When the Wright brothers first dreamt of the aerodynamics of flight, their designs looked different from planes today. We’re going to look more in-depth at varying types of aircraft propellers used on present-day aircraft.
The propeller is the airplane part that converts rotational energy generated by its power into propulsive force. Propellers are an essential part of any aircraft. Without them, an airplane cannot fly. It’s imperative that, when choosing propellers for your plane, you take all of the conditions of operation into consideration. These include takeoff, climb, cruising, and high speed.
Today, there are many types of aircraft propellers available. The simplest of these propellers is either the fixed-pitch or the ground-adjustable propeller. More complicated propellers include controllable-pitch and complex constant-speed systems (automatic systems).
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But first, let’s peek at some of the most common types of propellers – and their differences.
The constant-speed propeller utilizes a hydraulically- or electrically-operated pitch-changing mechanism controlled by the governor. The pilot, using the RPM lever in the cockpit, adjusts the setting of the governor. During operation, the constant-speed propeller will automatically change its angle to maintain constant engine speed. If engine power increases, the blade angle increases, allowing the propeller to absorb the additional energy while RPM remains constant. The same goes for the reverse. If the engine power decreases, the angle decreases, making the propellers take less air, steadying engine RPMs. The pilot selects the engine speed required for any particular type of operation.
Constant-speed propellers increase angle when the airplane dives and decrease angle when it climbs. These changes are due to the flight’s changing load. As such, the governor tries to keep the RPM constant. The propeller’s governor is the mechanism that allows a constant-speed propeller to work. A propeller governor senses the aircraft’s speed and changes the propeller’s angle to maintain a specific RPM. This change is accomplished by increasing or decreasing the oil pressure going to the propeller. A governor doesn’t consider the aircraft’s operational conditions.
As the plane goes into a dive, the propeller’s angle increases. This increase prevents overspeeding, and the power output doesn’t change – since there is no change to the throttle settings. The reverse happens during a climb. The governor will decrease the blade angle to keep the rpm at the desired setting.
High-quality constant-speed propeller systems respond to small variations to ensure constant engine RPM stays consistent throughout the flight.
Each constant-speed propeller needs an opposing force that operates against the governor’s oil pressure.
Multi engine and aerobatic propellers use counterweights mounted to the propeller blade to move in the high-pitch direction as the propeller turns. Some also use air pressure and springs to move the blades toward high pitch. Oil pressure from the governor will move the blades toward low pitch.
Most single engine propellers use springs and an aerodynamic twisting moment to move the blades toward low pitch and oil pressure to move the blades to high pitch.
As the name suggests, the pilot can change a controllable-pitch propeller‘s pitch or angle during flight while the propeller is still running. The advantage of this is the alteration of the propeller’s angle to meet flight conditions. The pilot can change the propeller’s pitch in flight or operate the engine using a pitch-changing mechanism operated hydraulically or electrically.
The controllable-pitch propeller allows for a change of angle while the propeller is still rotating. This change enables the propeller to assume an angle that gives particular flight conditions the best performance. The pitch positions may be limited in number, as they are with a two-position controllable propeller. Or the pilot can adjust the pitch to any angle between the minimum and maximum setting. The use of controllable-pitch propellers makes it possible to attain the desired engine RPM for any particular flight condition.
It is easy to confuse these controllable-pitch propellers with constant-speed propellers, but the two are very different.
Controllable-pitch propellers allow the propeller angle to be changed while the propeller is turning. But, the propeller must be changed manually by the pilot. The propeller’s angle will not change until the pilot alters it manually. The pitch on a constant-speed propeller can change automatically.
With the controllable-pitch propeller, the pilot changes the angle directly in flight. The angle will not change automatically, only when the pilot manually changes it.
Multi-engine aircraft use feathering propellers, reducing propeller drag to a minimum under engine failure conditions. A feathering propeller is a type of constant-speed propeller used on multi-engine aircraft.
Feathering propellers have a mechanism to change the pitch to an angle of approximately 90 degrees. Usually, a propeller is feathered when the engine fails to produce the power needed to turn the propeller. By angling the propeller parallel to the direction of flight, the drag on the aircraft reduces. With the propellers parallel to the flight line, the propeller stops turning, and minimum windmilling, if any, occurs.
Almost all small feathering propellers use oil pressure to take the propeller to a low pitch, while counterweights, springs, and compressed air take the propellers to a high pitch. Since the propellers would go to the feathered position during a shutdown, latches lock the propeller in the low-pitch position as the propeller slows down at shutdown. These can be external or internal, within the propeller hub. Centrifugal force holds the latches during a routine flight to ensure they don’t stop the propellers from feathering. Latches prevent excess load on the engine and starter at startup. If the propeller were in the feathered position during an engine start, it would place the engine under an undue burden during a time when the engine is already subject to wear.
Overwhelmed? In Need Of Propeller Repair? Just have questions?
Constant-speed, controllable-pitch, feathering… There’s no need to be overwhelmed at all of the different types of aircraft propellers! (Besides, we haven’t even covered all of them yet.) Please keep reading for our next blog post, where we’ll be continuing with more details about different types of propellers.
In the meantime, Stockton Propeller employs the experts in composite propeller repair. If you need to have your propellers inspected or overhauled, look no further than Stockton Propeller!