Autogyro Pitch Sensitivity in Low-Speed Forward Flight

A high accident rate among autogyros has prompted investigation of this aircraft type to increase understanding of its low-speed flight characteristics and safety limitations. Very little research has been carried out on this aircraft typr since the first half of the twentieth century. With the aim of increasing understanding of certain characteristics, and therefore safety, wind tunnel tests were conducted on a model autogyro to investigate its pitch-trim characteristics at very low forward speeds.

Autogyros are often built without horizontal tail surfaces or elevators, as pitch control is derived by adjusting the angle of the main rotor tip-path-plane. However at very low forward speeds, changing main rotor tilt-back can lead to possible irrecoverable loss of rotor speed, a dangerous situation for low-flying autogyros.

Wind tunnel tests were conducted on a model autogyro to investigate its pitch characteristics at very low steady forward speeds. The autogyro model was developed at the University of Bristol as a general-purpose tool for autogyro research. For this study, the model was modified to allow body incidence control and pitch moment measurement. The model was tested in the working section of the University’s five-foot by seven-foot wind tunnel, using the tunnel’s external balance. Utilizing this system, experiments were undertaken to find the minimum straight and level flying speeds at varying angles of attack and to investigate the effect of pitching moment on body incidence, elevator setting, horizontal tail position, thrust line height, and tunnel speed. Trade-offs associated with pitch control facilitated by placing a horizontal stabilizer with elevators in the propeller wake was also investigated.

The data suggest that a significant amount of pitching control can be achieved for autogyros in low-speed forward flight by the addition of elevators. Although an overly large or lengthy horizontal stabilizer with elevators could present problems during landing, the data indicate that placing these control surfaces behind the propeller provides a pitch control benefit. Slip-stream-mounted elevators could be used by a pilot for enhanced, delicate pitch control in the minimum straight-and-level speed flight regime, improving controllability.

The results of this study also suggest several avenues of inquiry that are currently being pursued. A comparison between the pitching moments with the horizontal tail in place and removed will differentiate between the pitch-stabilizing effects of the horizontal tail verses the pitch-controlling effect of the elevators. Further investigation into varying the thrust line height with respect to center-of-rotation is providing insight into how the main rotor and propeller aerodynamic interactions effect pitch control.

A re-design of the autogyro model is also underway, which to allow it to attain a more severe body incidence. This added pitch range will be exploited to find the true minimum model flight speed and further explore the pitch control implications reported here.