FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1932
1932 - 0099.PDF
JANUARY 29, 1932 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT primary importance. However, the climbing perform ance is not so susceptible to small changes in thickness to chord ratio as is the top speed condition. The induced drag forms the major portion of the wing resistance at climbing speed, and this is unaffected by profile drag. Another point to be remembered in connection with the relative climb of monoplane and biplane is this: — At climbing speed the thick highly cambered sections commonlv used on cantilever monoplanes work nearer the point of minimum profile drag than the thinner, less highly cambered sections common to biplanes. It is possible for the profile drag at climbing speed to be less tor the thick wing than for the thin wing. The sum total of this is that climb formulae developed for one type will apply equally well to both. The following table gives the estimated rates of climb at ground level for the various combinations chosen : — TABLE II. Bates of Climb at Ground Level (ft.jmin.). Apsect Ratio. 3 6 9 Weight, lbs.- 5,000 7,000 9,000 1,630 929 470 1,830 1,170 759 1,890 1.250 857 Bairstow's formula for rate of climb (Ref. 4) can be generalised as follows: — R = K, K, B.H.P. '• (1) Where R = Rate of climb at G.L. in f.p.m. W = Gross weight lbs. Vs = Stalling speed. B.H.P. = B.H.P. of engine at climbing speed. rj = " Net " airscrew efficiency. K, and K2 are constants for any aircraft. It is fairly obvious that the values of K, and K2 depend upon the L/D of the aircraft at climbing speed. The L/D is greatly influenced by the equivalent mono plane aspect ratio as well as the parasite drag of the aircraft. TABLE III. 3 6 9 110 6-4 4-9 1,480 2,500 3,250 The b.h.p. taken in evaluating K, and »;-K2 is the normal b.h.p. of the engine. The small change in b.h.p. delivered by the engine at the various best climbing speeds is allowed for in the values of K[ and J;K2. For the same reason the efficiency must be kept constant, and in this case it is taken as having a value of 69.5 per cent, corresponding to the normal loaded case for the aircraft. This expedient is necessary if the formulae are to be of use as indicated previously. In finding the rate of climb from the known climbing conditions at another weight, it is now not necessary to estimate the new r.p.m. and corresponding b.h.p. of the engine on climb, nor, as a matter of fact, is it necessary to know the efficiency in such a case, as from known conditions the value of (t;-K2-b.h.p.) can be obtained without separation. This is referred to later. By inserting the value of 69.5 per cent, for efficiency in the last column of Table III the value of K2 is obtained directly. The results are plotted on Fig. 5, where is will be noticed that the points do not lie on a straight line. Over that part of the curve most used, i.e., from an aspect ratio of 3 to 7, the value of K3 may be taken as having the following relationship:— K, 66 + 496 A (2) Fig. 6 shows the values of Kt plotted against the reciprocal of A. These points fall on a straight line and conform to the following law: — A (3) The substitution of 2 and 3 in equation (1) completes the climb equation, and two suggested applications are given in the worked examples at the end of the article. The next consideration is the climbing speed off the ground at any weight. To be of practical use the climb- Fig. 5.—Variation of K2, with Aspect Ratio. On right, Fig. 6—Variation of Ki with Aspect Ratio. The parasite drag cannot, of course, be generalised, but the effect of aspect ratio can to a great extent, thus reducing to a minimum the possible error in the estimate of rate of climb. The values of K, and K3 have been worked out to satisfy the rates of climb given in Table II. The results are tabulated in Table III: — ing speed must be expressed in quantities which can be measured on the full-scale aircraft. Two such quanti ties are the top speed and the stalling speed. Over a large change of weight the top speed of any aircraft is appreciably constant. As shown by Fig. 3 where for an increase in weight from 7,000 lb. to 9,000 lb. the top speed falls only 2 m.p.h. 96 c
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
