7. Cockpit controls description

Home build flight yoke for flight simulation

Gear lever

Gear up / down indicator

Parking brake

Speed brake

Flaps lever

Throttle, Prop-pitch and Mixture controls

Rudder pedals

Rudder and Elevator trim wheels

Rotary encoder to keyboard stroke interface circuit for rudder and elevator trim

Limitations of keyboard when driving with pulsed switches

 

 

Home build flight yoke for flight simulation

These drawings show the construction of a flight yoke for light aircraft. The yoke pitch and roll movement are roughly equal to the movements of Cessna Cardinal yoke (I got those from a friend in the States who owns one).

The tension adjustment for roll and pitch may take some experimenting with different types of springs (or bungee cord)

This flight yoke has been built into a wooden case, that is mounted like a drawer under the computer table. The wooden case can also house other control items like throttle, flaps and gear lever and various switches, to form a complete flight control console.

The interface for roll and pitch control to PC can be done via standard joystick port, using the X and Y coordinate of joystick A input.

I would suggest with starting with the wooden case, and then gradually continue with the metal parts. The drawing shows which dimensions are setting the pitch movement, so you can modify if needed.

The front tube that fits over the threaded 15mm rod must leave some room to pass the electric wiring from yoke handle to the case. How many wires you need, depends on the amount of switches. I used 40 wires (40 x 0.2mm f transformer wire stuck between tape) for the Hat and 17 switches.

The bearings for the control column should not be too tight as deformation in the wooden box would then hamper the column movement.

PCB (printed circuit board) plate is very strong material for all kinds of mounting plate uses. The PCB I used are the glass fiber reinforced type (CEM4). You can leave the copper layer on.

All yoke components

Detail of T-piece and roll centering

Detail of roll axis potmeter linkage

 

Front of controls case

Elevator linkage

Rudder movement linkage

 

Following section gives some examples of possible solutions for the different cockpit controls. All pieces are screwed to the front (or bottom) of the wooden cockpit case.

Gear lever

Moving the lever up and down flips the toggle switch. The toggle switch arm has been extended to get better switch action. Interfacing to keyboard via the toggle to momentary switch action circuit. The same switch can be connected to the gear light indication circuit as well.

Gear up / down indicator

When moving the gear lever up and down, the gear indicator lights are changing as well. For most light aircraft, there are three green gear down and locked indicators, and one red gear in transition indicator. Although the flightsim screen will also show them, I found it handy to have real lights next to the gear handle as well.

From the sim, you can check the light sequence and timing, and with a small circuit driven from the gear lever switch, you can make the four lights behave in the same manner.

 Gear up/down lights sequence

Gear lights circuit:

 

The above circuit shows the gear lights driving circuit and the connection to the gear lever switch. In shown position, the gear switch output is low. This should correspond to gear up position. The three upper LED’s and the red LED are dark.

When the gear switch is flipped to gear down position, the upper one-shot gets triggered, and produces a positive pulse of about 4.7seconds at the Q output. (determined by R*C of the upper one shot timing components) This pulse drives the lower transistor and the red LED will light for 4.7 seconds. At the same time, Q of the upper one-shot produces a low (going to 0V) pulse of 4.7 seconds. This pulse will pull down the base of the transistor that drives the three gear down LED’s. After 4.7 seconds, Q goes low, and Q goes high. The red light goes out, and the green lights light-up driven by the high output of the gear switch.

When the gear lever is again pushed in the gear up position, the switch output goes low, and the green lights go out. At the same time, the lower one-shot is triggered, and its Q output sends a high pulse of 4.7seconds to the red LED driver transistor.

You can change the gear transition time by changing the 470k resistor value or the 10uF capacitor value of upper and lower one shots: Upper determines up à down time, lower determines the downà up time.

(For info on the IC’s, see the Elevator Trim section)

Gear light indicator components and wiring setup:

 

Parking brake

Just a fun piece of mechanics that should give you the feeling of pulling or releasing the brake. Interfacing to keyboard via the toggle to momentary switch action circuit.

Shown in Parking Brake released position

 

Speed brake

Another simple mechanical construction. I’ve tried to make it resemble the Mooney speedbrake. Interfacing to keyboard via the toggle to momentary switch action circuit.

Shown in retracted position

 

Flaps lever

This flaps lever is a very simple one: It does not show the different flap settings, as many light aircraft have different number of flaps steps. The switches are momentary types (micro switches) so can be connected to the keyboard directly.

The U-piece and PCB "spring" keep the flap lever centered and provide more centering force. (The force or the micro switches was too little, and made the lever bounce when released.)

 

Throttle, Prop-pitch and Mixture controls

The lever throw distances are close to actual Cessna Cardinal. I used rotation type potmeters, but things could probably be simpler when using slide potmeters.

The throttle potmeter uses about 270o rotation, but prop and mixture use about 200o rotation. However, after calibration, the joystick calibration software will correct for this, and all levers will have full control range in the flight simulator cockpit controls.

 

Rudder pedals

 

The key to good rudder pedal action is smooth sliding parts and strong centering force. The sliding parts need to be some length, to avoid lifting the rear section when braking. Try to make use of (almost) the full potmeter rotation for better accuracy.

It is possible to use proportional toe brakes by converting the pedal tilt action into potmeter rotation action by means of gears. Potmeters should then be connected to a hacked PCB from USB joystick. I haven’t tried this, as I’m quite satisfied with on/off brake action.

 

Rudder and Elevator trim wheels

The rudder and especially the elevator trim wheel function is a very nice addition to the cockpit controls. Since elevator trim is used a lot, I have tried to make it come (somewhat) close the the real thing found in light aircraft. The whole assembly is mounted below the throttle/prop/mix section, and elevator trim is quite easy to reach. Rudder trim is somewhat low, as I needed to place the small keyboard for radio frequency input as well.

The trim interface is via rotary encoders, to a pulse shape circuit, to the keyboard trim up/down/left/right keys.

For the elevator trim, there is a small gear section that makes the turn sensitivity more realistic. Rudder wheel is directly mounted to the rotary encoder. The wheels need some friction, so I used dishwasher sponge (don’t laugh) in between the wheel and the wooden frame.

Rear view drawing of the trim wheel section

 

Rotary encoder to keyboard stroke interface circuit for rudder and elevator trim

In cockpits there are a number of rotating knobs, like radio tuning, OBS, etc. Also rudder and elevator trim are rotating inputs.

For radio tuning, I still use the numerical keypad as frequency input. Therefore I mounted a small keypad to the controls console, cut out of an old keyboard, and wired it to the main keyboard number keys. In Flight Unlimited III, OBS needs to be rotated by means of mouse. No keyboard input seems to exist.

For adjusting the elevator trim, you have to press pre-defined trim-up and -own keys repeatedly. Same for rudder trim. Real aircraft use trim disks that you have to rotate to change the trim setting.

Since elevator trim is used very often, it adds lots of realism in the sim when rotating the trim disk a couple of notches in stead of pushing a key a couple of times.

In the Cessna, the rudder and elevator trim disks need to be rotated 2.5 turns for trim limit to limit. In Flight Unlimited III, the elevator trim key needs to be pressed 180 - 200 times limit to limit, the rudder trim about 110 - 120 times limit to limit. (both seem to vary a bit over the different airplanes)

To make that amount of switch pulses over 2.5 turns is best done with a rotary encoder. This is a little device that looks like a potentiometer, with three connections, but internally has two wipers that pass a number of contacts when rotated. The mid connection is connected to the wipers’ center, while the outer connections are the contacts that each wiper passes. The reason for two wipers is the fact that the circuit that decodes the on/off cycles of the wipers also has to distinguish whether the rotary is turned clockwise or anti clockwise. This is accomplished by setting the inner and outer contacts 90o out of phase. I used Alps http://www3.alps.co.jp/index-e.html 12mm rotary encoder search for EC12E2420801. Below pictures show the encoder and the switching waveforms.

Rotary encoder

This type encoder produces 24 pulses per rotation. For the elevator, 2.5 turns should produce about 200 pulses. I added a gear ratio of 1: 2.5 between trim disk and encoder, giving 150 pulses for 2.5 turns. For the rudder trim I did not add any gear, as it is less used anyway.

Limitations of keyboard when driving with pulsed switches

After doing some experiments with the rotary encoder directly connected across the keyboard switches, I found that the keyboard controller IC (an old Intel P8049AH in my case) cannot detect too narrow pulses. (I checked this by rotating the encoder fast and checking with Notepad to see the key output). If key ON pulses are less than 20 msec, the key press is ignored or unstable. I also found that the time between two key presses cannot be too short either: The OFF time between two ON pulses needs to be more than 20 msec as well. This means that the rotary switch outputs need to undergo some pulse width modification before they can drive the keyboard. As mentioned before, the keyboard switches do not have a common ground, and therefore are best driven by means of a relay or Opto-coupler. For repeated switching, relays are too noisy. In this case, Opto-couplers are preferred.

The drawing above shows the rotary switch action (slow and fast turning) and required keyboard switch action to maintain stable key recognition. When the rotary encoder is turned very fast, the buffer circuit in between maintains the speed that the keyboard controller still can follow.

Circuit for elevator trim rotary encoder to elevator trim keyboard strokes

In the above circuit, the D-flipflop (one-half of CMOS HEF4013 IC) determines which direction the rotary encoder turns. When turning in one direction, HEF4013 Q output will be high and Q will be low. When turning the other direction, the situation reverses. When high, these outputs Q and Q enable upper and lower one-shot circuits build with IC HEF4538. These one shot circuits are used to re-shape the pulses from the rotary encoder as shown in the drawing. Since Q and Q are each others inverse, only one section will drive the keyboard via one opto-coupler, depending on the turning direction. For info on the IC’s do a search at http://www.semiconductors.philips.com/.

The HEF4538 IC is connected as two non-retriggerable one-shots.

The first one-shot has a twice longer time than the second. Both are triggered by the pulses of one of the switches of the rotary encoder. The first negative going pulse will produce a pulse at both Q outputs. The pulse time duration is equal to R*C as connected to the timing pins. The Q output pulse of the first one-shot lasts 0.07sec, that of the second 0.033sec. As long as the first one-shot output is high, neither one-shot can be re-triggered by additional pulses of the rotary encoder. Even if the encoder is turned very fast, the opto-coupler is driven with a pulse that is at least 0.033sec high and 0.032sec low. Since CMOS IC’s cannot deliver a lot of current, I added an emitter follower to drive the Opto-coupler. BC548C (or BC547) is a standard small signal NPN transistor. (see http://www.semiconductors.philips.com/)

The circuit is not perfect, as there are occasionally false pulses from the other section while tuning one direction. Adding 1 nF (0.001uF) capacitors at the rotary switches may help somewhat. (I did not use them). For elevator trim, I found the performance satisfactory.

In principle, the same circuit can be used for rudder trim. Since rudder trim is normally not rotated very fast, some simplifications can be made: I deleted the trigger inhibit after initial ON pulse, thereby saving one IC. The rotation direction detect circuit can be made from the other half of the HEF4013 IC. The circuit is shown below.

 

Elevator trim rotary encoder to keystroke converter circuit waveforms:

1&2: rotary encoder switches output (quick turn clockwise)

3: First one-shot output (HEF4538 pin 6)

4: Second one-shot output (pin10) that drives the opto-coupler

As can be seen, the very fast turn will give 32msec output pulses that the keyboard still can follow.

(Although you loose some pulses from the original encoder output, the final pulse count is the most you can get via the keyboard, and works much better than w/o pulse shape buffer).

Elevator and rudder trim electronics

Continue to next chapter or Back to Index