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Aviation History
1943
1943 - 1692.PDF
FLIGHT Notes from a Test Pilot's Diary—No. 11 JULY IST, 1943 Oil Pressure Why Test'bed Readings are Higher : Effect of Altitude By "WITNESS" ea feed, iametei many air-1 especially. HOW much simpler was the problem of oil pressurein the days before flights above 20,000ft. werecommon, and oil-flows through engines could be measured in pints rather than in thousands of gallons per hour. Thanks to the installation research which has been done in this country, particularly by the "Back-room Boys" of the R.A.F. in the Engine Flight at Farnborough and by the Rolls-Royce Flight Installation Department, enough is now known about what happens in oil systems at altitude to enable trouble-free layouts to be designed into new air- craft, rather than to modify, by trial and error, an installation after it has already been completed. Obviously, in these notes it would be improper to divulge publicly all that is latest and best in the design of oil systems, but there can be no harm in indicating where that knowledge existed those genuinely interested wish to avail themselvesofft. On the other hand, one can state the altitude/gjt^pressure problem in general terms, and an understandin^of this is of some value in test-flying. There are sevej&f fundamental reasons why an engine, when installed in a"n aircraft, shoul give lower oil pressure for equal temperatures as compa with the test-bed. On the test-bed it is-cpmmon practice to' oil tanks of considerably greater opacity than those in aircraft, at any rate in nghters//so that the surj" from which air, emulsified in the m\ Moreover, the oil tank usuallyjpas a\gravit pressure pump, and pipe-lines than those fitted in aircraft an craft the suction pump has to under climbing conditions. Again, on the return oil line fftorn theyfcavenge pump to the tank, back-pressure is not usuaS^-80 high on the benc as is caused by the compact aircraft oil-cooler, which designed to make the oil give ujp its heat with a minyimm of frontal area. Thus there is sufficient differenceJ^Rween bench and aircraft to account for n«yer pressure^^ading in the la.ter, even at ground level. A>fedud*(fnot about 5 to 10 1b. is a normal result. Frothing Effect In the air more changes occur which nearly all tend to accentuate the pressure drop. The most simple effect of altitude is that if the oil tank is vented to atmosphere, the reduced pressure of the air will lower the working efficiency of the oil pressure pump. Another important result is the increased frothing tendency of hot oil when tho air pressure does less and less to hold in the emulsified air. It is one thing for an oil pump to handle "solid oil" even when high temperature renders it very thin, but imagine the difficulty of trying to suck and maintain pressure with oil worked up into a froth of the same sort of consistency as the top-dressing of a pint of beer. Oil is a fluid and for practical purposes incompressible, but air is spongy stuff and thoroughly demoralising to any oil system. Once air gets the upper hand we suffer the unhappy results which most test pilots experience at one time or another—heavy breathing, hunting of the air-screw, loss of oil pressure and, in extreme cases, emptying of the oil tank accompanied by the airscrew flying into fully fine pitch. Air is bound to be pumped into the oil tank because, owing to the varying flight altitudes of the engine, the capacity of the scavenge pump or pumps must exceed that of the pressure pump by a substantial margin. It follows, therefore, that for every gallon of oil which is pumped into the engine, there is extraction capacity for, sav, a gallon and a half or two gallons, and the deficiency of oil must be made good by air. In this way a considerable volume of air must always be carried along in the scavenge line through the oil cooler to the tank. Air is, incidentally, a bad conductor and its presence tends to lower the rate of heat flow both into and out of the oil. Aeration Causes Aeration to the point of lowering the oil pressure exces- sively at height can have either of two main origins. Aera- tion in the engine may tend to occur especially at high oil temperature if the pumps are, through any reason such as wear or poor assembly, down in capacity. Some engine designs allow sufiiejent capacity in their pumps when the clearances are smau and before much wear has taken place, but there is notrenough margin to take care of the reduced effectiveness oinihe pump after some hundreds of hours run- ning?" Trfc»L/s one important point which recent flying experience^as proved: the oil pumps must have a suffi- cient exeats of\ capacity to be able still to cope with alti- tude aft^ allowing for fair wear and tear. n in the oil tank, the second possible source, is he product of sticking to certain fundamental rules nk dejfign. The "hot-pot" or partial circulating r is u&kfftunately the world's worst form of con- for de/aerating. The surface area on the top of the the "ifiot-pot" is small, whereas the largest possible ace frjjta which the air can be driven off is desirable. reove/T there is a. very rapid flow of oil into the " hot- *f the top and out again at the bottom, and a ten- encv^to form a whirlpool, like the last part of a bath un/fing out, may give air a chance to enter the suction hi quantity. With oil tanks, much progress has been made both by building de-aerators into the tank and incorporating some form of air sepa/Stor in the scavenge line. Conditions have also been assisted by pressurising the tank, so that instead of being vented to atmosphere, a valve is incorporated which allows the scavenge pump to build up a pre-deter- mined pressure in the tank before blowing off. For mili- tary purposes, pressurised tanks are obviously undesirable, and it is always the aim, when pressurisation is inevitable, to keep the pressure figure as low as practicable. These remarks would be incomplete without a warning concerning the two remedies which are most commonly applied after the pilot has reported lo\j**oil pressure. The first'is to screw up the pressure reteJfse valve. If this is done, the initial pressure of the oil will undoubtedly be raised, but once the oil has been warmed the pressure will fall to such a value that the relief valve closes, and no advantage will be found over the range of pressures below that. This is. however, subject to the qualification that some designs of pressure relief valves Behave in a peculiar way (more like viscosity than blow-off valves) with-aerated Oil and it is a matter of trial and error whether scre-wing^fp the valve will, or will not, show the slightest improjjftnent on any given engine. The other method sometimes adopted in the.hope of restoring pressure is" to make a rigid inspection of the suc- tion line for possible air-leaks at the joints. This may sound quite logical, especially as one well-known fighter has no less than twelve joints between the oil tank and the engine, but my own experience after many attempts to cure such imaginary leaks, is that this is generally a waste of time. The exceptions are, of course, when there tire outward signs of leaks or if a rapid loss of oil pressure occurs high up, accentuated by cooling of the oil. In the case of aB^Sir leak, unlike ali other pressure troubles, a lower oil temperature and more viscous oil will tend to break down the oil supply to the pressure pump.
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