TRANSITION AND HOVERING FLIGHT CHARACTERISTICS OF A TILT-DUCT VTOL RESEARCH AIRCRAFT

Flight measurements of various aerodynamic characteristics of a tilting ducted-fan, vertical-take-off-and-landing (VTOL) research aircraft have been recorded over the transition speed range during testing at the Langley Research Center and are presented in conjunction with pilots' observations.Test results cover a speed range of 35 knots to 115 knots and duct angles from 60 0 to 0 0 .Stalling of outer wing surfaces limited the rate of descent capabilities of the tilt -duct aircraft in the transition speed range.One possible method of reducing the adverse stall effects in the steady-state descent condition consisted in holding a constant angle of attack well below wing stall by raising the duct angle and dropping the nose below the horizontal.Static stability investigations (apparent directional stability and effective dihedral) indicated stability of varying degree throughout the range investigated, and pilots reported that the static stability went from marginally satisfactory at the high-speed end of the transition speed range to unsatisfactory at the low-speed end .Longitudinal and lateral-directional oscillations were well damped over the portion of the transition speed range investigated.Pilots' comments indicated that the damping of all oscillations studied was satisfactory.A limited hovering investigation indicated undesirably low control power and angular-velocity damping about the roll and yaw axes, which tends to confirm the necessity for this type of aircraft to meet minimum helicopter requirements.Motion of the aircraft about the pitch axis was controllable and was considered acceptable for research flying and, according to pilots' comments, would probably meet minimum helicopter requirements.Static rig measurements of the tiltduct configuration indicated that downwash from the ducted fans being reflected from the ground contributed in pa.rt, at least, to uncontrollable lateral destabilizing tendencies experienced during hovering flight.