Научный вестник МГТУ ГА (Aug 2019)
Electromechanical effect of lightning onto the fitting points of the aircraft skin
Abstract
One of the damage factors of the outer elements of aircraft structures by lightning strike is its electromechanical effect. The most typical and diverse types of damage caused by lightning to the fittings of the aircraft skin were discovered in December 2017 in the state of Louisiana (USA) after the lightning strike to the B-52 bomber during landing. On the ground the crew found the man-sized gash in the tail unit. Similar incidents took place with a number of Russian aircraft too. Researches, conducted at the Chair of Physics of Moscow State Technical University of Civil Aviation by a group of specialists in the field of lightning protection of aircraft, allow to explain the mechanism of this kind of damage with a high degree of reliability. This mechanism is of a complex nature. The Electromechanical effect of lightning on the conductive skin, due to the interaction of the lightning current spreading along the skin with the magnetic field created by it, is essential as well as the pinch pressure in the discharge channel. Loading of a conductive plating is caused by ponderomotive forces in the skin and inside the channel of the lightning discharge. Coming to the edge of the trim panel stresses wave from such loads, coupled with the impact of lightning born of the shock acoustic waves, adding up to the operational, staffing loading, can overload as at the contour of the trim panel, weakened by holes for fastening elements, and at the fastening elements too (rivets, screws). The calculation of the intensity of the stresses in the skin due to ponderomotor forces in the area of the rivet connection showed that only if the amplitude of the lightning current exceeds 100 kA, supercritical stresses can take place - as normal stresses in the skin along the rivet row and tangential stresses in the narrow part of the rivet flange. Since the statistics claims that the probability of the aircraft high-current lightning strike is negligible, it is obvious that these additional factors were active in the B-52 case. In the photo of the damaged part of the tail unit of the aircraft, both types of structural damage are clearly visible: the separation of one edge of the panel from the stringer along the rivet row (the flange remains in the place) and the damage of the other edge along the rivets with the flange.
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