December 19, 1965: Experimental Tiltrotor Aircraft Testing Terminated

  • Published
  • Air Force Flight Test Center

Following the crash of the first experimental air vehicle in New Jersey, Curtiss-Wright requested that Category I testing of the X-19 be canceled. Two days later, Aerospace Systems Division (ADS) concurred and the project was terminated. The X-19 was a four-engine tilt rotor S/VTOL vehicle that utilized radial lift force propellers. Four AFFTC pilots and engineers were part of the X-19’s Tri-Service Test Team.  A quadcopter or quadrotor was a type of helicopter with four rotors.  Although quadrotor helicopters and convertiplanes have long been flown experimentally, the configuration remained a curiosity until the arrival of the modern Unmanned Aerial Vehicle or drone. The small size and low inertia of drones allows use of a particularly simple flight control system, which has greatly increased the practicality of the small quadrotor in this application.  Each rotor produces both lift and torque about its center of rotation, as well as drag opposite to the vehicle's direction of flight.

Quadcopters generally have two rotors spinning clockwise (CW) and two counterclockwise (CCW). Flight control is provided by independent variation of the speed and hence lift and torque of each rotor. Pitch and roll are controlled by varying the net center of thrust, with yaw controlled by varying the net torque.  Unlike conventional helicopters, quadcopters do not usually have cyclic pitch control, in which the angle of the blades varies dynamically as they turn around the rotor hub. In the early days of flight, quadcopters (then referred to either as 'quadrotors' or simply as 'helicopters') were seen as a possible solution to some of the persistent problems in vertical flight. Torque-induced control issues (as well as efficiency issues originating from the tail rotor, which generates no useful lift) can be eliminated by counter-rotation, and the relatively short blades are much easier to construct. A number of manned designs appeared in the 1920s and 1930s. These vehicles were among the first successful heavier-than-air vertical take off and landing (VTOL) vehicles.  However, early prototypes suffered from poor performance, and latter prototypes required too much pilot work load, due to poor stability augmentation and limited control authority.

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