeuvre. This is the reason for 'Mach tuck' (in fact, this is the Mach tuck), and not the rubbish from the quoted Wikipedia passage.

2. Efficiency of the elevator fades. At lower subsonic speeds, the elevator effects the air not only above/below it, but also ahead of it. The approximate efficiency of the conventional elevator at such speeds can be expressed as

$$k_{\mathrm{elev}} = \sqrt{\frac{ S_{\mathrm{elev}}}{S_{\mathrm{stab}}}}$$

Therefore if the elevator $$S_{\mathrm{elev}}$$ occupies half of the stabiliser area $$S_{\mathrm{stab}}$$, its effect will be not $$0.5$$ but rather $$\sqrt{0.5} \approx 0.7$$ (compared to the all-moving surface). But at supersonic speeds such propagation upstream cannot happen, and the efficiency falls to the area ratio, i.e. $$0.5$$ in this example. This favous all-moving surfaces.

The second point affects the rudder just the same, of course. But the static directional stability (point 1) doesn't increase like it happens with pitch (in fact, it can fall), and the overall demand is lower, so conventional rudder is still acceptable. Nevertheless, there are a few aircraft with all-moving vertical tail, for example, Tu-160.

In other circumstances, conventional control surfaces are better (i.e. lighter), so they are used whenever possible.

(1) This should not be confused with the whole vertical tail sizing. This one is often determined by the stability dynamics, and directional stability of supersonic aircraft can be rather bad, demanding big tail - but not necessarily big rudder.