FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1914
1914 - 0577.PDF
MAY 29, 1914. [/QCHf THE FLYING MACHINE FROM AN ENGINEERING STANDPOINT. By FREDERICK In the first place, each aspect ratio should be explored by a number of determinations using aerofoils of varying camber ; secondly, the aerofoil section must not be uniform from end to end, the section must be " graded," or, as it is sometimes expressed, the camber must " wash out " at the extremities. Beyi.nd this, not one series must be tried but some dozen or more. The final " winner" for each aspect ratio is the aerofoil of greatest lift/drift. In my opinion, in the present unsatisfactory state of things, it is best (so far as the pressure constant is concerned) to assume a uniform value for all values of aspect ratio, say that given as appropriate to aspect ratio = 6 in the Table IV. Whether we •consider the N.P.L. result as valid or not, the salient fact is that we have at present no sufficient evidence that there is any change in the pressure constant worth taking into account. Alternatively we are not going far astray if we assume aerofoil pressures equal to half the pressure on the normal plane, as shown in Fig. 18. The most important fact with which we are immediately concerned in connection with the theory of least resistance is that the total aerofoil resistance for least value is almost constant in respect of velocity; in other words, provided that we design for least resistance, we know our traction coefficient in advance; it is virtually a constant, just as though the problem were that of an automobile required to ascend a hill of known gradient—an analogy which comprehends the fact that there is the direct wind resistance or body resistance additional in both cases. This constant is only within control, inasmuch as by careful design the effective value of the coefficient of skin friction, S, can be kept down, and a high aspect ratio adopted. Theoretical values of least gliding angle (that is to say, resistance coefficient), tabulated for values of £ and aspect ratio, are given in Table V. (From Aerial Flight, vol. i, p. 262.) It is of some interest to enquire to what extent these results are in agreement with modern experiment. TABLE V.—Least Gliding Angle (= 71) (Theoretical). WILLIAM LANCHESTER, M.Inst.C.E. (Continued from page 556.) It will be noted on referring to Table VI and Fig. 19 that the agreement is almost complete. The two cases of the Eiffel deter mination of the Voisin aerofoil and the K.A.F. 6 aerofoil are shown as outlying points, not being fully in agreement with the main run of the remaining ex- >/. 3 4 S to 7 8 (O [2 T S = 6-2S" 575" ST 5-ou 47° 4T 4-1° 3-8° have c 0-025. I : 9'2 1 : 10 I : 10-8 1 :n-s 1 : I2"2 1 : 12-8 I : 14 1:15 ollected e ?= 5'6° 5'*s" 47S° 4-5° 4"25° 4-0° 3-65° 3'42° xDerim 0-02. I : I0'2 I : II'I 1 : 12 I : 12-8 1 : I3-S I : 14-4 I : 15-8 I : 168 ;ntal data £=o-oi 4-«° 4-4° 4"i" 3'9J 3-b" 3-4" 3 "2° 3-o° from A 1 1 1 1 1 1 1 1 •ari v (2 13 14 147 i5-9 16-8 17-9 19 ous so E= 3-95° 3-c^ 3"4L 3-2L 3"o° 2-8° 2-6° 2-4° urces ; O'OIO. I :I4'S I : 157 I : 16-8 I :I7'9 I : I9'I I : 20-5 I : 22 I : 23/9 a series of aerofoils of Bleriot section—(Report of the Advisory Committee, 1911-12, p. 75),—aspect ratios vary from 3 to 8. Determinations of Voisin wing by Mr. Eiffel, aspect ratio 63. Aerofoil " R.A.F. 6," aspect ratio6. Aerofoils from my 1S94 model, aspect ratio 13-3, Fig. 16, independent determinations by N.P.L. and Gottingen Laboratories. The above are given in Table VI; columns 1, 2, and 3 give the aspect ratio, type and authority respectively ; column 4 gives the experimental determination, and my theoretical values are given in columns 5 and 6 for values of £ = 0-02 and I = 0-015. Table VI is shown plotted in Fig. 19, the relation of aspect ratio to lift /drift being represented by curves drawn through the observed and calculated values. TABLE VI. 2. 3- 4- 5: 6- _ Determina- TyPe- tion by Bleriot section N.P.L. ... 1. Aspect Ratio. 3-0 4"o 5-° 6-o 6-0 63 7-0 8-o IO'O I2'0 Experi- Calculated Calculated mental. g = o'02. £ = 0-015. R.A.F. 6 .. Voisin Bleriot section Eiffel N.P.L. f N.P.L." 13-3 Author, 1894^ I * Velocity 30 ft. per sec. t Velocity not stated. Gottingen: : IO'I n-5 I2'9 14 o H-5 14-0 IS*' 15 5 17-1 17-6 20'0 16-4 I7-3 102 II'I 12-0 12-8 13-5 I4-4 15-8 16-8 I2-0 I30 i4"o 14-8 I5'9 168 17-9 19-0 t Value at 50 ft. per sec. (computed by N.P.L.). perimental determina tions ; it will be noted, however, that the whole of the experi mental values lie between the two ad- j a c e n t theoretical curves given, and the general form of the experimental curve corresponds to the curves given by my equations. It is true that there is some thing in the nature of the hump on the ; experimental curve, ; the extremities of 't which correspond to ! a double surface co- ,. efficient of skin fric- 5 tion of 0'02, whereas • the central part of the a curve round about aspect ratio = 6 rises nearly to the upper curve. This pecu liarity of angular character of the curve may be a real feature, but I am disposed to think that is more probably due to the fact that a great deal more experimental work has been done in the region of the hump of the curve and so more highly per- <<"*' «IRD*nn IN - *. 1 LIFT/ O *»*IC1-«*T.O Fig. 18. fected forms have been available than for aspect ratios of greater or less value. It would appear probable that if equal diligence were dis played in designing and te>ting forms of other aspect ratios the upper theoretical curve (£ = 0-015) would be found to be very close to the m r <%—3* —-1 — ,— _ --j*- r 1 1 .». -M»l • 1 •*....•- r^x! f 1 i r-l . —1 1— ,—, ~ m — -;.".?'- 1—1 .. •...-; 1 1 t 1 1 1 1 j . ! 1 1 1 1 H 4 Fig. 19. truth ; some confirmation of this is found in the fact that the best value for the R.A.F. 6 aerofoil, a form that had been subject to considerable study both by the R.A.F. and the N.P.L., lies con siderably above the curve representing the run of other observations. In the N.P.L. report to me on the tests of my 1894 model, it is stated that if it had been found possible to employ a velocity of 50 feet-seconds instead of 30 feet-seconds, the figure obtained would probably have reached the neighbourhood of 20. This value is also plotted as an outstanding point in Fig. 19. (To be continued.) 577
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
