14
$\begingroup$

Modern commercial aircraft use stabilizer to trim instead of trim tabs. What are the advantages and disadvantages of this design choice?

Have aileron trim and rudder trim evolved in a similar way to move the entire control surface rather than aileron / rudder trim tabs?

$\endgroup$
  • 3
    $\begingroup$ "entire surface" is usually used for the all moving tail (or canard) plane that supersonic aircraft have where the surface moves as single piece. While trim using movable stabilizer (as most airliners have) means the surface is still composed of two parts and both move independently. $\endgroup$ – Jan Hudec May 27 '14 at 21:41
13
$\begingroup$

Trim is used to zero the stick forces at the desired flight condition, and with the small, slow early airplanes, an adjustable spring somewhere along the control cables would do. When speed increased, trim tabs were introduced, because their trim force goes up and down with dynamic pressure, whereas the spring would always create the same force. This would require re-adjustment with every speed change.

With the increasing size and speed of airplanes, manual control required ever more complicated means of force reduction (Flettner tabs, spring tabs), and the relative chord length of all moving surfaces shrank to allow for more control with less force. The moment around the hinge of a control surface increases with the square of the chord, but the lift change for a given deflection angle increases only with the square root of the chord.

In parallel, wing loadings increased, which required high-lift devices to keep take-off and landing speeds down. These add a lot of lift in the rear part of the wing, creating a strong nose-down moment, and now a simple control deflection would not suffice to trim the airplane. The reduced chord length meant that high deflection angles would have been required for trim, reaching far into the region of deflection angles with declining effectiveness. That is when adjustable stabilizers came up: The trim change with all configurations was simply too big to be covered by the elevator alone.

Transsonic flight was next, and here the adjustable tail incidence is crucial to trim the aircraft trough the transition from subsonic to supersonic flight. A contour break from a deflected elevator could cause shocks which could produce elevator reversal. The dynamic pressures of jet flight put an end to manual control - the last, famous examples were the Canberra or the Dash-8 (Boeing 707 prototype). Now hydraulic actuators were used, but still it makes sense to use aerodynamic means to reduce control forces, because the size and power requirements of the actuators can be reduced with clever hinge moment reduction. The trim tabs were initially moved by cables, but soon they used electric actuators.

Since rudder and ailerons are not affected when the high-lift devices (Fowler flaps, slotted flaps) are deployed, they still use simple trim tabs. Newest developments are electric control surface actuators which can help to save mass and avoid the messy hydraulics. The RQ-4 uses electric actuators because the hydraulic fluid would have to be pumped continuously through the lines in order to keep it warm enough at the high operating altitude. Only the landing gear still uses hydraulics for retraction.

$\endgroup$
  • 1
    $\begingroup$ I'd say that it was the widebody that put the true end to manual controls -- DC-9s (all the way up to the last B712 that rolled off the lines) still have manual cable-and-tab controls for most things...agreed that you need a movable tailplane of some flavor for transonic flight, though. $\endgroup$ – UnrecognizedFallingObject Jul 3 '15 at 4:16
  • $\begingroup$ @UnrecognizedFallingObject … but I bet they had boosted ailerons. The real art is to design ailerons such that no force multiplication is required. That was what I meant with "manual control", but of course you are right, a boosted manual control is still a manual control. $\endgroup$ – Peter Kämpf Jan 5 '18 at 0:20
  • $\begingroup$ the DC-9 ailerons are aerodynamically boosted, but require no hydraulic boost. $\endgroup$ – UnrecognizedFallingObject Jan 5 '18 at 0:31
7
$\begingroup$

One reason for moving the entire surface for trimming is efficiency. Deflecting the elevators or trim tabs creates more drag than adjusting the whole surface. The control surfaces are more aerodynamic without trim tab deflection.

The elevator is the main surface needed for trimming the aircraft. This is because things like fuel and payload will affect the center of gravity (CG), and the elevator is trimmed to balance the airplane. In addition to this, the external loads on the aircraft change throughout the flight from things like thrust settings and lift and drag from the wing. Different airspeeds and extension of flaps, speed brakes, landing gear, etc., will also affect the balance of the airplane.

Balance left and right is less of an issue, and can be addressed by changing the balance of fuel between tanks. Aerodynamic forces tend to stabilize the aircraft in yaw, so trimming in yaw is really only an issue in rare circumstances. Rudder or thrust settings can be used to adjust as necessary.

Another reason is the structure of the airplane. The horizontal stabilizer has much less load on it than the wings. The ability to move the tail surface adds weight and complexity to the design, and this would be a much greater challenge with the wing. The efficiency benefits make this design feasible for the horizontal stabilizer, but there aren't enough good reasons to move the vertical stabilizer or the wings to justify the costs.

Aircraft that actually use the entire stabilizer surface for control are in a different situation. These are generally military aircraft that need to maintain high maneuverability, especially at very low or very high airspeeds (see this question). These aircraft are also generally smaller, so moving a smaller surface is less of a challenge.

$\endgroup$
  • $\begingroup$ Flap deflections up to 10° do not increase drag, but add camber. This increases the maximum and minimum lift coefficients of a flapped control surface, which in my eyes make it aerodynamically better than a full flying surface: It can be smaller. Nothing reduces drag like less surface area. $\endgroup$ – Peter Kämpf May 30 '14 at 18:18
0
$\begingroup$

Trim tabs let the pilot move the control surface more easily. It is the elevators that actually affect the plane's attitude.

A all moving tail gives more control over the attitude compared to just the control surface. They also allow for more control in stall and transonic conditions (during flow separation) because the pilot can control the leading edge.

$\endgroup$
  • 2
    $\begingroup$ Not all trim tabs reduce control forces. Take the elevator of a DV-20 Katana: It moves such that control forces are increased. $\endgroup$ – Peter Kämpf May 29 '14 at 19:29
0
$\begingroup$

Advantages of full moving surfaces include: 1. Less drag 2. More authority

Disadvantages include: 1. The control surface can stall. 2. Requires move structure and more force to move.

$\endgroup$

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.