|In some airplanes, the trim wheel can be either rotated
manually, or via up/down trim switches on the flight yoke. Since I'm flying
many different planes, I have made a modification to the original Throttle
Quadrant trim wheel:
I have added a geared DC motor to the trim wheel to rotate it forward or backward, controlled by the trim switches on the yoke. Since it needs to be possible to rotate it by hand as well, an electric clutch has been added to decouple the trim wheel from the DC motor drive when the motor is not activated.
The electrical circuit is very simple: The electric clutch magnetizing coil has no polarity, so both DC motor and electric clutch coil are connected in parallel (both are 24VDC). The motor and coil are connected in H-bridge configuration. Below diagram shows the circuit, which I build from scrap components. The transistors are 60V/4A darlington types, so require little driving current.
The elevator trim-up and trim down switches control the H-bridge: When
both switches are off, both emitter followers Q1 and Q2 are pulled high. There is
no voltage delivered to the motor and coil.
For the mechanical solution, I have made use of junk components from old
The clutch has two gear wheels: the main gear drives the trim rotary encoder which sends trim-up/down commands to FS. The main gear is fixed to the inner shaft, brass bearing and trim wheel. The clutch gear is only fixed to the iron cylinder, and is driven by the DC motor.
In un-powered state, the clutch gear will freely rotate, it will have no
mechanical connection to the inner shaft. When the clutch coil is powered,
the magnetic field will make the iron cylinder stick to the iron ring on the
brass bearing. Now all gears are fixed together mechanically (with
sufficient torque), and when the DC motor drives the clutch gear, the inner
shaft, trim wheel and main gear will rotate together, and the trim rotary
will send out continuous pulses to FS. The rotation direction of the DC
motor determines trim-up or trim down action.
Google Sketchup file of the throttle quadrant with electric trim can be downloaded here. In this design I have made use of layers and groups, so you can check out each separate part individually.
A video of the electric trim action can be seen here.
Note: I noticed that Firefox will not always play the file correctly. If this happens, please download the file to your PC first and play from there.