Aerodynamic requirements for flare and landing of low-L/D glide vehicles.

The flare and landing performance of glide (unpowered) vehicles with low lift-to-drag ratios (L/D) is investigated analytically to determine the influence on landing characteristics of such factors as L/D, control range, and wing loading. An optimum-flare routine yielding minimum velocity loss for a specified reduction in sink rate is used to determine flare and landing maneuvers corresponding to a family of aerodynamic polars typical of lifting-body vehicles. Optimum flares are also computed for specific vehicles, including the X-15, the M2-F1 lifting body, and representative paraglider systems. The landing time at a given wing loading is found to increase rapidly with L/D and approach speed. For L/D below about 2.5, the landing time goes to zero. Data obtained from manned flights of a paraglider landing system indicate significant dispersion of touchdown sink rates due to wind shears. Criteria are derived for the flare margin, or landing time, required to offset these effects. It is shown that at wing loadings less than about 40 psf, an L/D greater than 3 is needed to compensate adequately for both flare-initiation tolerance and possible wind shears.