Excerpt from J.E. Gordon (The New Science of Strong Materials):
Problems of rot were always with us but there were other problems as well which were just as serious. The general structure of those aeroplanes was quite different from the old fabric biplanes. The main spar booms and other main structural members were sizable pieces of laminate wood, several inches square, and were generally boxed in on three sides by the plywood skin and shear webs. Now the spruce bar boom wanted to shrink and swell about twice as far as the plywood which was glued to it and this naturally gave rise to serious stresses near to where the two met along the glued joints.
Large pieces of timber take some considerable time to come to equilibrium with the surrounding humidity and, because the English weather changes so often, there was generally no time to build up dangerous differences in swelling strains so that we had comparatively little trouble from this cause, so long as the aircraft were in this country. When they were sent overseas the situation was different. In many climates there are long dry seasons followed by long wet seasons, each season giving ample chance for the wood to dry thoroughly and then, in due time to soak up a great deal of water and swell. In such places there was serious trouble. Big stresses were built up near the glue lines; if the glue was in bad condition it broke; if not, the wood failed near the glue. There was really no cure for this except to bring the aircraft home.
Proposed Mossie rebuild in uk - discussion - Page 9
de Havilland Mosquito
It was eventually determined that the problems were initially the result of a combination of poor gluing practices and poor mate-up of structural members. Worse, in the tropical conditions of the Far East, water soakage led to swelling and shrinkage that gave rise to wing skin delamination and spar failure.
The Mosquito difficulties caused the Squadron several stand-downs from December 1945 and ultimately
apparently contributed to its disbanding in March 1946.
I'll end my part in this with some more of Jeff Jefford, from his Flying Camels Annex K account of the problem:
The second problem concerned the adhesion between the spruce spar booms and various plywood components. Although there was some evidence of inadequate gluing in these cases, which were far less prevalent, it was concluded that the cause was "probably due to swelling of the top skin" causing the securing screws to pull through.
On balance it now appeared, in the case of the Mosquito, that both the manufacturer and the weather had contributed to the failures but it was now beginning to be appreciated that, in the latter case, the shrinkage which had led to separation of components was not so much a cause as an effect; the real damage mechanism was swelling.
These emergent conclusions were confirmed at an MAP meeting on 1st January 1945 which heard an explanation of the Mosquito's defects from Maj DeHavilland who had now returned from India. He was able to report that the manufacturer had conducted strength tests on the suspect scarf joint using partially glued specimens and these had shown that the strength factor in that region of the wing was adequate; surprisingly this was even the case when unglued samples were tested. The more critical failures were those concerning the mutual adhesion of spar booms (particularly the front ones), spar webs and wing skins. The trouble was "attributed to water soakage in conjunction with differential shrinkage and some unsatisfactory initial gluing."
The company undertook to improve manufacturing techniques among the contractors building Mosquito components which would take care of the inadequate "initial gluing" problem entirely. The "differential shrinkage" aspect was less easily resolved. The root cause of this was considered to be the ingress of water and it had become apparent that a major factor here was the deterioration of dope and sealant on the upper surfaces of the aircraft; a factor which had not been widely reported at first.
Repair of defective aircraft, of which there were about fifty in India, would involve replacement of the entire front spar and leading edge assemblies. Prevention of future occurrences was to be achieved by applying a plywood strip spanwise along the entire wing to seal the whole of the upper skin joint which ran the length of the front spar. This was subsequently introduced as Modification (Mod) 638. Surprisingly, since it altered the aerofoil section, Mod 638 appears to have had no adverse effect on either performance or handling. Finally, to improve the protective finish further, Major DeHavilland reactivated an earlier proposal that reflective silver paint be introduced. Although this had previously been ruled out on tactical grounds it was agreed that the suggestion would be re-examined and on 14th February 1945 a silver finish was authorised for all Mosquitos based in India.
DeHavillands had, at first, been understandably reluctant to acknowledge that their construction techniques were lacking but there seems little doubt that this had been the case in 1944, although this was a problem of quality control rather than a fundamental fault in the Mosquito's design and seems in any case not to have been critical. There is little reason to doubt, however, that the aircraft's greatest deficiency was the inherent inability of its wooden structure to stand up to the demands of the tropical climate and it appears to have been impossible to make the aeroplane waterproof. While Mod 638 may have been sufficient to keep the rain out in Europe, continuing post-war problems with late-build Mosquitos would indicate that it evidently failed to do the job in southern Asia.
With the advantage of hindsight an additional contributory factor suggests itself. It seems likely that the inherent tendency for the integrity of the Mosquito's wing to become degraded under tropical conditions was exacerbated by the stresses imposed by low-level attack operations.
Problems of rot were always with us but there were other problems as well which were just as serious. The general structure of those aeroplanes was quite different from the old fabric biplanes. The main spar booms and other main structural members were sizable pieces of laminate wood, several inches square, and were generally boxed in on three sides by the plywood skin and shear webs. Now the spruce bar boom wanted to shrink and swell about twice as far as the plywood which was glued to it and this naturally gave rise to serious stresses near to where the two met along the glued joints.
Large pieces of timber take some considerable time to come to equilibrium with the surrounding humidity and, because the English weather changes so often, there was generally no time to build up dangerous differences in swelling strains so that we had comparatively little trouble from this cause, so long as the aircraft were in this country. When they were sent overseas the situation was different. In many climates there are long dry seasons followed by long wet seasons, each season giving ample chance for the wood to dry thoroughly and then, in due time to soak up a great deal of water and swell. In such places there was serious trouble. Big stresses were built up near the glue lines; if the glue was in bad condition it broke; if not, the wood failed near the glue. There was really no cure for this except to bring the aircraft home.
Proposed Mossie rebuild in uk - discussion - Page 9
de Havilland Mosquito
It was eventually determined that the problems were initially the result of a combination of poor gluing practices and poor mate-up of structural members. Worse, in the tropical conditions of the Far East, water soakage led to swelling and shrinkage that gave rise to wing skin delamination and spar failure.
The Mosquito difficulties caused the Squadron several stand-downs from December 1945 and ultimately
apparently contributed to its disbanding in March 1946.
I'll end my part in this with some more of Jeff Jefford, from his Flying Camels Annex K account of the problem:
The second problem concerned the adhesion between the spruce spar booms and various plywood components. Although there was some evidence of inadequate gluing in these cases, which were far less prevalent, it was concluded that the cause was "probably due to swelling of the top skin" causing the securing screws to pull through.
On balance it now appeared, in the case of the Mosquito, that both the manufacturer and the weather had contributed to the failures but it was now beginning to be appreciated that, in the latter case, the shrinkage which had led to separation of components was not so much a cause as an effect; the real damage mechanism was swelling.
These emergent conclusions were confirmed at an MAP meeting on 1st January 1945 which heard an explanation of the Mosquito's defects from Maj DeHavilland who had now returned from India. He was able to report that the manufacturer had conducted strength tests on the suspect scarf joint using partially glued specimens and these had shown that the strength factor in that region of the wing was adequate; surprisingly this was even the case when unglued samples were tested. The more critical failures were those concerning the mutual adhesion of spar booms (particularly the front ones), spar webs and wing skins. The trouble was "attributed to water soakage in conjunction with differential shrinkage and some unsatisfactory initial gluing."
The company undertook to improve manufacturing techniques among the contractors building Mosquito components which would take care of the inadequate "initial gluing" problem entirely. The "differential shrinkage" aspect was less easily resolved. The root cause of this was considered to be the ingress of water and it had become apparent that a major factor here was the deterioration of dope and sealant on the upper surfaces of the aircraft; a factor which had not been widely reported at first.
Repair of defective aircraft, of which there were about fifty in India, would involve replacement of the entire front spar and leading edge assemblies. Prevention of future occurrences was to be achieved by applying a plywood strip spanwise along the entire wing to seal the whole of the upper skin joint which ran the length of the front spar. This was subsequently introduced as Modification (Mod) 638. Surprisingly, since it altered the aerofoil section, Mod 638 appears to have had no adverse effect on either performance or handling. Finally, to improve the protective finish further, Major DeHavilland reactivated an earlier proposal that reflective silver paint be introduced. Although this had previously been ruled out on tactical grounds it was agreed that the suggestion would be re-examined and on 14th February 1945 a silver finish was authorised for all Mosquitos based in India.
DeHavillands had, at first, been understandably reluctant to acknowledge that their construction techniques were lacking but there seems little doubt that this had been the case in 1944, although this was a problem of quality control rather than a fundamental fault in the Mosquito's design and seems in any case not to have been critical. There is little reason to doubt, however, that the aircraft's greatest deficiency was the inherent inability of its wooden structure to stand up to the demands of the tropical climate and it appears to have been impossible to make the aeroplane waterproof. While Mod 638 may have been sufficient to keep the rain out in Europe, continuing post-war problems with late-build Mosquitos would indicate that it evidently failed to do the job in southern Asia.
With the advantage of hindsight an additional contributory factor suggests itself. It seems likely that the inherent tendency for the integrity of the Mosquito's wing to become degraded under tropical conditions was exacerbated by the stresses imposed by low-level attack operations.
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