Aerodynamics of Ducted Propellers as Applied to the Platform Principle

Abstract : This report is intended to provide analytical means of predicting the performance spectrum for ducted propeller aircraft that maintain an equilibrium of forces by adjusting the angle between the vertical and the exit stream tube. Equations are derived relating the tilt angle, free stream velocity, propeller characteristics, and available power, to the net force produced by the platform and to the direction of this force. Particular emphasis has been placed on the relation between the duct exit velocity and propeller tip speed for fixed pitch propellers, because this variation determines the relation between the power available and the power required. It is impossible to determine the accuracy of the individual relations without further test data, because they must be combined to determine the overall performance, and the overall performance - not individual performance contributions - was all that was obtained by test. The net force calculated for the condition indicating the greatest difference between theory and test (V = 45 knots, tilt angle = 31 deg, and 100% power) produced a net thrust of 515 pounds directed 8.05 deg aft of the vertical. The test data of reference 3 for the same conditions of tilt angle and velocity indicate a net thrust of 624 pounds directed aft 12 deg. The difference between the theoretical net thrust and test net thrust is 17.5 deg. This is rather poor correlation, but the error cannot be completely attributed to the theory, because the net thrust is dependent upon the square root of the available power cubed. The test was conducted at full power, and it is not apparent exactly how the power available varied with RPM or what effect tilting the engines had on power output. The expressions for the moment are empirical and involve non dimensional parameters which were determined from the truck test data of Reference 3; therefore the moment equation cannot be checked against the experimental data to determine their accuracy.