The Theory and Practice of Model Aeroplaning Part 13

-- 1. Generally speaking, the model should be launched into the air _against the wind_.

-- 2. It should (theoretically) be launched into the air with a velocity equal to that with which it flies. If it launch with a velocity in excess of that it becomes at once unstable and has to "settle down" before a.s.suming its normal line of flight. If the velocity be insufficient, it may be unable to "pick up" its requisite velocity in time to prevent its falling to the ground. Models with wooden aerofoils and a high aspect ratio designed for swift flying, such as the well-known Clarke flyers, require to be practically "hurled" into the air.

Other fabric-covered models capable of sustentation at a velocity of 8 to 10 miles an hour, may just be "released."

-- 3. Light "featherweight" models designed for long flights when travelling with the wind should be launched with it. They will not advance into it--if there be anything of a breeze--but, if well designed, just "hover," finally sinking to earth on an even keel. Many ingenious pieces of apparatus have been designed to mechanically launch the model into the air. Fig. 50 is an ill.u.s.tration of a very simple but effective one.

-- 4. For large size power-driven models, unless provided with a cha.s.sis and wheels to enable them to run along and rise from the ground under their own power, the launching is a problem of considerable difficulty.

-- 5. 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 s.p.a.ce. 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.

[Ill.u.s.tration: FIG. 50.--MR. POYNTER'S LAUNCHING APPARATUS.

(_Reproduced by permission from the "Model Engineer."_)]

-- 6. 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. Such a 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. A rubber motor is not at all well adapted for the purpose just discussed.

-- 8. Professor Kress uses a polished plank (down which the models slip on cane skids) to launch his models.

-- 9. When launching a twin-screw model the model should be held by each propeller, or to speak more correctly, the two brackets holding the bearings in which the propeller shafts run should be held one in each hand in such a way, of course, as to prevent the propellers from revolving. Hold the machine vertically downwards, or, if too large for this, allow the nose to rest slightly on the ground; raise (or swing) the machine up into the air until a little more than horizontal position is attained, and boldly push the machine into the air (moving forward if necessary) and release both brackets and screws simultaneously.[46]

-- 10. In launching a model some prefer to allow the propellers to revolve for a few moments (a second, say) _before_ actually launching, contending that this gives a steadier initial flight. This is undoubtedly the case, see note on page 111.

-- 11. In any case, unless trying for a height prize, do not point the nose of the machine right up into the air with the idea that you will thereby obtain a better flight.

Launch it horizontally, or at a very small angle of inclination. When requiring a model to run along a field or a lawn and rise therefrom this is much facilitated by using a little strip of smooth oilcloth on which it can run. Remember that swift flying wooden and metal models require a high initial velocity, particularly if of large size and weight. If thrown steadily and at the proper angle they can scarcely be overthrown.

FOOTNOTE:

[46] Another and better way--supposing the model constructed with a central rod, or some suitable holdfast (this should be situated at the centre of gravity of the machine) by which it can be held in one hand--is to hold the machine with both hands above the head, the right hand grasping it ready to launch it, and the left holding the two propellers. Release the propellers and allow them a brief interval (about half a second) to start. Then launch boldly into the air. The writer has easily launched 1 lb. models by this means, even in a high wind. Never launch a model by one hand only.

CHAPTER XI.

HELICOPTER MODELS.

-- 1. There is no difficulty whatever about making successful model helicopters, whatever there may be about full-sized machines.

-- 2. The earliest flying models were helicopters. As early as 1796 Sir George Cayley constructed a perfectly successful helicopter model (see ch. iii.); it should be noticed the screws were superimposed and rotated in opposite directions.

-- 3. In 1842 a Mr. Phillips constructed a successful power-driven model helicopter. The model was made entirely of metal, and when complete and charged weighed 2 lb. It consisted of a boiler or steam generator and four fans supported between eight arms. The fans had an inclination to the horizon of 20, and through the arms the steam rushed on the principle of Hero's engines (Barker's Mill Principle probably). 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 alt.i.tude 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 pressure from the extremities of the eight arms.[47] This model and its flight (fully authenticated) is full of interest and should not be lost sight of, as in all probability being the first model actuated by steam which actually flew.

The helicopter is but a particular phase of the aeroplane.

-- 4. The simplest form of helicopter is that in which the torque of the propeller is resisted by a vertical loose fabric plane, so designed as itself to form a propeller, rotating in the opposite direction. These little toys can be bought at any good toy shop from about 6_d._ to 1_s._ Supposing we desire to construct a helicopter of a more ambitious and scientific character, possessing a vertically rotating propeller or propellers for horizontal propulsion, as well as horizontally rotating propellers for lifting purposes.

[Ill.u.s.tration: FIG. 51.--INCORRECT WAY OF ARRANGING SCREWS.]

-- 5. There is one essential point that must be carefully attended to, and that is, _that the horizontal propulsive thrust must be in the same plane as the vertical lift_, or the only effect will be to cause our model to turn somersaults. I speak from experience.

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.

-- 7. Instead of using two long vertical rods as well as one long horizontal one for the rubber strands, we might dispense with the two vertical ones altogether and use light gearing to turn the torque action through a right angle for the lifting screws, and use three separate horizontal rubber strands for the three propellers on a suitable light horizontal framework. Such should result in a considerable saving of weight.

[Ill.u.s.tration: FIG. 52.--CORRECT MANNER. A, B, C = Screws.]

-- 8. The model would require something in the nature of a vertical fin or keel to give the sense of direction. Four propellers, two for "lift" and two for "drift," would undoubtedly be a better arrangement.

FOOTNOTE:

[47] Report on First Exhibition of Aeronautical Society of Great Britain, held at Crystal Palace, June 1868.

CHAPTER XII.

EXPERIMENTAL RECORDS.

A model flying machine being a scientific invention and not a toy, every devotee to the science should make it his or her business to keep, as far as they are able, accurate and scientific records. For by such means as this, and the making known of the same, can a _science_ of model aeroplaning be finally evolved. The following experimental entry forms, left purposely blank to be filled in by the reader, are intended as suggestions only, and can, of course, be varied at the reader's discretion. When you _have_ obtained carefully established data, do not keep them to yourself, send them along to one of the aeronautical journals. Do not think them valueless; if carefully arranged they cannot be that, and may be very valuable.

EXPERIMENTAL DATA.

FORM I.

Column Headings:

A: Model B: Weight C: Area of Supporting Surface D: Aspect Ratio E: Average Length of Flight in Feet F: Maximum Flight G: Time of Flight, A. average H: M. maximum I: Kind and Direction of Wind J: Camber K: Angle of Inclination of Main Aerofoil to Line of Flight

-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- A | B | C | D | E | F | G | H | I | J | K -----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- | | | | | | A | M | | | 1 | | | | | | | | | | 2 | | | | | | | | | | 3 | | | | | | | | | | 4 | | | | | | | | | | 5 | | | | | | | | | | 6 | | | | | | | | | | 7 | | | | | | | | | | 8 | | | | | | | | | | 9 | | | | | | | | | | 10 | | | | | | | | | | 11 | | | | | | | | | | 12 | | | | | | | | | | | | | | | | | | | | -----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

FORM I.--_continued_.

Column Headings:

A: Model B: Weight of (Rubber) Motor C: Kind of Rubber, Flat, Square or Round D: Lenght in Inches and Number of Strands E: Number of Turns F: Condition at End of Flight G: Number of Propellers (No.) and Diameter (Diam.) H: Number of Blades I: Disc Area (DiscA.) and Pitch (Pitch) J: Percentage of Slip K: Thrust L: Torque in Inche-Ounces

----+----+----+-----+----+----+-----+----+-----+----+----+----+ A | B | C | D | E | F | G | H | I | J | K | L | ----+----+----+-----+----+----+-----+----+-----+----+----+----+ | | | | | | | | | | | | | | | 1 | | | | | | | | | | | | | | | 2 | | | | | | | | | | | | | | | 3 | | | | | | | | | | | | | | | 4 | | | | | | | | | | | | | | | 5 | | | | | | | | | | | | | | | 6 | | | | | | | | | | | | | | | 7 | | | | | | | | | | | | | | | 8 | | | | | | | | | | | | | | | 9 | | | | | | | | | | | | | | | 10 | | | | | | | | | | | | | | | 11 | | | | | | | | | | | | | | | 12 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | ----+----+----+-----+----+----+-----+----+-----+----+----+----+