How To...Build A Push Pull Throttle Quadrant
By Steve Sokolowski (29 October 2008)
FlightSim.Com has posted a number of construction articles dealing with the design and building of lever type throttle quadrants. These throttles are used mainly on commercial and larger general aviation aircraft. But what about the smaller Cessna Skyhawk, Skylane and other GA aircraft that make use what has become to be know as push-pull controls? Well, with this article I plan to show you, using readily available and inexpensive parts, how to do just that.
WHAT WE NEED
To build a throttle quadrant, we need to consider how the throttles are to operate; how are they connected to the computer and what components are needed to get the job done. First, the obvious choice for a push-pull throttle is how we will convert the throttle setting into a usable signal for the computer to sense and act upon. This can be easily done using a "slide potentiometer" (see Figure 1). As the name indicates, the slide pot has an internal lever that "slides" across a strip of carbon located within the body of the device, changing its value or "resistance" as the lever is pushed or pulled across it.
Slide pots come in a variety of styles and values; they also come in two main configurations; analog and audio. Audio pots are used in stereo equipment to adjust the volume of the music emanating from the speakers, because of their gradual increase or decrease in resistance value. This change is called: "logarithmic response"; thus making the audio slide pots unusable for flight simulator setups. So that leaves us with "analog". Analog slide pots give you a steady, even increase or decrease of resistance value at its output terminals. Just what we need for our throttle quadrant.
Resistance value is another consideration we must address in building the throttle. By selecting a slide pot with a resistance value too high will prevent small slide changes from being detected; too low will draw excessive current from your computer's power supply. The value of 100,000 ohms (or 100K ohms) is the perfect choice for all slide as well as rotary potentiometers used in your flight panel.
Analog taper; audio taper, resistance values, all might seem bit complicated for all of you with little on no electronics background. So to help with your assembly, Desktop Aviator (http://www.DesktopAviator.com) has a Model 1090 Throttle Slide Assembly (http://www.desktopaviator.com/Products/Slide/index.htm). The 1090 has all the needed hardware to build your throttle; you have the 100,000 ohm slide potentiometer, panel mounting "hub", 1/4 inch plastic rod and a knob. All you need do is to solder three wires to the pot and connect them to a device that senses the location of the lever and translates this reading into a usable USB output.
USB ANALOG INTERFACE
The most efficient way to interface the 1090 Slide Assembly to your computer is using the USB port. For this to happen, we need some sort of device that reads data from the slide assembly and converts it into an analog data stream in a form and resolution for the computer to handle. So in steps the Model 2097. (http://www.desktopaviator.com/Products/Model_2097/index.htm). The 2097 is a modified version of Desktop Aviator's popular 2090 Encoder Board (more on that board in a future article). The 2097 is a small circuit board that easily connects up to eight rotary and or slide potentiometers to your computer, using a 10-bit resolution. It easily plugs into an available USB port. It also provides four digital inputs thus allowing you to add a toggle or push button switch that can be assigned as flaps, landing gear (up/down), fuel tank select, parking brake, carb heat, etc.
WIRING THE SLIDE POTS
With your purchase of the 2097, you will also receive eight 3-pin female connectors and one 6 pin connector (Fig 4). These connectors provide the easiest way to connect the slide assemblies and toggle switches to the 2097
Figure 5 shows the underside of the 1090 Slide Assembly. Note there are four terminals; two of which are numbered the same. The two #2s indicate that these terminals are connected internally but can output different resistance values. For the wiring of our throttle quadrant, we will use the #2 terminal that is the closest to the mounting hub.
As seen in figure 5, solder a length of wire from the #2 terminal of the Slide Assembly (the one closest to the mounting hub) and connect it to the center terminal of one of the 3-pin female connectors. The #1 Slide Terminal is soldered to one of the outside terminals of the 3-pin connector and terminal #3 of the slide is soldered to the remaining terminal on the 3-pin connector. At this time, the only critical connection is the #2 terminal on the slide; make sure it's soldered to the center terminal on the 3-pin connector.
Try to keep the length of these wires to less then 15 inches. Longer wires might introduce "hum" into the USB adapter and cause erratic operation of the slide.
Depending on which Cessna model you are building, you will need at least two Slide Assemblies. For the Skyhawk two assemblies are required; one for the throttle and the second for fuel mixture; while the Cessna Skylane requires three assemblies; one for throttle, the second for fuel mixture and the third for propeller pitch.
MOUNTING THE SLIDE ASSEMBLIES
The easiest medium to shape and drill is plastic. It does not require any specialized cutting or drilling equipment. Figure 6 shows how three Slide Assemblies are mounted to a 1/8 inch thick plastic panel (also available from Desktop Aviator). Just measure and drill three 1/4 inch holes into the panel and mount the Assemblies using the lock washer and nut.
If you are planning on mounting the Slide Assemblies inside your own panel, it must be less then 1/4 inch thick. The mounting hubs on the Assemblies cannot be used on any panel with a thickness more then 1/4 inch.
CONNECTING THE SLIDE ASSEMBLIES TO THE 2097 BOARD
Figure 7 is a partial photo of the 2097. It shows the eight available analog inputs; of which we will be using three at this time. Take the three pin connector from the fuel mixture slide assembly (red knob) and plug it into the #1 position on the 2097 board. Now take the prop pitch (blue knob - if used) and connect it to the #4 position on the 2097. And finally, take the three pin connector from the throttle (black knob) and connect it to the #5 position on the 2097. That's it!
If you are anxious to see your throttle quadrant work, just plug the 2097 board into your computer's USB port; power up, then get into the "Game Controllers" window. Clicking on "USB Adapter" will display the window seen in Figure 8. Slide all assemblies in then out and observe the red calibration bars move up and down. This would also a good time to calibrate the throttle quadrant by clicking on "Settings" then "Calibrate".
If you need some additional help in calibrating your throttle you can go to: http://desktopaviator2.tripod.com/Instructions/Throttle/HiR/index.htm. Here you will have complete and accurate instructions on calibrating your throttle.
If you find that while activating the slide assemblies, the calibration bars for each is not in synchronization (all bars up with the slides push all the way in), just take the three pin connector from the offending slide and reverse it on the 2097 board.
PAINTING & INSTALLING THE KNOBS
With your purchase of the Slide Assemblies, you will also receive a wooden knob. The knobs need to be drilled with a 1/4 inch hole so that they can be pushed over the plastic rod on the slide assembly. But before installing the knobs; they need to be painted.
To distinguish the different functions of the assemblies, we need to paint the knobs in the standard black - blue - red format. Black being the throttle control; blue is used to adjust the prop pitch; while the red controls the fuel mixture. I find using a soft sponge dipped into the appropriate color will give very good results. When dried, the knobs should be painted with some sort of lacquer for protection.
When dry, press the knobs on the appropriate slide assembly.
The 2097 allows you to connect 4 switches. These switches can be programmed to any switch function you need. For this short article, lets concentrate on wiring two toggle switches to the 2097; one for flaps and the second for the landing gear.
A word about the toggle switches. A standard ON/OFF switch should not be used for the flaps control; rather a switch that has a spring return - center off type (momentary / off/ momentary). This toggle switch can be found at: Here for about $1.00. This switch can also be used for your landing gear control.
Figure 9 shows how you can wire the toggle switches to the six pin female connector. You may have noticed by now, we are using the first four pins of the six available. The last two pins are not used. Just take the wired six pin connector and plug it into the Male header connector on the 2097 board (Figure 10).
Again, these connections can be tested by going into the Calibration Window. Flip each toggle switch and take note of the lighting of the small red dots. When operating correctly, you can now assign the needed functions using your flight simulator program.
If you don't need toggle switches, Figure 11 shows you how to wire four normally open spring return switches to the 2097. The buttons are soldered to the same terminals as the toggle switches and placed on the same 6 pin male header.
The common ground connection for both the toggle switch and push button setup is made to the 2097 board at "GND".
ADDING A TRIM WHEEL
We still have five available analog inputs, so why now add a trim wheel? It's really easy. Figure 12 shows you how to wire a rotary type potentiometer (100,000 ohm) to the three pin connector. All you need to do is to mount some sort of plastic wheel to the shaft of the potentiometer and make these three easy wiring connections (you might consider using a shielded RCA cable). Then plug the connector into the 2097's analog input at position #8; then calibrate and assign the pot as your new trim wheel. For a more realistic trim wheel, you might consider using a multi-turn potentiometer in place of the single turn pot as shown.
With all the wiring and assembling complete, your new throttle quadrant will look like this (Figure 13).
Marlin P. Jones & Assoc.
Push Button Switches
Model 1090 Slide Assembly
Model 2097 USB Adapter
Push Button Switches