Most flight simulators have constant speed propellers but how much do computer pilots actually know about them and how to use them? Let's go into why we have them for a start. One of the problems with a fixed propeller is the given pitch of the propeller determines its peak operating efficiency. Beyond this speed the propeller efficiency drops off very steeply. This leads to a problem with aircraft that have better performance. To get the peak propeller performance over a much broader speed range we need to vary the pitch of the propeller to keep it operating at peak performance. This is what the CSU (Constant Speed Unit) does for us. Put simply, it keeps the speed of the propeller (RPMs) constant across a much wider speed range.

The introduction of CSUs into aircraft requires the introduction of a lever to control the RPM of the propeller. Previously the throttle had this function but the throttle is now used to control the manifold pressure. Adding further to your cockpit clutter, the manifold pressure also has a gauge. The new gauge is used to set the engine power. The combination of throttle, prop advance, and mixture sets the aircraft performance. Now, to increase power, we work right to left across the levers, mixture to rich, pitch to fine (or if you like, RPMs to max), then throttle to maximum. When decreasing power we work in the reverse. Set the manifold pressure first, RPMs next and then lean the engine.

With this additional technology comes added failure procedures. Now if the engine fails we loose oil pressure, which in turn means our CSU will change the propeller pitch to maximum coarse. To obtain maximum glide distance with the aircraft when you have an engine failure, the pitch lever should be set to maximum coarse (pull all the way out).

To give this information a more practical aspect let's look at the settings in a normally aspirated engine aircraft. To get the exact details for each aircraft you do need the aircraft manual; however, these settings work for most aircraft in flight simulators. They are a good guideline.

One of the issues you will find with this type of aircraft is the effect of the air becoming thinner as you climb and the corresponding drop in air pressure. This means the manifold pressure will drop with altitude. We say an aircraft is working at its peak efficiency when the throttle is at maximum and the manifold pressure is at the cruise level. In a normally aspirated engine it is impossible to get a manifold pressure greater than the atmospheric pressure. There is also a maximum height imposed by the falling pressure in the manifold.

CSUs are most commonly to be found in higher performance aircraft. To overcome the problems of altitude and falling pressure outlined above, many of these high performance aircraft will also have turbocharging. Turbocharging the engine allows the manifold pressure to exceed the atmospheric pressure. With a turbocharged engine there is a turbine on the exhaust system. This uses the exhaust gas to increase the pressure of the air flowing into the manifold. The aircraft engine defines the maximum manifold pressure and the aircraft uses a "waste gate" to control the turbo. Generally at lower altitudes the waste gate is open and gradually closes as the aircraft climbs.

The CSU simplifies the flying of the aircraft and improves the performance of the aircraft over a much broader speed range. Once you become familiar with the CSU's operation you will gain much greater control over the aircraft.