Rotating airfoils driven by an engine, which provide thrust to an aircraft.
Propellers have long been recognized as an efficient means of generating thrust. They were popularly used in aircraft design even before being used by Orville and Wilbur Wright to power the Wright Flyer in 1903. Leonardo da Vinci sketched propeller designs for helicopters in the 1500’s. Early propellers were based primarily on designs used for ships and windmills, but experiments soon found that long, thin airfoils provided better thrust than the shorter, thicker hydrofoil designs used in water.
The function of a propeller is to create thrust to accelerate an aircraft forward. Although a wing creates lift to overcome an aircraft’s weight, a propeller creates thrust to overcome its drag. This thrust keeps an aircraft moving. When the propeller’s thrust is equal to the aircraft’s drag, the aircraft travels at a constant speed. When thrust is greater than drag, the aircraft accelerates until drag is equal to the thrust. Likewise, when the propeller’s thrust is less than the aircraft’s drag, the vehicle decelerates until the drag and thrust are equal and the aircraft’s velocity becomes constant. Thus, varying the propeller’s thrust will change the aircraft’s velocity.
In a helicopter, the propeller is turned upward, so that the thrust is generated vertically to overcome the weight of the aircraft. When a propeller is oriented primarily to overcome weight instead of drag, it is usually called a rotor. The engine powering a propeller can be either a conventional piston (reciprocating) engine or a jet (gas turbine) engine. In the latter case, the propeller-and-engine combination is commonly referred to as a turboprop. Turboprops typically derive 95 percent of their thrust from the propeller, while the remainder comes from the jet-engine exhaust.
A propeller may be thought of as a severely twisted wing. In fact, the wings of many aircraft are twisted either to increase or decrease lift on certain portions of the wing by changing the local effective angle of attack. The propeller is twisted for a similar reason. Like an untwisted wing, a propeller could be designed without twist, as some of the first propellers were, but it would create less thrust than would a twisted propeller.
A propeller generates thrust in the same way that a wing generates lift. Instead of moving in a straight line, however, the propeller rotates about a hub that is turned by the engine shaft. A propeller actually traces out the shape of a helix as it travels around in flight. For this reason, propellers are often referred to as airscrews and are also analogous to the propeller screws found on a ship.
Both the rotating and forward movements of a propeller’s airfoil have an effect on how much thrust is developed. The velocity at each radial location of the propeller will be different, because the total velocity is the vector sum of the propeller radial velocity and the aircraft velocity. Because the propeller is rotating at a certain rotation rate, the propeller velocity at any distance from the axis of rotation is the rotational speed times the radial distance. Thus, the propeller velocity will be almost zero near the hub and a maximum near the tip. This difference in velocity requires that the cross sections of the propeller’s airfoil be twisted so that the chord line has a large angle of attack near the hub and a small angle of attack near the tip, in contrast to the airfoil of a wing that is nearly flat. The propeller’s chord line increasingly points in the direction of the aircraft motion, as the propeller airfoil sections progress toward the hub.
The angle between the chord line of the propeller and the propeller’s plane of rotation is called the pitch angle. To determine the local angle of attack of a propeller, one uses the propeller’s pitch angle at each blade section and subtracts the angle of attack of the incoming relative wind.
A propeller can be placed anywhere on an aircraft, either at the nose, tail, wings, or on a pod. In a tractor configuration, the propeller is placed facing forward, usually on the nose, and pulls the aircraft. In a pusher configuration, the propeller is placed facing the rear of the aircraft and pushes the aircraft forward. One design has no real benefit over the other. The tractor configuration is more common, because it allows a better balance of the aircraft’s center of gravity about the aerodynamic center of the wing with the engine placed near the nose. Pusher configurations are more common in canard aircraft for the same reason. In a tractor configuration, the slipstream from a propeller is often pushed over the wings, creating a faster flow over that part of the wing. This is sometimes used to generate more lift, but it is not commonly considered in aircraft design.
The propeller efficiency is a measure of how effectively a propeller transforms the engine power into propulsive power. It is measured by dividing the power output by the power input. The power output is the thrust generated by the propeller multiplied by the aircraft velocity. The power input is the amount of shaft power generated by the engine, measured in horsepower or watts. A propeller that is 100 percent efficient means that all of the power from the engine is transferred directly to the air. No propeller can achieve 100 percent efficiency, however, and is hindered by several factors. The propeller, as it rotates, adds energy to the air, and this energy is lost from the aircraft, because it remains with the air long after the aircraft has passed. Indeed, the most efficient propellers are the ones that take a large amount of air and increase the velocity of the air only slightly. Thus, all things being equal, larger-diameter propellers are more efficient than smaller ones. Also, the drag forces that act on the aircraft as a whole also act on the propeller. These forces include pressure drag, such as separation of the flow over a propeller, and friction drag, in which viscous effects of the air retard propeller motion.
Typical propellers have efficiencies in the 70 to 90 percent range. Fixed-pitch propellers have the lowest efficiency and can drop below 70 percent if they are operating at a velocity for which they were not designed.
The Wright brothers and Alexandre-Gustave Eiffel, among others, conducted early experiments on propellers. The Wright brothers were particularly concerned about maximum power output and thrust generation, because their early engines developed very little horsepower. They were able to design propeller blades with an efficiency of up to 70 percent, which was an extraordinary feat for the time. Eiffel, a French engineer and the builder of the Eiffel Tower in Paris, was also an ardent aerodynamicist who performed some of the first detailed wind-tunnel experiments on propellers. He was the first to show that propeller efficiency varied with the propeller’s rotation rate, diameter, and aircraft velocity. This parameter is now called the advance ratio and is used in propeller design, optimization, and selection.
Propellers can be used on aircraft in several different ways. In the fixed-pitch propeller, the propeller blade has a fixed angle of attack. Although the angle varies along the length of the propeller, the blade has a fixed orientation throughout its flight envelope, meaning that the propeller design has been optimized for a single speed. If the aircraft travels at another velocity, the propeller efficiency is reduced. Fixed-pitch propellers were used on all airplanes up to the 1930’s, when variable-pitch propellers were introduced.
The angle of attack of variable-pitch propellers can be changed by rotating the blade about the hub. This allows pilots to adjust the propellers’ relative angle of attack in flight to account for changes in the aircraft and wind velocity. A complex mechanism in the hub allows the pilot to change the propeller pitch in flight, thereby increasing overall performance. When variable-pitch propellers were introduced in the 1930’s, propeller efficiency across the range of flight conditions was greatly increased. A major drawback, however, was that as the pitch was altered, the torque on the engine was also changed. This would, in turn, change the rotation speed of the engine, resulting in a lower engine-power output.
Consequently, the constant-speed propeller was introduced in the 1940’s. It is a variant of the variable-pitch propeller in which the propeller pitch is changed automatically to keep the engine speed constant and to maximize total power output. Variable-pitch and constant-speed propellers may be feathered in flight during an engine-out scenario to minimize the propeller drag.
To keep the propeller efficiency from dropping, the velocity of the propeller tip must be kept lower than the speed of sound, or Mach 1. If this velocity is exceeded, shock waves form at the tip of the propeller, and the efficiency drops dramatically as the available power is reduced by pressure losses. Shock waves can create other problems, such as severe noise, vibration, and structural damage to the propeller. Because the velocity at the tip is a function of the propeller radius, engine-shaft rotational speed, and aircraft speed, these three factors come into play when determining what size propeller should be used. During the tradeoff analysis of an aircraft design, as the speed of an aircraft increases, the diameter of the propeller decreases.
To generate the same thrust for a smaller-diameter propeller given the same engine speed, an aircraft designer may opt to go with a larger number of propellers. The propeller must be balanced, and two blades are the minimum used. However, any number of blades greater than two may be chosen, as long as the blades are evenly spaced to maintain balance. Increasing the number of propeller blades means that to achieve the same thrust, a smaller diameter can be used. This is sometimes done to avoid the sonic tip speeds that may be encountered with long propeller blades on fast aircraft. Two-, three-, four-, and five-bladed propellers have been commonly used on aircraft throughout the twentieth century.
To overcome the drawback of the sonic tip speed limitation of propellers on some commercial aircraft using turboprops, the use of unducted fan propellers has been proposed. The unducted fan propeller is a many-bladed propeller with short, curved blades that allow craft to overcome the sonic tip concerns that plague high-speed aircraft using traditional propeller designs.
Anderson, J. A., Jr. A History of Aerodynamics and its Impact on Flying Machines. Cambridge, England: Cambridge University Press, 1997. An exhaustive and well-written history on the science of aerodynamics and how it affected the development of aircraft, including early propeller design. Milne-Thomson, L. M. Theoretical Aerodynamics. New York: Dover, 1958. A classic treatise on aerodynamics that includes detailed analysis of propeller thrust calculations. Raymer, Daniel P. Aircraft Design: A Conceptual Approach. Washington, D.C.: AIAA Press, 1992. An aircraft design guide that includes information on engine and propeller selection and sizing. Von Mises, Richard. Theory of Flight. New York: Dover, 1945. An explanation of the theoretical basis for aircraft flight that includes two chapters on propeller performance and theory.
Forces of flight