All of the VTOL research aircraft discussed in this paper have suc¬cessfully demonstrated conversion from hovering to airplane flight and vice versa. However, control about one or more axes of these aircraft has been inadequate in hovering flight. Furthermore, ground interference effects have been severe in some cases and have accentuated the inadequacy of control in hovering and very low speed flight.nStalling of wing surfaces has resulted in limitations in level-flight deceleration and in descent, particularly for the tilt-wing aircraft, which in this case is a very rudimentary type. Minor modifications to the wing leading edge have, however, produced surprisingly large and encouraging reductions in adverse stall effects.nHeight control in hovering and in low-speed flight has proved to be a problem for the aircraft not having direct control of the pitch of the rotors. The other systems have shown undesirable time lags in development of a thrust change.
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