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!

Rudders – a new take on Force-Feedback

Effective rudder control in the Spitfire is a must. In the immortal words of Mary Ellis who flew more than 400 Spitfires and sadly passed away recently at the age of 101, the Spitfire was “a lady in the air, but a bitch on the ground”.

Anyone who has flown the Mk.IX Spitfire in DCS World can attest to this fact. It is only with very accurate rudder action that the beast can be tamed.

Thus we have spent the last two weeks redesigning the rudder pedal action. Originally this was a simple spring based system. We looked at incorporating cams but given the data on rudder forces in the Spit we wanted to do better. At rest the Spitfire rudder pedals move around freely but with some inertia, you are moving a fair bit of weight, cables and pulleys around in the real thing. We thus settled on a pulley system with elastic bungees. Not only did this give the correct feel of inertia, it also opened up new possibilities.

Redesigned rudder system showing actuator and bungee for force feedback

Force feedback is something only the bravest will attempt. All the models I have come across are hugely complex designs with stepper motors and a multitude of pulleys, belts and gears. With the use of a bungee, all that is needed is to stretch it to apply greater force. The stretching is also quite simple with a relatively cheap electric linear actuator. I had previously used one of these in the design of my trim system for my replica Fieseler Storch (70% scale) and they are available with installed potentiometer to provide precise feedback on their position.

So by incorporating a 200mm stroke 12V linear actuator into the design we have provided a very simple means of accurate force feedback on the rudders. There is fortunately precise data on the stick and rudder forces of the Spitfire from tests conducted during WWII. For the rudder pedals which move some 100mm either way, this amounts to 12lb or 54N at full deflection at full speed. By using a 10mm bungee which provides 46N at 50% stretch and adjusting the moments appropriately, we can achieve the exact force with the actuator at full retraction. Conversely, at full extension the pedal forces are minimal but should give satisfying clunks and inertia when waving them about.

DCS-BIOS will provide the speed reading which will be interpreted by one of the Arduino’s. This will send a signal through a motor controller to handle the 12V and around 3A required for the actuator.

After the fiasco with the Melexis sensors we have now incorporated the Bi-Tech 6127 rotary Hall Effect sensor. This comes in an easy to use potentiometer format and is preprogrammed for various angle ranges. In our case we have selected the 60 degree unit for the rudder. This gives a very nice full range given the rudder pedal’s 57 degree full deflection.

Next we start looking at what can be done for the elevator.

Blueprint – get yours free!

Just for some weekend fun….

Blueprints were introduced in the early 1840’s to make the replication of drawings somewhat easier. The process used a ferric compound which was impregnated into paper. The process was normally that the original drawing would be made on Cartridge Paper. This was a very heavy paper made especially for rifle cartridges in the days before brass replaced them.

The drawing would then be traced onto a tracing paper with black Indian Ink. This would be placed on top of the treated paper and sandwiched between glass plates. The plates would then be paced in the sun for around 2 minutes (allow 10 minutes for a cloudy day…) and the copier could see the exposed paper turn blue (Prussian blue). When the desired depth of tint had been achieved the copy would be removed and washed. The areas underneath the ink would have remained white, the washing process would remove any remaining ferric compound and preserve the drawing. This would make any changes difficult to conceal and thus the “blueprint” of any plan would form the basis of construction/manufacture and seal whatever contract had been struck.

In order to celebrate the completion of the basic design, we have created a “Blueprint” of the Heritage Flight Simulator in AO size. Printed, this will create a most attractive poster. It’s a fairly large file of 16Mb. Feel welcome to head over to our shop and purchase it…for free!

HFS Simulator Blueprint

Auxiliary Controls Redone

Happy to announce that all the controls, other than the flying controls (Pitch/Roll/Yaw) have been checked and redone where necessary. This in order to ensure that they operate correctly with DCS-BIOS and to increase their robustness.

The Rudder Trim falls in the latter category. The original Spitfire rudder trim system had one turn from end to end, whereas the elevator trim had four turns. The easy way then was to directly connect a potentiometer to the knob. The risk here however is that the potentiometer can easily be damaged in the heat of battle through over-enthusiastic application. The updated design now works through a gear with a positive stop.

Another component which has been updated is the Chassis Control Mechanism (undercarriage lever).

We are replicating the somewhat idiosyncratic pneumatic action of the undercarriage as accurately as possible by mechanical means. In order to raise the gear, the pilot was required to “in one deliberate movement”, pull the lever down and to the left to remove it from the gate and without pausing, move it up to the top of the movement. Here he had to hold it until the pnuematics took over and plonked the lever across to the right and back into the gate. We achieve this through a set of clever (we hope, still to be proven!) springs, scissor press-plates, traps and triggers. Thus the pilot will pull the lever down which depresses the press-plate, move it across to the left which sets the trap, move it up then hold it for a few seconds and then release the lever. This lets the lever jump to the right which in turn triggers the trap which pushes the lever back into the gate.

Thats all very interesting but how does that affect this particular update? Well, for the undercarriage action DCS-BIOS will accept a toggle switch, pushbutton or rotary encoder. The length of time that the action of working the lever takes, say 7 seconds or so, would require immediate triggering of the action in the simulator when you start moving the lever down out of the gate. It was not possible to achieve this with two microswitches, I tried in a number of combinations, normally open, normally closed, parallel, series etc. It would only signal at the end of the cycle, which was 7 seconds too late. So I then devised a geared system for a rotary encoder, attached to the shaft. The clever bit here was to allow initial movement in a reverse direction without triggering the action incorrectly, ie. when you move the lever down out of the gate you don’t want the sim to think you are moving the gear lever down. So the wheel attached to the shaft has a slot which allows free movement for the extremities and only moves the gear on the central 60 degrees of the movement. This solves the problem and gets the gear moving in the sim at the appropriate moment. 

The final update was to the Remote Radio Control unit. Here the internals were completed and the whole front face now incorporates the engravings and switch retaining mechanism. There are three switch units:

  • Power on/off
  • 4 interlinked latching switches for channel selection
  • a 3-way switch to select radio mode
View of Radio Remote internals

What remains now on the controls is to redesign the main flight controls. They currently work directly against spring tension, which is not the most elegant way of doing things and provides poor feedback and centering. I wish to change this to a cam action, potentially with in-flight adjustable spring tension to provide force-feedback. 

But that is the subject of a future post! 🙂

Throttle Quadrant price reduced!

We have significantly reduced the price of the Spitfire Mk.IX Display Throttle Quadrant plan set. The new price is USD $35, down from USD $70.

In addition, purchase of this plan set will be fully redeemed against a purchase of the full set once that becomes available (targeted early 2019).

Feel free to head on over to our shop and pick up your plan set.

Spitfire Mk.IX Presentation Engine Hand Control Plan Set

Melexis perplexis

Melexis is a well known and reputable Belgian semiconductor manufacturer. When they recently announced that they were bringing out a new Tri-Axis magnetic sensor in analog format with standard, through-the-hole wiring, we believed this would provide an excellent consumer oriented product. At a reasonable cost, it offered great accuracy and a very compact footprint. It was the new MLX90371GVS-BCC-100-SP-ND.

It was therefore with great excitement and anticipation that we ordered twelve of these units from Digi-Key. We changed the designs of the Gunsight Base and Range settings to incorporate the new sensor and the Elevator Trim was also designed on this principle.

And then we tested it. Solid output of 2.48V, no matter how we moved the magnet. Refer to the oscilloscope diagram in the featured image above to see it flatline. Well it took many emails and to-ing and fro-ing between Digi-Key and Melexis before we obtained an answer. The units are pre-programmed to have both the lower limit and upper limit of the analog output at 50%! This effectively makes them useless unless you also happen to own a Melexis programming and daughterboard, available at the small sum of around $2000 🙁

I am waiting for an answer from Melexis why they have programmed them in this way, effectively making them unusable for the general consumers, hobbyists and makers. And why there is no explicit statement on this in their literature. I have yet to receive an answer.

Anyway, one has to move on from these little setbacks. We have updated the designs for the Gunsight rings and Elevator Trim with mini potentiometers. For the more critical controls of roll/pitch/yaw, we will be using the Bi-Tech 6127 Rotary Hall Effect Sensors. These come pre-programmed in a selection of operating angles which is great. Many thanks to Sokol1 on the SimHQ forums for pointing this one out. It is simply a drop-in replacement for potentiometers which makes installation and link-up extremely simple.

Some images of the changes that had to be made on the Elevator Trim design:

Elevator trim redesigned for a mini potentiometer
The elevator trim as it was with the Melexis MLX90371 sensor

Elevator Trim System Redesign

Just as a bit of a progress update, we have redesigned the elevator trim to provide 4 turns, utilising the same spiral principal as per the original. 

In the original, the smaller diameter section of the base contained a cog in order to move the chain which in turn pulled the elevator trim cables to and fro. This section is empty in our implementation. 

The interesting bit is the larger diameter section of the base. The original had a spiral with a follower which moved the cable to the indicator on the instrument panel, thus providing a mechanical means of showing the elevator trim position. The follower also limits the movement at either end of the spiral, resulting in 4 turns without being able to stress the elevator trim system beyond its limits.

We have taken this principal but instead of the follower moving a cable, in our case it rotates a spur gear which has a magnet attached to it. The movement of the magnet then gets interpreted by a rotary hall sensor mounted on the rear base cover.

Spiral, follower and spur gear in the elevator trim base
The Hall sensor mounted in the base cover
Assembled elevator trim unit