Got home only to discover been banished from the houseboat kitchen. I decided to use the time to try and learn a bit more about Tigers, the net is great, but there is a load of useless information to wade through; I expect the trick is knowing what to search for. Even so, it's a great tool. I found the following most instructive –
DH82 – and – Avilogs. Most DH 82 buffs will be all over it, but for a layman, they helped a great deal.
The geometry of the DH 82 flight load management is a testament to the KISS principal. I was fascinated with the amount of repair detail provided for the metal parts of the airframe, compared to the timber parts described in the -
Avilogs - 1947 Maintenance and repair sections. It was also interesting to note the 'terminology' used, the term 'see' rather than 'check' for example.
SvW # 70 –"Including the ends of the flying wires.
The inter-plane struts and their fittings must be the weakest points as most connect through a single bolt (from memory)".
There's a good start – I wondered what level of redundancy there is; i.e. how much of the 'bracing' can be lost before a wing 'breaks'. With apologies to the 'wizards' it looks as though there are 6 bracing wires; 2 flying, 2 landing and 2 inter-plane (cross bracing). The 'flying wires' attached to 6 'hard' points, the cross bracing to four within the inter-plane struts. That's 10 hard points in all, here the mathematics get difficult (for me) it's not possible to determine the ratio of load between the 'lift' bracing and 'landing' bracing, but the manuals indicate a higher percentage on the lift side during flight (which makes sense). So, the puzzle is, if a 'weak' point fitting had let go what is the gross effect?. To loose one of a possible four cross brace fitting is within the realms of probability, but would that cause the wing to break. The loss of one of a possible six flying wire fittings would be a more serious matter (mathematically speaking). I expect the ATSB will, in due course, provide the right answers, if at all possible. Previous reports into Tiger accidents have been very good, the 1998 one -
ATSB 199800648 - in particular. They were careful then to examine all the possibilities.
Cherry picked from ATSB 199800648, (simply to assist head scratching and muttering).
TMK was a single-bay biplane with a wood and metal structure covered by fabric. Metal, aerofoil shaped, flying and landing wires braced the wings. Wing slats were mounted on the outboard leading edge of the upper wings above the inter-plane strut attachment points. British Aerospace, the type certificate holder for the DH-82A, reported that this area underwent the greatest bending stresses when the wing was placed under aerodynamic load and, therefore, determined the ultimate load limit of the wing.
However, the slat-locking lever was found to be in the unlocked position. The investigation could not determine whether the slat-locking lever was unlocked during the aerobatics or became unlocked during the subsequent in flight break-up or ground impact. Both slats were bent upwards in a V-shape around the centre attachment. The outboard part of the right wing slat had additional deformation and contained a deep cut. The cut was consistent with the slat impacting either the right wing's flying or landing wires. Within the wreckage trail, the slats were found beyond the separated pieces of wing spar and internal structure.
The notion of a sudden load transfer combined with a loss of 'bracing' creating enough force to break a wing claimed to be within :: 7g to :: 5 g is intriguing.
Enough; time to mount my cunning plan to steal mince pies, cream and any loose choccy frogs the dogs didn't get. MC y'all.