Question on trim design.....

[Michel Lemieux]

At Hendon, I recently noticed something I had never remarked before.

Separate trim tabs on the same control surface.

Both the Beaufighter (amazing beast) elevators & the HE-111 ailerons (there are 2 per side, but the trims are on the same unit) have 2 separate trims with it seems 2 separate actuation systems.

Why would this be done? It seems to me that this over complexifies the systems and cabling.

Unless one is a fixed ground adjustable one.

Cheers!

[Cvairwerks]

One is a trim tab and the other is either a servo or anti-servo tab. The servo tab changes control forces with airspeed. Servo makes them lighter and anti makes them heavier.

[Michel Lemieux]

Thank you Cvairwerks! I know I would get the answer here.

So I guess the servos are automatically operated based on airspeed then?

Must be a fun system to adjust.....

BTW....are you into Corvairs? If yes.....that is my favorite classic car.

[Chris Brame]

One more for you: Some Curtiss C-46s had two trim tabs on the rudder.

[Cvairwerks]

Michel: Servo is somewhat of a misnomer. The servo tabs are connected to the wing or empanage by mechanical linkages and as the main surface moves, the tabs also move. For example, in servo usage, as the surface deflects one direction, the tab will deflect the opposite direction based on an amount designed into the tab control linkage system. Anti-servo means the tab deflects in the same direction as the surface and tries to move it back into the stream position.

[Michel Lemieux]

Got it! So it is a way of ''boosting'' the pilot input on the surface while reducing the pressure need to hold a certain position?

Kind of an evolution of a horn balance and an in between before real hydraulic servos?

Cheers.

[Cvairwerks]

It's all mechanical linkages. It can be used to reduce the pilot effort to move the control or increase his effort, based on how the designer configured them.