What are the advantages of using all-moving control surfaces? can someone give explanations based on aerodynamics?


The biggest two reasons to have a stabilator (the most common all-moving control surface) are stability at supersonic speeds and increased maneuverability.

As an airfoil approaches its critical Mach number, the diverted flow moving past the wing's leading edge, because it has to move faster than the wing itself to follow the contour, starts to exceed Mach 1 and then decelerates below it, causing a transsonic shockwave to form behind the wing's leading edge. This shockwave causes boundary separation of the airflow at the trailing edge of the wing, where traditional elevator surfaces are placed. This reduces the wing's lift and also the effectiveness of elevator surfaces, a phenomenon known as "Mach tuck". By instead moving the entire control surface, this problem is avoided as the entire airfoil, regardless of the quality of the flow of air over any point of it, is used to direct airflow and thus rate the aircraft's nose.

Second, and more intuitively, the larger the elevator surface, the more air it redirects and thus the more force it places on the tail section, in turn allowing the pilot to pitch the aircraft at higher rates. The ideal endpoint of this line of thought is that the entire stabilizer also becomes the elevator surface; a "stabilator".

Both of these considerations are critical to the design of fighter jets, which as of the fourth generation were almost all capable of Mach 1 (and a few could exceed Mach 2), and which also have to be highly maneuverable even as the moment of inertia of the airframe in the pitch plane increases (an unavoidable side effect of wrapping an airframe around two high-performance jet engines traversing the length of the fuselage).

  • $\begingroup$ I always thought it was the kink in the airfoil contour (in case of a elevator deflected upwards) that caused a lot of problems in terms of shock waves and that this was a reason to install all movable airfoils. Am I completely mistaken, or is this another reason for all-movable elevators? $\endgroup$ – ROIMaison Jun 29 '15 at 20:41
  • $\begingroup$ In fully supersonic flight that may well be the case; however, transsonic airflow causes the problems I mentioned long before the aircraft itself gets to Mach 1, and these problems are often encountered on aircraft with sufficient power to approach Mach 1 but which aren't designed for transsonic flight. $\endgroup$ – KeithS Jun 29 '15 at 20:48
  • $\begingroup$ But I guess a kink can also give problems before mach 1, as the increased local curvature will greatly increase the pressure, I don't think you need supersonic flow for this $\endgroup$ – ROIMaison Jun 29 '15 at 20:52
  • $\begingroup$ Many thanks for your great explanations KeithS! Apart from the reasons given by you (shockwaves, Mach tuck and more force), I have always wondered is there any aerodynamic theories or calculations than can be proved or related to those considerations? Does larger surface area on all moving control surfaces really provide more moment as compared to conventional tailplane (fixed+elevator)? Why most of commercial aircraft not using all moving control surfaces? $\endgroup$ – syahmi amir hamzah Jun 30 '15 at 9:57
  • $\begingroup$ @syahmiamirhamzah - All-moving control surfaces increase the maximum theoretical force the tailplane can provide because the force on the surface is a function of the relative angle of the airflow and the area that airflow impacts. The bigger the surface, the more force the tailplane can produce. In practice, you may not see a difference because there are other loading limits inherent in the airframe design. This is why commercial airliners don't use them; the forces produced exceed what the aircraft is required to do (and thus is built to withstand). A 747 doesn't need to execute a 9G turn. $\endgroup$ – KeithS Sep 23 '15 at 18:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.