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"Exponential" differential via mechanics
David J. Gall
Bob,toggle quoted messageShow quoted text
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
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.