Propeller Characterization: Variations in Pitch, Diameter, and Blade Number on Performance

1. Measuring propeller characteristics in a subsonic wind tunnel

1. Set up the propeller test rig in the subsonic wind tunnel using a 4-axis sting mount, as shown in Figure 3. A wind tunnel with a 2.6 ft x 3.7 ft test section and maximum dynamic pressure setting of 25 psf was used in this demonstration.

Figure 3. Propeller rig. Please click here to view a larger version of this figure.

2. Attach a 6-axis load cell to the rig. This will be used to measure thrust and torque.
3. Secure a brushless DC motor to the rig, then attach the first propeller.
4. Connect the DC motor to the electronic speed controller and the pulsed-width modulated signal generator, which controls the speed of the motor.
5. Connect a power analyzer, which will measure the current and voltage supplied to the motor, and connect it to a lithium polymer battery.
6. Use a spirit level to ensure that the sting-propeller setup is aligned in the direction of flow with zero pitch and zero yaw.
7. Secure the wind-tunnel doors and switch on the main power.
8. Turn on the wind tunnel, then switch on the signal generator and the load cell data acquisition system.
9. Zero the forces on the load cell using virtual instrument software.
10. Set the signal generator to run the motor at 10% throttle.
11. Start recording a zero-reading with the wind tunnel off. Record the speed following data:
    a. Propeller characteristics - number of blades, propeller diameter (in), and propeller pitch (in).
    b. Speed (in terms of percent throttle) based on the signal generator configuration.
    c. Dynamic pressure (psf) from the wind tunnel transducer.
    d. Voltage (V) and current (A) supplied to the BLDC motor from the power analyzer.
    e. Thrust (lb) and torque (in-lb) from the load cell.
    f. Propeller RPM (rotations per minute). Note that the RPM reading can only be extracted at the end of the experiment.
12. Turn on the wind tunnel and vary the dynamic pressure from 0 psf to 10 psf in steps of 0.5 psf.
13. At each setting, allow the wind tunnel to stabilize, then record the same data as listed above.
14. Continue to increase the dynamic pressure setting in increments of 0.5 psf up to a dynamic pressure setting at which the thrust and torque become negative. Record all data at each increment.
15. Reset the tunnel dynamic pressure back to zero, and turn off the wind tunnel
16. Set the motor speed to 50% throttle and repeat steps 1.11 - 1.15.
17. Set the motor speed to 100% throttle and repeat steps 1.11 - 1.15.
18. Repeat the above procedure for all propellers, making sure to test speeds of 10%, 50% and 100% throttle up to a dynamic pressure where thrust and torque become negative.
19. When all tests have been completed, plug-in the electronic speed controller to the programming kit, record all of the propeller RPM data.
20. Shut down all of the systems.

Table 1. Propellers tested.

Propeller Diameter x Pitch (in)	# of Blades	Material
18 x 8	2	APC
16 x 8	2	APC
15 x 8	2	APC
15 x 10	2	APC
15 x 12	2	APC
18 x 8	2	Wood
18 x 8	4	Wood

Note that the fixed pitch propellers used in this study are defined by their diameter and pitch in inches. For example, an 18 x 8 propeller is an 18 in diameter propeller with a geometric pitch of 8 in.