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Aviation History
1927
1927 - 0975.PDF
DECEMBER 29, 1927 79 THE AIRCRAFT ENGINEER SVFFLSMBNT TO FLIGHT from less than 0-02 to 0-7. In view of landings on steel •decks, the coefficients of friction between rubber and steel surfaces are of interest. Upon a wetted steel surface the coefficient is very low, and of the order of 0-1, but the deck •of an aircraft carrier is not perfectly smooth, and is slightly roughened by the application of a cement coating. There are no actual figures available for the coefficient of friction on such a surface, but they will be appreciably higher and approximately of the order of 0-7. Unfortunately there is, to the writer's knowledge, no reliable data concerning the coefficient of friction between aero tyres and landing fields, but from values deduced from actual landing tests, the coefficient appears to be quite small, particularly so on wet surfaces. The only information obtainable refers to tests on automo- bile tyres and we can assume that, for aero tyres with smooth treads, these values will be reduced. From comparison of tyres with smooth and patterned treads, it appears that the coefficient of the former is about 0-8 of the patterned (non- skid) type. The maximum coefficient for aircraft tyres probably lies in the neighbourhood of 0-5 with a mean value under normal conditions of 0-25. From previous considerations of the maximum braking permissible, it will be seen that even in the unlikely event of the wheels becoming locked, a tail-down landing can be made with absolute safety under the worst conditions and a tail-up landing under normal conditions. It has been definitely proved, by actual tests, that wheel brakes are perfectly safe in operation. Throughout the whole of the preceding work, no reference has been made to aerodynamic resistance. This drag will have the effect of permitting on iiicrease in the brake load, with a consequent reduction in length of run. At speeds below 40 ni.p.h. the drag falls off very rapidly and its effect is small compared with the braking produced by the wheels. It has been thought desirable to ignore this drag in view of the additional complications that would be involved by its inclusion. The magnitude of the braking load is only of interest in so far as the actual brake design and length of run is concerned. The maximum brake load obtainable is equal to the weight borne by the wheels and multiplied by the coefficient of friction l>etween the wheels and landing ground. The weight borne by the wheels has to be determined by subtracting from the total machine weisrht. that part carried by the air at any instant plus that part carried by the tail skid. The actual process for determining the weight borne by the air, is quite simple, but the lift experienced will be subject to large variations depending on the attitude of the machine during landing. The extreme cases will be represented in a tail-up and a tail-dovvn landing. Typical values for the lift, in terms of total machine weight, are given in Figs. 6 and 7. When the machine is in any intermediate position, the lift will range between these two sets of values. The results given in the above figures have been obtained from analysis of a particular machine and will serve to indicate the order of loads that may be expected. The excess of weight over lift represents the wheel load and this has been plotted on Figs. I) and 7. Tail skid loads, being of a small order, have been ignored in the tail-down case. Wheel loads have been obtained on the assumption that the machine has been rolling over a smooth surface and not subject to inertia loading. During the initial stages of alighting, wheel loads will be of a higher order, depending essentially on the vertical velocity of the aeroplane and the vertical travel of the wheel. ]f we consider the total weight of the aircraft to be air borne, then the wheel load (F/W) can be determined from : V2 0-0155 V2F ^y ==2ff T " T where T is the vertical wheel travel in feet. If none of the machine's weight is air borne, then F (-0155 V2) given by these two methods, depending on the proportion of the total weight carried by the air. In the case of shipboard landings where the deck is free from obstructions, the order of wheel loads during run to pull up. should approximate more nearly to those values given on Figs. 6 and 7, while on turf or macadam surfaces, the increase in loads will depend on the roughness of the surface and the forward speed of the machine. Some authorities assert that, on average landing grounds, loads up to three times the static load can be experienced during taxi-ing, but with well- designed shock-absorbing units such as present-day machines are normally equipped, the maximum load should not exceed twice the static load and with an average value of 1 • 1. These figures are confirmed by N.A.C.A. Report No. 249, which shows that under normal landing conditions the maximum load does not exceed 2G with a mean value, during run to pull up, of 1G. These results have been obtained by use of the N.A.C.A. accelerometer and the graphical record of the load alternations is of considerable interest. The reduction in length of run obtained by the use of wheel brakes cannot be treated in a simple manner. The actual process of calculating the run to pull up, is one of considerable complexity and with many variables entering into the problem. The experience and judgment of the pilot plays a major part in the length of run taken and since this human factor cannot be calculated, it is proposed to consider the retardation of a machine in as simple a manner as possible. The writer believes that the rather large assumptions made, will possibly provide no greater error than those involved in the more complex investigations. For general comparative purposes we can take the data given in N.A.C.A. Report No. 249. This gives particulars of length of run and landing speed for nine different types. These figures are given in Table 1. Columns 1. 2, 3 and 4 are self-explanatory and in Column 5 the average overall retarding coefficient has been determined. Table I. COL. l Machine. S.E.SaJ.N.6H. Curtiss Spad VIIVE.7 Vought I)H. 41)Co.4 Fokker ... Sperry MessengerM.B.3 Thos. Morse M.B.2 Martin Bom COL. 2 K"H4 1- 54 51 58 51 56.556 4457 58 COL. 3 a c • cC 450 575 485 800725 950 400 873 925 COL. 4 P"1 2900 •-'600 3360 2600 3200 3140 1940 3^150 3360 COL. 5 > .033 4 H .215 . 151 .232 .109 .147 .11.162 .12422 COL 6 .055 .047.072 .047.064 .062 .035 .068 .072 COL. 7 CM _,p* •<* CO o pf .355 .348 .372 .348 .364 .362 . 335 .368 .372 COL. 8 272 250 300 250 294 290 193 295 302 COL. 9 mi n •un . ~If 39.6 56.5 38.2 69. 0 59. 5 69. 5 52.0 66.4 65.0 The values given by the latter equation have been plotted on Fig. 8. The actual wheel loads will vary between the figures From analysis of several types of machines, in tail-down attitude, mean aerodynamic drag coefficients have been calculated for various initial landing speeds. The results are given on Fig. 9. The average aerodynamic drag coefficients taken from this figure are given in column 6. In order that we can forecast the diminution in length of run. by the application of wheel brakes, we can assume the nominal value of 0-3 as representing the coefficient of friction between the wheels (with tail skid) and ground. The total overall retardation coefficient is given in column 7 and the new length of run in column 8. The percentage reduction in lensth of run. due to the fitment of wheel brakes, is estimated, under this method, as 57-3. The benefit to be derived from wheel braking is fully demonstrated and the advantages are sufficiently great for efficient braking to be regarded as one of the essential qualities in aeroplane performance. Reports of the Boeing Air Mail machines say that action of the wheel brakes is astounding and the steering qualities so remarkable that a pilot is able to negotiate his Way between obstructions with the ease of a motor-car. Besides the advantages to be gained from quickness of pull up, many accidents occur through poor controllability on the ground, 880c
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