NASA Dryden Flight Research Center's new Propulsion Flight Test Fixture (PFTF), designed in house, is an airborne engine-testing facility that enables engineers to gather flight data on small experimental engines. Without the PFTF, it would be necessary to obtain such data from traditional wind tunnels, ground test stands, or laboratory test rigs.

Traditionally, flight testing is reserved for the last phase of engine development. Generally, engines that embody new propulsion concepts are not put into flight environments until their designs are mature: in such cases, either vehicles are designed around the engines or else the engines are mounted in or on missiles. However, a "captive carry" capability of the PFTF makes it possible to test engines that feature air-breathing designs (for example, designs based on the rocket-based combined cycle) economically in subscale experiments.

Figure 1. The PFTF Holds the Test Article underneath the F-15B airplane during flight.
The discovery of unknowns made evident through flight tests provides valuable information to engine designers early in development, before key design decisions are made, thereby potentially affording large benefits in the long term. This is especially true in the transonic region of flight (from mach 0.9 to around 1.2), where it can be difficult to obtain data from wind tunnels and computational fluid dynamics.

In January 2002, flight-envelope expansion to verify the design and capabilities of the PFTF was completed. The PFTF was flown on a specially equipped supersonic F-15B research testbed airplane, mounted on the airplane at a center-line attachment fixture, as shown in Figure 1.

NASA's F-15B testbed has been used for several years as a flight-research platform. Equipped with extensive research air-data, video, and other instrumentation systems, the airplane carries externally mounted test articles. Traditionally, the majority of test articles flown have been mounted at the center-line tank-attachment fixture, which is a hardpoint (essentially, a standardized weapon-mounting fixture). This hardpoint has large weight margins, and, because it is located near the center of gravity of the airplane, the weight of equipment mounted there exerts a minimal effect on the stability and controllability of the airplane.

Figure 2. The Interior of the PFTF accommodates instrumentation and fuel-system hardware needed for an experiment.
The PFTF (see Figure 2) includes a one-piece aluminum structure that contains space for instrumentation, propellant tanks, and feed-system components. The PFTF also houses a force balance, on which is mounted the subscale engine or other experimental apparatus that is to be the subject of a flight test. The force balance measures a combination of inertial and aerodynamic forces and moments acting on the experimental apparatus.

The PFTF instrumentation system is a slave to the instrumentation system of the F-15B airplane. At present, as many as 128 parameters can be monitored by use of the PFTF; however, it is possible to expand the capabilities of the PFTF to monitor more parameters, if necessary. These monitored parameters can include, but are not limited to, pressures, temperatures, accelerations, vibrations, and strains. Sample rates are variable, generally between 10 and 400 samples per second, but much higher data-acquisition rates are possible. Parameters can be recorded aboard the F-15B airplane by use of a digital recorder or telemetered to a control room.

An experimental apparatus as heavy as 500 lb (≈227 kg) and as long as 12 ft (≈3.7 m) can be mounted on the PFTF. The PFTF can accommodate experiments in which are produced thrusts or drags as large as 2,000 lb (≈8.9 kN) and side forces up to 500 lb (≈2.2 kN). For envelope-expansion flights, a surrogate engine-shape body denoted the cone drag experiment was flown attached to the force balance. The cone drag experiment inertially and spatially approximated a large engine test article. This cone drag experiment produced drag forces of up to 1,400 lb (≈6.2 kN) at high speeds. A top speed of mach 2.0 and a dynamic pressure of 1,100 psf (≈53 kPa) were attained in this configuration.

This work was done by Nate Palumbo, M. Jake Vachon, Dave Richwine, and Tim Moes of Dryden Flight Research Center and Gray Creech of AS&M. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Mechanics category. DRC-02-23.


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