E. Bruce Jackson (757) 864-4060
Aircraft Guidance and Controls Branch
505-64-52
RESEARCH OBJECTIVE: Future hypersonic and manned launch systems have envisioned an end-of-mission horizontal, possibly unpowered, landing on a conventional runway. An attempt to appreciate the significance of the vehicle lift-to-drag ratio upon manual landing difficulty was undertaken, utilizing the Langley Transport Systems Research Vehicle simulator cockpit.
APPROACH: The simulation was based upon a preliminary version of the HL-20 vehicle. A range of vehicle configurations with maximum lift-to-drag ratios between 2.7 and 5.2 were simulated by added to or subtracting from the nominal drag coefficient. A three-part landing approach trajectory, consisting of a steep outer glideslope, a preflare portion, and a shallow inner glideslope, was defined. A final flare maneuver was performed at the end of the inner glideslope. Outer glideslope angle, preflare load factor, and preflare initiation altitudes were predetermined for each lift-to-drag configuration to satisfy the energy requirements. A computer-generated image of a typical runway was presented to the pilot for outside visual cues, and included Precision Approach Path indicator lights as an outer glideslope aimpoint reference. Guidance information in the form of a "fly-to" box on the head-down electronic attitude indicator display was provided. No head-up display was provided.
ACCOMPLISHMENTS: Three different test pilots with appropriate experience were asked to evaluate a series of approach-and-landings. Following a set of five familiarization approach & landings for each configuration, each pilot repeated the task several times for each configuration and provided a numerical pilot opinion rating of the perceived difficulty of performing the landing. The results for each configuration were then averaged and plotted as shown in the figure. The figure clearly shows the trend of a reduction in perceived difficulty as maximum lift-to-drag ratio was increased. The data indicated that fair to good opinions (pilot ratings of 3.5 or less) may be obtained for configurations with a lift-to-drag ratio above approximately 3.8 . Subsequently, additional analysis of the data indicates a strong correlation between time available for the pilot to perform the landing maneuver and maximum lift-to-drag.
SIGNIFICANCE: This result is significant in that it gives indication of a lower bound on maximum lift-to-drag ratio to achieve successful landings of an unpowered lifting body vehicle.
FUTURE PLANS: Based upon results from this study, an effort to improve the HL-20 basic lift-to-drag ratio has begun.
(circa 1990)
Publications relating to this work:
Rivers, Robert A.; and Jackson, E. Bruce: Preliminary Piloted Simulation Studies of the HL-20 Lifting Body. J. Aircraft, vol. 31, no. 3, May-June 1994.
Jackson, E. Bruce; Rivers, Robert A.; and Bailey, Melvin L.: Effect of Lift-to-Drag Ratio in Pilot Rating of the HL-20 Landing Task. J. Spacecraft and Rockets, vol. 30, no. 5, Sept-Oct 1993, pp. 543-548.
Jackson, E. Bruce; Bailey, Melvin L.; Rivers, Robert A.: Effect of Lift-to-Drag Ratio Upon Pilot Rating for a Preliminary Version of the HL-20 Lifting Body . Presented at the 1991 AIAA Atmospheric Flight Mechanics Conference, held in New Orleans, LA, August 18-21, 1991. AIAA Paper No. 91-2090.