The ‘tumble’ departure mode in weightshift-controlled microlight aircraft

Abstract

The cost of private or recreational flying is high for most conventional aircraft types. During the last 25 years, however, an alternative has become available in the form of the microlight aircraft. It has a relatively low cost of ownership and has opened up flying to a greater audience. However, there have been a number of accidents, usually fatal, to this class of aircraft, which could not be explained through any conventional understanding. The reason for these accidents, which involve a departure from controlled flight followed by aircraft structural failure (generally including mechanical failure of the basebar, wingtips and leading edge), has become known as the “tumble”, owing to the basic motion of the aircraft. This paper analyses the tumble mode, from its initiation through to the steady rotation condition. History has shown that the tumble mode has always resulted in the destruction of an aircraft. In consequence, it is the authors' opinion that consideration of the tumble during the approval of new designs should concentrate upon avoidance, since there is no identifiable recovery mechanism from the established mode without the use of an external safety device. A programme of research into this phenomenon was initiated in 1997. The peculiar nature of the tumble motion has required the aerodynamic modelling to address the effects of a high pitch rate, which has led to the introduction of unsteady aerodynamic effects. This paper sets out to describe and explain the mode, leading to a model that might reasonably be developed to produce relatively tumble resistant aircraft. Finally, the methods of possible entry are explained; it is intended that this information may be used in pilot training to ensure the avoidance of the tumble instability.