The probable theory of the collapse of the wing after the collision with the birch
The wing structure:
cross section on the damaged position:
A-A cross section:
W(1) = wide of the wing on the damaged position before collision with the birch (original wide)
W(2) = wide of the wing on the damage position after collision with the birch.
T= Thickness of the wing on the damaged position
L = Length of the wing from the top to the damaged position
F = lifting forces
the one critical lifting forces acts on the top of the wing:
stress = (Moment/ inertia) x neutral axle = Moment/section modulus = (Fx L)/ SM
allowable stress aluminium structure = 400 MPa
section modulus of original cross section:
SM(1) = (W(1) x T^2)/6
section modulus of damaged cross section:
SM (2) = (W(2) x T^2)/6
stress after collision = (F x L) / SM(2)
if W(2) = 2/3 W(1), then stress after collision = 3/2 x stress before collision
It means, the stress increase is proportional to reduce of the wide of the wing cross section
At the moment of collision with the birch, the small area of the wing are damaged only. This damage leads to the reduce of the wing wide at the collision position and to the increase of the stress on the structure. The higher stress causes the development of cracks and in end-effect to the break of the residual wing structure along the collision position. Also the instable manoeuvre in the critical flight moment can cause to increase of moments and in end-effect to the disproportionately stress increase.
The stress rises proportionally to the lowering of the wing wide and to the increase of the moments. If the allowable stress of the aluminium material is reached, the structure of the wind breaks and destroys along the collision position.
Also in case of reach the stress up to 400 MPa after the collision, the local structure is disproportionately loaded and can cause in to the small cracks and in to the totally destroy of the structure along the wing. The fatigue aspects and the fatigue conditions have a high influence on the stress behaviour. Therefore the total collapse of the wing- structure under 400 MPa is possible.
For the detailed conclusions and for the compliance this theory the investigation of the damaged structure, rivets, screws and the whole structure also due to fatigue conditions is necessary.
trybuna.de 11.12.2015 Autor:AS