Design and Analysis

# How Aircraft Propellers Work

11 May 2017
Moisture condensing into visible water vapor in the low pressure vortices formed by the propellers of an Airbus A400M. Credit: Ramon Jordi

Aircraft propellers are a key component in the design of propeller-driven planes. They generate the thrust necessary to keep the aircraft in the sky; without the forward motion produced by propellers, there would not be sufficient air flowing past the aircraft’s wings to generate lift.

A propeller is made up of one or more blades attached to a central hub powered by a driveshaft. The blades have the cross-sectional shape of an airfoil, similar to a wing. The physical function of an aircraft propeller is to transform the rotary motion produced by the engine into forward motion of the aircraft.

Generation of Lift

A propeller blade generates lift by changing the direction of the air that it comes into contact with. Consistent with Newton’s first law of motion, air is turned by the blade, applying a force to the fluid that changes its direction. Following Newton’s third law, a force is applied to the blade in response. This force can be resolved into drag and lift components. Drag is in the same direction as the initial fluid movement and lift is perpendicular to the fluid motion.

Airfoil generating lift. Credit: University of Tennesse

The reason the air flow turns is complex and includes the conservation of mass, momentum and energy. All of these quantities must be conserved in the physical system of a propeller blade passing through air. In a fluid like air, mass can redistribute itself freely as long as it conserves momentum and energy. There are three spatial dimensions to consider for the vector quantities of velocity and momentum, and momentum in each of them must be conserved. In a fluid, a change in velocity in one direction can cause a change in velocity in a perpendicular direction. We observe the effects of the air flow turning in the pressure and velocity variations around the blade, in the vorticity it sheds, and in the downwash it generates.

Balancing Forces with Twist

A propeller blade’s tip travels much faster than its root near the hub. This is because for any rotation of the propeller, the tip travels a greater distance in the same amount of time as the root. As a result, forces along the blade vary substantially and must be accounted for in propeller design. To produce uniform lift along the blade, a twist is introduced along its length from tip to root, with a low angle near the fast-moving tip and a higher angle near the relatively slow root. This gives the tip a low angle of attack so that it doesn’t stall at high speed, while the root has a higher angle of attack to ensure lift is produced even at its low speed.

A propeller’s overall angle of incidence (or pitch), can be either of fixed or variable design. A fixed-pitch propeller has its angle set by the manufacturer, and cannot be altered. It is targeted for optimal efficiency in a specific operating regime such as climb or cruise.

A propeller blade’s angle of incidence (pitch) varies from hub to tip. Credit: Federal Aviation Administration

A variable-pitch propeller, on the other hand, is capable of altering its pitch for optimal efficiency through a range of operating conditions. Early adjustable-pitch propellers had two settings that could only be adjusted on the ground. Today, many variable-pitch propellers are capable of adjustment in flight over a wide range of settings. Constant-speed propellers are a type of variable-pitch propeller that automatically adjust pitch with a governor to maintain constant engine RPM through a range of air loads. This type of propeller can maintain high efficiency by allowing the pilot to select the most efficient engine RPM for the current flight conditions.

Camber and Efficiency

Another aspect of propeller blade design is the camber of the blade’s cross-sectional airfoil shape. Camber refers to the characteristic curves of the airfoil’s upper and lower surfaces, and the asymmetry between them. Altering these curves will produce different performance characteristics of the blade, allowing engineers to tailor the shape for optimal efficiency in the desired operating regime.

Propeller efficiency is the ratio of the power output by the propeller as useful thrust to the power input to the propeller from the engine. Efficiency varies with airspeed and engine RPM, and is often plotted versus a term known as the propeller advance ratio. The advance ratio relates the aircraft’s airspeed to the propeller’s tip speed. High advance ratios occur when the aircraft is moving at high speed or the propeller is rotating slowly. Modern propeller’s often exceed 80% to 85% efficiency over a certain range of advance ratios. This range of high efficiency can be extended in variable-pitch propellers by adjusting the blade angle of attack.

Resources

Design of Optimum Propellers - AIAA Journal of Propulsion and Power

Performance of Propellers - MIT Lecture Notes - Unified: Thermodynamics and Propulsion