Supermarine Spitfire flight controls
29/4/2015 This week I started on a set of Spitfire controls for a client who lives in Melbourne, Australia. The first part I have made is the brass fire button for the replica spade grip which came from the UK. Whilst it seems such a small part the fire button is probably the most difficult part of the project, machining internal threads is never much fun! It is built with five components. Three brass pieces, the switch and a black nylon cap on the back. The design drawing and machining took a full eight hours. I have encased the back of the switch in hot glue to protect the terminals and wiring.
Day two begins with an accurate full scale drawing of the spade grip to calculate the shape of the brake lever handle. This will be cut from 10mm aluminium plate on the CNC mill then the handle will be smoothed off on the linishing belt. The brake handle will pull on a cable to operate the brake switch. It will also have a strong return spring.
Day three – making and installing the brake lever.
I have now made the yoke column from some 40mm OD aluminium pipe, (34mm ID). Some 1/2? pitch chain around an idler sprocket provides a good linear motion that is connected to the spring centering mechanism. When the spade grip is tilted, the spring that is not being stretched moves down over the slotted anchor plate. These plates are bolted to the column also on a slot so that the correct neutral (upright) position can be set.
At the end of three full days in the workshop the column is nearly completed. After installing the spade grip which is quite heavy, I discovered that the return springs needed to be a bit stronger so I replaced them. The real aircraft doesn’t have any centering on the spade grip, it relies on airflow over the ailerons but a simulator needs to have positive centering. The side to side movement is light and it should be easy to fly with. The chain guard is made in three pieces and it can be removed easily for maintenance. Once completed the column will have to be dismantled again for painting. I’ve had a local auto paint shop make up some “Supermarine green” spray paint made up so the assembly will match the colour of the spade grip.
The next part of the build is the pivot block and main chassis. Two 40mm square hollow sections form the main rails and the pitch centering device will reside between the rails. The pivot block is machined from 75mm diameter nylon and this unit will allow height adjustment of the spade grip and also removal of the column for shipping and maintenance.
The new chassis design has worked extremely well. The challenge was to make a pitch centering device below floor level for the control column that is long and quite heavy. I used the same principle as the centering cams that I use on my steering tillers but on a much larger scale. The force required for pitch movement is quite strong and soon there will be hydraulic dampers fitted to prevent a fast return to neutral. A linear 10Kohm potentiometer will be used to transmit pitch.
After spending quite some time trying to source some fluted aluminium tubing to use on the pedals, I had to give up! Most tube suppliers I contacted were only interested in minimum order of multiple lengths of stock, even a local ladder manufacture would not help with one metre off-cut! I spent half a day milling flutes on some round tube and flat plate, the flat plate was then curved to sit in slots machined into the 6mm side plates. This was a lot more work but at least the upper and lower grips match perfectly and pedals are certainly heavy duty. The next job is to mount them on the rudder rails.
The ends of the tubes are fitted with 8mm stainless steel guide pins secured in a Delrin adapter. The 50mm x 50mm aluminium angle brace is the mount for a guide bush also made from Delrin. The compression springs provide rudder centering and the travel limit is set when the Delrin components make contact. The rudder pedals move 100mm through full travel.
The brake plate is now fitted and connected to the spade grip lever. When the lever is pulled half travel, the micro switch will connect the earth wires from the independent brake switches to the BU0836X control card. When left rudder is applied fully the left wheel brake will come on and the same for right brake. When the spade grip lever is pulled to max, the second switch will apply both wheel brakes.
With only 22 degrees of total pitch movement on the yoke, the best option for pitch transmission is a slide potentiometer connected to the centering mechanism.
Here is the potentiometer for rudder position transmission. The rudder bar only moves through 25 degrees but rotates the potentiometer about 270 degrees for good resolution.
The completed unit ready for shipping!