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Aviation History
1949
1949 - 1834.PDF
598 FLIGHT, 3 November 1949 GAS-TURBINE FUEL SYSTEMS . . . in the well-known bob-weight governor. The effect of this rate is, in most cases, to cause an increase in the controlled parameter with altitude. Some form of correc- tion is therefore desirable, and this may be (a) a deliberate counter-error scheduled into the system; (b) a direct com- pensation related to fuel flow; or (c) a complete rough- scheduling system overridden by the direct control If the second of these is used, a time-lag must be introduced to maintain the instantaneous rate, for stability. The com- plete rough-scheduling system has the advantages that it limits the duty of the direct system, and also provides a separate system for use in emergency. It does, however, considerably enlarge the fuel system. Passing on to our third objective, we can list the main conditions to be avoided as: excessive r.p.m.; excessive flame temperature; and extremely low idling speed. The maximum r.p.m. is limited by a top-speed governor, one form of which is shown in the Lucas pump (Fig. 5) where a centrifugal pressure difference proportional to (r.p.m.)2, derived from radial drillings in the rotor, is applied to a spring-loaded diaphragm. A tappet on the diaphragm lifts a half-ball valve in the pump servo system, thereby limiting the speed to that required to move the diaphragm. The scheduling system used with this pump on British engines provides a correction on the rate. Top-speed governing, indirectly, gives a measure of flame-temperature limitation. If required, a closer limit can be imposed by direct control. Owing to the high flame-temperatures, it is a convenient approximation to control jet-pipe temperature, which is lower, but related. This has been done by use of an electrical bridge system incorporating a thermocouple in one arm, the out-of-balance signal being employed to operate a variable in the fuel supply. Mechanical systems have also been devised, using differential expansion as a measure of temperature. What has been said about rate correction and stability on r.p.m. control applies in the main to temperature governing. The necessity for temperature control is debat- able, since much can be done by r.p.m. scheduling to keep within temperature limits. Moreover, the temperature distribution across the airflow in any gas turbine is never absolutely uniform, so that some degree of Compromise is necessary in any case. British units in service do not at present employ temperature controls, responsibility being imposed on the pilot not to exceed a given limit at the particular conditions when the scheduling might make it possible- The dangers of too low an idling speed, particularly at altitude, are flame-extinction, overheating, and difficulty in pulling away. At extreme altitudes the range of air-to- fuel ratio consistent with combustion at low air-flows is reduced. This implies a limit on minimum idling speed if sufficient overfuelling for reasonable acceleration is to be possible without overheating or flame extinction due to over-richness. There also exists a minimum steady r.p.m. at which the unit will run. If speed falls below this, the r.p.m. will slowly fade out and gas temperatures will rise rapidly. All these ill-effects can be avoided by imposing a limit on mini- mum fuel-flow. Such a limit may be inherent in the control system, but this is unlikely. A simple idling con- trol is the Lucas minimum burner-pressure control seen in Fig. 5. Fuel is supplied from pump delivery into a capsule chamber. The capsule controls a half-ball valve, throttling the inlet, to maintain the chamber pressure constant. A reducing non-return valve between the chamber and the burner pressure line ensures a minimum burner pressure and consequently a minimum idling flow. [In the preceding article on this subject the root quantity ofequation IV included a multiplication sign instead of a plus sign. It should have read F= V -1140 + 760?,. Also, theauthor points out, in the second paragraph of col. 2, p. 513, he should have used the words "system flow number" in-stead of "throttle flow number."—ED.] AMERICA'S OFFICIAL HISTORY-II "The Army Air Forces in World War II." Volume two:"Europe—Torch to Pointblank." Published by the Univer- sity of Chicago Press ($6.00), and in Britain by the Cam-bridge University Press (48s). 897 pp. WHEN we reviewed Vol. I of this official history of theU.S.A.A.F. at war (Flight, May 26th, 1949), we com- mended its honesty and absence of bunk. Vol. II, which carries the story of the Air Force's part in the war jn Europe up to the end of 1943, enhances that opinion. In fact, the Editors' determination to tell '' the truth, the whole truth and nothing but'' has gained for this volume an unfortunate notoriety, for it contains revised figures for the number of German fighters shot down by American air gunners in their early battles with the Luftwaffe which prove that '' Eighth Air Force claims were far more exaggerated than even their severest critics had assumed." They show, for example, that in the October, 1942, raid on Lille, two enemy fighters were destroyed, not 102 as originally claimed. The revised figures have been published, unfortunately, by the Press on both sides of the Atlantic, without any attempt to explain the impossibility of accurately assessing victories during combat. The Editors of this history are guilty of no such neglect. They point out that " It was hard for crews in a large formation to determine which bomber had been respon- sible for a destroyed German plane, so that each gunner who had fired at the enemy fighter from a reasonable range was likely to claim it." They add, also, that "the promised award of a decoration for his first kill did little to dissuade him from the not unnatural belief that it had been his bullet which had scored. This over-estimation of German losses was undoubtedly serious: it might easily have been fatal, for it was realized by. October, 1943, that, while the cost of the American daylight offensive was proving almost disastrous, its bombing suc- cesses remained problematical. The deciding factor- for con- tinuing the daylight offensive was the supposed decimation of the Luftwaffe by American gunners, as this was a vital pre- requisite for the invasion of Europe. That the offensive did finally succeed, as Vol. Ill will recount, was due to the gal- lantry and determination of American aircrews, who smashed through to their targets in the face of staggering losses im- posed by the Luftwaffe which, in fact, grew stronger daily. That is the real story, not the fact that those same crews claimed more aircraft than they actually shot down. Volume II also describes in detail U.S.A.A.F. participation in the campaigns in North Africa, Sicily and Italy, and in the grim, unending war against the U-boats in the Battle of the Atlantic. But it is in the story of the gradual build-up of the Eighth Air Force in Britain that we can best see the U.S.A.A.F. growing in stature, to such effect that it would eventually demand its independence as an equal partner of the U.S. Navy and Army. This struggle for independence was as relentless as the war against the Luftwafie, for the Navy in particular deeply resented the growing importance of the Air Force, especially its participation in the war at sea. That resentment showed itself during the invasion of Sicily, when the Navy complained that "close support by aircraft, as understood by the Navy, did not exist." In fact, instead of "the Navy's anticipated loss of up to 300 ships, there stood the actual loss to enemy air action of only twelve vessels." The British Admiralty commented that the small losses were "the result of a very high degree of air superiority." J. W. R. T. AIR FRANCE'S LOSS WIILST making what would have been the 1,973rd crossingof the Atlantic by Air France since the route was opened for regular services on July 1st, 1946, one of the company's Constellations crashed in the Azores on October 28th. There were no survivors amongst 48 passengers and crew. The regular Air France service between Paris and New York stops at the Azores when weather favours the southern route and in this case the Constellation took off from Orly at 2000 G.M.T. on October 27th and sent a message at 0229 G.M.T. that it was approaching to land at Santa Maria. That was the last message received and the aircraft hit the ground near the top of Alvargia Peak (3,600ft), on the island of San Miguel. The pilot. Chief Pilot Jean de la None, had mide 88 crossings of the Atlantic as captain. D 16
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