LIGHT PLANE POWER PLANTS.
The Air-Cooled Four-in-Line is the Pioneer of Post-War Low-Powered Engines with many Records to its Credit.
NO factor has contributed more to the successful evolution of the light ‘plane than the reliability found in the aero engine of the present day. The accumulated experience and data which has so painstakingly been acquired during the past decade has enabled designers to produce machines which have none of the many vices of their prototypes so that flying is now a comparatively simple business, calling for no exceptional skill, or sense of touch, which was formerly required when machines had marked tendencies to swing violently, stall with little or no warning, develop spins on turns and so forth. But high as these aerodynamic qualities are, flying would still be a precarious mode of travel were it not for the splendid standard of dependability of the engine.
This dependability has been demonstrated in the most convincing manner and in a variety of ways—by the historic flights to India, Australia and Africa, for example, and by the excellent record of service of machines in daily use by flying clubs, schools and in commercial aviation in various quarters of the world.
The type of engine which has figured most prominently in this long list of achievements has been the air-cooled four-cylinder in line and it is interesting to find that until Major Halford designed the “Cirrus,” specially for installation in the first D.H. ” Moth,” this type of power unit had never before been evolved or utilised for air work.
The original ” Cirrus ” was rated at 27-60 h.p. having a bore and stroke of 105 mm. and 130 mm., and a characteristic of the design was its robustness throughout. Cylinder barrels were of cast iron with detachable aluminium heads, the whole assembly being secured to the crankcase by long studs passing through lugs on the cylinder heads. Valves were overhead and push-rod operated and the pistons were of aluminium. The crankshaft was carried on five bearings and the camshaft on three. Ignition was by means of one B.T.H. magneto incorporating an impulse starter while the carburettor was an P.S. 42 type Zenith. The weight of the ” Cirrus ” without oil was 268 lbs. and fuel and oil consumption were 0.66 pints per h.p. hour and 0.013 pints per h.p. hour respectively. The ” Cirrus ” was by no means a high speed unit and gave of its normal h.p. at 1,800 r.p.m., a factor which in conjunction with its sturdy construction contributed to its reliability.
After the ” Cirrus” had been turned out in considerable quantities, its manufacturers produced, in response to the demand for an engine of more power, which came with the transition of the light ‘plane from the simple to the more elaborate type, a larger edition known as the Cirrus Mark II. As a result of the experience gained with the Mark I, new features were incorporated in the design which raised the power output to 80 h.p. and made both for an increase in reliability and a reduction in maintenance costs.
It was with an engine of this type that Mr. Bert Hinkler successfully carried out his great joruney in his Avro Avian to Australia—a distance of 12,000 miles in 15i days. Throughout this flight no part required replacement and at the conclusion, when the engine was dismantled, everything was found to be in excellent condition.
Other types which have been added to the ” Cirrus ” range are the Mark III and the “Hermes.” The former is of 85-95 h.p. rating, the dimensions of the bore and stroke being 110 mm. x 130 mm., giving a total swept volume of 4,939 c.c.
All the salient points of design found in the other types of ” Cirrus” have been retained, and while the weight per h.p. has been lowered appreciably, the general robustness of construction remains.
The “Hermes “, which was introduced in July of this year, represents, perhaps, the very latest in aero engine design and it is noteworthy that its power weight ratio is less than 3 lbs. per h.p. Of 105-115 h.p. rating, it has a cubic capacity of 5,717 c.c. (114 m.m. bore x 140 m.m. stroke), and there are many points incorporated in the general layout which indicate that considerable thought and ingenuity has been expended. Normal requirements as regards aerobatics are, for example, allowed for by the provision of a special oil tray which raps the oil during the execution of” acute” manceuvres ; provision is made too for the attachment of a starter gear and fuel pump drives without structural alteration. Particular care has been taken also, to reduce the overall dimensions of the unit, so that the “Hermes ” can be installed in machines designed for ” Cirrus ” engines, the bearer centres being identical. The two magnetos are both mounted at the rear of the engine in such a position as to allow of easy accessibility and adjustment of the distr butors and contact breakers
and the H.T. wires are of minimum length. The results of the Air Ministry 100 hours test (which the” Hermes” passed easily at first attempt) showed that with a compression ratio of 5.1 to 1, the petrol and oil consumption was .58 pints per h.p. hour and 1 pint per hour—very moderate figures for an engine developing 105 h.p. Another 4-cylinder in-line engine which has quickly made a name for itself and which has recently completed one of the most remarkable tests of endurance to which any aero engine has ever been subjected is the 100 h.p. D.H. “Gypsy.” This engine has a bore and stroke of 114 m.m. and 125 min., and its capacity is 5226 c.c. Designed jointly by Major Halford and Captain De Havilland it bears a strong resemblance to the” Cirrus” family. The cylinders are cast iron with detachable aluminium heads, each housing one inlet and one exhaust valve which operate on bronze seatings, screwed into the head. The crankshaft is of particularly robust section and is carried on five plain bearings. Lubrication is by gear type pump in conjunction with a ” wet ” sump which has a capacity of two gallons. The sump has cooling fins cast on its exterior surface so that the propeller slip strLarn, impinging on it at a high velocity, maintains a low oil temperature. The camshaft, like the crankshaft, is carried on five substantial plain bearings and the cams operate the valves through tappets and tubular duralimin push rods which have hardened steel ball joints at both ends. The camshaft is driven through the medium of spur wheels which incorporate a ver flier device which permits a very fine adjustment in timing setting. An intermediate gear in the timing train incorporates a worm drive for the two magnetos which are mounted transversely on the engine. Carburation is through a single Zenith carburettor bolted to an orthodox type of induction pipe, so placed as to be conveniently jacketed and warmed by the exhaust manifold. The maximum r.p.m. of the “Gypsy “is 2,100 at which speed a full 100 h.p. is given off ; its normal running is 1,900 r.p.m. II
Reverting to the endurance test previously mentioned, this was begun in December of last year and continued for nine months, the project embarked upon being the compilation of a total of 600 hours in the air, without overhaul, repair or replacements ; the ” Moth” in which the ” Gypsy ” was installed, to cruise, as far as possible, at a constant speed of 85 m.p.h. This ambition was fully realised, and at the conclusion of the predetermined 600 hours the seals which were affixed to numerous parts of the engine, were still unbroken. Facts and figures of the tour are as follows :—Flying time : 600 hours ; total running time, including bench tests, taxying and running up : 661 hours, 5 minutes ; total distance covered in the air : 51,000 miles ; petrol consumption including bench tests, taxying and running
up on occasion of each flight : 4i, gallons per hour, or 19i miles per gallon ; oil consumption : 0.45 pint per hour, or 1,400 miles per gallon. Bench tests : three bench tests were given on the Heenan and Fronde brake : (1) When the engine was new, (2) after 500 flying hours and (3) after 600 flying hours.
Commenting on this remarkable achievement, Major Halford points out that comparing the performance with that of the average car engine, the mileage flown (51,000) represents about 5 years (at 10,000 miles per annum) of car mileage, incidentally, without failure, without overhaul and without even decarbonising.