V. E. Johnson

“ Bamboo.—Bamboo has per pound weight a greater resilience than any other suitable substance (silk and rubber are obviously useless as parts of the framework of an aeroplane) . On full-sized machines the difficulty of making sufficiently strong connections and a liability to split, in the larger sizes, are sufficient reasons for its not being made more use of; but it makes an almost ideal material for model construction. ”

“ Simplicity should be the first thing aimed at—simplicity means efficiency, it means it in full-sized machines, still more does it mean it in models—and this very question of simplicity brings us to that most important question of all, namely, the question of weight. ”

“ In full-sized machines, those in which the distance between the main aerofoil and balancers is considerable (like the Farman) show considerable automatic longitudinal stability, and those in which it is short (like the Wright) are purposely made so with the idea of doing away with it, and rendering the machine quicker and more sensitive to personal control. In the case of the Stringfellow and Tatin models we have the extreme case—practically the bird entirely volitional and personal—which is the opposite in every way to what we desire on a model under no personal or volitional control at all. ”

“ Now, the first requirement of a model aeroplane, or flying machine, is that it shall FLY.
As will be seen later on—unless the machine be of large size, 10 feet and more spread—the only motor at our disposal is the motor of twisted rubber strands, and this to be efficient requires to be long, and is of practically uniform weight throughout; this alone alters the entire distribution of weight on the machine and makes:
§ 6. "Model Aeroplaning an Art in itself," and as such we propose to consider it in the following pages. ”

“ We have said that the first requisite of a model aeroplane is that it shall fly, but there is no necessity, nor is it indeed always to be desired, that this should be its only one, unless it be built with the express purpose of obtaining a record length of flight. ”

“ One-cylinder Petrol Motors.—So far as the writer is aware no success has as yet attended the use of a single-cylinder petrol motor on a model aeroplane. Undoubtedly the vibration is excessive; but this should not be an insuperable difficulty. It is true it is heavier in proportion than a two-cylinder one, and not so efficient; and so far has not proved successful. The question of vibration on a model aeroplane is one of considerable importance. A badly balanced propeller even will seriously interfere with and often greatly curtail the length of flight. ”

“ Now the higher the aspect ratio the greater should be the efficiency. Air escaping by the sides represents loss, and the length of the sides should be kept short. A broader aerofoil means a steeper angle of inclination, less stability, unnecessary waste of power, and is totally unsuited for a model—to say nothing of a full-sized machine. ”

“ To give you some idea how slight a thing will make a model behave badly, I fitted a skid to protect the propeller underneath the aeroplane, and the result in retarding flight could be seen very quickly, although the weight of the skid was almost nil. [3] To all model makers who wish to make a success I would say, strip all that useless and heavy chassis off, cut down the 'good, honest stick' that you have for a backbone to half its thickness, stay it with wire if it bends under the strain of the rubber, put light silk on the planes, and use an aluminium [4] propeller. ”

“ The choice of materials for model aeroplane construction is more or less limited, if the best results are to be obtained. The lightness absolutely essential to success necessitates—in addition to skilful building and best disposition of the materials—materials of no undue weight relative to their strength, of great elasticity, and especially of great resilience (capacity to absorb shock without injury) . ”

“ There is a science of model aeroplaning—just as there is a science of model yachting and model steam and electric traction, and an endeavour is made in the following pages to do in some measure for model aeroplanes what has already been done for model yachts and locomotives. To achieve the best results, theory and practice must go hand in hand. ”

“ In marine propulsion the pitch ratio is generally 1·3 for a slow-speed propeller, decreasing to 0·9 for a high-speed one. In the case of rubber-driven model aeroplanes the pitch ratio is often carried much higher, even to over 3.
Mr. T.W.K. Clarke recommends a pitch angle of 45°, or less, at the tips, and a pitch ratio of 3-1/7 (with an angle of 45°) . Within limits the higher the pitch ratio the better the efficiency. The higher the pitch ratio the slower may be the rate of revolution. Now in a rubber motor we do not want the rubber to untwist (run out) too quickly ”

“ When, for instance, a screw is said to have a pitch of 1 ft., or 12 in., it means that the model would advance 1 ft. through the air for each revolution of the screw, provided that the propeller blade were mounted in solid guides, like a nut on a bolt with one thread per foot. ”

“ By increasing the number of blades the skin friction due to the resistance that has to be overcome in rotating the propeller through the air is added to.
Moreover a fan is stationary, whilst a propeller is constantly advancing as well as rotating through the air.
The action of a fan blower is to move a small quantity of air at a high velocity; whereas the action of a propeller is, or should be, to move a large quantity of air at a small velocity, for the function of a screw is to create thrust. ”

“ In the case of rubber-driven models desired to run along and rise from the ground under their own power, this rising must be accomplished quickly and in a short space. A model requiring a 50 ft. run is useless, as the motor would be practically run out by that time. Ten or twelve feet is the limit; now, in order to rise quickly the machine must be light and carry considerable surface, or, in other words, its velocity of sustentation must be a low one. ”

“ It is so easy to write: "Its weight is negligible, and it can be provided free of cost; all that is necessary is to work a bicycle pump for as many minutes as the motor is desired to run. This stored-up energy can be contained in a mere tube, of aluminium or magnalium, forming the central rib of the machine, and the engine mechanism necessary for conveying this stored-up energy to the revolving propeller need weigh only a few ounces." Another writer recommends "a pressure of 300 lb." ”

“ On the score of efficiency, when it is desired to make a limited number of turns give the longest flight (which is the problem one always has to face when using a rubber motor) it is better to make use of an abnormal diameter, say, more than half the span, and using a tip pitch angle of 25°, than to make use of an abnormal tip pitch 45° and more, and large blade area. ”

“ In practice some prefer to put the centre of gravity in models slightly above the centre of head resistance, the reason being that, generally speaking, wind gusts have a [16] "lifting" action on the machine. It must be carefully borne in mind, however, that if the centre of wind pressure on the aerofoil surface and the centre of gravity do not coincide, no matter at what point propulsive action be applied, it can be proved by quite elementary mechanics that such an arrangement, known as "acentric," produces a couple tending to upset the machine. ”

“ The rear of the model is the proper place for the propeller, in the centre of greatest air disturbance; in such a position it will recover a portion of the energy lost in imparting a forward movement to the air, caused by the resistance, the model generally running in such air—the slip of the screw is reduced to a corresponding degree—may even vanish altogether, and what is known as negative slip occur. ”

“ Herein model aeroplanes differ from model yachts and model locomotives. An extremely small model locomotive built to scale will still work, just as a very small yacht built to scale will sail; but when you try to build a scale model of an "Antoinette" monoplane, including engine, it cannot be made to fly unless the scale be a very large one. If, for instance, you endeavoured to make a 1/10 scale model, your model petrol motor would be compelled to have eight cylinders, each 0·52 bore, and your magneto of such size as easily to pass through a ring half an inch in diameter. ”

“ It will not do to tip up the elevator to a large angle to make it rise quickly, because when once off the ground the angle of the elevator is wrong for actual flight and the model will probably turn a somersault and land on its back. I have often seen this happen. If the elevator be set at an increased angle to get it to rise quickly, then what is required is a little mechanical device which sets the elevator at its proper flight angle when it leaves the ground. ”

“ In a large pitch angle so much energy is wasted, not in dynamic thrust, but in transverse upsetting torque. On no propeller out of dozens and dozens that I [77] have tested have I ever found a tip-pitch of more than 35° give a good dynamic thrust; and for length of flight velocity due to dynamic thrust must be given due weight, as well as the duration of running down of the rubber motor. ”

“ Geared Rubber Motors.—It is quite a mistake to suppose that any advantage can be obtained by using a four to one gearing, say; all that you do obtain is one-fourth of the power minus the increased friction, minus the added weight. This presumes, of course, you make no alteration in your rubber strands. ”

“ Experiments in marine propulsion also show that the thrust depends more on the disk [62] area than on the width of the blades. All the facts tend to show that for efficiency the blades of the propeller should be narrow, in order that the air may not be acted on for too long a time, and so put too much in motion, and the blades be so separated that one blade does not disturb the molecules of air upon which the next following one must act. ”

“ Electric Motors.—No attempt should on any account be made to use electric motors for model aeroplanes. They are altogether too heavy, apart even from the accumulator or source of electric energy, for the power derivable from them. To take an extreme case, and supposing we use a 2-oz. electric motor capable of driving a propeller giving a static thrust of 3 oz., [23] on weighing one of the smallest size accumulators without case, etc., I find its weight is 41⁄2 oz. ”

“ A light form of skid, made of steel pianoforte wire, should be used. As to the weight and size of the model, the now famous "one-ouncers" have made some long flights of over 300 yards [43] ; but the machine claiming the record, half a mile, [44] weighs about 10 oz. And apart from this latter consideration altogether the writer is inclined to think that from 5 oz. to 10 oz. is likely to prove the most suitable. It is not too large to experiment with without difficulty, nor is it so small as to require the skill of a jeweller almost to build the necessary mechanism. ”

“ When the horizontally revolving propellers are driven in a horizontal direction their "lifting" powers will be materially increased, as they will (like an ordinary aeroplane) be advancing on to fresh undisturbed air.
§ 6. I have not for ordinary purposes advocated very light weight wire framework fabric-covered screws, but in a case like this where the thrust from the propeller has to be more than the total weight of the machine, these might possibly be used with advantage. ”

“ Admitting, then, that this wake does exist, it follows that a propulsive screw is better than a tractor. In a matter of this kind constructional considerations, or "ease of launching," and "ability to land without damage," must be given due weight.
In the case of model aeroplanes constructional details incline the balance neither one way nor the other; but "ease in launching" and "ability to land without damage" weigh the balance down most decidedly in favour of a driving or propulsive screw. ”

“ The solution of the problem of steering by the use of two propellers is only partially satisfactory and reliable, in fact, it is no solution at all. [45] The torque of the propeller and consequent tilting of the aeroplane is not the only cause at work diverting the machine from its course.
§ 4. As it progresses through the air it is constantly [107] meeting air currents of varying velocity and direction, all tending to make the model deviate more or less from its course ”

“ By the escape of steam from the arms the fans were caused to revolve with immense energy, so much so that the model rose to an immense altitude and flew across two fields before it alighted. The motive power employed was obtained from the combustion of charcoal, nitre and gypsum, as used in the original fire annihilator; the products of combustion mixing with water in the boiler and forming gas-charged steam, which was delivered at high [114] pressure from the extremities of the eight arms. ”

“ From time to time flying competitions are arranged for model aeroplanes. Sometimes these competitions are entirely open, but more generally they are arranged by local clubs with both closed and open events.
No two programmes are probably exactly alike, but the following may be taken as fairly representative:—
1. Longest flight measured in a straight line (sometimes both with and against the wind) . [48]
2. Stability (both longitudinal and transverse) .
3. Longest glide when launched from a given height without power, but with motor and propeller attached.
4. Steering. ”

“ Aluminium and Magnalium.—Two substances about which a great deal has been heard in connection with model aeroplaning; but the writer does not recommend their use save in the case of fittings for scale models, not [91] actual flyers, unless especially light ones meant to fly with the wind. Neither can compare with steel. Steel, it is true, is three times as heavy as aluminium, but it has four or five times its strength; and whereas aluminium and magnalium may with safety be given a permissible breaking strength of 60 per cent. and 80 per cent. ”

“ Rubber has a much greater superiority over steel or other springs, because in stretch-twisted rubber far [40] more energy can be stored up weight for weight. One pound weight of elastic can be made to store up some 320 ft.-lb. of energy, and steel only some 65 lb. And in addition to this there is the question of gearing, involving extra weight and friction; that is, if flat steel springs similar to those used in clockwork mechanism be made use of, as is generally the case. ”

“ Such a [111] device does not present any great mechanical difficulties; and I leave it to the mechanical ingenuity of my readers to devise a simple little device which shall maintain the elevator at a comparatively large angle while the model is on the ground, but allowing of this angle being reduced when free flight is commenced.
§ 7. The propeller most suitable to "get the machine off the ground" is one giving considerable statical thrust. A small propeller of fine pitch quickly starts a machine, but is not, of course, so efficient when the model is in actual flight. ”