How engines that give a fraction of one horse-power propel model aircraft weighing several pounds
WINNER OF THE SIR JOHN SHELLEY CUP IN 1937, this petrol-driven model was designed by Captain C. E. Bowden. It is a typical example of a petrol-driven model using an engine with a capacity of six cubic centimetres. The photograph shows the model gliding down to land after its winning flight.
THE story of the evolution and development of the aeroplane is familiar to many people, but the part played by the model aeroplane does not seem to be nearly so well known.
A number of well-known aviators carried out their pioneering work with the aid of models. Sir Alliott Verdon-Roe, for example, founder of the firm of A. V. Roe & Co., Ltd., spent much time and money on building models in the early part of this century. From these models he derived valuable experience.
With the widespread publicity attending the development of the aeroplane it was inevitable that a number of people should turn their attention to the building, purely as a hobby, of replicas of the full-sized machines. Although there has been, and no doubt always will be, strong support for “solid” non-flying replicas, it is with the flying model that the greatest development has been, and is now being made.
With the exception of one or two attempts with steam, and a small number of compressed-air machines, virtually all model aircraft up to about 1934 or 1935 were powered by a “motor” consisting of twisted strands of rubber. As long ago as 1848, however, John Stringfellow built a model aeroplane propelled by steam (see the chapter “The First Powered Aeroplane”).
The “motor” of twisted rubber had the advantages of cheapness, flexibility of arrangement and consequent adaptability to any size of model, and a favourable power-weight ratio; but it suffered from the handicap that the rate of power delivered was not uniform. When the motor was fully wound up the torque would be considerably greater than after a few seconds of unwinding. Eventually the power would diminish until it was insufficient to fly the model, which would then glide to earth.
Model aircraft, specially designed for duration flights, have flown for from four to six minutes and have soared with the aid of rising currents of air for periods in excess of an hour. But these models are not replicas of the full-sized aircraft.
To build scale models, considerable weight must be put into them in reproducing the correct shapes of wheel coverings, engine cowls and so on of the full-sized aircraft. The performance of the model therefore suffers and the duration of flight is reduced to about one minute.
It is not surprising, therefore, to find that the keen aero modeller should have turned to the internal combustion engine as a means of providing the motive power. With such a power unit the duration of flight would then be limited solely by the amount of petrol that could be carried. Moreover, the power-weight ratio is considerably better than that of the rubber motor.
The earliest attempts in this direction were made as long ago as 1912 when a biplane of 7-feet span, weighing 10¾ lb. and powered by a 52 cc, engine, made an officially timed flight of 51 seconds. This flight was to the credit of Mr. D. Stanger.
This record stood for eighteen years. In 1930 the duration was increased to 71 seconds by another biplane, built by Captain C. E. Bowden. This model was of similar span and weight and was powered by a 30 cc. engine built by Mr. E. T. Westbury.
These engines, though small in comparison with motor-cycle engines, were large as compared with those commonly in use today for power units in model aircraft. Their size thus necessitated a fairly large model, and therefore the cost of the whole machine was beyond the means of the majority of modellers. Only a few could afford to experiment with them.
HAND LAUNCHING a petrol-driven model aeroplane. In competitions the model aeroplanes are generally started from the ground and taxi along for a short distance before taking off. The engine of the model illustrated has a capacity of six cubic centimetres. Ignition is supplied by a miniature sparking-plug operated from a flash-lamp battery and a small electric induction coil.
A few years ago the Americans, foreseeing a considerable future for ultra-small petrol engines, set about the design and manufacture in considerable quantities of really miniature petrol engines. The Americans were so successful that there are now available a number of petrol engines of from 2.3 to 18 cc. capacity, of exceptionally light weight, and capable of flying model aircraft weighing from about 1½ to 15 lb.
A popular size is 6 cc; such an engine has a bore of ¾-in. and a stroke of 13/16 in. The crankcase and cylinder are of aluminium or even of electron, with a cylinder liner and piston of steel. Variable ignition is fitted; there are a miniature sparking plug less than 1 in. high and a carburettor with needle jet. The parts are machined to fine limits of accuracy. A special lightweight coil, weighing some 2 oz., is operated from a 4-volts pocket flash lamp battery and provides the spark.
Such an engine, complete with coil and petrol tank, weighs some 10 oz., to which must be added a further 4 oz. for the flash lamp battery. The complete power unit thus weighs under a pound, develops about 4 horse-power, and will fly a machine of 6 feet span and weighing about 5 lb. Such a petrol-driven model aeroplane is shown in the accompanying photograph, about to be hand launched.
An example of a larger type of engine, and one of British manufacture, is the 18 cc. Comet. This engine will drive an 18-in. diameter airscrew at some 3,500 revolutions a minute, and develops approximately ½ horse-power.
The engine is made entirely of electron except for the crankshaft and cylinder liner, which are of steel. It weighs just under 1¼ lb, including the streamline cowling on which the engine is mounted and inside of which is the petrol tank.
Model petrol-driven aircraft fly at speeds ranging from 12 to 25 miles an hour. The speed is dependent on the wing loading (see the chapter “Wing Loading Problems”).
The flight duration of these model aeroplanes is controlled by a small “time-switch”, usually a cheap clock mechanism which has been rebuilt, or by a “self-timer”, as used for operating the triggers of cameras. In each instance the purpose is to actuate a switch placed in the electrical circuit which, after a predetermined interval of time, cuts out the engine. The model then glides to a safe landing within the aerodrome or field.
The modern petrol model is anything but a toy. It requires to be designed in all essential parts as carefully as a full-sized machine, and, as far as the power unit is concerned, it entirely follows out full-size practice. Only in the matter of control while in the air does the technique differ as, without a pilot’s guiding hand, the model must be so trimmed as to be stable in flight and in the glide back to earth after the engine has been cut off. This is achieved by the use of control surfaces somewhat larger than those found on full-sized aircraft, thus making the control particularly sensitive, and virtually automatic.
The high degree of control that is possible is best illustrated by the fact that competitions for petrol aeroplanes are usually organized on the basis of the competitors having to time the length of flight as near to a specified figure as possible. By this is meant the time that elapses from the moment that the machine is released on the ground to the moment that it touches down at the end of the glide. Suppose the time specified is forty-five seconds. The competitor has to allow several seconds for the machine to run along the ground getting up speed to take off. Then he must allow so many seconds for the length of time for the engine to be running and, finally, a length of time for the glide after the engine is stopped.
It is obvious that the competitor must have an accurate knowledge of the performance of his model. In estimating the length of time to allow the engine to run, he must take into consideration the state of the ground from which the model has to rise and the strength and direction of the wind. So high is the degree of control obtainable that the winner of a competition of this nature will control his model to land within one or two seconds of the stipulated time.
It is possible to make petrol aeroplanes fly in right-hand or left-hand circles, fly to some considerable heights, or keep just above the ground. Recently a considerable amount of research has been done in connexion with radio-controlled models. A 14-feet span model built by an American — Leo Weiss — has been successfully controlled by radio. The model can be turned to right or left and brought down, under power, at the will of the controller.
Easy to Build
Model petrol aeroplanes are constructed in a similar manner to full-sized aircraft. The airframes are built up of stringers on bulkheads, the stringers being either of birch or balsa and the bulkheads of three-ply. In addition, monocoque construction has recently been developed. In this, sheets of thin balsa are used in a similar manner to the stressed skin construction common today in full-sized aircraft (see the chapter “All-Metal Construction”). Wings are built up with ribs and bracings of wood and then covered with silk, doped and painted.
MONOCOQUE CONSTRUCTION is often used for the fuselages of petrol-driven models. This photograph shows the inside of an American model, built for radio-control, which has a wing span of 14 feet. The main plane fits into the cutaway part of the fuselage.
Undercarriages in many instances are fully sprung, either with rubber or with coil spring suspensions. The wheels are fitted with tyres that are genuinely pneumatic. The wheels are obtainable in sizes ranging from 2 in. to 8 in. diameter, and are provided with valves so that the inflation pressure can be regulated.
The photograph at the top of this page shows a typical petrol aeroplane in the 6-cc. class. It was designed by Capt. C. E. Bowden. This model was winner of the Sir John Shelley Cup in 1937 and the photograph shows the model gliding in to land after its winning flight. This type of model is easy to build, is virtually foolproof and crashproof. It is not uncommon to hear that several hundred flights have been made during one season, without serious damage to the machine and without any damage to person or property.
“Flying Season”, April to October
A more ambitious type of model has been built by Mr. D. A. Russell. This model has a span of some 10 feet and weighs 15 lb. The fuselage is built entirely of three-ply birch of only in. thickness, laid up as a stressed skin over longerons and formers, also of birch. This model is beautifully finished; it is fitted with windows and doors, fully sprung landing chassis and pneumatic tyres.
By arrangement with the Royal Aero Club, the Society of Model Aeronautical Engineers governs the flying of all types of model aircraft in Great Britain. This body is organized and controlled by a council of representatives elected from members of model aircraft clubs throughout the country. There are well over one hundred of these clubs and the total membership is some thousands. In addition, there is a great number of modellers who, for various reasons, do not belong to clubs.
Throughout the winter season, meetings for lectures and exchange of idea;s are held weekly or fortnightly by many of the clubs. During the “flying season”, which extends from April till October, meetings are arranged every week-end in all parts of the country.
A WING SPAN OF ABOUT TEN FEET makes this model a particularly fine example of petrol-driven model aircraft. It weighs 15 lb. and is built entirely of three-ply birch, only one thirty-second of an inch thick. Stressed-skin construction and pneumatic tyres are used. The model is shown being launched from a moving car, the forward speed of the car ensuring that the model will be properly airborne from the moment of release.
Some of the larger clubs organize well-attended “rallies” to which several hundreds of modellers come from ail parts of the country. At these meetings competitions for all classes of models are held under rules drawn up by the Society of Model Aeronautical Engineers, and under the control of official timekeepers who have been appointed by the Society.
To safeguard competitors and public, stringent rules have been drawn up by the Society about the flying of petrol aeroplanes. No flying may be done in competitions under the control of the Society unless the aircraft has been inspected by its officials and a third-party insurance policy is in existence covering each machine. This is purely a precautionary measure. No accident to person or property has ever occurred in connexion with the flying of model petrol aeroplanes in Great Britain. The smallness of the risk may be appreciated from the fact that full third-party protection to an unlimited amount may be obtained for the outlay of a few shillings a year.
If the development of the model petrol aeroplane during the past few years has been rapid, the future development will undoubtedly be more rapid still. Already the Government and research workers in various fields have carried out preliminary investigations with a view to using such model aircraft in times of war.
A fleet of radio-controlled model petrol aeroplanes carrying wire “streamers” could form an effective barrage against raiding aircraft. A fleet of 1,000 could be produced at the cost of two or three modern bombers. The use of model petrol aeroplanes as attackers, each carrying light highly explosive bombs, is regarded as equally possible.
As a means of transporting important messages or plans across difficult tracks of country or short stretches of water, the petrol aeroplane has distinct possibilities that are available at the present time. Flights of up to thirty miles have already been accomplished without difficulty.
Another direction in which development work is being undertaken is in the use of a petrol engine to drive a small generator as used with a portable transmitting wireless set. The whole unit weighs only some 5 or 6 lb. and is capable of being carried by hand or slung on the back of an explorer. A patent embodying this idea has been taken out for military application. According to this patent the unit could form part of a soldier’s equipment.
Let us hope, however, that these warlike usages will not eventuate. There is plenty of scope for development in ways of peace, such as the carrying of advertisement banners. As a means of providing many hours of instructive pleasure the model petrol aeroplane is rapidly gaining support in all parts of Great Britain and is helping to promote international goodwill.
In 1937, for the first time, there was held an international competition for model petrol aeroplanes for a cup presented by Capt. C. E. Bowden. The cup was won by the United States of America. Although open to the world, this competition must always be held in Great Britain.
THE CAVALIER, an American-built model of semi-monocoque design. Petrol-driven model aeroplanes require to be designed in all essential parts as carefully as full-sized machines. The speed varies from twelve to twenty-five miles an hour and is dependent on the wing loading. Automatic switches to cut off the engine are fitted.