Propellers, or props for short, are one of the most misunderstood aircraft devices. While it may be obvious to some, aircraft propellers are designed to transfer the engine’s horsepower into thrust by moving large amounts of air to the rear of the airplane. Upon looking at a propeller blade cross section, you will realize that it actually serves as the airfoil of an aircraft. Looking more closely, the airfoil section varies with each blade to provide the most efficient AOA (angle of attack) for both rotational and forward speed. The thickest part of the blade’s airfoil is at the propeller hub which tapers down to the tip, reflecting the range of airspeeds the blade experiences in operation.
Similar to any threaded device, the propeller’s pitch is the forward distance it has the ability of moving in one revolution. This is termed “effective pitch,” as there will be some slippage. By contrast, “coarse pitch” allows the aircraft to move forward further in one revolution. Typically, aircraft are equipped with certain engine and propeller combinations that provide the most efficient operation in any flight setting.
Different Types of Aircraft Propellers
There are several different types of aircraft propellers, those of which we will outline in this section.
Fixed-pitch propellers are the most common variety and consist of a single piece of aluminum alloy or wood. Other propeller types are generally designed with blades that pivot in the propeller hub, so that different AOAs, including feathered, flat pitch, and reverse movements can be achieved. As their name implies, controllable pitch propellers allow pitch to be controlled by the pilot. They are different from constant-speed propellers, which automatically employ a governor to provide constant rpm by changing the blade angle within a preset range. Full-feathering propellers enable the blades to be rotated to a high positive angle that is aligned parallel to the slipstream and stop their rotation after an engine is shut down on a multi-engine aircraft.
Controlling Propeller Blade Angle
Controlling the propeller blade angle is done with the help of hydraulics, but electrical components are also used. Usually, engine oil pressure is boosted, enabling the propeller governor to change blade angles. Meanwhile, constant rpm is maintained by adding or relieving pressure to obtain the proper blade angle for the selected rpm. It is important to note that controllable pitch, constant-speed, and feathering propellers take advantage of springs, counterweights, accumulators, and centrifugal force, alongside hydraulic pressure to control their function.
Carrying out a good preflight check involves an in-depth assessment of the propeller and aviation spinners before operating the engine. Corrosion and defects on such components are leading causes of propeller failure. Even minor dings and dents in the blades need to be looked over and remedied according to the manufacturer’s maintenance recommendations. Furthermore, cracks, oil, or grease leaks, as well as damaged spinners, rotated blades in the hub, and missing screws are also reasons to ask for professional input. Pre-takeoff checks of the pitch change mechanisms, feathering, and governor operation should also be carried out.
For unexpected aircraft speed changes or sluggish response to rpm, one must check the propeller or governor for issues. On a multi-engine aircraft, a propeller that feathers itself without being commanded to do so is an indication of trouble. It may indicate a serious crack in the assembly or a deteriorated O ring or seal. Excessive propeller vibration may also be an indicator that your aircraft needs maintenance, as it can damage other components like the alternator, magnetos, or vacuum pump.
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