Wind in your face
Home In most open aircraft, there will be considerable wind in your face, either due to prop-wash (single prop engine) or airspeed (sailplanes). Also some simple closed canopy aircraft have ventilation system that is taken from air-inlets at left and right wing sides.

I took an intro flight in a sailplane once, and noticed the ventilation / airspeed relation. Even more, when the pilot did a slip to loose altitude, the airflow came only from one side. (quite powerful too, I had to hold my glasses from having them blown off my face).

I have tried to simulate this effect by adding two electric fans at simpit left and right side. The fan speed can be linked to airspeed, to get airflow proportional to airspeed. You can also link it to engine RPM, to simulate prop-wash.  I have made the control independent for the two fans, so you can even mix in some slip effect, derived from the turn coordinator.

The block diagram above shows the possibilities for Fan-drive. If you omit the side-slip differential fan drive feature, you only need one DAC and power buffer, that drives 2 fans in parallel.

The fans are 120x120mm centrifugal brushless blower type, rated 12V/1.3A. Mine were some obscure Chinese brand, but for reference you could check here.
When used in close proximity, they have a narrow but powerful airflow range. I mounted them in such a way, that the left and right airflow strikes the pilot's head and shoulders and upper arm, as illustrated in the drawings. 

For the first experiments, I have used the same DAC channel that I also used for the DIY Force Feedback Interface as they need almost identical parameter input (airspeed and engine RPM).
The opto-coupler diode is driven by a current that is proportional to airspeed. Engine RPM can be mixed in for prop-wash effect on open aircraft. A current similar to the opto diode will also flow through the opto-coupler transistor. The 2k2 potmeter can be used to adjust the max speed of the Fan at max airspeed/RPM condition.

The Fan power buffer consists of two emitter followers driven by a differential amplifier. The amplifier input voltage is  controlled by the opto-coupler. At zero opto current, the amplifier biasing  will result in about 5.5V across the fan. This voltage can be adjusted by the 1k potmeter, to fine-tune the starting voltage of the Fan. (which was in my case around 5.8V)
When more current flows through the opto transistor, the voltage at input will rise, and so will the fan voltage. At about 5mA of opto-current, the fan voltage will reach its maximum, about 15V. (about max limit for a 12V fan)
The switch can be used to switch-off the proportional fan drive, and a constant fan speed can be set by the offset potmeter for normal ventilation purpose. 
Note that if you want differential L/R fan drive, you need to build two of the above circuits, each with its dedicated DAC. I build a dual unit, but for my experiments I have used single channel operation.

Below some pictures of the actual application.

Cockpit with 2  fan holes at the L/R side.   Side-view of fan mount.    Dual fan driver

First findings: Test flight in the Cessna)
First of all: Some ventilation was really needed, as the enclosed motion cabin gets kind of stifling. It is pretty cool to feel the increased airflow when throttling up and increasing speed on the runway. I mixed RPM and Airspeed 50/50, and increased the sensitivity to have 75% fan speed at max throttle / cruising speed. I may add some adjustable air-guides to be able to direct the airflow. The fan rush adds a bit realism to the cockpit wind noise. Shutdown is also more realistic, as everything including the ventilation will stop.  

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