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Aviation History
1940
1940 - 2114.PDF
SUPPLEMENT TOFLIGHT 72 30 THE AIRCRAFT ENGINEER JULY 25, 1940 h - DROP HEIGHT. I - LENGTH OF AIR COLUMN (EXTENDED). V - RATIO OF VERTICAL MOVEMENT OF WHEEL TO MOVEMENT OF STRUT. b ASSUMED-0-5 WHERE b • LENGTH OF f ° AIR COLUMN (STATIC). K / W\O-9V SMAX. WHERE <5 MAX - MAX.*• -I p) * fT" DEFLECTION OF TYRE (ie. WHEN FUaY SQUASHED). P • DYNAMIC LOAD REQUIRED TO SQUASH TYRE. W • ACTUAL STATIC LOAD PER WHEEL. . ^ • c. . G t a RATIO- APL Fig. 2.—Shock absorber load factor curves. In the absenceof data, take P = W, where W = design static load for tyre (i.e. load required to give 33.33 per cent, static deflection).For military aircraft the tyre is usually selected to give W = 1.1W at 35 lb./sq. in. pressure. In the absence ofdata take 8 max. = 0.75 tyre width. Tyre Characteristics The " tyre efficiency " is defined in the same way as the " shock absorber efficiency " above. It can be taken at 47 per cent, (this and other tyre data taken from Dunlop curves). The " design static load " of the tyre is the static load required to produce 33J per cent, compression of the tyre, usually at 35 lb./in.2 pressure. Other important quantities are the maximum tyre de- flection (i.e. deflection when fully squashed) and the dynamic load required to produce this. In the absence of data this load can be taken at 3.6 times the design static load, and the full deflection at 0.75 of the tyre width. Let V = vertical landing velocity. V2 h — drop height = —28 W = static load per wheel (actual).n — ratio of maximum ground reaction per wheel to W. r = ratio of vertical movement of wheel to strut closure. / = length of air column in strut in extended position. s = actual strut stroke. sm = available strut stroke. TJS = shock absorber efficiency (assumed at 0.80). b = length of air column in strut in static position. p I air pressure in strut, in static extended and p j compressed positions respectively. P = dynamic load required to squash tyre. (Take P = 3.6 times design static load in absence of data). 8 max = max. tyre deflection (fully squashed), (take hmax = 0.75 x tyre width in absence of data), ijj = tyre efficiency (assumed at 0.47). r = I lor wheel mounted directly on vertical shock absorber. N.B. : Care must be taken to use consistent units. RATIO \ ASSUMED - % FOR KEY TO SYMBOLS, SEE FK3 2 Case i.—Strut closure known (i.e. actual stroke = avail- able stroke). Energy to be absorbed per wheel = W X h. Let E, be energy absorbed by tyre, then energy absorbed by strut = Wh — E, — rn\X X r;s x Sm WA —Et h — Et/Wn = If deflection and energy absorption curves for tyre are available (the latter can be constructed from the former) n can be found accurately by a process of successive approxi- mation, as follows : Assume a value for n, then load on tyre = nW, from which tyre deflection and E( can be found, giving a second value for n. The process is repeated until the value assumed and the value found agree. Once n is known the degree of inflation of the strut required to produce the nominal stroke can be found from the air compression laws (cf. next paragraph). Case 2.—Strut stroke unknown. The tyre deflection can be represented by the equation /wW\°-98 = 8 max x —— S = ACTUAL STROKE I - LENGTH OF AIR COLUMN I (EXTENDED) b- LENGTH OF AIR COLUMN (STATIC) CURVES REPRESENT LAW "I 7|V1'3 = CONSTANT I.e. ft = b/lHP 1•50 -60 70 -60 Fig. 4.—Shock absorber load factor curves. Energy absorbed by tyre : /W\0-9E, = 7j, X 5 X wW = 17, X S max X wl-*W X (— Energy absorbed by strut + max x W\°-< dividing through by W and substituting assumed values for 17, & 17, h = 0.8 rns + 0.47 «'•• x 8 max ( —) .. (1) RATIO Fig. 3.—Shock absorber load factor curves. When the strut is compressed it is instantaneously at a standstill and therefore the load due to the oil is zero. Hence the ratio of the air pressure in the compressed position to the static air pressure is equal to n, i.e. : _ P^ (This assumes that maximum reaction occurs ~~ Ps at end of stroke). The air is compressed according to an approximately adiabatic law, which will be assumed to be : pyi-3 = constant*. *See Jones, Aircraft Engineering, Vol. X, No. 113 (1938).
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