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Aviation History
1927
1927 - 0974.PDF
TOFLIGHT 78 THE AIRCRAFT ENGINEER DECEMBER 29, 1927 Let /W = Load borne by wheels. i?i = Distance of wheel ahead of C.G. R2 = Distance of C.G. above ground. fi = Coefficient friction between tyre and ground. The maximum brake load is given by /a/W, and conse- quently, upon application of the brake load, there is a pitching couple of value ^/\VR2, and a counteracting couple of fW Bi. In order that the aeroplane may not nose over The limiting value of M is therefore :— fx = tan a. This angle is usually between 12° and 13°, on normal undercarriages, but where wheel brakes are fitted American practice is to increase this angle to 17°. These statements are borne out by actual tests,* which have shown that, with a well-proportioned undercarriage, the wheels may be locked and the machine landed without difficulty. Coefficient of Friction. In the whole question of brake design there is, perhaps,, no more arbitrary point than the coefficient of friction between tyre and ground. This is unfortunate because it is the datum line from which brake design starts. The coefficient varies between wide limits depending upon the nature and conditions of the surface, inflation pressure of the tyres and type of tread. If the tyres are very soft and the surface uneven, then it has been found that coefficients of friction in excess of unity can be obtained, due to inter- locking of the tyre with the interstices of the ground. For any given tyre, increase of inflation pressure produces a slijrht DISTRIBUTED LOAD ON WHEELS & TAIL SKID COMD/TWN OF LAND/AIG TAIL DO>VA/ v VELOCITY WHEEL LOADS v WHEEL TRAVELASSUMPTIONS I) MACMW5 FALL/MS F/fS£ir/''VOr/l//fSO/f/¥£j (Z) CWFO/f/V / (S/STAHCf THROUGHOUT TRAVEL VERTICAL TRAVEL WHEEL (INS) 10 20 30 VELOCITY (M.P.H.) 50 LOAD ON WHEELS v. VELOCITY COHO/r/O/V Of LAND/N6 TA/i Uff : ' i ! | FIP 71 Ib. / -. , , , ..... —•—r*T^* I i ! t | ^^ i • ao 30 VELOCITY (M.RH.) 40 50 •07 ^ 06 05 AVERAGE MEAN AERODYNAMIC DRAG v LANDING SPEED 10 20 30 40 50 INITIAL LANDING SPEED (MPM) In order that the value of braking may be appreciated, the tangents of angles 12° to 1 * 12° 13° tan a 0-212 0-230 are given below. 14° 15° 16° 17° 0-249 0-267 0-286 0-305 A disadvantage may be found in placing the wheel forward, in view of increased tail loads, but 17° has been found to be a sound compromise, permitting of excellent braking. In a tail-do «vn landing angle a will be increased by angle p. Angle & is usually about 12°, and the braking can be con- siderably increased. a -f /8 24° 25° 26° 27° 28° 29° 30° tan(a-H8) 0-445 0-466 0-487 0-509 0-532 0-554 0-577 In the cage of a machine with wheels disposed such that angle a is 17°, then the wheels may be locked and a safe tail- up landing made, providing the coefficient of friction between tyre and ground does not exceed 0-3. If the more usual three-point landing is made then, under similar conditions, the coefficient can reach 0 • 5 with safety. improvement on the braking effect, since the area of contact is reduced and the pressure per unit area increased. In a paper dealing with four-wheel brakes and read before the Inst. Auto. Eng., Mr. F. A. Stepney Acres has shown that over a series of tests on road surfaces, the coefficient varies between 0-46 and 1 -3, with an average value of 0-7. Enquiries from the Dunlop Rubber Co. produced the following information :— Coefficient Friction between Tyres and Various Surfaczs. Surface Condition n Authority Granite setts ... Greasy 0-2 Dunlop Rubber Co. Macadam ... ... Dry 0-7 Do. Steel (smooth) ...Wet 0-1 Do. Steel (smooth) ...Dry 0-6 Do. It will be noticed that the coefficient of friction between a rubber tread and ordinary types of road surface may vary * " Airplane Brakes," by Weaver. " .Slipstream," Nov., 1927. 8806
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