Notes on Carburetor Installation in Airplanes
A writer in “The Aeroplane,” an English publication, discourses on some features of carburetor installation that may be of interest to the aviation student, so portions of the dissertation are reproduced herewith.
“Users of airplanes fitted with ordinary type carburetors will do well to note carefully the way in which these are fitted, for several costly machines have been burnt lately through the sheer carelessness of their users. These particular machines were fitted with a high powered V-type engine, made by a firm which is famous as manufacturers of automobiles de luxe. In these engines there are four carburetors, mounted in the V between the cylinders. When the engine is fitted as a tractor, the float chambers are in front of the jet chambers. Consequently, when the tail of the machine is resting on the ground, the jets are lower than the level of the gasoline in the float chamber.
“Quite naturally, the gasoline runs out of the jet, if it is left turned on when the machine is standing in its normal position, and trickles into the V at the top of the crank-case. Thence it runs down to the tail of the engine, where the magnetos are fitted, and saturates them. If left long enough, the gasoline manages to soak well into the fuselage before evaporating. And what does evaporate makes an inflammable gas in the forward cockpit. Then some one comes along and starts up the engine. The spark-gap of the magneto gives one flash, and the whole front of the machine proceeds to give a Fourth of July performance forthwith. Naturally, one safeguard is to turn the petrol off directly the machine lands. Another is never to turn it on till the engine is actually being started up.
“One would be asking too much of the human boy—who is officially regarded as the only person fit to fly an aeroplane—if one depended upon his memory of such a detail to save his machine, though one might perhaps reasonably expect the older pilots to remember not to forget. Even so, other means of prevention are preferable, for fire is quite as likely to occur from just the same cause if the engine happens to be a trifle obstinate in starting, and so gives the carburetors several minutes in which to drip—in which operation they would probably be assisted by air-mechanics ‘tickling’ them.
“One way out of the trouble is to fit drip tins under the jet chamber to catch the gasoline as it falls. This is all very well just to prevent fire while the machine is being started up, but it will not save it if it is left standing with the tail on the ground and the petrol turned on, for the drip tins will then fill up and run over. And if it catches then, the contents of the drip tins merely add fuel to the fire.
Reversing Carburetors
“Yet another way is to turn the carburetors round, so that the float chambers are behind the jets, and so come below them when the tail is on the ground, thus cutting off the gasoline low down in the jets. There seems to be no particular mechanical difficulty about this, though I must confess that I did not note very carefully whether the reversal of the float chambers would make them foul any other fittings on the engine. It has been argued, however, that doing this would starve the engine of gasoline when climbing at a steep angle, as the gasoline would then be lowered in the jets and need more suction to get into the cylinders. This is rather a pretty point of amateur motor mechanics to discuss, for, obviously, when the same engine is used as a ‘pusher’ instead of a tractor, the jets are in front of the floats, and there seems to be no falling off in power.
Starvation of Mixture
“Moreover, the higher a machine goes the lower is the atmospheric pressure, and, consequently, the less is the amount of air sucked in at each induction stroke. This means, of course, that with the gasoline supply the mixture at high altitudes is too rich, so that, in order to get precisely the right mixture when very high up, it is necessary to reduce the gasoline supply by screwing down the needle valve between the tank and the carburetor—at least, that has been the experience of various high-flying pilots. No doubt something might be done in the way of forced air feed to compensate for reduced atmospheric pressure, but it remains to be proved whether the extra weight of mechanism involved would pay for the extra power obtained. Variable compression might do something, also, to even things up, but here, also, weight of mechanism has to be considered.
“In any case, at present, the higher one goes the more the power of the engine is reduced, for less air means a less volume of mixture per cylinder, and as the petrol feed has to be starved to suit the smaller amount of air available, this means further loss of power. I do not know whether anyone has evolved a carburetor which automatically starves the gasoline feed when high up, but it seems possible that when an airplane is sagging about ’up against the ceiling’—as a French pilot described the absolute limit of climb for his particular machine—it might be a good thing to have the jets in front of the float chamber, for then a certain amount of automatic starvation would take place.
“When a machine is right up at its limiting height, and the pilot is doing his best to make it go higher still, it is probably flying with its tail as low as the pilot dares to let it go, and the lateral and longitudinal controls are on the verge of vanishing, so that if the carburetor jets are behind the float chambers there is bound to be an over-rich mixture in any case. There is even a possibility of a careless or ignorant pilot carrying on in this tail-down position till one set of cylinders cuts out altogether, in which case the carburetor feeding that set may flood over, just as if the machine were on the ground, and the whole thing may catch fire. Whereas, with the jets in front of the floats, though the mixture may starve a trifle, there is, at any rate, no danger of fire through climbing with the tail down.
A Diving Danger
“On the other hand, in a ‘pusher’ with this type of engine, if the jets are in their normal position—which is in front of the floats—there is danger of fire in a dive. That is to say, if the pilot throttles right down, or switches off and relies on air pressure on his propeller to start the engine again, so that the gasoline is flooding over out of the jets instead of being sucked into the engine, there may be flooding over the magnetos if the dive is very steep and prolonged. In any case, a long dive will mean a certain amount of flooding, and, probably, a good deal of choking and spitting by the engine before it gets rid of the over-rich mixture and picks up steady firing again. Which may indicate to young pilots that it is not good to come down too low under such circumstances, trusting entirely to their engines to pick up at once and get going before they hit the ground.
“On the whole, it seems that it might be better practice to set the carburetors thwartwise of engines, for then jets and floats would always be at approximately the same level, no matter what the longitudinal position of the machine, and it is never long enough in one position at a big lateral angle to raise any serious carburetor troubles. Car manufacturers who dive cheerfully into the troubled waters of aero-engine designs are a trifle apt to forget that their engines are put into positions on airplanes which would be positively indecent in a motor car. An angle of 1 in 10 is the exception on a car, but it is common on an airplane, and no one ever heard of a car going down a hill of 10 to 1—which is not quite a vertical dive. Therefore, there is every excuse for a well-designed and properly brought-up carburetor misbehaving itself in an aeroplane.
“It seems, then, that it is up to the manufacturers to produce better carburetors—say, with the jet central with the float. But it also behooves the user to show ordinary common sense in handling the material at present available, and not to make a practice of burning up $25,000 worth or so of airplane just because he is too lazy to turn off his gasoline, or to have the tail of his machine lifted up while he is tinkering with his engines.”