• How To...Build Your Own Modular Desktop Flight Controls Part 4

    How To...Build Your Own Modular Desktop Flight Controls

    By Ferry Herfst

    Throttle Quadrant

    You have probably forgotten that I started this project to build a throttle quadrant which ended up being the last thing I built. Due to a feature of FSUIPC which enabled me to alter the way FS responded to the different axes I did not need to build a complicated mechanical solution. I could use a direct drive between the levers and the potentiometers. Otherwise I had to think of a way to let the rotation of the potentiometer increase in the feather region of the prop lever? Why, you ask. That can be read in the additional info.

    I first tried to draw the PA31 quadrant from my memory. But I knew I also wanted to be able to use reverse with a detent or lock between throttle and reverse. I figured I was better off with a King Air throttle quadrant. Also because I chose the Beech style for the switch layout on the yoke. And the King Air has reverse. Also more realistic than adding reverse to a piston aircraft quadrant. I paid a visit to a company where I got my twin rating. They owned two King Airs. One was in maintenance and they let me spend some time in the cockpit. I used paper and pen to draw the outlines of the entire thing on a number of sheets. I copied the distance between the slots where the levers run through and the length of the slots. The way the levers diverged away from each other. I also drew the shape of the prop and condition knobs to be able to duplicate these. When I got home I worked out my drawings and chose exact measurements.

    The quadrant is actually part of a cylinder. I could make the round part out of sheet metal. The sides, back and bottom would be made of 9 mm plywood, the same as used for the yoke. I used 40 x 3 mm aluminium strips for the levers. In hindsight I might have opted to use 4 mm. The 3 mm ones are not very stiff. The King Air levers are nicely shaped. The throttles move away from the prop levers and change from broad to narrow near the knob, from 40 mm to 20 mm. The prop levers lean towards each other and are tapered the same way as the throttles. The mixture or condition levers move away from the prop levers. These are also tapered. While making them, I tapered the strips first and I marked where they had to be bent. I bent the levers after that.

    But before I was able to do all that I had to figure out how I would position this in the quadrant box. All levers would share one common axis. That way I could add a simple adjustable throttle friction by tightening a nut on one end of the axis. The awfully weak (non adjustable) friction of a brand new elite quadrant had wondered me some years before. I would use the same nylon gear wheels as I used in the yoke (I actually bought these with the throttles in mind) and I would use nylon rings on either side of each lever for smooth action. I would attach one gearwheel to the lower end of each lever with the common axis running trough the center of the gearwheel. The potentiometer will be driven by a smaller gear resulting in a greater angle of movement in the potentiometer compared to the 60 degrees of travel by the lever.

    The power lever has a greater travel and needed a smaller reduction. I had to experiment with the potentiometer travel needed by the joystick to be able to calibrate it. The Wingman Extreme Digital 3D was kind of specific in this respect. The game pads were not that critical. In the end I had to enlarge the prop and mixture travel just a bit to be able to calibrate them. I used the X and Y axes of one joystick for the prop axes. The X and Y of the other one were used for the condition levers. The throttle axes of both joysticks were used for the throttles. The Z (twist) axes were used for trim and spoiler.

    One set of levers looked like this.

    The blue lines are the nylon rings. The green lines are the gear wheels. There are in fact rings between the gear wheels and the levers to keep the gear wheels from being squeezed. The grey lines are aluminum. Both the levers and the L shaped mounts. The yellow bar is the copper alloy threaded rod used as a common axis. The red lines are the cross section of a copper alloy tube. It runs through the lever and gearwheel and keeps them together. They are not supposed to touch the mounts. The gears I had to use were a bit larger than I planned so I had to make a groove in the wood under the lever gear.

    The prop levers in detail.

    Because I wanted to be able to put this assembly under a little tension to increase friction or decrease friction I used a nut on both sides. Tightening the nut would increase friction of course. In the actual quadrant there are three of these lever sets. I used copper alloy tubes between the lever assemblies. The length of these tubes is very important. It took a couple of efforts to get it right. Otherwise the friction will vary a lot between the separate levers. The positioning of the L shaped mounts is a factor in this as well. On the actual quadrant I placed the friction nut on the outside of the quadrant. Enlarging the nut with a wooden disk. I did not place the potentiometers all in front of the axis. I wanted to but there was just not enough room so the staggered position was a necessity.

    One thing that you have to pay attention to is that the potentiometers of both the power, both the prop and both the mixture axes are in the same position when the lever is in halfway position. It will show when calibrating. If the potentiometers are offset, the line drawn in the calibration square in windows is not straight. It took some experimenting to get it right. It's no big deal but will result in the levers being offset with the flightisim values being equal. A certain degree of offset is acceptable since I have not seen an aircraft without offset levers during flight.


    I added a feature to this quadrant that can not be found on real King Airs--a spoiler lever. Because I want to be able to fly jets as well I added the spoiler lever right of the mixture. To accommodate this I made the box a little wider than the original quadrant. I tried to maintain the Beech appearance so it would not seem out of place.


    Reverse is another nice feature which is available in the King Air. The throttles are limited to idle by a detent. You have to lift the levers to be able to go into reverse, about 1.5 cm. The levers actually extend and stay in this extended position throughout the reverse range (or beta range). I had to think of a way to extend the throttle levers. I split the original lever a few centimetres above the gear wheel. I bolted a 3 mm aluminium strip to it which extends up. Using 3 bolts on this strip and 3 slots in the original lever I could slide the lever up and down. The 3 slots provide guidance and stops. I used locking nuts to be able to tighten them without leaving sideways play in the lever but not too tight. Otherwise pulling the levers out would be too hard. An additional bolt with a sunken head through the power lever would provide the stop for idle, and would move up with the lever and run along a guidance in beta range. The bolts of the two levers are close to one another.

    The power lever close up in beta range. The bolt running along the beta guidance can be seen. The lever stops when the bolt meets the stop. The trim gear wheel is held by a thick nylon ring. This provides a nice resistance but still runs smooth. The potentiometer gear is allowed to slip on its axis when the trim pot reaches maximum deflection.

    Elevator Trim

    The trim wheel is another real aircraft part. This one is from a Rallye, A French single engined aircraft. The gearwheel on the trim wheel was compatible with my gearwheels. A nice coincidence. About 2.5 revolutions of the trim wheel are needed from full up to full down trim.

    Mounting It

    The throttles need to be below desk level. Or just a little above. The throttle box can be linked to the same type of clamps used on the yoke. But this time I used strips to keep a distance between the box and the clamps. The lower part of the box is level with one of these strips which ends at the lower part of the clamp. The box will be in front of and below the desk. The throttle knobs are just a bit above desk level. This can be changed according to personal taste or the throttles can be incorporated into a permanent set-up. The clamps are very strong and the whole set-up is very stable.

    The finished quadrant. The power levers in idle position, props full fine and condition to cut-off. The spoilers are in retracted position. A nice view of the clamps as well.

    The depth of the box is 15.5 cm measured at the bottom (so actually a little more). Height at the back is 12.5 (greatest height is also a little more). The center of the axis is located 2.8 cm from the bottom and 5 cm from the back. Width is 23 cm. The throttles extend 14 cm, the prop levers 12 cm, and the condition levers 10 cm above the quadrant.

    Connecting It

    A 25 pin printer-port like connecter was used to connect the throttle quadrant to the radio panel since the joysticks circuits were inside this box. A cable with 26 wires was used. That way I had some wires left to use for the flap switch, the planned aileron and rudder trim and a to/ga button.

    Finishing Touches

    I made all the placards that are on the real quadrant. I used a Dymo label printer with clear film and white text. I looks very professional, at least I think so ;-) The power, prop and mixture knobs were made out of a broomstick. The knobs, white lines and red feather area were painted using Humbrol model paint. I still want to make a nice spoiler knob on the handle. I added the trim switch to the quadrant just below the condition levers. All the knobs in the real King Air are actually black. But I liked them better in black, blue and red as on most piston aircraft.

    Links To All Article Parts

    Part 1: Introduction And Radio Panel
    Part 2: A Bit On Electronics
    Part 3: Building The Yoke
    Part 4: Building The Throttle Quadrant
    Part 5: Modifying The Rudder And Adding Brakes
    Part 6: The Finished Project And Some Modifications
    Part 7: Additional Information

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