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Aviation History
1927
1927 - 0724.PDF
SUPPLEMENT TO FLIGHT SEPTEMBER 22, 1927 THE AIRCRAFT ENGINEER Surface per Horse-power or Span2 per Horse-power. It has always been the writer's practice to compare the speed of aircraft on a basis of wing area per horse-power.* This is very convenient for biplanes of normal proportions, but it will not give a good comparison between biplanes and monoplanes. The reasons for this are fairly obvious : for instance, a monoplane and biplane when built for the same purpose have about the same drag, while the surface of the monoplane is. in general, smaller. Examples of this might be quoted, but it is better not to appeal to examples too much, as statistics can always be produced to prove anything. It is essential, above all. that any formula for racing should be based on sound theoretical grounds ; it should also be free from the necessity of reconciling past examples, and it is for this reason that the writer is of the opinion that the span and brake horse-power should be the basis for formula racing. Before passing on to this, however, the greatest drawback of the wing area basis may be indicated. Every competitor will do his best to defeat the formula, and formula? in general tend to produce freakish craft. Xow. mere area may be increased to a very large extent without affecting the machine structurally, or lowering its speed materially, by adding largely to the chord and keeping the spar depth the same. An area'horse-power formula would, undoubtedly, tend to produce rather grotesque aspect ratios, and the machines which were cood on formula would carry a lot of surface of which no effective use could be made beyond getting a good handicap. The case is very different as regards span, which will now be considered. Span and Brake Horse-pmcer Span ifs the dimension for which there is no substitute. As was shown by Lanchester a long time ago. the drag of an aeroplane consists of two parts, the frictional drasr and that drag which is inmrred because a certain weight is carried at a certain speed on a certain span (now called induced drag). It is the first part which is under the control of the designer, the second part being a law of nature. This second part depends on no other dimension but the span. If an aeroplane has a big span for its B.H.P.. it ran carry a big load for its B.H.P. If it has a small span it will have a high speed, and low load capacity. Both tvpes. from racing to heavy load carrving. should have their chance on the formula, provided the frictional drag has been kept as low as possible. Xow if an aeroplane is found to have too hi<_'h a loading to "get off" and climb well on its W. H.P., in- creasing the area by extending the chord will have little effect. The induced drag (which is the dominating part in this condition) remains the same at the same forward speed. Put shortly, and without all the qualifying statements, increase of chord is no cure for an over-loaded machine, but increase of span is. In other words, the general capacity of an aircraft hi any direction is governed by its span. In a speed contest, then, that machine which goes fastest for its span and its B.H.P. should win (the square of the span is the quantity involved). As regards the possibility of defeating the formula by artkJe* This in the quantity callsd • Wins; power " by Professsor Everting In:i(4 in THE AIRCRAFT E>~(,L\~EEII of JSov. 25. 1926.— Kli. Fig-1-—A represents a monoplane of 20 ft. span carrying some load which causes a certain induced drag. In B is shown two separate non-interfering planes dividing the load between them and having the same induced drag as the monoplane. Cases A and B should be equally handicapped, for they are inherently capable of doing the same job for the same induced drag. ADDE D ING " 1030 I gj/50-20 » IS 1 <* ii. t- 0 Jill TTT TTTT TTTT k I II I I II I II I I I I I II I I | I I k in 0-6 0-6 04 SPAN OF SMALLER PLANE SPAN OF GREATER PLANE. 02 Fig 2.—The general scale connecting gap-span ratio of equal wing biplanes is taken from American N.A.C.A. Report No. 151 by Max Munk. increasing the span, this is on quite a different footing from an increase of chord. Stresses cannot be dodged on wing extensions which must be looked well after structurally, and which will add rortistanc-e. If anything very freakish tended Area to appear a new definition of span such as TT TT, , would Mean ( hord put it right. What has been said so far amounts to thi* : That it is desirable and justifiable to handicap by means of dimensions, that there is only one dimension—the span—which is funda- mentally concerned in all the capabilities, other than racing, of an aircraft, iind that since the idea of a formula is to admit not onlv pure speed machines, thev should be handicapped bv the dimension which determines their other capabilities. Monoplanes and liipUinnt If span is accepted as the criterion, it will be necessary to define the term. It will be evident that the overall span of a monoplane and a biplane have not the same signficance. The extra wing of the biplane does something towards reduc- ing the induced drag and should, therefore, get some allow- ance in the formula. If the basic idea of the formula be accepted, namely, that any aeroplanes having the same induced drag when doing the same job. should have the same handicap, the portion of the biplane's extra win<r that should be added to give the equivalent monoplane span, is easily found. In Fig. 1, A represents a monoplane of 20 ft. span carry- ing some load at a speed which causes a certain induced drag. In B is shown two separate non-intrrfering planes dividing the load between them and having the same induced drag as the monoplane. Since the induced drag depends on the square of the span, each of these wings must have a span of 14-14 ft., for the combination to have the same induced drag as the monoplane. This is the case of the infinite gap equal-wing biplane, and eases A and B should l>e equally handicapped, for they are inherently capable of doing the same job for the same induced drag. If the " span for handicapping " of A is 20 ft., and the biplane is assessed by adding a suitable fraction of one wing to the other, the " span for handicap " of B is Sx -f a fractior of S2, which will total 20 ft., or S, 4- 0-41 S2 for the biplam without any interference. This is one extreme case, tht other being where the gap is infinitely small, and then Case t- is considered to be two 20-ft. planes superposed. In thi case Sj + O X S2 = 20 ft., HO that the allowance for th other plane of the biplane must be something between S1 --• 0-41 S2 and SI, depending on the (iap/Span ratio. There i also the unequal wing biplane to be considered. Theo 6686
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