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Power Absorption

A propeller must be able to absorb all the shaft power developed by the engine and also operate with maximum efficiency throughout the required performance envelope of the aircraft. The critical factor is tip velocity. If tip velocity is too high, the blade tips will approach the local speed of sound and compressibility effects will decrease thrust and increase rotational drag. Supersonic tip speed will considerably reduce the efficiency of a propeller and greatly increase the noise it generates.

This imposes a limit on propeller diameter and RPM, and the TAS at which it can be used.

Other limitations on propeller diameter are the need to maintain adequate ground clearance and the need to mount the engines of a multi-engine aircraft as close to the fuselage as possible to minimize the thrust arm. Increasing the propeller diameter requires the engine to be mounted further out on the wing to maintain adequate fuselage clearance. To keep VMC within acceptable limits, the available rudder moment would have to be increased. Clearly, increasing the propeller diameter to increase power absorption is not the preferred option.

Solidity

To increase power absorption, several characteristics of the propeller can be adjusted. The usual method is to increase the ‘solidity’ of the propeller. Propeller solidity is the ratio of the total frontal area of the blades to the area of the propeller disc. It can be seen from Figure 16.18 that an increase in solidity can be achieved by:

•Increasing the chord of each blade. This increases the solidity, but blade aspect ratio is reduced, making the propeller less efficient.

•Increasing the number of blades. Power absorption is increased without increasing tip speed or reducing the aspect ratio.

Increasing the number of blades beyond a certain number (five or six) will reduce overall efficiency.

PROPELLER DISC

Figure 16.18 Solidity of a propeller

Thrust is generated by accelerating air rearwards. Making the disk too solid will reduce the mass of air that can be drawn through the propeller and accelerated. To increase the number of blades efficiently, two propellers rotating in opposite directions on the same shaft are used. These are called contra-rotating propellers.

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Moments and Forces Generated by a Propeller

Due to its rotation a propeller generates yawing, rolling and pitching moments. These are due to several different causes:

•Torque reaction.

•Gyroscopic precession.

•Spiral (asymmetric) slipstream effect.

•Asymmetric blade effect.

Note: The majority of modern engines are fitted with propellers which rotate clockwise when viewed from the rear, so called “right-hand” propellers. The exceptions are small twin piston engine aircraft, which often have the propeller of the right engine rotating anti-clockwise to eliminate the disadvantage of having a “critical engine” (see Chapter 12), plus some older aircraft.

Torque Reaction

Because the propeller rotates clockwise, the equal and opposite reaction (torque) will give the aircraft an anti-clockwise rolling moment about the longitudinal axis. During take-off this will apply a greater down load to the left wheel, Figure 16.19, causing more rolling resistance on the left wheel making the aircraft want to yaw to the left. In flight, torque reaction will also make the aircraft want to roll to the left. Torque reaction will be greatest during high power, low airspeed (IAS) flight conditions. Low IAS will reduce the power of the controls to counter the “turning” moment due to torque.

	16		PROPELLER
		TORQUE
	Propellers		ROTATION

Figure 16.19 Torque reaction giving left turn during take-off

Torque reaction can be eliminated by fitting contra-rotating propellers. Torque from the two propellers, rotating in opposite directions on the same shaft, will cancel each other out. Co-rotating propellers on a small twin will not normally give a torque reaction until one engine fails. A left “turning” tendency would then occur. Counter-rotating propellers on a small twin will reduce the torque reaction following an engine failure.

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Gyroscopic Effect

A rotating propeller has the properties of a gyroscope - rigidity in space and precession. The characteristic which produces “gyroscopic effect” is precession. Gyroscopic precession is the reaction that occurs when a force is applied to the rim of a rotating disc. When a force is applied to the rim of a propeller, the reaction occurs 90° ahead in the direction of rotation and in the same direction as the applied force. As the aircraft is pitched up or down or yawed left or right, a force is applied to the rim of the spinning propeller disc.

Note: Gyroscopic effect only occurs when the aircraft pitches and/or yaws.

For example, if an aircraft with a clockwise rotating propeller is pitched nose-up, imagine that a forward force has been applied to the bottom of the propeller disc. The force will “emerge” at 90° in the direction of rotation, i.e. a right yawing moment. Gyroscopic effect can be easily determined when the point of application of the imagined forward force on the propeller disc is considered.

Pitch down - forward force on the top, force emerges 90° clockwise, left yaw.

Left yaw - forward force on the right, force emerges 90° clockwise, pitch up.

Right yaw - forward force on the left, force emerges 90° clockwise, pitch down.

Gyroscopic effect will be cancelled if the propellers are contra-rotating.

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Spiral Slipstream Effect

As the propeller rotates it produces a backward flow of air, or slipstream, which rotates around the aircraft, as illustrated in Figure 16.20. This spiral slipstream causes a change in airflow around the fin (vertical stabilizer). Due to the direction of propeller rotation (clockwise) the spiral slipstream meets the fin at an angle from the left, producing a sideways force on the fin to the right.

Spiral slipstream effect gives the aircraft a yawing moment to the left.

The amount of rotation given to the air will depend on the throttle and RPM setting. Spiral slipstream effect can be reduced by:

•the use of contra or counter-rotating propellers.

•a small fixed tab on the rudder.

•the engine thrust line inclined slightly to the right.

•offsetting the fin slightly.

PROPELLER ROTATION

SPIRAL SLIPSTREAM

LEFT YAW

Figure 16.20 Spiral slipstream effect

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