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Are all-moving rudders more stealthy? If so, how come YF22 which is just as fixated with rear body stealthiness as the YF23 comes with conventional ones?

Also, all-moving rudders have fixed camber, which means they are far more efficient during supersonic flight, right? Another reason for an all-moving design.

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As a general rule when you see an all-moving surface vs a fixed fin and control surface it's because you get more control authority per unit of surface area, so the surface itself can be smaller. Better stealth, less drag. You lose the passive weathervaning effect that provides basic yaw stability you get with a fixed fin, but with a fully active artificial stability system, that might be acceptable to the designer (although maybe not to the Air Force).

The 23 went with what amounts to a V tail, on top of all-moving surfaces, with the surfaces providing yaw and pitch authority at same time, and a fully active artificial stability system to make up for the lack of fixed surfaces. This would have had much less drag than the regular horizontal and vertical surfaces, and probably a better radar signature to boot.

Why it lost out to the 22? According to Wikipedia, it was faster and had better stealth, as your would expect from the tail arrangement, but was less agile, so we know what the Air Force's priorities were. And there was probably a range of other factors that made the 22 a better choice overall (it could be that the 23 was just a bit too radical, which may have impacted maintainability, reliability, and cost factors in a way that made the 22 look like a safer bet).

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  • $\begingroup$ Sure, but why LM decided to go with a conventional rudder? the f22 relies on a computer to be flyable and has all moving elevators anyway. $\endgroup$ – Meatball Princess Jan 17 at 17:40
  • $\begingroup$ Maybe they decided they wanted the passive yaw stability of a fixed fin. You see very few all moving rudders. The only other ones that spring to mind is the SR71 and the Zenith homebuilts although I'm sure there are others. $\endgroup$ – John K Jan 17 at 17:57
  • $\begingroup$ @JohnK the Sukhoi PAK FA also has all moving rudders; however, the Russians weren't too concerned about stealth. All the hype about it being more maneuverable than the F-22 goes to the wayside when you consider that it's more a turn-fighter with stealth as an afterthought rather than a stealthy fighter with good maneuverability. Point is, you're right. The buyer's priorities in the ATF competition was practicality first, and honestly militaries should all be putting practicality first. i.e. the T-14 sucks even though everybody's crying about how "amazing" it is. $\endgroup$ – Jihyun Jan 18 at 4:59
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    $\begingroup$ @JohnK in the case of non-reversible hydraulic control systems I don't think an all-moving rudder is free-floating; therefore it does provide the same stabilizing yaw effect ("weathervane" effect) as does a fixed fin of the same size. We see the same in radio-controlled planes-- some rc gliders have all-moving vertical fins and many have all-moving horizontal stabilizers and since the surfaces are connected to servos and not free-floating, the dynamics are quite different than we'd see with an all-moving surface connected by cables and pulleys to a control stick or pedals. Food for thought-- $\endgroup$ – quiet flyer Jun 16 at 16:42
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    $\begingroup$ @quietflyer: At the maximum dynamic pressure of an F-22/23 the yaw stability is determined a lot by aeroelasticity, which gives a fixed vertical an edge. With active stabilisation, however, both flight dynamics and aeroelasticity can be compensated, and the remaining advantages of the all-moving vertical are indeed only at supersonic speed and in stealth. Given the higher complexity and possibilities for failure of active stabilisation, the fixed vertical is the more robust solution. $\endgroup$ – Peter Kämpf Jun 16 at 20:47

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