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Aviation History
1949
1949 - 1471.PDF
AUGUST 25TH, 1949 FLIGHT 235 members of the air frame are most heavily loaded during the performance of given manoeuvres, whilst other members are most severely stressed during entirely different con- ditions. As it is virtually impossible to consider all the manoeuvres to which an aircraft may be subjected, it is usual in the design of civil aircraft to consider only eleven. The following list of these eleven conditions is not, however, exhaustive ; the full design requirements as published by the Air Ministry contain several sub-divisions as well as many subsidiary requirements. The ultimate load factors are based on measurements made with an accelerometer in the aircraft, but are also dependent to a great extent on design experience. When referring in general terms to the load factors to which an aircraft has been designed, the highest ultimate load factor is implied—in this case, 9. The higher the load factor, the heavier and stronger the aircraft and, consequently, the less the useful load. This being the case, it would patently be uneconomic to make the load factor unnecessarily high : the usual figure for heavy transport aircraft is 5, but such machines are, of course, not intended to perform aerobatics. The Eleven Conditions Centre of pressure forward : ultimate load factor = g. This manoeuvre corresponds to that of an aircraft travelling at top speed and suddenly assuming the stalled position without appreciable loss of speed. The centre of pressure would thence be in the forward position. This would occur if an aircraft diving vertically at its terminal velocity were pulled out of the dive very suddenly. Centre of pressure back : u.l.f. = 6. In this case, it is assumed that the aircraft diving at its terminal velocity is pulled out more gradually than before and assumes its normal flying attitude without appreciable loss of speed. In this attitude, the centre of pressure will be about half-way along the chord ; it is, of course, not such a severe man- oeuvre as the foregoing and therefore the load factor can be lower. Up gusts and down gusts : u.l.f. = 2. It is assumed that the aircraft flying at top speed in a horizontal attitude encounters a gust with a vertical component of 25ft/sec. Fast glide : u.l.f. = 2. The aircraft is considered to be gliding at i| times its top speed with engine off. Landing : tail up (u.l.f. = 1.15) ; tail down (u.l.f. = 4) : one wheel (u.l.f. = 3) : braked undercarriage (u.l.f = 4). These conditions are introduced primarily to cover the requirements of the landing gear but, in addition. necessarily affect the fuselage and wings of the aircraft. Diving: u.l.f. =2. . The aircraft is assumed to be diving at its terminal velocity, but not greater than 450 m.p.h. Inverted flight : u.l.f. = 4.5. It is assumed that the angle of attack of the aircraft when upside down is suddenly increased so that the centre of pressure moves to the forward position. Once more considering a bracing wire of circular cross- section, three typical conditions of flight can be taken for a working example. It may be assumed that it is required to determine the diameter of the wire to meet the most extreme flight condition, i.e., c.p. forward = 900 lb ; ultimate load factor = 9. The ultimate tensile strength is assumed to be 50 tons/sq in, and the 0.1 per cent proof stress to be 35 tons/sq in. It is known that the maximum design load is 8,100 lb (900 x 9) and it must then be decided whether to design on the ultimate tensile strength or the proof stress, and as I of 50 tons/sq in = 37.5 tons/ sq in, and the proof stress is less than this (35 tons/sq in), it is clear that the wire must be designed on the latter. Proof load factor = § of 9 = 6.75 Area of wire basic load x proof load factorproof stress 900 x 6.75= — sq in 2240 X 35 . = 0.0775 sq in •"• — = 0.0775 4 • - d2 = 0.0775 x - = 0.00S.7 ' : IT d = v 0.09S7 = 0.3142m diameter Whilst high speeds in themselves impose no strain on the human body, the average pilot cannot withstand a manoeuvre in which the aircraft is stressed to more than about 4 or $g, although cases are not unknown of pilots who can cope with -jg before blacking out. The limitations of the average pilot necessarily affect the design of the aircraft, as there is obviously no advantage to be gained in building an aircraft capable of carrying out manoeuvres much more severe than a pilot can endure. If a pilot and aircraft are to have the same limitations, then a limit ol 5g for the pilot would mean designing the aircraft to an ultimate load factor of 10. ANTARCTIC "AIR LIFT" THE Governor of the Falkland Islands, Sir Miles Clifford,has ordered from Canada a Norseman aircraft, with floats and skis, for the rescue of the seven British explorers andscientists at Marguerite Bay, Graham Land, if the Antarctic supply ship John Biscoe fails to force a passage through thepack ice early next year. The aircraft will be shipped from Lcndon when the JohnRiscoe. sails this November on her annual rounds of British outposts in the Antarctic. She will also carry two Mk 5 Austeraircraft equipped with floats. The men at Marguerite Bay have had no mail since the JohnBiscoc called there in February, 1948. The earliest the supply- ship can hope to force the ice pack is February, 1950, and inall probability the Norseman will be used to fly Christmas mail to the Base. Sir Miles Clifford, who is responsible for all British outpostsin that section of the Antarctic administered by the Falkland Islands (on behalf of the Colonial Office) is taking no chanceson the ship failing to force the ice-pack two years running. If she fails, and if ice-conditions permit, the Norseman will beused to evacuate the Marguerite Bay base. There are vast areas of smooth ice between the islands offthe coast of Graham Land, and they offer ideal take-off con- ditions. Even after the spring break-up in November thereare large areas of shelf-ice—ice which has not broken away from the coast. Sometimes, however, the shelf-ice at Mar-guerite Bay breaks away during the spring break-up, and this would present the Norseman with impossible landing and take-<>tf conditions. It is possible that the aircraft will operate from Hope Bay B 29 • (on the north-west coast of Graham Land) which was evacu-ated earlier this year following a disastrous fire at the base hut. If the conditions of the, ice prevent the Norseman fromlanding at Marguerite Bay to evacuate explorers' personnel, it will be used to parachute supplies to them—supplies ofwhich, by then, they will probably be in great need. FORTHCOMING EVENTS Aug. I7th-27th. Model Engineer Exhibition, New Horticultural Hall, London, S.W.I. Aug 21st co 28th.—British Gliding Association : National Gilding Con- tests, Gt. Hucklow, Derbyshire. Aug. 28th.—Cowes Aero Club : Air Display. Sept. 3rd. Association of British Light Aero Club and Centres : Summer Conference, White Waltham. Sept. 6th to 8th.—R.A.F. Golfing Society : Autumn Meeting and Ladies' Section Championship at Walton Heath Golf Club. Sept. 7th to llth.—S.B.A.C. Annual Flying Display and Exhibition, Farn- borough. Sept. 7th.—R.Ae.S. (Luton) : " Air Liner Operation." Sept 12th to 18th.—Battle of Britain Week R.A.F. " At Home " day Sept. 17th. Sept 14th.—Inter-Services Swimming Championships, Seymour Hall, Marytebone, London. Sept. 15th.—R.Ae.S. ; Fifth British Commonwealth and Empire Lecture, " Inter-City Transport Development on the Common- wealth Routes," by E. H. Atkin, F.R.Ae.S. - _ Oct. 5th.—R.Ae.S. (Luton); Film Show. Nov. 2nd. R.Ae.S. (Luton) ; " Gas Turbine Development.' Nov. 23rd.—R.Ae.S. (Luton) ; " Brains Trust." Dec 7th.—R Ae.S. (Luton) : " Role of Aircraft in Future Warfare," Air Marshal Sir Robwt Soundby, K.B.E., C.B., M.C., D.F.C •'"•• AFC.
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