105: Propulsion Fundamentals

105.1 State the type and model of the engine used on the C-2A aircraft.

Two ALLISON T56-A-425 turboprop engines power the aircraft with Hamilton-Standard constant speed propellers. The engines each develops a maximum of 4,600 indicated horsepower at takeoff.

105.2 State the five forces acting on a propeller.

Centrifugal Force The greatest force acting upon the propeller blade is centrifugal force. This force tends to pull the blade of a spinning propeller out of its hub. To prevent the blades from breaking into fragments or flying off into space, the blade is thicker near the hub. The hub is made from a strong steel forging.

Thrust Bending Force Thrust bending force causes a rotating propeller to try to pull away from the aircraft. Because it is held back by the hub and the load of the aircraft it is pulling, the blade tips, which are thinner and lighter than the blade shank, bend forward. The sum of these bending forces on the blades is carried at or near the hub. Hence, the section of the blade at the hub must be proportionately thicker. Centrifugal force counteracts thrust-bending force by its tendency to pull the blades in a straight line.

Torque Bending Force Torque bending force is the tendency for a blade to bend backwards, throughout its length, in a direction opposite rotation. The density of the air creates this bending force.

Aerodynamic Twisting Force Aerodynamic twisting force tries to rotate the blades in the hub to an increased blade angle. The point at which this force is exerted most strongly on the chord of the airfoil is known as the center of pressure. During normal cruise conditions, this center of pressure is nearer the leading edge of the propeller, so the force tends to rotate the blades to a higher pitch.

Centrifugal Twisting Force The centrifugal twisting force on the blades tends to twist them to a lower pitch angle. This occurs because all parts of the propeller try to remain in a plane parallel to the plane of rotation

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