A study has been made to determine the effect of variable propeller gearing on the low-speed performance of modern thin-blade propellers. Inasmuch as experimental propeller data over a suitable range of operating conditions are currently lacking, and because of the scarcity of suitable airfoil data, it was necessary to devise methods of estimating the per¬formance. The methods used are based on optimum loading considerations.nResults from the analysis of a sample flight problem indicated that, for propellers having a design advance ratio of 2.0, peak values of static thrust and take-off thrust occur at approximately the same rotational speed used for high speed. If the design advance ratio is 3.0 or higher, variable propeller gearing should be used for low forward speeds because the rotational speed corresponding to high speed is too low and hence the propellers are overloaded at low speeds. By incorporating variable gearing to increase the rotational speed, the propeller loading is reduced with a consequent gain in thrust. This gain in low-speed thrust due to reducing the propeller loading increases as the design advance ratio is increased; for a design advance ratio of 4.0, the gain in take-off thrust amounts to 30 percent or more. When a given power is absorbed at optimum rotational speeds, the static and take-off thrusts of propellers having high design advance ratios are higher than the static and take-off thrusts of propellers having lower design advance ratios. For a given power requirement this optimum rotational speed becomes lower as the design advance ratio is increased.
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