AEDC support aids F-15 in reaching 50-year milestone

  • Published
  • By Bradley Hicks
  • AEDC Public Affairs

Perhaps better than any other military aircraft, the F-15 Eagle exemplifies the definitions of versatile, revolutionary, indomitable and enduring. 

The fighter, first deployed in late July 1972, recently turned 50. To mark the golden anniversary of the aircraft that has never been downed in air-to-air combat, celebrations were held across the Air Force and the aerospace industry.

The relationship between the F-15 and Arnold Engineering Development Complex dates back to the very origins of the fighter. AEDC helped the F-15 get off the ground, and AEDC support has since played a considerable role in its longevity.

With unprecedented maneuverability and acceleration, range, weapons and avionics, the F-15 was designed with the purpose of gaining and maintaining air supremacy over the battlefield. AEDC boasts a significant part in the pursuit of this objective, as test cells across the complex have been instrumental in the success of the F-15 over the years. This work includes an early focus on aircraft design during the developmental stages of the F-15 and the engine and munitions testing that has continued throughout the half-century life of the fighter.

Work on the F-15 began at Arnold Air Force Base, headquarters of AEDC, prior to the selection of the final design by McDonnell Douglas, which began designing the aircraft in the late 1960s. Scale models of various configurations proposed for the F-15 were tested in Arnold wind tunnels. This work was done to help determine optimal aircraft design, and data obtained aided engineers in making design alterations and refinements to ensure peak performance of the F-15.

When McDonnell Douglas unveiled the F-15 in 1972, it featured several of the changes that had been validated during testing at Arnold. Among these were updates to the rear of both the wings and the horizontal and vertical tail surfaces.

While Arnold engineers helped nail down its final design, their work with the F-15 was just getting started.

AEDC F-15 engine testing support

In the late 1960s, two companies – Pratt & Whitney and General Electric – were competing for an Air Force contract to build the engine for the F-15. Using engine test facilities at Arnold, the Air Force was able to evaluate the systems side-by-side in identical conditions. The contract was awarded to Pratt & Whitney in 1970.

Around the beginning of the 1970s, the power plant of the F-15 – the Pratt & Whitney F100 engine – would undergo evaluation at Arnold. Tests in the 16-foot transonic and 16-foot supersonic wind tunnels demonstrated the compatibility of the engine with the variable geometry inlet of the F-15 prior to the aircraft’s first flight. The test section itself was not large enough to accommodate a full-sized nose section, so a special fuselage segment was designed to produce inlet airflow conditions the F-15 would encounter in flight.

In 1972, one of the largest test articles installed in an Arnold jet engine test cell during the 1970s was placed in the J-1 test cell to support the F-15 flight test program. This article was comprised of an F100 engine attached to the J-1 40-ton inlet simulator which simulated the aircraft’s variable air inlet and the ducting that separates the inlet from the engine.

The purpose of these tests was to eliminate engine inlet incompatibility issues before the aircraft was flown. Over the course of several weeks, tunnel conditions of speed, altitude and attitude were varied to match conditions the engine was expected to encounter in flight to ensure the quality of air reaching the face of the engine would not cause it to stall.

By 1973, testing in Arnold turbine engine test facilities had figured in all phases of the F100 development cycle. The F100-PW-100 engine was the initial engine installed in the F-15. More than 23,000 hours of engine tests followed these early efforts on all variant upgrades to the F100 engine.

In April 1985, the F100 was used in the C-1 test cell to support the checkout of the new Aeropropulsion Systems Test Facility to verify the plant could safely conduct testing. During the successful checkout, which occurred around six months after ASTF was dedicated, an F100 engine was run in the test cell for more than an hour at a simulated flight condition of 16,000 feet altitude and Mach 0.8. The engine was operated at power levels from idle to afterburner ignition. This checkout demonstrated the successful integration of the facility equipment required to operate an engine.

The F100 was chosen for this checkout due to the volume of test data available on it and it was a representative engine used by the Air Force operational fighter fleet, which includes the F-15.

The following year, new reversing and vectoring engines were tested for the first time by the Air Force in ASTF. An F100 was fitted with the thrust-vectoring and –reversing exhaust nozzle. The nozzle was designed to deflect exhaust and reverse thrust both in flight and on the ground thereby significantly increasing maneuverability while reducing landing distances under both normal and icy conditions. The tests were performed to confirm these concepts were feasible.

In 1988, the Pratt & Whitney F100-PW-229 Increased Performance Engine, an evolution of the standard F100 engine, completed Initial Flight Release Testing at Arnold. More than 170 hours of simulated altitude testing were conducted over a nine-week period to clear the engine for flight testing in an F-15. Some of these tests lasted up to 22 hours, during which more than 900 engine parameters were recorded. The F100-PW-229 was subjected to simulated altitudes of up to 50,000 feet and speeds exceeding Mach 2.

By 1992, 5,000 total accumulated cycles, or TACs, which are units of measurement for major rotating components tracked during an engine’s operational life, were completed on the engine to substantiate the latest modification to its fourth stage turbine blades.

The F100 Super Pacer engine underwent RAM accelerated mission testing in 1996 to determine how the F100 would age. Accelerated mission testing, or AMT, is used to rapidly age an engine. Through this method of testing, several years of normal life can be accumulated in a short amount of time, allowing issues to be identified and corrected before occurring in normal use. RAM AMT is a variation of AMT developed by the Air Force to provide additional stress on an engine. The term “RAM” refers to the use of test facility compressors to increase the engine inlet pressure and temperature to more closely simulate actual flight conditions.

AEDC testing of the F100 has continued into the new millennium.

In the early 2000s, testing in the T-1 test cell validated proposed improvements to the Pratt & Whitney F100-PW-220 engine, which could be applied to the F-15, in preparation for flight testing. During an 18-month test phase, engineers used the Non-intrusive Stress Measurement System, or NSMS, to measure the amount of blade deflection or bending that occurs during engine operations.

August 2001 saw the conclusion of a 12-month test program in which the F100-PW-229 performed more than 3,500 sea level AMTs and nearly 1,000 RAM AMT cycles in the Sea Level Test Cells SL-2 and SL-3 at Arnold.

Around two years later, the F100-PW-229 returned to SL-3 for testing to validate the durability and integrity of the engine hardware.

In 2013, the F100-220 underwent sea level RAM testing in the SL-2 cell and, prior to this, had completed testing in the J-2 test cell to support the Aircraft Engine Component Improvement Program.

The F100 was also involved in what was, at the time, AEDC’s longest-running jet engine test. Testing on the F100-PW-229 Engine Enhancement Package began in SL-3 on Oct. 19, 2009, and concluded on Sept. 21, 2010. This AMT subjected the engine to 6,000 TACs to simulate the operational stresses it would experience in flight between depot overhaul visits. The F100-PW-229 engines were typically rated at 4,300 TACs.

Conducted with the goal of extending the operational life of the engine while reducing maintenance time and cost, this effort involved more than 2,000 test hours. The program consumed nearly 3 million gallons of fuel.

Altitude testing was conducted on the F100 in 2011 as part of a Continuous Improvement Program project, during which new parts installed in the engine were assessed.

A 15-month AMT of the F100-PW-220 began in SL-3 in January 2013 and ran until the spring of 2014. This test helped extend the life of the F100-PW-220 by two years – from 8 years to 10.

AEDC conducts F-15 store testing

Several years after the initial engine tests, advanced techniques were employed in tests in the AEDC 4-foot transonic wind tunnel, known as 4T, to determine whether the F-15 could jettison its auxiliary fuel tank during supersonic flight. This model tank was successfully dropped from the parent aircraft and, along with the free-drop technique, the computer-controlled support system of the wind tunnel was used to measure forces on the fuel tank model to ensure it would respond to these forces the same way an actual fuel tank would when released.

In 1974, the same year the fuel tank drop test occurred, miniature stores of the AIM-7 Sparrow and AIM-9 Sidewinder air intercept missiles, along with various air-to-surface stores, including that of a 500-pound bomb, were assessed in 4T at speeds ranging from about 450 to 1,000 mph. These tests were conducted using a 1/20th scale model of the F-15, as these represented potential payloads for the fighter. This work was performed to determine the effects of external payloads on the flow field characteristics beneath the armament stations of the F-15 and was done in preparation for weapons separation flight tests scheduled at another Air Force installation.

The F-15 is renowned for its versatility when it comes to the number of different rockets, bombs and external loads it can carry. Munitions testing would continue to make up a large part of work involving the fighter occurring in Arnold wind tunnels. Over the years, hundreds of store combinations have been evaluated to help ensure the ability of the F-15 to safely accomplish air-to-ground and air-to-air missions.

As the 1990s were coming to a close, weapons separation tests completed in 4T marked the first steps in certifying the Air-Guided Missile 154A Joint Standoff Weapon and the Joint Direct Attack Munition Guided Bomb Unit 31 for use on the F-15E Strike Eagle, a variant of the F-15 introduced in the latter part of the 1980s. This test, sponsored by the Air Force Seek Eagle Office at Eglin Air Force Base, Florida, was part of an ongoing effort that began in 1989 to certify various munitions for the F-15E. Arnold personnel built 1/20th scale models of the F-15E, the AGM-154AJSOW and JDAM GBU-31 for the test which measured aerodynamic force and moment data in the flow field of the F-15E.

During testing that concluded in September 2006, the separation characterizations of the Joint Air to Surface Standoff Missile from the F15E were evaluated in 4T. This test provided direct support to the ability of the Air Force to conduct long-range strike operations.

In the early 2010s, a scale model of the F-15E was used in 4T to conduct store separation and aerodynamic testing on a powerful precision-guided bomb that would enable the fighter to find and destroy moving targets under challenging conditions.

F-15 models, engines and related equipment have occupied AEDC facilities off and on over the past 50-plus years, but the aircraft holds a permanent spot on the grounds of the Arnold AFB. Located at the Main Gate is a full-size static display of an F-15. In August 2007, this display was dedicated in memory of Maj. Jim Duricy, a test pilot who was killed in April 2002 when he was forced to eject at a high speed as the F-15C he piloted crashed into the Gulf of Mexico.