FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1934
1934 - 1464.PDF
SUPPLEMENT TOFLIGHT 76. THE AIRCRAFT ENGINEER NOVEMBER I, 1934 "V Wwhence, or, introducing the " power factor," L V \V ^^Ws'WF) (9) where, V = any speed level flight, in m.p.h., at any height Hft. P/(/i) = b.h.p. with engine throttled to give THPA = THPR at speed V. Again note carefully the significance attached to the power term. In the case of the High-speed Figure we were dealing only with one specific point in the power curves, i.e. the intersection of the curves for THPA at full throttle and THPR as indicating the maximum speed level flight at the particular height considered. At some speed Vx substantially below the maximum speed there will be excess power available at full throttle, and the machine will climb. To maintain level flight at speed Vx it is thus necessary to throttle the engine so that THPA (throttled) equals THPR at speed Vx. This process can be repeated for all speeds and all heights within the flying range, so that obviously almost in- numerable THPR and THPA (throttled) curves may be calculated. Only one pair of these, however, will deter- mine the Distance Figure, the pair required being, as previously stated, that which gives the maximum ratio of speed level flight to power. This ratio is really analogous to the " miles per gallon " figure so familiar to motorists. The reader will now appreciate why the Distance Figure is seldom, if ever, referred to in type technical descriptions, an intimate knowledge of both machine and engine performance being essential if the Distance Figure estimate is to serve any useful purpose as a comparison. Such data is, of course, quite outside the scope of the usual technical descriptions. 10,-The Altitude Figure For steady climbing flight, Vc = ^-(THPA -THPK) or, by previous reasoning, .. (17) If a climb be commenced at sea level with the aeroplane in the attitude corresponding to minimum power required, and this altitude be maintained throughout the climb right up to the absolute ceiling, the optimum rate of climb will not be realised at low altitudes, but the final altitude reached will remain unaffected. Assuming (17) to apply to such sea level conditions, then, V = speed level flight, in m.p.h., corresponding to minimum power required. P = b.h.p. at full throttle at engine r.p.m. appropriate to V. Since the altitude of the aeroplane is assumed to remain constant throughout the climb, the overall drag coefficient, kD, will also remain constant. But the mass density of the air will decrease with height, hence, for constant resistance, the speed level flight will increase with height. Therefore, at the ceiling the speed level flight, V, will have increased to V/\/a. Also, as previously, power will fall oft with height in accordance with f(h). Thus, at the absolute ceiling, (17) becomes. At the ceiling, •whence, vPf(h) =W = O DV 375 y/ . t: For steady climbing flight, approximately, W = L hence, 1,-Pfjh) DV I_ whence, By definition, L V^ W 1?f7=375 .. (18) V = W or, allowing for the difference between British and Conti-nental units, V = Substituting in (18), KLP/2S_ W _ I 375 or. /W" tl-- 375P2 Pf(h) whence, writing p — 0.0051 for m.p.h. units, 18.95 P/(A) ~J^ where, P = b.h.p. at full throttle at sea level at engine r.p.m. appropriate to the ceiling speed. o and f(h) have values appropriate to the absolute ceiling. 11.-The Lift Ratio This is a figure of merit which the writer is suggesting here for, he believes, the first time. At least, he has never seen anything of a similar nature quoted previously in technical descriptions of new types of aeroplanes. The suggested figure of merit has the following significance, KLmax (from machine characteristics)Lift Ratio = By definition. KLmax (from aerofoil characteristics)L = K,.pSY- L = W or, Substantially, Then, Whence, from machine characteristics. Surface loading KLmax.= p(V landing)*' The other value, K,max. from aerofoil characteristics, is the full scale maximum lift coefficient for the basic aerofoil independent of such auxiliaries as slots, flaps, etc., and is determinate, with appropriate corrections, from tunnel data. The object of the Lift Ratio is to indicate to what extent the designer has cheated the basic laws of aerofoils by using flaps and/or slots, and by general layout of his wing truss to avoid loss of lift by interference and to take advantage of cushioning effects near the ground, short landing run, etc. It is suggested that the ratio might be useful in making preliminary estimates for a proposed new design by taking the landing speed as a datum and deriving the other quantities therefrom. In any case it would add to the value of type technical descriptions. (To be concluded in our next issue.) •,;...!/'.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
