Revised Elevator Mechanism Complete

With all the excitement of establishing the design collaboration with FlyingIron Simulations (you must check out this link!) mostly over and the necessary information exchanged, we were able able to continue the elevator mechanism redesign. We have abandoned the simple opposing spring mechanism and opted instead for a more precise and smoother cam based system.

Profile of the new cam based elevator mechanism

Initially the thinking was to also incorporate force feedback, however the stick forces on the elevator in the real Spitfire are very light. We are fortunate in having the results of extensive tests done by NACA (National Advisory Committee for Aeronautics) during 1942. They show that the aircraft has neutral static longitudinal stability, as shown by the fact that no change in elevator deflection was required to trim throughout the unstalled speed range. Typically only 3 degrees up-elevator movement was required to go from level flight to the first signs of the stall. Even when pulling 4g, stick force was only at 13lb. When the aircraft was static, the stick force induced by friction  to pull the stick all the way back already sat at 6lb. All in all then force feedback would be almost imperceptible and not worth implementing.

Design of the Elevator Cam

The elevator cam design turned out to be an interesting exercise. The Spitfire Control Column, when pushed fully forward, is in the upright position. The normal neutral position is 11 degrees back from upright, something very often missed by simulation designers. To move the stick to the fully back position requires another 14 degrees. Any cam design then must take into account this differential.

The cam and its follower are tensioned with a spring which can be exchanged to fine tune the stick forces.

Bellcrank assembly with tensioning spring

The support assembly follows the design of the original bellcrank mechanism situated at the bottom of Frame 11.

Frame 11 with the Bellcrank assembly

Next we start on the aileron redesign. This should prove to be a good challenge as the NACA test reports indicate very large influence through speed, to the extent that it was not possible to move the spade grip left or right more than half way (20 degrees) at speeds greater than 180mph. This of course affected roll rate too. But more on that next time!

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