Computer-generated image of Apex high-altitude research sailplane in flight.
The Apex Project was designed to prove the feasibility of extended duration flight at altitudes near 100,000 ft. More information is needed about the aerodynamics of this previously unexplored flight regime if those aircraft are going to be successful.
Research into low Reynolds number, high subsonic Mach number aerodynamics had produced some promising, if somewhat limited, results (see references). Some questions remained unanswered, and flight testing offered an attractive means of acquiring some of those answers. A preliminary design study was undertaken several years ago to find a satisfactory method for achieving trimmed flight at or near 100,000 feet altitude. The study identified several possible techniques which might have allowed an aircraft to achieve trimmed flight beginning between 95,000 and 100,000 ft (depending on the launch method used.)
The objectives of the Apex Project were:
- To validate high-altitude airfoil design methodologies by measuring airfoil characteristics at low Reynolds numbers and high subsonic Mach numbers in a low turbulence (flight) environment.
- To establish a high-altitude testbed aircraft.
Apex high-altitude research sailplane mock-up. Experiment Description
The Apex test vehicle was to have been a remotely piloted, highly modified commercial sailplane. It was to have been carried to an altitude in excess of 100,000 feet by a high-altitude balloon, then be released vertically and maneuvered to achieve horizontal flight (see drawing). Flight testing was to have been made at altitudes between 100,000 and 95,000 feet. The unpowered craft was intended to collect aerodynamic data down to an altitude of about 70,000 ft and then glide to a landing at Edwards AFB. The wing had been designed to incorporate an airfoil suitable for low Reynolds number flight at high subsonic Mach numbers (less than 0.65 Mach). A portion of the unswept/untapered wing was to have been served as a test section. The wing instrumentation was designed to be embedded during construction. The following measurements were to have been made to satisfy the stated objectives:
- Measure boundary layer profiles at several locations streamwise.
- Measure chordwise pressure distribution.
- Measure boundary-layer state with hot-film anemometers.
- Measure section drag with a fixed wake rake.
- Measure freestream airdata information.
The experiment was to have used a PCM and telemetry system to send the signals to a ground-based recording system. Flight test maneuvers were planned to be limited duration push-overs to lower-than-trim angles of attack, and stabilized turns to achieve higher-than-trim angles of attack. Other maneuvers could have included constant alpha descents, constant qbar descents or constant Mach number descents.
Due to the extraordinary flight conditions that Apex was being designed for, there were several questions that came up on a regular basis. The project compiled a list of some of the most frequent questions and answers to them. The Apex Project completed several parts of the airframe, but the effort was canceled before they were assembled and flown.