FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1912
1912 - 1074.PDF
l/0GHf NOVEMBER 23, 1912. FACTOR OF SAFETY AND NORMAL SPEED. By W. 0. THE articles recently appearing in FLIGHT on the subject of the factor of safety in aeroplane design were certainly well timed, and doubtless others beside myself have been led to think more closely on this matter now that we are asked to commit ourselves to the support of some numerical value. For my own part, I have been looking into the matter and have found, as is invariably the case, that even a brief study reveals aspects not hitherto appreciated. Thus, I now raise the query whether the factor of safety should not vary inversely as the normal speed of the machine, if aeroplanes of different kinds are to be equally strong under the same weather conditions. The argument that I am about to put forward makes no pretence at accuracy in detail, nor does it pretend to take into account all of the many considerations that are properly related to a complete study of the subject. On the other hand, I think it can claim some measure of interest as a line of thought, and as such I present it for discussion to other readers of FLIGHT. As an hypothesis I assume that the basis on which the factor of safety should be established may be represented by assuming that the abnormal stress in flight results from the machine being struck by a gust equal to («) times its normal velocity, and that the additional expected stress is in the ratio of V2:(V + nV)-. Resolvingthisexpressiontoitssimplestterm, (V + nV)- _ V2 + 2nV- + «2 V' V2 ~ V2 find that the factor of safety becomes a function of n + i)2, which may be written thus: kiy + ij- where fl=gust and V = normal speed, both in m.p.h., and k is a constant. Now it is necessary to adopt some arbitrary standard as a basis that is acceptable to all, and for the sake of argument I will suppose that the standard is a factor of safety of 6 at 60 miles an hour. The next point to consider is the probable maximum value of «, and at first sight one is inclined to adopt as a basis the idea of a maximum gust being a percentage of the normal flight speed. This, however, would essentially differentiate between the weatherliness of slow and fast machines in a manner that would not only be unjust to the slower craft, but would put an arbitrary premium on high speed in a way that is hardly proper. Moreover, the weather is not under control, and if you glance for a moment at the wind charts published on pp. 792-3 of FLIGHT for August 31st, 1912, you will see that a more reasonable assumption is to adopt some arbitrary numerical value for the gust that may strike any machine, whether its normal speed is high or low. Again, for the sake of argument, I will take the numerical value of 20 as a reasonable gust value and transposing it in the formula I find that a constant k MANNING, A.F.Ae.S. increase on the speed for which the size of the parts have already been calculated, whereas with the 30 m.p.h. machine the percentage increase is twice as much. The question arises, therefore, what factor of safety should the 30 m.p.h. machine possess at 30 m.p.h. if it is to have the same effective margin of strength as a 60 m.p.h. machine, when both are struck by a 20 m.p.h. gust ? I have prepared a curve from the formula, which shows this relationship graphically, and if the basis is that the 60 m.p.h. machine is to have a factor of safety of 6 at its normal flight speed, then to be equally strong in the presence of a 20 m.p.h. gust, the 30 m.p.h. machine must be designed for a factor of safety of 9*4 in respect to its normal speed. As I said in the first instance, this is more a line of thought to be discussed than a definite suggestion for a basis of present design, but it is conceivable that it might take shape as such in future if the criticism of others fails as follows = ri- + 2n+ 1, we in order to provide a factor of safety The formula therefore becoming 3-4 3-4 is necessary 6 at 60 m.p.h. .2 of 20 V Now the machine that is built with a normal speed of 30 m.p.h. will ordinarily have its parts calculated on a basis of 30'- whereas a 60 m.p.h. machine will ordinarily be designed with the figure 60- in view (I am ignoring for the moment the question of gusts and am merely assuming that the design is based on the flight speed itself). Thus if the 30 m.p.h. machine so designed is struck by a 20 m.p.h. gust it will be relatively weaker than the 60 m.p.h. machine under the same circumstances, because a 20 m.p.h. gust only represents a 30 per cent. OF F • 3 4 £f "I" ao MILE.S PE.R HOUR to disclose any radical error in the argument. The subject of the factor of safety is certainly one of vital consequence to designers, and the idea put forward in FLIGHT that we should endeavour to establish some basis for mutual moral support is at least sufficient reason why the technology of the subject should receive the serious attention of those interested; I hope, there fore, that others will take the matter up. When I was on the Plain, Mr. de Havilland told me that he had carried out experiments with BE 2, with a view to discovering the maximum increased strain that could be produced in the wires by taking the machine out in a very bad wind, and controlling her in a way likely to produce maximum stress, the wires to be tested having been fitted with an instrument for determining these strains. I understand that the maximum strain observed was twice the normal. Under these conditions V + = 2 v V + 1 = 1-41; ^ = '41. But we know the maximum speed of BE 2 to V V be about 70 m.p.h.; hence the stress was that equivalent to a head gust of 287 m.p.h. This points rather to the advisability of v being taken as 30 m.p.h. in the case of military machines and others which might be taken out in very bad weather. BE 2's factor of safety should be, /30 \2 therefore, 3-4! — + 1) =7, which, I believe, is sub stantially correct. 1074
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
