# What are the pros/cons of all-moving rudder?

All-moving flight control surface (stabilator instead of a stabilisator+elevator) are common on supersonic aircrafts but this is usually used for pitch control only. For rudder, this design (one all-moving piece instead of 2 separated pieces, the rear one being the one moving) is not common. The only example I think of is the SR-71. What are the reason for adopting this all-moving rudder on the SR-71? Why is it not spread? In short, what are the pros and cons of all moving control surface for yaw control?

• The PAK FA/T-50 has all-moving stabilators, and so does China's J-20. Jul 29, 2015 at 17:09

All-moving control surfaces are like regular surfaces, only that the moveable part extends over 100% of the chord. Since the effectivity of a control surface grows with the square root of the chord fraction $\lambda$ of its moveable part, an all-moving (also called full-flying) control surface has twice the effectivity (defined as normal force change per degree of movement) of one with a 25% chord length of its moveable part.

Effectivity of a control surface over chord fraction $\lambda$

Hermann Glauert was the first to calculate the inviscid lift due to control deflection, and the result of his work is shown above. The truth (as defined by wind tunnel results) is somewhere between his curve and that of my simple approximation.

## Control surface in subsonic flow

Deflecting a flap causes the camber of the control surface to change. This is a positive effect in subsonic flow and allows the resulting airfoil to create more lift in the desired direction. Also, the orientation of the leading edge is maintained, causing much smaller suction peaks there than when the full surface is deflected. Suction peaks require a long pressure rise, which promotes flow separation when its gradient is too steep.

## Control surface in supersonic flow

In supersonic flow, camber increases wave drag, so the full-flying control surface is more effective. Pure supersonic flow has no suction peaks, and flow separation is less of a concern. Therefore, full-flying surfaces are better for supersonic flight, while hinged surfaces are the better choice in subsonic flow. Especially in transsonic flow, at least a moveable stabilizer is necessary to avoid control reversal due to the contour break of a hinged control surface.

## Stealth aircraft

Stealth aircraft benefit from large, coherent surfaces, and the gap of a control hinge will increase their RCS. Even subsonic stealth aircraft like the Lockheed F-117 and the Northrop Tacit Blue use full-flying control surfaces for their ruddervators, as does the supersonic Northrop YF-23.

## Why the SR-71 rudders are special

An intake unstart at Mach 3.2 is a violent event, and the SR-71 needed all the yaw control authority available to compensate for the sudden asymmetric loss of thrust. Its rudders were optimized for supersonic use, and the actuator could be driven to its extreme position by an explosive charge which was fired automatically when an intake unstart was detected. To ensure optimum intake operation, the engines were positioned at mid-span, and the yawing moment of thrust changes was much higher than that of conventional fighter aircraft which have their engines close to the centerline for better roll agility. In addition, they use their rudders mostly in subsonic flight; only the size of the vertical tail is determined by yaw stability at top speed.

• @ManuH: Those details are from Ben Rich's biography. I guess most if not all of it can now be found on SR-71 fansites. Jul 31, 2015 at 12:06

Pros:

• Higher rudder sensitivity; moving the entire vertical stabilizer increases the surface area redirecting airflow, allowing higher yaw rates using rudder only.
• Better performance at low airspeeds, for much the same reason.

Cons:

• Increased forces on the vertical stabilizer at higher speeds, possibly in excess of materials strength; requires a limiter system such as FBW to account for airspeed and reduce stabilizer rotation at higher speed to produce a constant yaw force for a given rudder pedal input and to prevent overstress.
• Increased interference drag. Interference drag occurs when aerodynamic surfaces meet at perpendicular angles; air being moved in one direction encounters air being moved perpendicularly at the same time, causing energy to be spent compressing the air. All-moving control surfaces typically preclude the use of fairings to reduce this phenomenon.
• Causes more of a roll moment in the direction opposite the yaw. Stabilators on fighters are also often used as "elevons", producing roll as well as pitch control. A vertical all-moving surface in the same configuration as an ordinary tail would provide an unbalanced force on the top of the airframe to roll the aircraft in the opposite direction to the yaw, only magnified by the increased surface area redirecting airflow. You could correct this with an equal or at least offsetting vertical all-moving surface below the empennage.
• Interference with horizontal stabilators. Traditional elevators/rudder are easier to keep out of each others' way, but when the entire stabilizer rotates, this becomes more difficult. Typically pitch is emphasized over yaw, and so an all-moving vertical stabilizer would be limited in total travel even at low speed to prevent binding the horizontal stabilator. You could stagger horizontal and vertical surfaces but that reduces the moment arm that the closer one has, offsetting the gains of an all-moving surface.

Other reasons it's not used when stabilators are include the fact that emphasis in aerobatic airframe design is typically on maximizing pitch and roll. Rudder is typically used to augment yaw produced by bank in a coordinated turn and so the amount of force the rudder provides is of least overall importance compared to the other primary control surfaces.

• You can control rudder forces easily by picking the right hinge line. There is no need for FBW when control forces need to be tailored. Jul 29, 2015 at 17:06
• Having flown an aircraft with an all-moving stabilator and no anti-servo tab, a more sensitive all-moving rudder is definitively not a pro, but a con. The rudder is a blunt force instrument to keep the airplane coordinated, and doesn't require the fine grained control of the elevator or aileron to maintain precise altitude and heading.
– rbp
Jul 29, 2015 at 19:11