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Aviation History
1944
1944 - 0765.PDF
APRIL 13TH, 1944 FLIGHT 399 v CORRESPONDENCE The Editor does not hold himself •.esponsible for the views expressed by correspondents. The names and addresses of the writers not necessarily for publication, must in all cases accompany letters. THE NAPIER SABRE II Too Complicated and Vulnerable YOUR very excellent review of the Napier Sabre II aircraft engine calls for congratulations to the draughtsman who made those magnificent drawings showing every detail so very clearly. The drawing office gets very^ little publicity "or praise' either in war or peace, and yet it is from that source that modern wars are won or lost to a large extent, i" might add that the work of a draughtsman is most gruelling and trying, bent oveT*a drawing-board day in and day out, and often working with very trying artificial light in these black-out days. As regards the actual design of the Sabre II, it does not appeal to me in any sense as a war job. In the first place it seems to pie that it is far too complicated and has too many gears and working parts to be easily produced in large quan tities. In other words, it takes far too many man-hours to produce and all very delicate work to fine limits. J Again, its "percentage of vulnerability " from flak or bullets appears to be very high. This is a very important point in wartime. Then, of course, there are the chances of failure of any of these numerous working parts and their lubrication requirements. Napiers, having built under licence in peacetime, the two- crankshaft Jumo diesel aircraft engine, appear to me to have missed the mark in not developing that really perfect system as a two stroke petrol aircraft engine. The scavenger pump would in that case become the petrol vapour pump supplying the cylinders with the necessary mixture from a carburettor. We should then have an engine greatly simplified without valves and their attendant drive mechanism and also the entire elimination of cylinder heads and gaskets and holding- down holts. This represents a lot of weight saved. Having double the number of power strokes you need only have half the number of cylinders, again saving a colossal amount of weight. Having two pistons in one cylinder you would then only require one sparking plug to two pistons. So that instead of having 48 sparking plugs as shown in the Sabre II you would *, only require 12. Again a great saving in weight and mechanism. •• You would also only have six long cylinders against the 24 cylinders shown in the Sabre II to get the same power. J. R. GOULD (Major, late R.A.F.). Top Overhauls Very Simple MAY I express my appreciation of the article on the Napier Sabre II? (Flight, March 23rd). I consider that we should all take off our hats to Major Halford and team for this contribution to the war effort, although it was not designed in waitime. There was, I believe, one technical error in the article. The writer refers to fully floating gudgeon pins which are secured by circlips. Fully floating gudgeon pins are fully floating and, as a rule, have bronze or aluminium end pads. I kave seen cases where gudgeon pins have done more than float, because an end pad lias been left out during assembly and caused a beautiful score in a cylinder bore. [The gudgeon pins are fully floating and are retained by circlips in the bosses.—ED.] Xo doubt some readers will remember Major Halford at motor race meetings at Brooklands and elsewhere, with a modified car which he called the " Halford Special." I forget the original make of the car, but it was a good performer in its "hotted up ' state. I am afraid I cannot agree with your correspondent in Flight, March 30th, C. A. Glasshouse, who suggests that " Sabre " maintenance must always be a headache. To begin with, top overhauls must be very simple. Everything appears to be readily accessible, plugs, auxiliaries, etc. There are no tappet adjustments, etc., to be made, with their rather complicated rockers and camshaft, poppet valves and springs. These do give a certain amount of trouble, which sleeve valves eliminate. I am not trying to draw comparison with Rolls-Royce engines to which we owe so much. In a major overhaul, although sleeves have to be treated with great care, I believe, I would say that the sleeve valve gine would be easier to work on, and a far smaller number of components have to be stripped down and reassembled. Some years ago Bristol's gave an illustrated comparison of a poppet valve engine and a sleeve valve type with all parts laid out and photographed, which greatly impressed me. Of course, the sleeve valve liquid-cooled motor with cylinder blocks is not quite so simple to strip down as an air-cooled radial, but the Sabre would seem to score heavily over, say. a 12 cylinder inverted vee motor with petrol injection. At the moment, so far as 1 know, the Sabre has not the additional complication of two-stage, two-speed blower with intercooler. Internal combustion engines are now reaching a very high state 0/ efficiency and every improvement entails additional mechanical complication, which has been very capably handled by designers both from production and maintenance points of view. This race for high efficiency in aircraft engines has not yet reached the same peak as racing motor cycles and cars, which have given over 100 h.p. per litre quite regularly and with great reliability compared with the 50-odd h.p. of the modern aircraft engine. It is noticeable that the Sabre has motor racing in its blood, the short piston stroke and attendant high crankshaft revs. The shorter the stroke the lower the piston speed. High revs do not necessarily mean greater wear on parts, as has been proved by motor racing; in fact, the reverse is the case, low revs, have produced more wear, especially on cylinder bores. The 24 little "pots" of the Sabre will probably be made to give a great deal more power yet, and I can almost imagine the chagrin German technicians must have felt when analysing the Sabre (as they probably have by now). N. V. BRITTAIN. JET PROPULSION FOR AIRCRAFT Gunpowder's Low Calorific Value YOUR correspondent who suggests gunpowder as being superior to petroleum fuel for the jet propulsion of aircraft (Flight, March 16th) is evidently overlooking the very im portant matter of calorific value. Whereas petroleum fuel has a calorific value of approximately 20,000 B.T.U. per lb., the heat content of gunpowder is only about 1,325 B.T.U./lb., i.e., one-fifteenth that of petrol.. (Even T.N.T., which 1 imagine could not 4>p used in any case, has a calorific value of only 6,500 B.T.U./lb.—less than one-third that of petrol.) Thus, even though some energy is used for compressing the air to burn petrol, a jet-propelled aircraft using gunpowder would. I submit, have to carry at least ten times the weight of fuel to have the same range as one using petroleum fuel. The explanation for the relatively low heat content of ex plosives is, of course, because they carry their own oxygen for combustion, whereas petrol, consisting entirely of hydrogen and carbon, obtains the necessary oxygen from the air and thus benefits by a form of continual flight refuelling free, gratis and for nothing. A. R. OGSTON, F.Inst.Pet. . What Mr. C. G. Grey Said Years Ago IN Flight of February 24th "Astro-Aeronaut's" reference to "the same age-old mistake . . . that propulsive effort is obtained by the action of the exhaust gases pushing upon the atmosphere" reminds me of some correspondence 1 had "with Mr. C. G. Grey about nine years ago, in which he held that the recoil of a rifle was caused by the exploded gases pushing against the air after the bullet was shot Out. I have avoided becoming involved with jet arguers ever since, but your corre spondent " S. H.-A.," in Flight of March 10th, is really one too many. Let us take a simple rocket consisting of a tube closed at one end in which fuel is burned very rap'idly. The pressure against the closed end may be about 200 lb./sq. in., whereas there is no other end to be pressed. Apparently it must be pointed out that the gases once exhausted are not connected to anything, so that pressing with them against the air may be rather fun but quite superfluous expenditure. How is the pressure against the closed end maintained ? First, by the tube itself against which the pressure gradually falls atong its length from 200 lb./sq. in. to 15 lb./sq. in. in air or zero in vacuum. Second, by the inertia of the burned gases which are being constantly accelerated towards the open
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