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In Chuck Yeager's autobiography, he went into a little bit of detail as to why the X-1 was able to break the sound barrier safely where as the P-51D he flew during WWII could not have safely broken the sound barrier. He mentioned it had to do with the controls... Something about flow separation, as I recall, and the elevator not working as a result in most aircraft before the X-1.

I was wondering if someone could explain this though? What advances where made during X-1 development that made it stable at Mach 1?

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2 Answers 2

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The control problems are caused by a shift of the aerodynamic forces. In subsonic flight, they are mostly created in the forward part of the wing (the lift increase with angle of attack works at the quarter chord point of a wing, regardless of sweep), while in supersonic flow they work equally over the full chord. The center of lift in pure supersonic flow is at mid chord, and in reality the aerodynamic center shifts slowly back when the aircraft accelerates through Mach 1 and more of the surface is exposed to supersonic flow.

Since the c.g. position is mostly fixed (Concorde pumped fuel between tanks to shift the c.g.), this shift must be compensated with elevator deflection. The elevator must move trailing edge up, which creates a break in the airfoil contour, which in turn creates a heavy shockwave at high subsonic and supersonic flight speed. This shock causes flow separation and can lead to elevator reversal. Only when the fixed part of the horizontal tail can be moved, too, or you have a full-flying tail (the whole stabilizer is moved, not only the rear part of it) to create the needed lift change without a contour break, the aircraft can be trimmed for both subsonic and supersonic speed.

The benefit of the X-1 was a stabilizer with variable incidence, which the P-51 lacked. Sweep would have helped, too, but was not really necessary.

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I posted a question about chord particularly to help me understand the answer to this one...if you have a moment... And while I'm at it, what is "incidence" and how do you make it variable versus...fixed, I 'd guess? –  Jay Carr May 20 at 13:32
    
@JayCarr for the variable incidence of the horizontal tail look at horizontal stabilators. In these tails the whole horizontal stabilizer can rotate, not just the elevator. In the EMB-145 the elevator trim adjusts the angle of the stabilator rather than moving a trim tab on the control surface. –  casey May 20 at 15:31
    
@casey That does make some sense, but I'm not sure what you mean by "incidence". It sounds like horizontal tail did something embarrassing it would rather not talk about to my ears... –  Jay Carr May 20 at 17:59
    
@JayCarr en.m.wikipedia.org/wiki/Angle_of_incidence –  casey May 20 at 18:01
    
@casey Ah...just to double check my work: the angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the craft body? –  Jay Carr May 20 at 18:05

The phenomenon you are describing is called "Mach tuck." As Peter described, the location of the lift forces changes, which can be more than the elevator can compensate for.

There is also another factor at work. This image shows what is happening on a wing or tail surface near supersonic speeds. At the higher speeds, a flow separation is shown. This means that the flow is no longer "sticking" to the surface, instead creating turbulence in that area. This flow separation occurs at the trailing edge, exactly where the control surfaces tend to be. This means that instead of deflecting into smooth flow, they enter turbulence. This can cause buffeting in the controls, and makes the control surfaces much less effective.

The solution to this problem was to move the entire tail surface instead of just a trailing edge portion. Since this surface now acts as both the horizontal stabilizer and the elevator, it is called a "stabilator." You will notice this feature on most supersonic fighter jets.

Supersonic flow on airfoil

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So, basically you end up compensating for decreased flow over the back of the rear vertical stabilizer by simply moving the whole thing? Is that right? –  Jay Carr May 20 at 15:44
    
Yes, this is primarily for the horizontal stabilizer but can also be the vertical stabilizer. –  fooot May 20 at 15:46
    
I always forget if the "horizontal" or "vertical" refers to the angle of the surface or the axis being stabalized... Sorry, got that backwards. –  Jay Carr May 20 at 15:48

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