Low L/D Approach and Landing Trainers

In addition to the need to simulate the handling qualities of a new air­plane, a need to accurately duplicate the approach and landing perfor­mance also evolved. The air-launched, rocket-powered research airplane concept, pioneered by the X-1, allowed quick access to high-speed flight

for research purposes. It also brought with it unpowered, gliding land­ings, after the rocket fuel was expended. For the X-1 series of airplanes, the landings were not particular stressful because most landings were on the 7-mile dry lakebed at Edwards AFB and the approach glide angles were 8 degrees or less (lift-to-drag (L/D) ratios of about 8). As the rocket – powered airplanes reached toward higher speeds and altitudes, the landing approach angles increased rather dramatically. The approach glide angle for the X-15 was predicted to be between 15 and 20 degrees (lift-to-drag ratios between 2.8 and 4.25) primarily because of the larger base area at the rear of the fuselage. The L/D was further reduced to about 2.5 after landing gear and flap deployment. These steep unpowered approaches prompted a reassessment of the piloting technique to be used. Higher – than-normal approach speeds were suggested as well as a delay of the land­ing gear and flap deployment until after completion of the landing flare. These new landing methods also indicated a need for a training "simula­tor” that could duplicate the landing performance of the X-15 in order to explore different landing techniques and train test pilots.

Out-of-the-cockpit, simulated visual displays available at that time were of very poor quality and were not even considered for the X-15 fixed-base simulator. Simulated missions on the X-15 fixed-base simula­tor were flown to a high-key location over the lakebed using the cockpit instruments, but the simulation was not considered valid for the landing pattern or the actual landing, which was to be done using visual, out-of- the-window references.

North American added a small drag chute to one of its F-100s to allow its pilots to fly landing approaches simulating the X-15. Additionally, both the Air Force and NASA began to survey available jet aircraft that could match the expected X-15 landing maneuver so that the Government pilots could develop a consistent landing method and identify what external cues were necessary to perform accurate landings. The F-104 had just entered the inventory at the AFFTC and NASA. Flight-testing showed that it was an excellent candidate for duplicating the X-15 landing pattern.[735]

Various combinations of landing gear and flap settings, plus partial power on the engine, could be used to simulate the entire X-15 land­ing trajectory from high key to touchdown. F-104s were used through­out the program for chase, for training new X-15 pilots, for practicing approaches prior to each flight, and also for practicing approaches into uprange emergency lakebeds. The combination of the X-15 fixed-base simulator and the F-104 in-flight landing simulation worked very well for pilot training and emergency planning over the entire X-15 test pro­gram, and the F-104 did yeoman work supporting the subsequent lift­ing body research effort as well, through the X-24B.

In the late 1960s, engineers at the Air Force Flight Dynamics Laboratory had evolved a family of reentry shapes (particularly the AFFDL 5, 7, and 8) that blended a lifting body approach with an exten­sible variable-sweep wing for terminal approach and landing. In support of these studies, in 1969, the Air Force Flight Test Center undertook a series of low L/D approach tests using a General Dynamics F-111A as a surrogate for a variable-sweep Space Shuttle-like craft returning from orbit. The supersonic variable-sweep F-111 could emulate the track of such a design from Mach 2 and 50,000 feet down to landing, and its sophisticated navigation system and two-crew-member layout enabled a flight-test engineer/navigator to undertake terminal area navigation. The result of these tests demonstrated conclusively that a trained crew could fly unpowered instrument approaches from Mach 2 and 50,000 feet down to a precise runway landing, even at night, an important con­fidence-building milestone on the path to the development of practical lifting reentry logistical spacecraft such as the Shuttle.[736]

Notice that the landing-pattern simulators discussed above did not duplicate the handling qualities of the simulated airplane, only the perfor­mance and landing trajectory. Early in the Space Shuttle program, man­agement decided to create a Shuttle Training Aircraft (STA). A Grumman G II was selected as the host airplane. Modifications were made to this unique airplane to not only duplicate the orbiter’s handling qualities (a variable-stability airplane), but also to duplicate the landing trajectory and the out-of-the-window visibility from the orbiter cockpit. This NASA training device represents the ultimate in a complete electronic and

motion-based training simulator. The success of the gliding entries and landings of the Space Shuttle orbiter confirm the value of this trainer.