Propulsion System

The propeller is a rotating airfoil. It is subject to drag, stalls and other aerodynamic factors that apply to any airfoil. The propeller provides the thrust to pull the aircraft through the air. As seen at right, the cross section near the hub of the propeller is thick, and has a fairly large angle of attack. The angle of attack and the thickness decreases toward the tip of the blade. Since the linear speed at the tip is much faster than at the hub the change in angle of attack provides uniform thrust along the surface of the blade.

The propeller is normally connected directly to the engine crankshaft. Some aircraft , however, employ gear arrangements between the engine and the propeller.

Propellers fall into two main categories:

• Fixed Pitch
• Controllable Pitch (Constant Speed)

Controllable pitch propellers allow the pilot to set the pitch of the blades, either directly or via a governor, to the best angle for the flight condition and performance desired. Usually for takeoff, a fairly “flat” angle of attack and high engine RPN is used to produce maximum horsepower and thrust. As altitude is gained the pilot can reduce RPM and increase pitch for a cruise climb condition. Once cruise altitude is reached the throttle, mixture and propeller pitch can be adjusted for the desired cruise performance.

The pilot has only one method of controlling thrust on fixed pitch propellers; that being adjusting engine RPM. With controllable pitch propellers, the pilot can adjust two controls; these being RPM (throttle) and Manifold Pressure (propeller pitch control). The Tachometer indicates RPM and the Manifold Pressure Gauge indicates the manifold pressure.

On constant speed propellers, a governor automatically adjusts the pitch of the propeller blade whenever the engine throttle setting is changed. Low RPM and High Manifold pressure should be avoided, as this places undue stress on engine components, and can lead to eventual engine failure.

For any given blade angle, the propeller has an ideal geometric pitch. It is designed to travel a certain distance in one revolution. However, due to slippage, the ideal geometric pitch is never attained. Therefore the effective pitch is always less than the geometric pitch. The propeller is never 100% efficient.