An aircraft with a fixed pitch propeller has only two engine control levers: throttle and mixture. You set the engine power output using the engine RPM. If your aircraft is fitted with a variable pitch propeller, an additional control lever and instrument are required. A variable pitch propeller can also be called a constant speed propeller. The throttle adjusts the amount of fuel going to the engine, but instead of RPM, the engine power output is monitored with a manifold absolute pressure gauge. This gauge is calibrated in inches of mercury, so it's common practice to refer to engine power output simply as inches, written as inches MAP.

he additional control lever is labeled RPM but is commonly called the prop lever, even by the examining authority. The forward position of the prop lever is used to increase RPM, while the aft position is used to decrease RPM. However, it's not quite that simple. As you learned when studying piston engines, the prop lever is only connected to the spring in the constant speed unit. It's better to think of the prop lever as a request lever. The constant speed unit will always try to give you the RPM you want, but under some situations, it won't be able to, as we will see shortly.

When an aircraft with a variable pitch propeller is on the ground, the prop lever is always positioned fully forward. The throttle is used to adjust thrust for taxiing as normal. The illustration shows an aircraft lined up on the runway ready to take off. The brakes are set, and the engine is idling. Under these conditions, the propeller will be in the fully fine position with the blades on their fine pitch stops. Gently open the throttle, and you can see both the RPM and the manifold pressure increasing to the takeoff values. Because the blades are at the optimum angle of attack, the propeller will give maximum efficiency.

Now, release the brakes. As soon as the aircraft begins to move forwards, the constant speed unit will start to increase the pitch; otherwise, the RPM would increase. This is why the variable pitch propeller is also called a constant speed propeller. The mechanism maintains a constant RPM when the true airspeed changes. There is a maximum time limit of five minutes for full takeoff thrust, so as soon as any obstacles are cleared and the aircraft is established in the climb, the engine power can be reduced to maximum continuous. In this example, the manifold pressure is reduced to 35 inches MAP.

Climb is then continued to the chosen cruise altitude. You can see the blade angle being increased to maintain the optimum angle of attack. You now have to level off and reduce power output to the cruise settings. In this example, we'll use 23 inches and 2,300 RPM for the cruise. The recommended practice when reducing the engine power is to reduce the manifold pressure first. Gently use the throttle to give 23 inches of manifold pressure. Now, gently pull the prop lever back to reduce the RPM to 2,300.

The advantage of a variable pitch propeller is that under most normal operating conditions, the optimum blade angle of attack is maintained, ensuring the propeller operates with maximum efficiency. But what would happen if the throttle is closed or the engine fails? There is now no shaft power trying to maintain the requested 2,300 RPM. The constant speed unit will fine off the blades, but it can only do so until they reach the fine pitch stop, after which the RPM will decrease. The propeller is now generating drag instead of thrust, and torque is acting to keep the propeller rotating. In fact, the forward motion of the aircraft is rotating the propeller. The propeller is said to be windmilling. The drag from a windmilling propeller is known as windmilling drag.

On a single-engine aircraft with a variable pitch propeller, there is no mechanism to feather the propeller to reduce windmilling drag. However, it is possible to reduce windmilling drag on a single-engine aircraft by pulling back the prop lever. You have requested a lower RPM, so the constant speed unit will drive the propeller blades towards coarse. This reduces windmilling drag.

On a multi-engine aircraft, windmilling drag on the failed engine must be reduced to keep VMC as slow as possible, so a mechanism is incorporated to drive the propeller past the coarse pitch stop into the feathered position. The blades are now at a zero lift angle of attack. There is no torque because the airflow is no longer able to rotate the propeller, and drag from the stationary propeller is minimised, thus reducing asymmetric thrust.

We know that a propeller generates thrust by accelerating air rearwards. Therefore, a more powerful engine will require a propeller that can accelerate a greater mass of air rearwards. A propeller can accelerate more air rearwards by increasing the RPM, but this will give increased tip speed. Increasing the blade length can also be used to accelerate more air rearwards, but this will also give increased tip speed. If the tip speed exceeds the local speed of sound, shockwaves will decrease thrust and increase the rotational drag. Supersonic tip speed will also greatly increase the noise generated by the propeller.

So maximum tip speed imposes a limit on propeller diameter and RPM. However, there are other limitations on propeller diameter. Adequate ground clearance is one consideration, and fuselage interference on multi-engine aircraft is another. With these restrictions in mind, increased power absorption from a propeller can be obtained by increasing the number of blades. A three-blade propeller can accelerate a greater mass of air rearwards without excessive tip speed or problems with ground clearance or fuselage interference. Increasing the number of blades increases the solidity of the propeller disc.

As you can see, a more powerful engine will require a propeller with four blades, which will further increase the solidity of the propeller disc. An even more powerful engine might require as many as five blades. For a conventional propeller, five blades is the maximum number. Beyond five blades, the solidity of the disc is so high that not enough air can pass between the blades to be accelerated, so propeller efficiency begins to decrease. Any further increase in engine power would require contra-rotating propellers, two propellers rotating in opposite directions on the same shaft. Contra-rotating propellers are not very common.

However, a type of propeller very common on light twin-piston engine aircraft is counter-rotating propellers. These have nothing to do with power absorption, but as their name is very similar, it is worth reviewing the difference. Counter-rotating propellers are two propellers rotating in opposite directions on different shafts. Counter-rotating propellers are fitted to eliminate a critical engine. The next lesson will cover more on this topic.