Why are pitch trim wheels always so large ?

[Styggron]

Hello everyone,

Was wondering why pitch trim wheels are always so large. huge wheels. Especially on airliners but even on GA, the wheels are still "large"
How come there are not buttons or smaller wheels etc or a small slider etc ?

Just curious.

[speedy70]

because they are more accurate that way.

Cheers Chris

[Paul K]

because they are more accurate that way.

Yes, I think you are right. The greater the diameter of the wheel, the easier it is to make small and precise adjustments to the trim.

[Piper_EEWL]

Yes I think that's exactly the reason. Also on old planes (especially big bombers or airliners) there was no hydraulic system to support the pilot with force. So big trim wheels mean big leverage means smaller forces.

[Nick - A2A]

So big trim wheels mean big leverage means smaller forces.

Yes, this is the key I believe, and not just on big planes.

For instance, on something like a Cessna 172, that largish trim wheel will be directly fixed to a much smaller sprocket. This will give the pilot the mechanical advantage needed to move the trim tab. (The sprocket moves a length of 'bicycle chain' connected to an actuator which is in turns moves control cables.) As Sebastian says, the bigger the trim wheel in relation to the sprocket, the less force needs to be applied by the pilot.

Think of it as a bit like a bicycle in fact. If the bike was placed upside down and someone very strong was trying to hold the pedals motionless, you could easily overpower them by rotating that large back wheel by hand!

Nick

[AKar]

How come there are not buttons or smaller wheels etc or a small slider etc ?

It is fairly common these days to have buttons for the trim, typically on the yoke but sometimes somewhere in the panel. These use an electric motor (or some other means) to drive the trim system, most often the wheels are retained for manual use if required. Actually, in just about every airliner that's how it is done, with a buttons on the stick. The notable exception being Airbuses which lack any electrical trim switch, being either auto-trimmed or manually by turning the wheels (the wheels in big planes most often are not actually directly driving the trim but via a hydraulic system).

Some smaller airplanes, gliders notably, use trim levers or re-positionable buttons which re-center the springs in the control system.

-Esa

[Piper_EEWL]

Some smaller airplanes, gliders notably, use trim levers or re-positionable buttons which re-center the springs in the control system.

Yes that's right. The gliders that are equipped with the trim button are usually more modern models (the oldest one I've flown with that feature is a Glasflügel H206 Hornet from 1976). And it's very comfortable. You basically adjust your glide to where you want it to be. Wait till the speed stabilizes push the button and the plane will be trimmed for that glideslope. Very nice if you fly a lot in an out of thermals since that usually requires a lot of trimming forth and back.

[FishermanIvan]

They're not. Look at the trim wheel on a Seminole, and it's tiny.

[DC3]

[quote="AKar" (the wheels in big planes most often are not actually directly driving the trim but via a hydraulic system)[/quote]

I would guess that on some modern airliners the trim wheels use encoders to tell the computer systems where to set the trim (via hydraulics, servos, or ...)

However, I actually have no knowledge of this, it is only speculation based on knowledge of other modern day control systems. Feedback, i.e. turning the trim wheel when the autopilot is controlling it) might be through a stepper motor or some similar item.

Just my speculation....

[AKar]

I would guess that on some modern airliners the trim wheels use encoders to tell the computer systems where to set the trim (via hydraulics, servos, or ...)

However, I actually have no knowledge of this, it is only speculation based on knowledge of other modern day control systems. Feedback, i.e. turning the trim wheel when the autopilot is controlling it) might be through a stepper motor or some similar item.

Just my speculation....

I don't know every airplane there is of course, but typically the trim wheels, where included, are parts of mechanical linkage. In A320 for instance, cables run from the trim control wheels all the way back to the THS actuator. There is an override mechanism, that allows the manual command to cancel any electrical command driving the THS, the THS itself is actuated by hydraulic motors driving a ball screw. The position sensing is done at the actuator.

When we come to A350, the manual trim wheels are removed, and there is no manual pitch trim possibility in flight, under normal control laws. There are manual pitch trim switches that operate only on ground, or in case the auto-trim is lost. The THS is driven electrically, by 540 VDC Motor Brake Assemblies, as they are called.

-Esa