15-12-2012, 05:05 PM
A 6-Degree of Freedom Static Thrust Stand Developed for RC-Scale Jet Engines
Freedom Static.pdf (Size: 1.84 MB / Downloads: 27)
ABSTRACT
The description of a portable 6-degree of freedom static thrust stand for RC-scale jet engines is
reported. The stand includes three axial and three lateral load cells measuring static thrust with six degrees
of freedom. A pitot probe with single axis position control placed perpendicular to exhaust flow measures
stagnation pressure along the nozzle centerline. A pitot probe open to the inside of the engine nozzle
normal to the exhaust flow and near the outlet measures static pressure. A digital scale measures fuel
consumption. The engine is mounted with exhaust gases exiting upward avoiding ground effects and thrust
acting downward. The test stand instrumentation is interfaced with a laptop computer running National
Instruments® LabView 9.0. The report includes descriptions for test stand structure, hardware, mounting
instructions, instrumentation specifications, references to download instrumentation software, complete
wiring diagrams, and the test procedures used for testing an RC jet engine. Some testing results are
included for a JF-170 Rhino RC-scale jet engine including graphs for static thrust, fuel consumption, and
exit plume pressure profiles. A momentum defect is found in the exit plume of the JF-170 engine.
INTRODUCTION
The test stand to be described in this paper was specially fabricated to support a student-lead design
effort to build and fly a small scale (~1/10 scale) research vehicle that reproduces many of the capabilities
demonstrated by the 1960s-era Lunar Landing Research Vehicle (LLRV) and Lunar Landing Training
Vehicles (LLTV).1 The Utah State University student design team named the free flying vehicle the Lunar
or Planetary Surface Landing Research Vehicle (LPSLRV).
The approach for this project, whenever possible, was to replace 1960s-era analog designs with proven
and reliable modern digital computer-aided technologies. This sub-scale (~1/10th scale) vehicle produced
by this work simulates the reduced-gravity (i.e., lunar or planetary surface environment) using a verticallythrusting
jet engine to partially offset the vehicle weight. The function of the gravity-offset system is to lift
5/6th of the vehicle weight without contributing to horizontal linear acceleration. For this project a small
RC-scale jet engine was used to provide the gravity offset features of this vehicle. The engine selected for
the gravity offset system was Jet-Central® JF-170 Rhino centrifugal turbine engine.2 The engine features a
single shaft turbojet with an annular combustor. A single stage axial flow turbine drives a single stage
centrifugal compressor. The shaft is supported by 2 fuel/oil lubricated, annular contact bearings. The
turbine speed is controlled by the amount of fuel received from the fuel pump, which is controlled by a fullauthority
digital engine control system (FADEC). The turbine runs on both jet-A fuel and K-1 grade
kerosene. The engine was maintained in a vertical state using turning vanes inserted in the exhaust plume,
the turning vanes are seen in Figure 1. The actual thrust vectoring prototype mounted to the JF-170 jet
engine in the 6-degree of freedom load balance is seen in the “Testing Results” section of this report in
Figure 13. Figure 2 shows a solid model of the JF-170 jet engine installed in the load balance with the
coordinate system defined.
Brackets
Three brackets are needed to mount the jet engine and the load cells to the triangle. The bracket
dimensions are given in Figure 7. The assembly of the bracket, load cells and triangle are seen in Figure 5.
The bolts used for bracket attachment measured 0.5 (12.7 ). The slotted design seen in Figure 7
allows for adjustment in engine height based on load cell length.
Pitot Probe
Pressure measurements during engine testing are taken using two pitot probes – one static and one
dynamic. The static probe is open to the inside of the engine nozzle, normal to the exhaust flow and near
the outlet. The dynamic pressure sensor is on a track that enables position control during testing. While
the engine is running, the dynamic probe is swept across the midline of the engine nozzle perpendicular to
the exhaust flow. The dynamic pitot probe required a sufficiently rigid pitot tube to avoid the extreme
velocities and temperatures it encountered during testing. Each probe was originally connected to a Micro
Switch USA pressure transducer but later changed to Omegadyne pressure transducers. The stagnation
pressure is sensed with a 0-30 Omegadyne PX-142 pressure transducer. The static pressure at the
nozzle exit plane is sensed with an identical transducer ranged from 0-
CONCLUSIONS
The necessity to fully characterize small engines for space or atmospheric flight vehicles creates the
need for thorough engine testing. Commercial grade testing stands are not available for small scale jet
engines leading to the development of this test stand. This report gave the description of a test stand used at
Utah State University to characterize a small jet engine. This stand gave accurate results and provided the
characterization needed to build an atmospheric flight vehicle.10 All hardware and instrumentation used in
constructing this engine test stand can be changed according to project needs.