The Eclipse project was comprised of three phases: wake turbulence assessment, airdata calibration, and tethered operations. The first two phases have been completed successfully. The tethered operations were initiated in late 1997.
The wake turbulence produced by the C-141A and the handling qualities of the QF-106 in that wake were evaluated and assessed in several flights. From the results of these flights it was determined that proper positioning of the QF-106 behind the C-141A provided stable, controllable flight conditions. For one flight test, smoke-generating devices were placed under the C-141A's wings to enable visualization of the aircraft's wake vortices. Also, one of Dryden's F-18 chase aircraft flew at various distances and lateral positions behind the C-141A to probe the wake in an effort to define the wake turbulence environment. Finally, this probe test was replicated by the unmodified QF-106. It was found that both aircraft, particularly the QF-106, were very controllable even in wake turbulence. Testing also confirmed viability of the chosen tow rope length and a low tow position.
The modified EXD-01 was flown in the fall of 1997 to obtain the airdata calibration with the modified noseboom, which has been shortened to prevent interference with the tow rope. The tethered flights began in late 1997.
The QF-106 was selected by KST because the aircraft has a delta wing planform representative of the Astroliner spacecraft that the company plans to build. The QF-106 is a rugged, reliable aircraft which was available from the Air Force's drone target aircraft inventory. The C-141A was chosen because it can be configured as a tow aircraft with no modification to the airframe.
Dryden added a research instrumentation system to obtain airspeed, aircraft motion, tow rope tension, and tow rope angle measurements. Modifications to the QF-106 included shortening the nose pitot boom and addition of a tow rope attachment and release mechanism. The cockpit was modified to provide the pilot with a tow rope tension display and also the two (an electrical and a mechanical) tow rope releases. A video camera was installed near the aircraft's nose to provide the control room with a view of the tow rope during the flight. These aircraft modifications were performed by Dryden personnel. No sigificant modifications were made to the C-141A. All of the towing and tow rope jettison equipment will be placed on a standard cargo pallet secured in the rear of the aircraft. A video display of the tow rope and EXD-01 was installed, as well as a flight test instrumentation pallet to obtain C-141A aircraft parameters. Differential GPS will be used to determine the separation distance between the two aircraft.
To enhance flight safety and reduce the number of unknowns during flight tests, the Eclipse project used a high fidelity simulator. Full nonlinear mathematical models of the EXD-01 aircraft, C-141A aircraft, and the tow rope were modeled in the Eclipse simulator which had both a piloted and an off-line, batch version. The former was used for pilot training for normal and emergency operations, while the latter was used for dynamic analyses and for validation of major design decisions.
As the flight project progressed the simulation was be validated with flight data. An additional benefit of the fully validated simulation was the ability to extrapolate the Eclipse tow dynamics to larger, future tow launch concepts.
Flight Test Approach
A build up approach was used in the tow demonstration flight phase. That meant each test mission build upon the knowledge and experience gained from the prior test. The first steps were to validate all test and flight procedures. Initial missions would also validate predicted performance of both aircraft, particularly during takeoff and climbout. For subsequent missions, the EXD-01's performance and handling qualities were evaluated at various flight configurations. At first, the EXD-01 was flown in a high drag configuration with landing gear and speed brakes extended, whereas final flights were conducted in a 'clean' or "landing gear up" configuration.
Two tow rope configurations were used. The first flight tests employed a tow rope that consisted of three primary elements: a 1000 foot Vectran rope that is bisected by a 50 foot section of 8-ply nylon strap. Damping characteristics of the tow rope were significantly improved by the nylon segment. Then flight testing used a tow rope that was made of a single 1,000 foot Vectran element. The two airplanes were staged on the runway, during which the hookup to the tow rope was made. The C-141A then added tension to the tow rope by taxiing forward slowly, then it accelerated, taking off at 120 knots airspeed. In tow, with engines at idle, EXD-01 rotated at 130 knots and lifted off at 165 knots. The EXD-01 pilot then positioned the airplane in a 'low tow' position at a -20 degrees elevation angle throughout the tow. The EXD-01's engine were at idle power throughout the towed portion of the mission, to enable it to 'power up' rapidly after release for a conventional landing. All towed flights concluded with the release of the EXD-01 from tow at the target altitude.