The Theory and Practice of Model Aeroplaning Part 12

-- 16. Apart from or in conjunction with skids we have what are termed "shock absorbers" to lessen the shock on landing--the same substances can be used--steel wire in the form of a loop is very effectual; whalebone and steel springs have a knack of snapping. These shock absorbers should be so attached as to "give all ways" for a part side and part front landing as well as a direct front landing. For this purpose they should be lashed to the main frame by thin indiarubber cord.

-- 17. In the case of a biplane model the "gap" must not be less than the "chord"--preferably greater.

In a double monoplane (of the Langley type) there is considerable "interference," i.e. the rear plane is moving in air already acted on by the front one, and therefore moving in a downward direction. This means decreased efficiency. It can be overcome, more or less, by varying the dihedral angle at which the two planes are set; but cannot be got rid of altogether, or by placing them far apart. In biplanes not possessing a dihedral angle--the propeller can be placed _slightly_ to one side--in order to neutralise the torque of the propeller--the best portion should be found by experiment--unless the pitch be very large; with a well designed propeller this is not by any means essential. If the propeller revolve clockwise, place it towards the right hand of the machine, and vice versa.

-- 18. In designing a model to fly the longest possible distance the monoplane type should be chosen, and when desiring to build one that shall remain the longest time in the air the biplane or triplane type should be adopted.[40] For the longest possible flight twin propellers revolving in opposite directions[41] are essential. To take a concrete case--one of the writer's models weighed complete with a single propeller 8 oz. It was then altered and fitted with two propellers (same diameter and weight); this complete with double rubber weighed 10 oz. The advantage double the power. Weight increased only 20 per cent., resistance about 10 per cent., total 30 per cent. Gain 70 per cent. Or if the method of gearing advocated (see Geared Motors) be adopted then we shall have four bunches of rubber instead of two, and can thereby obtain so many more turns.[42] The length of the strands should be such as to render possible at least a thousand turns.

The propellers should be of large diameter and pitch (not less than 35 at the tips), of curved shape, as advocated in -- 22 ch. v.; the aerofoil surface of as high an aspect ratio as possible, and but slight camber if any; this is a very difficult question, the question of camber, and the writer feels bound to admit he has obtained as long flights with surfaces practically flat, but which do, of course, camber slightly in a suitable wind, as with stiffer cambered surfaces.

Wind cambered surfaces are, however, totally unsuitable in gusty weather, when the wind has frequently a downward trend, which has the effect of cambering the surface the wrong way about, and placing the machine flat on the ground. Oiled or specially prepared silk of the lightest kind should be used for surfacing the aerofoils. Some form of keel, or fin, is essential to a.s.sist in keeping the machine in a straight course, combined with a rudder and universally jointed elevator.

The manner of winding up the propellers has already been referred to (_see_ chap. iii., -- 9). A winder is essential.

Another form of aerofoil is one of wood (as in Clarke's flyers) or metal, such a machine relying more on the swiftness of its flight than on its duration. In this the gearing would possibly not be so advantageous--but experiment alone could decide.

The weight of the machine would require to be an absolute minimum, and everything not absolutely essential omitted.

It is quite possible to build a twin-screw model on one central stick alone; but the isosceles triangular form of framework, with two propellers at the base corners, and the rubber motors running along the two sides and terminating at the vertex, is preferred by most model makers. It entails, of course, extra weight. 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. The propeller speed has already been discussed (_see_ ch.

v., -- 15). The model will, of course, be flown with the wind. The _total_ length of the model should be at least twice the span of the main aerofoil.

FOOTNOTES:

[39] This is a good plan--not a rule. Good flying models can, of course, be made in which this does not hold.

[40] This is in theory only: in practice the monoplane holds both records.

[41] The best position for the propellers appears to be one in front and one behind, when extreme lightness is the chief thing desired.

[42] Because the number of strands of rubber in each bunch will be much less.

[43] Mr. Burge Webb claims a record of 500 yards for one of his.

[44] Flying, of course, with the wind. _Note._--In the "Model Engineer" of July 7, 1910, will be found an interesting account (with ill.u.s.trations) of Mr. W.G. Aston's 1 oz. model, which has remained in the air for over a minute.

CHAPTER IX.

THE STEERING OF THE MODEL.

-- 1. Of all the various sections of model aeroplaning that which is the least satisfactory is the above.

The torque of the propeller naturally exerts a twisting or tilting effect upon the model as a whole, the effect of which is to cause it to fly in (roughly speaking) a circular course, the direction depending on whether the pitch of the screw be a right or left handed one. There are various devices by which the torque may be (approximately) got rid of.

-- 2. In the case of a monoplane, by not placing the rod carrying the rubber motor in the exact centre of the main aerofoil, but slightly to one side, the exact position to be determined by experiment.

In a biplane the same result is obtained by keeping the rod in the centre, but placing the bracket carrying the bearing in which the propeller shaft runs at right angles horizontally to the rod to obtain the same effect.

-- 3. The most obvious solution of the problem is to use _two_ equal propellers (as in the Wright biplane) of equal and opposite pitch, driven by two rubber motors of equal strength.

Theoretically this idea is perfect. In practice it is not so. It is quite possible, of course, to use two rubber motors of an equal number of strands (equality should be first tested by _weighing_). It should be possible to obtain two propellers of equal and opposite pitch, etc., and it is also possible to give the rubber motors the same number of turns. In practice one is always wound up before the other.

This is the first mistake. They should be wound up _at the same time_, using a double winder made for the purpose.

The fact that this is _not_ done is quite sufficient to give an unequal torsion. The friction in both cases must also be exactly equal. Both propellers must be released at exactly the same instant.

Supposing _all_ these conditions fulfilled (in practice they never are), supposing also the propellers connected by gearing (prohibitive on account of the weight), and the air quite calm (which it never is), then the machine should and undoubtedly would _fly straight_.

For steering purposes by winding up one propeller _many more times_ than the other, the aeroplane can generally speaking be steered to the right or left; but from what I have both seen and tried twin-screw model aeroplanes are _not_ the success they are often made out to be, and they are much more troublesome to deal with, in spite of what some say to the contrary.

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 meeting air currents of varying velocity and direction, all tending to make the model deviate more or less from its course; the best way, in fact, the only way, to successfully overcome such is by means of _speed_, by giving the aeroplane a high velocity, not of ten or twelve to fifteen miles an hour, as is usual in built up fabric-covered aerofoils, but a velocity of twenty to thirty miles an hour, attainable only in models (petrol or steam driven) or by means of wooden or metal aerofoils.

-- 5. Amongst devices used for horizontal steering are vertical "FINS."

These should be placed in the rear above the centre of gravity. They should not be large, and can be made of fabric tightly stretched over a wire frame, or of a piece of sheet magnalium or aluminium, turning on a pivot at the front edge, adjustment being made by simply twisting the fin round to the desired angle. As to the size, think of rudder and the size of a boat, but allow for the difference of medium. The frame carrying the pivot and fin should be made to slide along the rod or backbone of the model in order to find the most efficient position.

-- 6. Steering may also be attempted by means of little balancing tips, or ailerons, fixed to or near the main aerofoil, and pivoted (either centrally or otherwise) in such a manner that they can be rotated one in one direction (tilted) and the other in the other (dipped), so as to raise one side and depress the other.

-- 7. The model can also be steered by giving it a cant to one side by weighting the tip of the aerofoil on that side on which it is desired it should turn, but this method is both clumsy and "weighty."

-- 8. Another way is by means of the elevator; and this method, since it entails no additional surfaces entailing extra resistance and weight, is perhaps the most satisfactory of all.

It is necessary that the elevator be mounted on some kind of universal joint, in order that it may not only be "tipped" or "dipped," but also canted sideways for horizontal steering.

-- 9. A vertical fin in the rear, or something in the nature of a "keel," i.e. a vertical fin running down the backbone of the machine, greatly a.s.sists this movement.

If the model be of the tractor screw and tail (Bleriot) type, then the above remarks _re_ elevator apply _mutatis mutandis_ to the tail.

-- 10. It is of the most vital importance that the propeller torque should be, as far as possible, correctly balanced. This can be tested by balancing the model transversely on a knife edge, winding up the propeller, and allowing it to run down, and adjusting matters until the torque and compensatory apparatus balance. As the torque varies the mean should be used.

In the case of twin propellers, suspend the model by its centre of gravity, wind up the propellers, and when running down if the model is drawn forward without rotation the thrust is equal; if not adjustment must be made till it does. The easiest way to do this _may_ be by placing one propeller, the one giving the greater thrust, slightly nearer the centre.

In the case of two propellers rotating in opposite directions (by suitable gearing) on the common centre of two axes, one of the axes being, of course, hollow, and turning on the other--the rear propeller working in air already driven back by the other will require a coa.r.s.er pitch or larger diameter to be equally efficient.

FOOTNOTE:

[45] These remarks apply to rubber driven motors. In the case of two-power driven propellers in which the power was automatically adjusted, say, by a gyroscope as in the case of a torpedo--and the _speed_ of each propeller varied accordingly--the machine could, of course, be easily steered by such means; but the model to carry such power and appliances would certainly weigh from 40 lb. to 60 lb.

CHAPTER X.

THE LAUNCHING OF THE MODEL.