Aileron Force Feedback Design

The Spitfire handling was unusual in the lack of control force harmony. Perhaps it was this very idiosyncrasy which made it such a pleasure in the air.

Now with the Heritage Flight Simulation Spitfire Mk.IX we will be able to put this to the test.

Spitfire aileron forces as recorded during NACA tests (1942) 

As can be seen from the graph above, the aileron stick force at 110mph was already over 22 lb for full deflection. This increased rapidly to a point where 130mph it required a force of 40 lb to get full deflection and at higher speeds it simply became impossible to move the spade grip left or right to its full extent. At 300mph it took 40 lb to move the stick halfway or 20 degrees. It must be remembered however that as the airflow over the ailerons increased, so did their effectiveness. Therefore it took less movement of the stick at higher speeds to achieve the same roll rate.

Flight controls complete – elevator, rudders and aileron

Our challenge has been to incorporate this behaviour in our simulator. To achieve that requires some means of force feedback. I have previously written about how we achieved this with the rudders in a novel and much simplified manner from the norm. Similarly we discussed the use of a spring loaded cam system for the elevator as, given the very sensitive and light forces in the elevator, force feedback was not required.

Mechanism in port wingroot

The forces involved in the roll axis are very high however. We therefore needed to find a way of replicating these without placing undue stress on the airframe and preferably using the same actuator used for the rudders. All the while keeping the construction simple and low cost. To arrive at the optimal solution took many days of intense effort; calculating, testing, simplifying and redoing.

The aileron force feedback is coupled to the same actuator used for the rudder and the mechanisms have been moved down into the fuselage underpan

The design utilises the wing roots and fuselage underpan to contain the mechanism. We have used four carefully selected expansion springs rigged in series with pulleys to provide a 4 to 1 mechanical advantage. The control lines have been routed back to the 200mm stroke actuator and the forces have been calculated to provide a full spectrum from 0 to 280mph. A fifth expansion limited spring provides low forces typical of the aircraft at standstill, running at 5.6 lb at full deflection. As the airspeed builds, this rapidly ramps up to a full 41 lb at 280mph. (It is unlikely such a force would ever be applied by a pilot in the normal run of things, but it will still be hard to move the stick part of the way.)

Flight controls with aileron force springs and pulleys in port wing root

We will now be incorporating these changes into the cutting patterns for the plywood and aluminium plate and doing final refinements. Then the task of consolidating all the assemblies will commence and all parts will be properly named and numbered. Final drawing preparation will then be done and cutting, bending and assembly of the prototype can kick off.

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