"Exponential" differential via mechanics

David J. Gall


Larry Hamm's suggestion is good but it requires significant angular
displacement of the belcrank to get any substantial differential.

Consider this alternative: Make your tailcone belcrank in the shape of the
letter 'K' with the angled legs pointing forward. The rudder pedal cables
connect to the angled legs, but the rudder and tailwheel cables connect to
the straight leg. This gives a differential since the angular displacement
of the belcrank is increased for any given linear displacement of the cable
the more the angled belcrank leg moves forward in its arc [d-theta/d-x goes
as 1/cos(theta)].

Similarly, move the cable attachment points on the rudder pedals aft of the
plane of the rudder pedal pivot so that as the rudder pedal is pressed
forward, the attachment point arm becomes more perpendicular to the line of
travel of the cable.

Either of these geometries will induce a differential movement in the
belcrank; both together will give even more differential.

The resulting angular differential can be amplified or reduced by varying
the ratio between the length of the angled legs of the belcrank and the
effective lengths of the rudder pedal arms (and the desired throw of the
pedals forward of neutral). The ratio of the length of the angled legs of
the belcrank to the straight legs and, finally, to the length of the rudder
and tailwheel belhorns will control the total angle of the rudder and
tailwheel deflections with rudder pedal displacement.

David J. Gall
P.S. Larry's suggestion does not have to be fabricated as an oval or
ellipse; a simple diamond or even a rectangle will work.

-----Original Message-----
From: Q-LIST@yahoogroups.com [mailto:Q-LIST@yahoogroups.com]
On Behalf Of Bob Farnam
Sent: Friday, October 20, 2006 10:08 AM
To: Q-LIST@yahoogroups.com
Subject: RE: [Q-LIST] Taxiing before first flight. (long editorial)

My ratio is not as much as I would like, but is limited by my
own requirement that I be able to reach the unlock detent on
the full swivelling tailwheel at full rudder. This so I can
pivot around a wheel on the ground.
The result is that my airplane is less sensitive than the
original design - enough that I can fairly easily steer it
straight at takeoff speed, but still sensitive. I would
really like to have what the RC guys refer to as
"exponential" control, where the response is low in the
center part of the travel, but increases at full rudder
input. Easy to do with an RC transmitter which has it
builtin, but I haven't yet figured out a simple and durable
mechanical way to make it happen. Anyone have a sudden flash
of insight?

Bob F
EAA Flight Advisor

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