A CRUISING SPEED OF 100 MILES AN HOUR is provided by the Chilton monoplane with an engine developing thirty-two horse-power at 3,500 revolutions. The aircraft is a low-wing monoplane fitted with flaps which may be adjusted during flight to give different gliding angles. Wooden airframe construction is used and the wings are covered partly with plywood and partly with fabric.
THE lower the cruising speed of an aeroplane, the greater the effect of wind speed on the mileage that the aircraft will cover in an hour. If the cruising speed is 60 miles an hour, a wind speed of 30 miles an hour will give the aeroplane a ground speed of 90 miles an hour when it is travelling with the wind. Against the wind the ground speed will be only 30 miles an hour. This speed is less than that averaged by the majority of cars on the road, and is of little use for air touring.
An aeroplane with a cruising speed of 100 miles an hour would be able to cover the ground at 70 miles an hour even against a wind of 30 miles an hour. The percentage increase in speed when flying with the wind would not be so great with the faster aeroplane, but the more practicable aircraft is the one that is able to maintain a reasonable speed whether it flies with or against the wind.
It is the high performance of the Chilton light monoplane that makes it an essentially useful aircraft, apart from the economy of its maintenance and running costs. Many ultra-light aeroplanes are designed as an inexpensive means of getting into the air for local flying. The object behind the Chilton monoplane, which can correctly be called an ultra-light aircraft, was to design what the makers described as “a real aeroplane in miniature”. Aircraft of the Chilton type and other single-seater ultra-light aeroplanes are exempt from the normal Certificate of Airworthiness procedure applying to all other aircraft. This considerably cheapens their construction and maintenance, because adjustments and repairs do not have to be carried out by engineers holding Air Ministry licences. The question of the safety of this class of aeroplane is left in the hands of the company which underwrites the necessary third-party insurance. Third-party cover is required before the Air Ministry will issue registration letters for the aeroplane. The insurance company is naturally careful to ensure that the aircraft covered are competently built and designed.
The Chilton monoplane is a low-wing aircraft with a four-cylinder engine, spats and split trailing-edge flaps. Aircraft of this class are not officially permitted to perform aerobatics, but the Chilton monoplane, to ensure its complete safety, is stressed to a degree suitable for aerobatics. It is of wooden construction, with plywood covering for the greater part of the wings and for the fuselage. The control surfaces are fabric-covered.
The speed-range ratio of 3·3 to 1 is obtained with the aid of the flaps, which permit a landing speed as low as 35 miles an hour. The maximum speed is 112 miles an hour and the cruising speed is 100 miles an hour. The use of the flaps also steepens the glide of the aeroplane which, because of the clean design, is particularly flat without the flaps in use.
There are intermediate positions for the flaps between off and fully on, and they may be moved during the glide to control overshooting or undershooting when approaching the aerodrome. The flaps start at the ailerons on either side of the aeroplane, and are continuous for their whole length, passing underneath the fuselage.
The flaps are not used during the take-off, which is, however, as short as 80 yards. The landing run is only 50 yards. The fuel consumption is 1⅔ gallons an hour when cruising at 100 miles an hour, and the running costs are estimated at less than a halfpenny a mile. The range of the aircraft in still air is 500 miles.
The wing span is 24 feet, the length of the aircraft 18 feet and the height 4 ft 10-in. The unusually wide track of 6 feet for a small machine assists stability when landing. The wing area is 77 square feet which, with an all-up weight of 640 lb, gives a wing loading of approximately 8·3 lb to the square foot. Although the maximum permissible all-up weight is 700 lb, 640 lb is the maximum normally desirable. This weight is made up as follows: weight of aeroplane empty, 395 lb; pilot, 160 lb; luggage, 20 lb; and fuel, 65 lb.
There are four inches of spring travel for the wheels, which have low-pressure tyres. The tail skid is sprung. The control cables are enclosed and the ailerons are fully differential. That means that when the stick is put over towards the right, the left aileron is only slightly depressed, while the right one is raised considerably. There is thus, for example, little of that aileron drag to the left which would tend to turn the aeroplane in a direction opposite to that desired when a turn to the right was to be made.
Easy to Fly
The engine unit is a converted Ford 10 engine. This is a water-cooled four-cylinder four-stroke unit. Dual ignition is fitted, and the developed horse-power is 32 at 3,500 revolutions a minute. The engine is fitted in an upright position in the nose of the aircraft, with the four short exhaust manifolds projecting through the right-hand side of the cowling. The radiator is fitted immediately below the engine, an oval opening in the front of the cowling allowing the air to reach it.
The weight of the engine is 141 lb, including the two magnetos, the airscrew hub and the eight sparking plugs - two to each cylinder. Many of the parts of the engine have been altered. An aluminium head is fitted and also a special crankshaft with a heavy-duty ball bearing to take the thrust of the propeller. A number of subsidiary light-alloy castings are used to reduce the weight. The bore of the cylinders is 63·5 and the stroke of the pistons 92·5 millimetres. This gives a total engine capacity of 1,172 cubic centimetres.
Although the cockpit appears small from the outside of the aeroplane, the pilot finds there is ample room when he is seated. The cockpit is comfortable and well protected by the windscreen. Two of the features first noticed when the aircraft is taxied are the smoothness and comparative quietness of the engine. These qualities are largely due to the high number of revolutions a minute at which the engine develops its maximum power. Taxying is particularly easy, the aircraft answering to the rudder almost as though wheel brakes were in use ; the wide track makes sudden turns quite safe.
Like the majority of clean low-wing monoplanes, the Chilton requires an easing back of the stick to lift it off the ground as soon as flying speed is attained. With the tail up in the position of level flight the view from the cockpit is good, and encouraging for a pilot with but little experience. In spite of the high performance of this aircraft, it is not designed as an expert’s aeroplane; it has a sensitive response to the controls, but is easy to fly.
THE FOUR-CYLINDER-IN-LINE ENGINE is based on a small motor-car power unit. This power unit is modified to be suitable for aircraft use. It is provided with a new cylinder head and a number of lighter castings. Dual ignition is added and a special crankshaft, fitted with a bearing to take the propeller thrust, is used.
The throttle works in the opposite direction to those on most British aircraft; it is pulled to open and pushed to close. Pilots who are used to the orthodox arrangement, however, do not find this disconcerting, because the shape of the Chilton control is unusual. Instead of being a knob-topped handle, it is a disk-shaped button on the instrument board.
The best climbing speed is about 60 miles an hour; after the aeroplane has been cruising level at 100 miles an hour, the nose of the machine has to be pulled up surprisingly high before the speed drops back to this figure. At this speed the monoplane climbs at the rate of 650 feet a minute.
All the controls are powerful and the response to them is immediate. Sideslipping is possible, although it cannot be performed as steeply and slowly as on, say, a training biplane. This, however, is normal with a clean monoplane, and is no drawback when effective flaps are provided which have four different positions. These provide the best way of shortening or lengthening the glide of the Chilton during an approach.
Without the flaps in use the aircraft will glide flatly and would float a long way across an aerodrome before a landing could be accomplished. With the flaps fully down, the rate of sink is rapid and the. forward touch-down speed slow. The two intermediate positions provide an advanced form of glide adjustment which is not found, even to-day, on a great many full-size and expensive aircraft.
Cantilever Wing Construction
If the aircraft is brought in slowly with the flaps fully down, the landing has to be made quickly at the end of the glide because of the rapid sink. Most pilots find the machine easiest to land when the flaps are right down, and they bring it in faster than necessary, or if they bring it in with the flaps set in the position before the fully down one. It is largely a matter of the type of aircraft with which the pilot is familiar. In a single-seater aircraft the pilot has to find out the landing characteristics for himself, because no dual instruction is possible. But several pilots with well under fifty hours’ solo flying experience have successfully flown the Chilton monoplane.
The wing construction is of the cantilever type with two box-section spars. The wings are in three parts. A centre part with parallel leading and trailing edges is let into the underside of the fuselage and has the undercarriage attached to it. The two outer parts are tapered and are detachable from the centre part.
The fuselage is a semi-monocoque ply-covered wooden construction with spruce longerons. The cross section of the fuselage is rectangular, with a rounded three-ply top fairing.
In the same way as the throttle control, the rudder controls are unusual so far as most power aeroplanes are concerned. The rudder controls, of the pedal type, are hinged to the floor of the cockpit, and are similar to those often used on sailplanes. It is unusual features such as these, both visible and unseen, which give the Chilton its individuality. The aircraft is the successful outcome of an attempt to produce a safe high-performance monoplane with a low-powered engine.
Two young designers, the Hon. A. W. H. Dalrymple and Mr. A. R. Ward, were entirely responsible for the design of the Chilton monoplane. Both had previously studied aircraft design and construction at the De Havilland Aeronautical Technical School at Hatfield, and the Chilton aeroplane is the first one to be built to their designs. Work on the design was begun early in 1936, and in May of that year the firm of Chilton Aircraft was formed to build the prototype machine. The prototype was flown in April 1937 and work on production models began soon afterwards. In August 1937 the firm took over the production of the Carden-Ford aero engine - the unit with which the Chilton monoplane is powered.
The aircraft has been designed to be strong enough to take power units, other than the Carden-Ford, with horsepower up to 50. The designers have considered fitting a 44 horse-power inverted Train engine. It is calculated that with this engine the cruising speed would be increased to 112 miles an hour and the maximum speed to 125 miles an hour. The climb is calculated as the extraordinarily good figure of 1,000 feet a minute.
AN UNUSUAL THROTTLE is fitted to the Chilton monoplane. It is a small knob, seen at the bottom of the instrument board and on the extreme left. This knob is pulled outwards to open the throttle and pushed in to close it. Doors at the sides of the cockpit are lowered to simplify entrance; when the doors are closed the cockpit is well protected from the wind and propeller slipstream.
