Propulsion and Thrust

Overview

Source: Alexander S Rattner; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA

Aircraft, rockets, and ships produce propulsion by accelerating surrounding fluid or high temperature combustion products to high velocity. Because of the principle of conservation of momentum, the increased fluid velocity results in an effective thrust force on the vehicle. The thrust capabilities of propulsion systems are often measured with static thrust tests. In these tests, propulsion systems are mounted and operated on fixed, instrumented platforms, and the holding force on the mounts is measured as the thrust

In this experiment, a small-scale static thrust measurement facility will be constructed and modeled. The thrust curves for two model aircraft motors and propeller systems and a computer cooling fan will be measured. Thrust efficiencies will also be evaluated (thrust force / electrical power input). Measured thrust values will be compared with theoretical predictions based on measured air velocities.

Procedure

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

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.
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.
Cut tw

Results

In Fig. 3a, the thrust vs. power curves are presented for the three propulsion devices evaluated in this experiment. The fan achieves the highest thrust, reaching 0.68 ± 0.02 N at 11.83 ± 0.08 W input power. The smaller propeller produces slightly more thrust per input power than the larger propeller, but reaches its maximum operating voltage at 2.66 ± 0.04 W. Fig. 3b presents the thrust efficiency for the three devices. For the small propeller and fan, the efficiency generally decreases with increasing po

Application and Summary

This experiment introduced the basic operating principles of fluid propulsion devices found in aircraft and watercraft. A static thrust test platform was constructed to measure the propulsion capability of model aircraft propellers and a pc cooling fan. The resulting thrusts and propulsion efficiencies (thrust per input power) were measured and compared. Theoretical thrust values were also estimated based on downstream jet velocities. Measurement and rating of propulsion system performance, as demonstrated here at small