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Aviation History
1941
1941 - 2172.PDF
170 FLIGHT SEPTEMBER I8TH, 1941. A typical De Havilland v.p. a<rscrew. Hollow steel should be superior evento dural from the point of view of re- sistance to abrasion by stones, butmay suffer more from damage due to bullets or cannon shells. It has alsoyet to be shown that the early troubles of fatigue cracking of hollow steelblades, due to panting of the sheeting, have been completely overcome,though there is no doubt that ttie situation in this respect has im-proved enormously during the past few years. Magnesium requires very carefulservicing in order to remove any in- dentations as soon as they are formed ;otherwise, owing to its notch seusi tivity, fatigue cracks are likely tostart. It is also extremely subject to corrosion. On most installations,therefore, it cannot be considered par- ticularly good from the point of viewof wear and reliability, but on machines where careful attention canbe paid to servicing, and vibration stresses in the blades are low. magnesium can be used. Reparability This is a factor which is oftenneglected when different materials or designs axe being compared, but inwartime it is of paramount impor- tance, and even in peacetime it shouldby no means be overlooked. Its im- portance can, perhaps, be judged from The hub and blade roots of the newDe Havilland four-blader.. the fact that of all the airscrews whichare returned to De Havillands for re- pair, including blades which havebeen pierced by bullets, and blades and hubs which have been bent ordamaged in crashes, over 80 per cent, of the blades and 85 per cent, of thehub parts are repaired and returned to service. Damage marks in the lead- AIRSCREW BLADE MATERIALS ing edges of dural blades caused bycontact with the ground in a crash, or by abrasion from stones on aero-dromes, can simply be smoothed out, and even bullet holes can be cleanedup and faired out, and the blades can be returned to service. Blades whichhave been bent can nearly always be straightened, and, if the bend is nottoo serious, it is even possible to do this work without re-heat treatment. Here, again, magnesium comparesvery unfavourably with dural, as the extent of possible repair is less, andthe susceptibility to damage is greater. Wood shows up better in thisrespect than it did a few years ago; it has now been found possible to re-pair broken blades by splicing on a new outer portion. It should be re-membered, however, that this form of repair will have to be carried outon every blade which is involved in a crash, as the blades will always bebroken and not merely bent, as in the case of a dural blade. It thereforemeans that a complete new outer section of the blade has to be manu-factured, and that after attaching to the old root the whole blade must bere-covered. From the point of view of repar-ability, hollow steel shows up perhaps worse than any other material be-cause a crash will almost certainly in volve buckling of the steel sheeting,and although suggestions have been made from time to time that it mightbe possible to straighten blades in this condition, it seems unlikely that thiswill ever be a really practicable job.
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