Dryden's F-15B Research Testbed aircraft project team has completed initial checkout flights of their new in-house designed Propulsion Flight Test Fixture (PFTF). The PFTF is an airborne engine test facility that allows engineers to glean actual flight data on small experimental engines that would otherwise have to be gathered from traditional wind tunnels, ground test stands or laboratory setups.

Traditionally, flight test is reserved for the last phase of engine development. Putting new propulsion concepts into a flight environment is generally set aside for mature engine technologies and involves either designing a vehicle around them or mounting them to a missile. However, due to the "captive carry" capability of the PFTF, new air-breathing propulsion schemes, such as Rocket Based Combined Cycle engines, can be economically flight-tested using sub-scale experiments.

The PFTF flew mated to Dryden's specially equipped supersonic F-15B research aircraft. "The PFTF combined with the extensive research instrumentation capability of the F-15B provides a unique facility to mature advanced propulsion concepts from the laboratory to flight quickly and cost effectively," said F-15B project manager Dave Richwine.

Discovering the "unknown unknowns" of flight test provides valuable information to engine technology de-signers early in development, before key design decisions are made. This can produce large benefits down the road. This is especially true in the transonic region of flight (from Mach 0.9 to around 1.2), where wind tunnel and computational fluid dynamics data can be difficult to obtain.

Here is a closer look at the new test fixture.

"The majority of the design of the PFTF was done in-house utilizing years of flight research experience on the F-15B as well as some of the latest finite element and computational fluid dynamics design codes," said Nate Palumbo, PFTF principal investigator.

The PFTF, carried on the F-15B's centerline attachment point, underwent in-flight checkout, known as flight envelope expansion, in order to verify its design and capabilities.

Envelope expansion for the PFTF included envelope clearance, which involves maximum performance testing. Top speed of the F-15B with the PFTF is Mach 2.0. Other elements of envelope clearance are flying qualities assessment and flutter analysis.

Another part of flight envelope expansion was checkout of the force balance. An important part of the PFTF, this force balance is analogous to the balance used in a classic wind tunnel, and serves as the actual mounting point for flight experiments. Instrumentation on the force balance measures thrust, inlet drag, aerodynamic movement such as yaw, and drag of experimental engines. Instrumentation also measures aerodynamic pressures, internal and external temperatures, strain, and acceleration.

Calibration of the F-15 and PFTF air data systems was completed as part of the envelope expansion, and included airflow visualization. Airflow visualization of the PFTF and a "stand-in" test engine was accomplished by attaching small tufts of nylon on them and videotaping the flow patterns revealed during flight.

A surrogate experimental engine shape, called the cone tube, was flown attached to the force balance on the PFTF. The cone tube emulated the dimensional and mass properties of the maximum design load the PFTF can carry. As the F-15B put the PFTF and the attached cone tube through its paces, accurate data were garnered, allowing engineers to fully verify PFTF and force balance capabilities in real flight conditions. When the first actual experimental engine is ready to fly on the F-15B/PFTF, engineers will have full confidence and knowledge of what they can accomplish with this "flying engine test stand."