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VARIABLE PITCH
To overcome the deficiencies of high speed, coarse pitch propellers at take off or in the climb, in full-sized aviation the variable pitch propeller is widely used. In this a fairly complex mechanism allows the pilot to select the blade pitch required to give efficient propeller performance, and hence good thrust, over the whole speed range. This is done by rotating the blades axially at the hub; it is not feasible to change the twist of the blades themselves. Hence such a propeller will still lose some efficiency at speeds other than the design point, but this detracts only slightly from the all-round improvement in efficiency and safety. Model aircraft have been flown with variable pitch propellers and experimentation continues. The cost is likely to be high if a propeller is broken and the danger of shedding blades requires a great deal of care in design of the hub and gearing. Rubber driven models offer scope for improvement here too, and variable pitch propellers have shown up very well in experiments although not, as yet, widely adopted.
A constant speed propeller is one in which the pitch is varied automatically to maintain a constant engine r. p.m. The engine has a most economical speed and fuel can be saved if the propeller pitch is matched to this under all or most flight conditions.
14.3 THE DIAMETER; PITCH RATIO
Propellers for models are ‘fixed pitch’ and are normally marketed with a stated diameter to pitch ratio. The ratio itself is independent of the units and dimensions used, since a 28 x 18 cm propeller, with D/P ratio of 1.56 is practically the same as an 11 x 7 inch propeller, with die exact ratio of 1.57. A propeller with a diameter of 40 cm and pitch of 26 cm would have a D/P ratio of almost the same value. Since the resistance of a propeller to the air depends greatly on the diameter as well as the pitch, if the diameter is increased, the pitch must be reduced if the engine is to drive the new propeller at the same r. p.m. That is, for a given power input, the diameter/pitch ratio must be increased.
