spl3 - DC Motor / Propeller Characterization 24 Feb 06 Lab...

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Motor / Propeller Characterization 24 Feb 06 Lab 3 Lecture Notes Nomenclature r radial coordinate R prop tip radius V flight velocity Ω motor and propeller rotation rate v motor terminal voltage i motor current η overall propulsive efficiency P P B number of blades D prop diameter ( 2 R ) P prop pitch T prop thrust Q prop torque thrust thrust power ( TV ) elec electrical power ( iv ) Motor/Propeller Characterization The combination of a propeller and an electric motor, shown in Figure 1, can be viewed as a propulsion unit whose thrust T ( V,v ) and current i ( V,v ) both depend on the air velocity V into the propeller, and the voltage v applied to the motor terminals. We can then de±ne the thrust power P thrust , the electrical power P elec , and the overall propulsive efficiency. P thrust TV P η ( V,v ) = = (1) elec i v + υ V υ υ T (V, ) i (V, ) Figure 1: Propulsion unit consisting of a propeller driven by an electric motor. For any given propeller and motor, the thrust T and current i depend only the oncoming air velocity V and applied voltage v . Calculation of the T ( V,v ) and i ( V,v ) functions for any given propeller and motor combination is rather complicated, and is beyond scope here. However, these can be readily measured in a motor/prop test stand in a wind tunnel. Typical behavior of these functions is shown in Figure 2. The rotation speed Ω( V,v ) is also shown for interest. Each thrust curve in Figure 2 is seen to drop to zero at a particular aerodynamic pitch speed V o ( v ), which is roughly proportional to the applied voltage v . The reason for this drop to zero is primarily due to the aerodynamic characteristics of the propeller, although the motor characteristics also come into play to a smaller extent. An electric aircraft in level flight requires T > 0, and hence its maximum level-flight speed clearly cannot exceed the pitch speed. Hence, to increase the aircraft’s maximum speed, it is necessary to examine what features of the propeller and motor combination might be changed to increase V o (in addition to the obvious solution of increasing the voltage v ). 1
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spl3 - DC Motor / Propeller Characterization 24 Feb 06 Lab...

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