How lightweight flying models are built for use in distance-flying competitions
THE GENERAL DESIGN of a duration type model aeroplane has many pints of similarity to the design of a full-size aeroplane. Wooden frameworks are built for the fuselage and wings and are covered with special tissue paper which is drawn taut by shrinkage. The wings obtain their lift from a true aerofoil section and are built up of ribs attached to spars.
IN aviation, as in many other scientific activities, the model exercises an undeniable fascination, apart from its value in research. This chapter deals with the “duration” or “endurance” type of model.
The development of the duration type has recently passed through a difficult phase. The type has been saved from degeneration only by the timely action of the Society of Model Aeronautical Engineers, often referred to as the S.M.A.E.
To understand the lines on which development has taken place it is necessary to remember that the heavier a model is for a given wing area, the faster it must fly to remain airborne and the steeper will be the gliding angle when the propeller has ceased turning. Conversely, the lighter the wing loading, the “flatter” will be the gliding angle, and less power will be required to fly the model.
In conformity with this principle, two varieties of the type have been evolved. The first of these was a fairly small model of which more than half the total weight was taken up . by the rubber motor. This variety was therefore a fairly fast flier and obtained its “duration” by a steep climb to a considerable height, followed by a fairly steep glide to earth. Such a machine was more liable to damage, because of its speed, and has not been greatly favoured.
The second variety of model was the ultra-light machine, of comparatively large wing area. The motor weight was only about one-third of the total, and the “duration” performance was obtained by a slow climb, followed by a long “flat” glide.
On hot sunny days fast upmoving currents of air, known as “thermals” (see the chapter “Modern Soaring Flight”) are found to exist in hilly parts of the country. Thermals, with an upward velocity of from 1 to 2 feet a second, are found on the sunny side of most buildings or even of trees. Enthusiasts therefore were not slow to realize that, if their models could be launched towards these uprising currents of air, the “duration” of their models would be prolonged; in other words, the glide, after the propeller had stopped, could be extended. Further, it followed that, if the wing loading of the model could be so reduced that the sinking speed (rate of vertical descent per second) could be made equal to the speed of the upmoving currents of air, a model would neither rise nor fall, but would be carried slowly along at a level height until it dropped, or even climbed, according to the force of the upmoving current of air.
This led to the development of a “freak” class of model — built of small-sectioned wood, covered with the lightest of tissue paper, and powered with a small motor. Thus was produced an extremely fragile machine which virtually floated along in the breeze.
In no class of rubber-driven model yet built has the run of the propeller exceeded about two minutes; yet freak machines of the type described above have “flown” for more than an hour. In reality they have merely floated about, rising or falling as they drifted (by chance) into or out of these upmoving — or thermal — currents of air. Such a state of affairs was bound to affect adversely model aeronautics in general and the design of duration type models in particular. Timely action by the Society of Model Aeronautical Engineers has induced a return to a more reasonable design. The action took the form of a set of rules.
The most important of these rules is that the area of the main wing must not exceed 210 square inches, and that the weight of the model must not be less than eight ounces. Thus the wing loading must be at least some 5½ ounces per square foot of wing area and the production of a freak model is impossible.
Two further rules are that the cross-sectional area of the fuselage must not be less than a minimum calculated from the formula L²/100 — where L=the overall length of the model in inches, and that the tailplane area must not exceed 33 per cent of that of the main plane.
A model built in conformity with these rules would be eligible for all the main competitions organized by clubs affiliated to the S.M.A.E. and, in particular, would be eligible for the Wakefield Cup, an annual international competition devised to advance the flying of model aeroplanes.
It is not difficult to build an endurance type model aeroplane, but considerable care is required in the details of the work. Patience is necessary in the assembly of the many small pieces of wood which have to be glued together. There are two chief ways in which the construction of an endurance type model aeroplane may be undertaken: a kit of materials may be bought or the aeroplane may be built direct from full-size blueprints. The use of a kit is desirable when the experience of the builder in model making is limited. The cost of a kit is from two or three shillings upwards, according to the type of model to be built. All wood, tissue paper, glue, rubber, wheels and wire arc included in the kit with detailed instructions and a full-size drawing. When the model is built direct from blueprints, the builder has himself to measure the quantities of the various materials required. He is able, however, to choose the quality he prefers and even to make small modifications should he so desire.
Light but Strong Wood
Balsa wood is invariably used for model aircraft because it is extremely light and yet sufficiently strong. Balsa wood is obtained from South America. It weighs only about half as much as cork, and is approximately one-sixth the weight of woods such as deal or birch. It is not so strong as these woods, but lightness is of considerably more importance than great strength.
The glue used to hold the various pieces of balsa wood together is a quick-drying type of the cellulose variety. Rapid assembly of the model is thus possible because no time is wasted waiting for the various sections to become firm.
In place of the fabric used on full-size aircraft, a special form of tissue paper is used. This paper is available in a variety of colours. It has the special property of shrinking if sprayed with water and then allowed to dry slowly. Thin strips of bamboo are used for certain parts of the model where sharp curves are required.
The general design of a duration type model is similar in many ways to that of a full-size aeroplane. There are frameworks of wood for the fuselage, wings and tail unit; the whole is covered by the special tissue paper in place of fabric. The tissue paper is even doped after the model is complete. For this purpose a liquid known as banana oil is used; it gives a final tightening to the tissue and also provides waterproofing. Before the covering is applied the wooden framework is sandpapered until a smooth surface is presented at all points where the tissue paper will touch.
Strands of rubber are used to provide motive power for the propeller. These strands run along the inside of the fuselage; to give access to them the tail unit may be made detachable from the fuselage. Certain parts of the model, where extra strength is required, may be covered with thin wood instead of tissue paper. Such points might be the leading edges of wings or the points of attachment of the wings and undercarriage. The wood used for the purpose is balsa, which may be as thin as one sixty-fourth of an inch.
A ROUNDED FUSELAGE is used on this duration type modal. Dihadral is given to the win’s to provide this aeroplane with lateral stability and the wheels are fixed to a sprung undercarriage to absorb the shocks of landing. The fin on the tail has a small rudder, or trimming tab, so that adjustments may be made to ensure straight flight.
The undercarriage bears resemblance to a full-size aeroplane in that it is sprung to withstand the shocks of landing. The springing is obtained by the use of rubber bands.
Fuselages for endurance type models may be rectangular or rounded in cross section. The rounded ones require more skill in construction. Strips of balsa wood one-eighth to one-quarter of an inch thick are used for longerons. These are formed to the desired shape by being placed over the plan and held in position with pins driven into the bench alongside the longerons. Cross-pieces which join the longerons are then glued into position. In this way the four sides of a rectangular fuselage are constructed; afterwards they are joined together. Sides which are similar in shape may be built one on top of the other to ensure their being equal in every way. The two sides may be split apart with a razor blade should the glue stick them together.
When the fuselage has been completed the tissue paper is gummed into position and smoothed as tightly as possible. When dry it is ready for spraying with water through a fine nozzle such as that of a scent spray. The tail unit completes the after end of the fuselage; the fore end is finished with a shaped block of hard balsa wood. Through this block is drilled a hole for the spindle of the propeller; washers are fitted between the hard block and the propeller to act as bearings.
For the wings flat strips of balsa wood are used for ribs and give the wings a true aerofoil section. The ribs are glued to the spars of balsa wood; when completed the wings are smoothed down and covered in the same way as the fuselage. Sometimes holes are drilled in the wider parts of the ribs to lighten them.
How Straight Flight is Ensured
When the wings are joined to the fuselage — usually by some form of wire fixing — they are given dihedral to provide the model with lateral stability. The tail plane is also built with an aerofoil section and in a similar manner to the wings. Incorporated in the combined tail fin and rudder, there is generally a small movable section to act as a trimming tab. By moving this tab a little to one side or the other it is possible to correct any tendency the model shows to turn to the right or left when in flight.
By steaming bamboo it is possible to bend it to the desired shape for the tail fin or other parts. When dry, the bamboo will retain the shape to which it has been bent and will not have any tendency to spring straight again.
Propellers are supplied ready made with some model kits, in others the wood is partly shaped. The preparation of a propeller is most important because a badly shaped propeller may spoil an otherwise good model, whereas a poor model will generally fly with a good propeller. Several strands of 3/16-in. by 1/20-in. rubber are generally used to provide the motive power of an endurance type model. A special liquid lubricant is used for the rubber. Before a flight the propeller is twisted backwards for a large number of times; in this way the rubber strands are wound up. When the model is released the rubber strands unwind and so turn the propeller.
The propeller is fitted with a freewheel device to permit it to continue turning freely after the driving power of the rubber strands is exhausted. If the free-wheel were not provided, when the rubber strands had completely unwound there would be a tendency for the momentum of the propeller to rewind the rubber in the opposite direction. The wind passing by the propeller would also tend to rewind the rubber in the opposite direction. The propeller would thus offer considerable wind resistance and might greatly upset the glide of the aircraft after the motive power was exhausted.
The propeller is fitted so that it exerts a downward pull in relation to the fuselage. This counteracts the tendency of the model to climb too steeply with the first rush of power. If too steep a climb were permitted, there would be danger of the model stalling when the motive power ceased.
The undercarriage is built of wire and wood, thread being used to bind various items together. Different types of wheels may be used from those made with thin celluloid to heavier pneumatic wheels. Pneumatic wheels as small as one inch in diameter are available.
LAUNCHING A DURATION MODEL. The propeller is held until just before the launch to prevent the rubber strands which provide the motive power from unwinding. The aeroplane is not thrown forward, but a steady forward movement is made by the right hand as the propeller speeds up after being released by the left hand. The model shown in this photograph is fitted with a ski undercarriage instead of wheels.