FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1925
1925 - 0278.PDF
MAY 7, 1925 SOME ASPECTS OF THE COMPARISON OF MODEL AND FULL-SCALE TESTS THE Wilbur Wright Memorial Lecture given by D. W. Taylor, E.D., D.Sc, LL.D., before the Royal Aeronautical Society on April 30, dealt with the fundamental mathematical treatment of the principle of dimensional homogeneity, first fully enunciated by Riabouchinski, and Dr. Taylor followed mainly the methods developed by his compatriot, Dr. Buck- ingham. Dr, Taylor, it should be pointed out, is a member and secretary of the United States National Advisory Com- mittee for Aeronautics, and it will be recollected that a few years ago another great American expert in the subject of aerodynamics, Dr. Ames, also read a paper before the Royal Aeronautical Society. Dr. Taylor's paper was one of very considerable length, and it is quite out of the question for us to publish it in full. From the very nature of the subject it will be realised that the paper does not lend itself to summary treatment, and we can, therefore, refer to only one aspect of it, i.e. that dealing with certain results obtained by the Americans in their new variable-density wind tunnel. This subject has appeared of particular interest to us in view of the fact that a similar tunnel has not yet been operated in this country, although it would seem a most valuable means of getting to the bottom of the phenomenon known as " scale effect" and other im- portant subjects. " After a detailed statement of the derivation of formulae expressing in terms of m, I and t (mass, length, and time) the various quantities to be considered. Dr. Taylor said : " Having then reduced our original broad formula to R = PL*V*Ft LVjv) involving the density, the size, the speed, and some unknown function of LVjv, the well-known Reynolds' number, we need to form some conception of the effect of Reynolds' number, commonly called the scale effect. While we do not know the form of the function, we do know for the flow of water, oil, and air in pipes the relative experimental values. The original wonderful experiments by Reynolds have been repeated and amplified by others since 1880, and it seems established that at low speeds where the fluid flows smoothly F3 has one set of values, and at high speeds when the motion is completely turbulent there is another well-defined set of values, while for intermediate speeds values are rather inde- terminate. Wind tunnel investigations on such objects as cylindrical wires, struts and streamline wires show that the Reynolds No 27.O 175,000 72.5 456,000900,000 1,690,000J,570,000 -8° ~4 Lift and drag characteristics OS tested in the -9^-LVanable Density Wind Tunnel from I to IS. 32 atmospheres. resistance departs appreciably from the Law of the Square with variation of Reynolds' number. " When we come to such objects as an airplane, however, we have difficulty with the ordinary wind tunnel. For constant Reynolds' number to test a model, say one-twentieth scale, would require wind tunnel speed twenty times the actual flying speed, and there are no wind tunnels that can come in sight of this performance. Such speeds would be greater than the velocity of sound. There appears to be only one practicable solution of the difficulty, namely, the use of a testing tunnel where we vary the density of the air and hence the value of v. " The kinematic viscosity coefficient v for air varies in- versely as the pressure and decreases with temperature according to a somewhat complicated relation. Table I below gives numerical values when the unit of length is the centimetre and the unit of time the second. Temp. 50 40 30 20 10 0 — 10 — 20 — 30 — 40 — 50 Kinematic Cent. 1 1 1 1 1 1 1 1 1 1 1 1/10 •284 •292 •300 •308 •318 •329 •340 •351 •364 •378 •392 TABLE I. Viscosity <Z oefneients v in cm2ysec Pressure in atmospheres. 1 0-12840-1292 0-1300 0-1308 0-1318 0-1329 0 1340 0-1351 0 1364 0-1378 0-1392 5. 0-025680-02584 0-02600 0-02616 0-02636 0-02658 0-02680 0•02702 0-02728 0-02756 0-02784 10 0 012840-01292 0-01300 0-01308 0-01318 0-01329 001340 0-01351 0-01364 0-01378 0 01392 20 0-006420-00646 0-00650 0-00654 0-00659 0-00665 0-00670 0-00676 0-00682 0-00689 0-00696 " The variable density wind tunnel of the National Advisory Committee for Aeronautics, as originally suggested by Dr. Munk of our staff, was described to the Society two years ago and a few sample results given. A good deal of experience has been had since then with the appliance. One lesson of experience has been that when we are working under a pressure of 20 atmospheres it takes but a small electrical spark to kindle a substantial fire. However, these little practical difficulties have been overcome, and experience in testing a 1.6- I Sy Test 103 -i-'ymbol Atm. q~ kg/m * Reynolds No ij- o I 00 286 I3S,C"~ '•* * 2.51 72.7 a 5. IQ 152.6 o 10.15 335.O - a 20.00 6300 85,000 . 461,000 950,000 2060.000 3,580.000, ,24 28 Lifi and drag characteristics os tested in the —J-—Variable Density Wind Tunnel from I to 20.0 atmospheres 278
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
