The static margin is a function of the pitch stability of the aircraft and is defined as sigma = -Cm,alpha/CL,alpha. If the aircraft is stable in pitch, it will resist any distrubance in pitch and therefore naturally return to neutral. However, humans (and computers in the case of UAVs) are quite capable of controlling unstable motions. For example, walking and biking are unstable activities we do every day quite naturally. Therefore, although it is nice for a pilot to have an aircraft that is stable in pitch, it is not necessary. In fact, Perkins [1] suggests that many of the early successful airplanes, including the Wright flyer, were most likely unstable in pitch.
The dynamic margin, on the other hand, is a function of both pitch stability as well as pitch damping. The pitch damping opposes any sudden change in pitch. Therefore, if an aircraft's pitch damping is large enough compared to its pitch stability, we as humans are perfectly capable of controlling the aircraft. The dynamic margin can be directly related to pilot opinion, and hence, handling qualities. This means that there are known bounds for the dynamic margin that will result in level 1 handling qualities. For an excellent paper on the subject, see Phillips [2].
It is always wise to design the aircraft for an appropriate dynamic margin rather than the static margin rule of thumb.
References
[1] Perkins, C. D., "Development of Airplane Stability and Control Technology," Journal of Aircraft, Vol. 7, No. 4, 1970, pp. 290-301.[2] Phillips, W. F., and Niewoehner, R. J., "Characteristic Length and Dynamic Time Scale Associated with Aicraft Pitching Motion, " Journal of Aircraft, Vol. 46, No. 2, pp. 572-582.
No comments:
Post a Comment