View of the PRINCIPAL COMPONENTS.
I had “dabbled” with various versions of Microsoft’s flight sim over the years, but I wasn’t aware of its versatility until
I recently visited an old friend. He has built up a replica cockpit of an Airbus A-320. Suitably inspired, I went and
purchased a Logitech joystick. This, of course, proved lots better than trying to fly with a keyboard! The next step was to
build up a set of rudder pedals. I decided to go all the way and incorporate differential brakes.
One potential hassle I envisaged was the problem of converting the angular movement of rudder pedals and brakes to a rotary
motion in order to drive a potentiometer .. and thus derive the required analogue input that flight sim required. I managed
to find a source of plastic rack and pinion gears, which, to me, seemed an ideal solution. What follows, is a description of
how I proceeded to build up a suitable mechanism. I can visualise how the same mechanism could be incorporated into other air
craft controls from control columns to throttle, mixture, prop pitch levers etc.
View of the PRINCIPAL COMPONENTS.
Above, are shown the basic components used to construct this mechanism.
(a) Potentiometer (pot) .. 10k ohm, 300 degree rotation, 16mm diameter body.
(b) Knob .. which fits spline of pot.
(c) Pinion.
(d) Rack .. the rack and pinion I managed to source are made from plastic. Initially, I was somewhat concerned regarding
their durability. However, during my initial tests with my rudder pedals I subjected those components with quite a degree
of stress with no ill effect.
(e) An approximate 100mm (4”) long section of aluminium angle measuring 12 x 12mm x 1mm thick.
During my initial tests with my rudder pedals, a problem became apparent with the pot. These devices are assembled using a
sticky, viscous grease. This adds a degree of friction in the bearing which, for normal electronic type applications, gives
a nice “feel” to the control. The effect I had was that things were satisfactory when the rudder pedals were operated slowly
(as our flight instructor would dictate), but sudden, quick movements (which made our flight instructor unhappy) caused the
rack and pinion gears to unmesh and skate, one over the other. My flying often requires those sudden, quick movements, so
the problem needed resolving. I added more and more spring tension to try and overcome the problem, but the amount of tension
needed was, in my view, grossly over stressing the gears and pot bearings. The problem was caused by that friction in the
pot bearings not letting the pot shaft accelerate quickly. The solution was to partially dismantle the pot and clean out the
bulk of that sticky grease.
View of the COMPONENTS modified and partially assembled.
(a) the pot looks the same but now turns a lot easier.
(b) the knob has had a lot of its bulk machined away .. thus takes up less space, further more, there is no need for it to look
like a knob! A 3.2mm (1/8") hole has been drilled concentric with the splined hole.
(c) the pinion hole has been tapped 1/8" Whitworth. A countersunk 1/8" metal threaded screw is fitted inside the knob, and
attaches the pinion to the knob. A 1/8" nut is fitted as a lock nut.
(d) the rack has been attached at one end of the aluminium angle. Note that it may be attached to either leg of the angle, at
various positions on that leg (or surface), so thought should be given regarding the correct place to suit your application.
View of the basic CONCEPT.
The aluminium angle with the rack attached will be fixed to the lever so that it may pivot and be held in mesh with the
pinion by a light spring.
The pot will be somehow attached to a "fixed" part of the mechanism. In this case an angle bracket is shown, but different
shaped brackets will be required for different applications. I can visualise a "Z" shaped bracket suiting a number of
situations.
View of the COMPONENTS fitted to my rudder pedals.
The pivot point for the rudder bar is at the domed nut.
To the right of the rack and pinion mechanism is the right pedal. They pivot on the rudder bar so that the pedal is always
presented squarely to the foot even with maximum rudder applied.
The pedals pivot, for application of brakes, at the two locked nuts visible in the image. The rack and pinion mechanism for
the brakes is located inside two pieces of channel and is thus difficult to photograph.
There is no electrical wiring to the pot. at this stage.
It is obvious that it is necessary to get the measurements correct for different angular movements of the lever. This may be:
(a) calculated mathematically.
(b) drawing the assembly (say) full size with the lever in each extreme position and measure dimensions from the drawing.
(c) build a "mock up" of the lever assembly from light metal/plywood/or whatever, fit the components and find the correct
dimensions by trial and error.
Some basic thoughts:
It is essential that solid mechanical stops need to be fitted in your mechanism which will limit the movement of the "lever"
prior to the potentiometer reaching its internal stops. This obviously protects the pot and the gears from damage.
The pot is limited to 300 degree rotation. I worked at fitting the mechanical stops to utilise 270 odd degrees rotation.
Thus, there is that bit of tolerance at each limit of the movement of the "lever".
To achieve 300 degree rotation, I measured the distance of travel for the rack as 25 mm (1").
Therefore, for 270 degrees, I calculate the rack travel to be 22.8 mm (0.9").
I did some measurements of my rudder pedals and brakes which I will include.
Angular travel of rudder bar +/- 18 degrees. Thus 36 degrees total movement.
Distance from rudder bar pivot point to top of teeth of pinion .. 25mm (1").
Distance from rudder bar pivot point to pivot point on aluminium angle .. 36.5mm (1.44").
Brake pedal movement .. 14 degrees.
Distance from brake pivot point to ? (need to check this) 63 mm (2.5").
Note re thread types. I have suggested the rather dated 1/8" Whitworth screw to assemble the pinion to the knob .. mainly
because I had the screws and the taps. Any other thread type like 3mm metric or #4-40 American can of course be substituted.
From extensive past experience, I have found it's one thing to obtain this sort of construction article, but another thing
to source the necessary components. Also, it's a problem for some people to do, or have done, the necessary machining.
So, to assist others, I am willing to obtain components, rework the pot., and make up the knob/pinion assembly as illustrated
in the second image .. "components modified". Just drop me an e-mail to enquire.
Click here to go to that page.
There is a "Send an e-mail to Ron" link at the bottom of that HOMEPAGE.