How Aircraft Propellers Generate Thrust

Propeller blades are similar to aircraft wings, with cambered and flat sides, chord lines that run throughout the blade length, and leading & trailing edges. Aircraft propulsion generated by rotating propellers requires the use of a highly complicated mechanism since the propeller takes on the features of a rotating wing. There are differential aerodynamic properties between the top and bottom of the propeller blades. Propeller blades tend to be long and thin, and because of their rotation, the speed of the blade tips tends to be faster than that of the near-hub sections. Consequently, analyzing the differential airflow generated around the rotating blades becomes difficult. This blog aims to deconstruct the complex working mechanism of propeller propulsion into a more simplified explanation for ease of understanding. 

Factors That Determine Propeller Thrust Complexity

Although aircraft propellers work like a regular screw, the movement of the propeller blades in a continuous airstream compared to the action of a screw in a solid medium makes all the difference in terms of their working principles. Since the propeller blades are twisted at an angle, they experience drag while rotating in the airstream. The amount of propeller drag is proportional to the degree of the angle and twist in the blades. Another differentiating geometrical parameter between propellers and screws is that pitch characterizes the propeller design. Propeller pitch can be defined as the distance that a rotating propeller can move in the forward direction through a soft solid in one revolution. Propeller wings are designed like airfoil wings, which sport an angle of attack with the horizontal axis of the wing, and they generate lift when accelerating air downward because of their backward tilt. However, when two airfoil-like blades such as these are attached to a central hub, they produce a combined "screwing" effect to push the aircraft forward at the appropriate speed instead of delivering the desired lift.

Since aircraft propeller speed is different at varying sections along the blade length, the angle of attack of each section has an independent value to generate uniform thrust. Thus, the angle of attack is most prominent near the center of the assembly where the blade's speed is slowest, and it is the smallest at the tips where the blade's speed is the highest. Due to the difference in the angles of attack, there is an apparent twist in the blade to correct the differential thrust it might experience.

The Dynamics Between Propeller Thrust and Aircraft Acceleration

Propellers are intended to use the engine's horsepower to generate linear thrust, with the rotating shaft acting as the intermediary which transfers that power. The aircraft will enter accelerated motion if the propeller's thrust is greater than the drag it experiences. Although increasing the number of revolutions per minute (RPM) of the propeller and the engine's power can increase the aircraft's acceleration, there will be a simultaneous decrease in thrust. Research suggests that at low airspeeds, propeller efficiency is extremely low, and it rises with an increase in airspeed until a maximum peak is achieved around 85-87% efficiency, especially in the case of fixed pitch propellers. Beyond this point, the efficiency starts to decline, and this airspeed-to-efficiency relationship is known as the Maximum Efficiency Envelope.

The Association Between Propeller Types and Thrust

Fixed-pitch propellers are the simplest type of aircraft propellers as the blades are fixed at a particular angle in relation to the central hub. They are primarily used in training aircraft because of their simplicity. When the blades have a shallow angle in the incident airstream, the propeller experiences less drag and greater thrust by spinning faster, especially at the time of takeoff. However, since fixed-pitch propellers are designed to be efficient only during ascent or takeoff, they are destined to be much less efficient while they are airborne. 

On the other hand, variable pitch propellers are designed to improve the mechanical simplicity of fixed pitch propellers. Variable pitch propellers can work over various airspeeds and engine power settings. This convenient feature was readily exploited in fighter planes used in World War II. The propeller blades were designed to reach a shallow angle to have the chord line parallel with oncoming air, following a phenomenon called "feathering" so that the drag experienced by the plane could be reduced in the event of an engine failure. Moreover, variable pitch propellers are designed so that the path traversed by the blade tip is helical. At the same time, the net velocity results from the rotational and translational velocity vectors experienced by the rotating blade, those of which are also dubbed as "helical tip" velocities. Thus, the pilot can adjust variable pitch propellers, either on the ground or when airborne, to adapt to different airspeeds and altitudes.

To Sum Up

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