• How To...Build A Push Pull Throttle Quadrant

    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.


    Figure 1

    Figure 2

    Figure 3

    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.


    Figure 4

    Figure 5

    Figure 6

    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.


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