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Propeller

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Airscrew

Description

An aircraft propeller is an aerodynamic device which converts rotational energy into propulsive force creating thrust which is approximately perpendicular to its plane of rotation. The rotational energy can be produced by a piston or gas turbine engine or, in limited applications, by an electric motor. A propeller can be attached directly to the crankshaft of a piston engine, as is the case in many light aircraft, or it might be powered through a reduction gear box (RGB) attached to a piston or jet engine. In this case, the RGB converts the high rotation speed of the engine to one that is more appropriate for propeller operation. Propellers have two or more blades spaced evenly around the hub and are available in fixed pitch or in variable pitch configurations. More sophisticated propeller designs include those of the constant speed, contra-rotating and counter-rotating types.

Propeller Design

The cross section of a propeller is similar to that of a low drag wing and is subject to the same aerodynamic issues such as angle of attack, stall, drag and transonic air flow. There is a twist along the length of a propeller blade because the blade speed is much higher at the tip than it is at the root. The twist is necessary to maintain a more or less constant angle of attack along the length of the blade. Like a wing, propeller performance is degraded when it is not at its optimum angle of attack. To overcome this deficiency, many propellers use a variable pitch mechanism to adjust the blade pitch angle as the engine speed and aircraft velocity change.

Propeller design considerations include the number and shape of the blades but compromises are required. As an example, increasing the aspect ratio of the blade will reduce drag. However, as the amount of thrust that is produced by a propeller is proportional to the blade area, increasing the aspect ratio means that either longer blades or more blades are required to maintain equivalent thrust. Longer blades will approach transonic tip speed at a lower RPM than shorter ones and increasing the number of blades also results in an increase in the blade to blade interference effects.

The performance of a propeller diminishes greatly as the blade nears transonic speed. The relative airspeed at any point on a propeller is a vector sum of the tangential rotational speed of the propeller and the aircraft speed. As a result, the propeller blade tip will reach transonic speed well before the aircraft. At the critical speed, shock waves result in a significant increase in both drag and noise. Swept back, scimitar shaped propellers are used in some installations to increase the critical propeller speed and to reduce shock wave formation.

six-bladed turboprop engines
RCAF C130J six-bladed turboprop engines

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