Stationary Versus Windmilling Propeller Drag

       During the summer of 2001 I investigated the drag force of propellers using the wind tunnel owned by the Goshen College Physics Department. The goal was to determine whether a propeller held stationary has more drag than one that is allowed to windmill or vice versa. The variables investigated were the pitch, wind velocity, length, and rotational frequency of the propeller. A brief discription of the results is shown below. For a complete picture of the investigation, read my paper.
Thesis.doc (651k)
Thesis.htm (101k)

      I was first introduced to the question when a friend did a paper on the topic. It was only a literature research paper; he did not do any experimental work himself. All of his sources claimed that a windmilling propeller has more drag than one held stationary, except one. His father carried out this experiment in his own plane and found that a stationary propeller has more drag than a windmilling one.

Mounting The Propeller (25k)
Looking Down the Wind Tunnel (21k)
Diagram of the setup (5k)
Close-up of the pin mounting (15k)

      It was found that the characteristics of the propeller determined whether a stationary propeller has more drag than windmill one. The results shown below focus on the crossover point where it switched, i.e. when the stationary and windmilling drag force is the same.

      The graph to the right shows the drag force versus the pitch of the propeller. The data is normalized so that the force of a windmilling propeller with pitch 3 is zero. It appears that a propeller of pitch 4.5 would have equal stationary and windmilling drag forces. For lower pitches, there is more drag on a windmilling propeller and at higher pitches there is more drag on a stationary propeller.

 

        The graph at right shows the force vs. the length for pitch 3 and pitch 8 propellers. It is not possible to determine whether the force increases linearly or quadratically with length due to the narrow range of data. Notice that for the pitch 8 propeller, at longer lengths there is more drag on a stationary propeller than on a windmilling one, but at shorter lengths it is the other way around. Also notice that for the pitch 3 propeller, the ratio between the windmilling and stationary drag force becomes larger with length. This suggests that the crossover point also depends on the length.

Drag Force vs. Wind Velocity

       This graph looks specifically at the effect of wind velocity on the crossover point. The data appear to be constant for different wind velocities. This suggests that the crossover point is independent of wind velocity. Somewhere between pitch 4 and 5 the ratio is one. This is in agreement with the drag vs. pitch data.