FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1920
1920 - 0603.PDF
JUNE 3, 1920 and it was possible for the members of the Advisory Com-mittee to keep all the facts in mind while devoting the majority of their time to other duties. They then providedthe necessary co-ordinating factor. This is no' longer possible, and the function could best be discharged by the staff ofthe school working under their director with a view to co- ordinating and making available all the knowledge in eachbranch of the work as existing at the moment. But I am wandering too far from the work of the AdvisoryCommittee and of its successor ; let me return to the latter. We have discussed somewhat .Cully the manner in which itmay promote education in aeronautics. Its main work, however, is to advise on scientific and technical problems,and to undertake or supervise research and experimental ' work in aeronautics. Let us consider its procedure an'i someof the more important, problems calling for solution. As in the past, the work will be carried on mainly through thesub-committees, and the legacies left by the Advisory Com- - mittee are by no means inconsiderable. Recently somegeneral questions relating to fluid motion have been under the consideration of the Advisory Committee. When fluidis flowing steadily along a tube of diameter d and, at a uniform •"•" mean speed v thence up to a definite value of the quantityvdjv where v is the coefficient of kinematic viscosity, the fluid is at rest along the walls of the tube, its velocity increasesas you move away from the walls according to a parabolic law and the friction between the fluid and the tube is pro-portional to v. As the' quantity vdjv increases beyond the critical value, this law breaks down, and ultimately thefriction reaches a value approximately proportionate to v*. It is of importance to know what exactly is the state of themotion at the surface. Is the fluid there still at rest, or does the laminas motion still persist close to the boundary ? Dr. Stanton has recently made some experiments on theflow close to the wall in tubes of 0.269, 0.714 and 12.7 cm. in diameter, and for values of vd/v of 460 to 325,000. Themeasurements were made with a special Pitot tube, one side of which was the wall of the tube, while the other was a kindof small lip 0-05 mm. in thickness, which could be screwed outwards from the wall. By this means it was possibleto make measurements of the friction on the walls and the velocity of flow for very small openings of the Pitot tube.In one series of measurements with the 12-7 cm. pipe the position of the centre of the Pitot tube ranged from 0-013 mm.to o • 178 mm. From the results it appears certain that there is always between the limits of vdfy indicated a very thinlayer along the walls of the tube which is in a state of laminas flow. The velocity is zero over the walls, and the friction isgiven by the limit of the quantity pdv/dx when x is zero, x being measured at right angles to the surface. This resultis one of the very greatest importance in the theory of fluid motion applied to aeronautics, establishing as it does theconditions which must be assumed to hold at the surface of aircraft. Along with this, perhaps, should be classed asfundamental for aerodynamical theory some recent work of Messrs. Cowley and Levy and some investigations on whichit is understood that Professor Bairstow is engaged on the equations of motion of a viscous fluid. The theory of the airscrew offers a wide field for investiga-tion. Recent experiments of Mr. Fage have shown that the well-known Froude momentum- theory when supplementedby the " inflow " effect gives a very good general account of the behaviour of an airscrew, and that the aerofoil theorywhen corrected for the interference of the blades in accordance with some very ingenious experimental work at the R.A.E.enables the form of a propeller to be designed with fair accuracy. But before a further advance can be made itis necessary to know more about the distribution of the air pressure over the blades. Experiments to determine thisin the case of a model airscrew have been made by Mr. Fage, and the results will be before the Committee at an earlydate ; there is little doubt that they will permit a further step in theory of great value. Meanwhile at the R.A.E.the apparatus for plotting the pressure over an airscrew in flight are well advanced, as also is the apparatus formeasuring the thrust of the screw under the same conditions. More recently still a programme of tests on a family of pro-pellers of varying pitch diameter values and varying .aspect values has been laid down by a panel of the AerodynamicsCommittee appointed for the purpose, and it has been arranged to give this programme precedence over all other airscrewtests in one of the 7-ft. channels at the N.P.L., while experi- ments on a reversible propeller are in progress Amongother airscrew questions is the effect of the high lift speed which is now reached. With various engines such experimentsas have been made have shown that with a stationary air- screw as the top speed reaches the velocity of sound thecharacter of the air flow round the propeller entirely alters ; the slip stream disappears ; the air appears to be drawn inat the centre and driven radially outward at the tips of the blades. Probably the thrust had almost entirely disappeared,and the propeller became very inefficient. In wind channel work perhaps the most interesting in-vestigation in the immediate future will be a series of com- parative tests which is being arranged between the nationalchannels in this country, America, France and Italy. Model aerofoils of standard section will be tested in all the channels,and it is hoped to include a test on at least one complete model. Details are now under discussion between therepresentatives of the various channels. M. Toussaint has made the very valuable suggestion that among the modelsshould be at least one which has already been tested on the aerodrome track at St. Cyr. Methods for facilitating or improving the accuracy ofchannel work are continually under discussion, and in con- nection with the demand for a full investigation into thestability of any new type of aeroplane methods for measuring the rotary derivatives have become increasingly important.Here again is ample room for research and investigation. Some further experience will show how, given the valuesof the coefficients, the necessary calculations can be best effected. Recent papers by Miss Cave-Browne-Cave and byMr. Relf have thrown much light on this somewhat intricate matter. The importance of stability is generally recognised,and the researches now in progress as to the stability charac- teristics of many of the best-known types of aircraft arefull of interest. Another matter calling for attention is the investigation ofthe aerodynamic properties of special forms of wings, especially high lift wings. A number of interesting results have beenattained. It remains to compare these, and possibly to extend them, in directions which may offer promise of advance.Aerofoils suitable for airscrew design also offer a field ol useful investigation. A problem to which the A.C.A. have recently devotedconsiderable attention is that of the prevention of fire on aircraft. It appears from the records of the AccidentsDepartment that fires in the air are very rare ; five were recorded in a period prior to December 31, 1918, during whichover ^oojooo hours were flown, while in the next six months the figures were four and 36,000 hours. The fires on crashvary greatly with the type of the machine, ranging from one in 35 crashes in one type to one in 4 4 crashes in another.A striking difference between rotary and stationary engined types of machines was noted, the latter firing four timesas often as the other. Investigation showed that with rotary engines a fire-resisting bulkhead had almost alwaysbeen inserted between the engine and the rest of the machine. This led the Committee dealing with the matter to recommendthe insertion of such a bulkhead as a general rule. They have made other preliminary recommendations, and are con-tinuing experiments with a view to improved safety. One obvious precaution is the use of a fuel having a much higherflash point than petrol. Thus there would be less tendency to catch fire through the tank bursting on crash and the fuelsplashing over the engine or otherwise coming in contact with something capable of igniting it. A number of suchfuels have been prepared and tested. They are all somewhat heavier than petrol and less efficient for equal weights, butthe experiments are still in progress. The question how fires occur on crash is one of difficulty.The tank usually bursts, and the petrol is splashed about. Sparks from the magneto may be a possible cause, andaccordingly steps have been taken to secure that the high tension system should be reasonably safe and the magnetofireproof. Again, the liability to burst differs greatly in various types of tanks, and recent experiments have showna reasonable probability of designing a tank which will not be unduly heavy in proportion to its contents, and yet willremain intact when a crash occurs. A still other line of anquiry has been the attempt to providea system of jets through which some fire-quenching liquid could be sprayed did a fire occur, and this, though notcompleted, has met with a fair amount of success. With a view to showing how effect could be given to the suggestionsof the Committee an aeroplane at Farnborough has been modified so as to be practically immune from many of therisks common to other aircraft. The importance of all this to the safety of future passengers is very obvious, and thereis ample scope for further work. When dealing with fuel and engines, note should be taken ofthe experiments on engines bearing other fuels than petrol. 603
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
