Propulsion and Thrust

1. Fabrication of static thrust test system (see schematics and photograph, Fig. 2)

1. Form two cylindrical bushings on a lathe with outer diameter 42.16 mm, length ~10 mm, and bore through the center axis of 9.50 mm.
2. Press one flanged ball bearing into the bore on each bushing. Insert the bushings flush into the two parallel ports of the 4-way tee fitting, with the bearings on the outside. The bushings should fit snugly in the tee fitting. (See the pivot assembly schematic in Fig. 2b.
3. Cut two 100 mm long lengths of the aluminum right-angle extrusion. Drill a 3.2 mm hole in the middle of the longer side of the extrusions, ~45 mm up from the base. Drill two mounting holes near the ends of the shorter sides of the extrusion.\
4. Insert the shaft through the two bearings in the 4-way tee fitting. Even lengths should be exposed on each end. Slide the right-angle extrusions onto the exposed shaft ends. Screw the right angle extrusion to the work surface through the mounting holes. Install the shaft collars on the exposed ends of the shaft to keep the assembly centered between the right-angle brackets.
5. Cut short (~18 mm) and long (~36 cm) lengths of 42.16 mm outer diameter PVC pipes. Insert the short length into the horizontal port on the 4-way tee fitting, and the long length into the vertical port. Insert a pipe cap on the end of the horizontal length.
6. Position a precision digital scale (±0.1 or ±0.01 g recommended) under the horizontal pipe arm cap.
7. Mount the propeller motors and fan on pipe caps. The propellers should be offset so that the caps do not block the airflow. It is recommended that the propeller motors are glued to the heads of thin screws installed on pipe caps (Fig. 2c).

2. Performing experiments

1. Mount the smallest propeller and motor pipe cap onto the vertical pipe arm.
2. Record the distances (moment arms) from the pivot axis to the propeller motor axis (L_prop) and from the pivot axis to the contact point of the horizontal arm on the scale.
3. Connect the propeller motor to a variable voltage DC power supply (turned off).
4. Turn on the scale, and tare (zero) the reading.
5. Turn on the power supply and vary the voltage in ~0.4 V increments up to 3.8 V. For each case, record the voltage, supplied current, scale reading (in grams), and scale range during steady operation (typically oscillates by ~0.3 - 5.0 g). It may be necessary to tap the propeller blade to start it spinning. Ensure that the airflow is in the right direction (flowing toward the rear of the motor). If not, reverse the positive and negative leads on the power supply.
6. If available, use a thermal anemometer to measure the air velocity just behind (downstream) the propeller at a few conditions. The velocity varies over the propeller face area, so this is only an order-of-magnitude measurement.
7. Repeat Steps 2.1 - 2.6 for the other motor and propeller and the PC cooling fan. The fan can operate up to 12 V.

3. Analysis

1. Using Eqn. 4, calculate the propeller and fan thrusts (T) for each measured case. The major source of uncertainty is the variation/oscillation in the scale reading during operation. Substitute this range (Step 2.5) for m in Eqn. 4 to determine the thrust uncertainty.
2. For each case, compute the input power . The uncertainty can be estimated as , where ΔI and ΔV are the current and voltage measurement uncertainties (0.005 A and 0.005 V here).
3. For each case compute the thrust efficiency . The uncertainty for thrust efficiency would be .
4. Compare the measured thrusts with estimated theoretical values using the anemometer velocities (Eqn. 3). Here the outlet area can be estimated as the propeller/fan face area, less the hub or motor area: . How do these compare with measured values?