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.

Ellipsoid beauty and cradles

The beauty of the Spitfire lies not only in its ellipsoidal wings. As wonderfully illustrated by our featured image, it is amplified by the sinuous curves of the wing fillets. In a touch of brilliance these mould the wings gently into the wing roots and onto the fuselage. Those wide, sweeping fillets  saved huge amounts of interference drag at these critical intersections.

It would be remiss of us not to capture some of this in our design. Simply having a cigar-like structure does not sufficiently evoke the true magic of this aircraft. We are therefore proud to unveil the full shape of our design. If you were wondering how the fuselage was going to be supported, postulate no more. The fuselage sits in a cradle which captures some of the beauty of the wing roots and fillets.

The Cradle

The design accurately follows the original wing roots and fillets, providing a walkway access from the wing into the cockpit, just as if you were climbing into the real aircraft.

Plan view showing the fillets and wing root walkway

Not only does the design provide space for replicating the ailerons with full force feedback installed in the wing roots, it functions as a removable base. This allows the fuselage to be moved through doorways and set down in the cradle before fitment of the fillets.

Wing roots will contain the aileron force feedback mechanism

As pointed out in our previous article, aileron forces in the Spitfire are extremely high, with only half the normal motion available at speeds over 180mph. This, together with the very light elevator forces provided a unique piloting experience which we wish to replicate.

The sensuous curves of the wing roots, fillets and belly

It is intended to leave the rear of the simulator cockpit open so as to allow visitors to gain a unique view into the interior.

View from below showing the support box

The support box will form a solid base for the simulator. It will also serve as the mounting point for a motion simulator should that be employed. Alternatively, placing on a castored pallet will provide mobility where required, for example on museum floors.

The removable fuselage cradle

A very worthwhile read on how the elegant wing and empennage designs for the Spitfire were developed can be found here. I was previously unaware of the important role played by the Canadian, Beverley Strahan Shenstone, in its formulation and design. Truly, a remarkable man and one of a most talented team to be working with Mitchell on this icon at Supermarine.

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!

Design Collaboration with FlyingIron Simulations for X-Plane 11!

We are excited to announce that we have entered into a design collaboration with FlyingIron Simulations. Using our detailed interior design, they will be producing the most accurate simulation of the Supermarine Spitfire Mk.IX for X-Plane to date. For those of you who are unfamiliar with their work, head on over to the store to check out their magnificent P-47N Thunderbolt.

For their debut project this design house has tackled one of the most complex system WWII aircraft. All systems are functional and the artwork is a thing of beauty.

Hence we can think of no better team to tackle this challenge. This will make available to operators of the Heritage Flight Simulation Spitfire another option besides DCS World. It will allow pilots to experience flying the Spitfire in their own environs, no matter where they are based. For museums who would be operating the simulator the cost of a professional licence for X-Plane runs at a once off payment of $750. This in contrast to the DCS World quoted figure of €250 PER MONTH for professional operators!

Now the DCS World Spitfire is a fine simulation, and very reasonable for private users at just a once off payment of $50. But we are extremely pleased that we will soon have more options available to our customers!