FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1941
1941 - 2173.PDF
SEPTEMBER I8TH, 1941. FLIGHT 171 AIRSCREW BLADE MATERIALS Octopus corner. Most 01 thesedamaged De Havilland airscrews can be repaired and put into service again. It therefore appears that it will be necessary to allow for 100 per cent, scrap on hollow steel blades which are involved in a crash. Weight It was on the basis of weight saving that wooden blades were first pro- posed as an alternative to dural; as a general figure it used to be quoted that a wooden-bladed airscrew would be some 20 per cent, lighter than a corresponding dural one. A few years ago this was, in fact, the situation, but recently progress has been made in the reduction of weight of dural blades. What has made possible this reduction is the introduction of means for measuring the vibration stresses in dural blades while they are actually operating on an engine. When a dural blade fails, the failure can almost in- variably he traced to fatigue and not to any effect of steady stress, so that it used to be necessary to design a duralumin blade to withstand the steady stresses imposed on it, and then vibration stresses, which, at that time, were a completely unknown quantity For some years past now it has been De Havilland's practice to carry out strain gauge measurements on their airscrews on each new engine application. If the vibration stresses are found to be high, it has often been found possible, with the co-operation of the engine manufacturer, to make changes to the engine which smooth out the most severe vibrations, thus reducing the stress level not only in the airscrew but also within the engine itself. In other cases, where no change in the engine could be made or was desired, a modification could be made to the blade which would alter the r.p.m. at which it resonated with the excitation from the engine, and so ensure that the conditions of high stress occurred outside the running range of r.p.m. In one particular case, high stresses were recorded in the tips of a blade, and a change in the design was made in which the Wade was thinned and its weight re- duced by some 5 lb., and at the same time the vibration stresses reduced by some 30 per cent. This is typical of the way in which vibration stress measurements help the designer of a dural blade to reduce weight. Now that these stresses can he measured and, if need be. reduced t>v a modification to the engine or air- screw, very much thinner blade sec- tions can be used than would have been considered safe a few years ago,a 'id, what is more important from the point of view of weight reduction, the diameter of the shank of the blade can be reduced considerably. The extent of this saving can, perhaps, best be judged by the fact that a blade shank size which was originally de- signed for an airscrew to absorb 750 h.p. is to-day running most satisfac- torily on engines of the 2,000 horse- power class. No corresponding developments have taken place to reduce the weight of wooden blades, and it is indeed difficult to see where any major reduc- tion can be made. The dimensions of a wooden blade are dictated much less by vibratory stresses, owing to the in- herent damping in the material itself, and the sizes are determined by the steady stresses. The only way to re- duce the weight of a wooden blade, therefore, seems to be to increase the strength of the material without in- creasing the density, or to reduce the density without reducing the strength. Since during the past developments in improved wood, weight and strength have always been increased or re- duced together, there does not seem to be very much hope of obtaining any startling improvements in this respect. The indication, therefore, is that, while some further reduction in weight per horse power on dural airscrews is still possible as vibration research pro- ceeds further, 't is unlikely that wood will develop any faster and most prob- ably not so fast. To day the weights of a dural or a wooden-bladed airscrew for the same installation are almost identical. Sometimes wood may be a few per cent, lighter, but in many cases the dural airscrew will actually be lighter than the wooden one. Magnesium was first introduced be- cause it seemed to offer a weight sav- ing over duralumin, and such a sav- ing was, in fact, obtained. The saving is not quite so great as would at first be thought, because the sections have to be made larger in order to give the greater factor of safety required by the notch-sensitivity of magnesium. It is, however, possible, if the vibration stresses on the installation can be made sufficiently low, to build a magnesium airscrew which shows a weight saving of some 10 to 15 per cent, as compared with dural. Hollow steel appears also to ofler the possibility of a saving in weight over dural, particularly for very large sizes of airscrew. Considerations of the strength and stiffness of the struc- ture in buckling seem, however, to offset the apparent initial advantage, and on typical present-day installa- tions the weights for dural and hollow steel appear to be very similar. It is possible that further development of the hollow steel blade may enable further advantage to be taken of its apparent possibilities in weight saving. Adaptability One of the great advantages of a dural over a wooden blade is that it can be made with a relatively small root diameter, thus giving easy adapt- ability to any type of hub without making the hub of an excessive size. This is of particular importance in multi-bladed airscrews. A four-bladed airscrew of large shank diameter will mean that it is necessary to have quite a large separation between the four blades to avoid interference between the roots, while a small shank diameter, such as is obtainable with the .'.ural blade, enables the roots to be kept close in to the air- screw shaft. Magnesium again is a little bit worse than dural, though the difference is not very great. Hollow steel construction, as used in present designs, appears to give root sizes rather similar to dural blades, but here, again, it seems that some
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
