A variety of propeller configurations used on military and civilian airplanes.

Figure shows thrust and torque produced by a propeller blade.

Propeller blade sections. An a (alpha) denotes angle of attack of airfoil sections.

Propeller terminology.

Use of pitch control.

These three aircraft use contrarotating propellers.

Helicopter forward motion.

##### Propellers and Rotors

The propeller on an aircraft converts the turning power of an engine's crankshaft into the thrust force. This thrust is equal to the mass of air forced backward by the propeller per unit time multiplied by the added velocity imparted to this air. If one has ever stood behind a spinning propeller while the airplane was at rest on the ground, this backward moving air, called the slipstream, is very noticeable.

Basically, a propeller blade is a small wing producing a resultant aerodynamic force that may be resolved into a force pointing along the axis of the airplane (thrust), and a force in the plane of the propeller blades (the torque force). The torque force opposes the rotary motion of the engine by acting as a "drag" on it. In equilibrium, the propeller rotates at a constant rate determined by the engine torque that is equal and opposite to the propeller torque.

The propeller blade consists of a set of airfoil-shaped sections that may vary in outline from the tip to the root of the blade. Although a wing is fixed with respect to the airplane and sees only the relative free-stream flow of air, the propeller is also rotating with respect to the airplane, and it sees an oncoming flow of air which is the vector sum of the airplane free-stream velocity and the propeller rotational velocity. The angle between this relative velocity and the plane of the propeller rotation is called the helix angle or angle of advance. For a particular airplane velocity, this helix angle varies from the root to the tip since the tip sections of the propeller are revolving faster than the root sections. As one approaches the root, the helix angle approaches 90°.

To obtain an aerodynamic force, the airfoil blade section is placed at an angle of attack to the relative velocity vector. Thus, the total angle from the plane of the propeller rotation to the chord line of the blade section is the sum of the helix angle and angle of attack for that section. This is known as the blade angle. A propeller blade appears to be twisted with the tip sections with small blade angles and the root sections with large blade angles due in main to the increase in helix angle.

The blade angle is also called the pitch angle. This pitch angle may be fixed for a propeller blade, hence a fixed-pitch propeller. or may be adjustable by hand on the ground (adjustable pitch propeller) or controlled automatically in the air (controllable pitch propeller). The efficiency of a propeller is power output divided by power input and would desirably be as close to a value of one (or 100 percent) as possible. The efficiency is proportional to the free-stream velocity and for maximum efficiency requires a different pitch-angle setting. For takeoff, an airplane uses a fine or low pitch (flat blade angle or small angle of attack) to provide a high number of revolutions per minute (rpms). A coarse or high pitch is used for cruising and gives a low number of rpms.

Some propellers may be feathered in flight. This means that the blades are turned so that the leading edges of the airfoil sections are aligned to the free-stream velocity. Feathering is used on a stopped propeller to avoid damaging an engine and to decrease the propeller drag. Some propellers have reversible pitch for use as a landing brake. In this case, negative thrust is obtained by turning the blade to a large negative angle of attack.

The design of a propeller, like an airplane, is influenced by many factors, some of which cause contradictions in design. The overall shape is determined by compromise and is largely dependent on the mission to be performed. For low speeds, the propeller blade is usually slender with rounded tips. For high speeds, larger paddle-shaped blades are used or more propeller blades are used.

The slipstream is produced by a propeller producing thrust by forcing air backwards. It is a cylindrical core of spiraling air that flows back over the fuselage and wings. The fact that it strikes the wings has important effects—some detrimental and some beneficial. The slipstream flow is faster than the free-stream flow; this means that the drag of the fuselage, wings, and other parts exposed to it is larger. The slipstream moves over the wings and is beneficial in providing for effective control by the tail surfaces since the aerodynamic forces produced by these surfaces are dependent on the square of the velocity of the air moving over them. This is important in the cases of taxiing or takeoff when the free-stream velocities may be low.

The rotary motion of the slipstream, however, causes the air to strike the tailplane at an angle and not directly head on. This may have an effect on the stability and control of the airplane. The effects of the rotary motion of the propeller may be counteracted by using contrarotating propellers (spinning in opposite directions).

For a helicopter, the rotor is the lift-producing device. The blades of the rotor are airfoil-shaped and are long and slender (large aspect ratio). The number of blades varies with the design. Generally, for heavier helicopters, more blades are used to reduce the load that each must carry.

As for the airplane propeller, the helicopter rotor blades have a pitch angle defined as the angle between the plane of rotation of the blades and the chord line of the blades. The pitch of the blades may be controlled collectively (collective pitch) or controlled individually (cyclic pitch).

Collective pitch changes the pitch of all blades together and with changes in engine power settings, produces the lift necessary for the helicopter to takeoff, hover, climb, and descend.

Cyclic pitch is controlled by the swashplate of the rotor head, which allows the pitch of individual blades to vary as they rotate about the hub. When a pilot wishes to fly forward, the swashplate is tilted forward. As each rotor blade approaches the forward position (toward the direction of flight) of its cycle, its pitch decreases, the blade lift is reduced, and its flight path descends. As the blade rotates to the rear, the pitch is increased, the blade lift is increased, and the flight path ascends. The net effect is to tilt the whole rotor disk forward to the desired angle, the total lift vector is rotated and a forward thrust component is given to the helicopter.

The rotation of the main rotor blades produces a reactive torque that tends to rotate the helicopter body in the opposite direction. Directional control is accomplished by a tail rotor. It provides sufficient thrust to counteract this rotational tendency. Additionally, by controlling the thrust of the tail rotor, the heading of the helicopter may be controlled.

—Adapted from Talay, Theodore A. Introduction to the Aerodynamics of Flight. SP-367, Scientific and Technical Information Office, National Aeronautics and Space Administration, Washington, D.C. 1975. Available at http://history.nasa.gov/SP-367/cover367.htm