F. J. Camm

“ One of the most important units of an aeroplane, whether full-size or model, is the screw, since excellence of design with regard to the other portions of the machine are rendered void if the means of converting the power of the engine into work are inefficient.
The action of an air-screw may be likened to a bolt turning in a nut (the screw being the bolt and the air the nut) , the difference being that whereas one turn of a bolt with, say, a Whitworth pitch of 14 threads per inch in a nut is bound to advance a distance equal to the pitch = 1/14 in., an air-screw may only advance 75 per cent. ”

“ It can now be taken as a general rule that a plant will fly a machine three times the weight of its thrust. Hence a plant developing 3-oz. thrust would fly a machine weighing 9 oz. or 10 oz. But, since the thrust of a compressed-air engine is not constant, gradually diminishing as the pressure in the container grows less, for a machine weighing 9 oz. (assuming the machine to be built first) the plant will require to develop about 5 oz. initial thrust. The diameter of the propeller is dependent on the most efficient speed of the particular motor employed. ”

“ On certain types such a simple motor is not possible. In order to concentrate the weight more at one point, the rubber and its struts have to be shortened; and to get the necessary number of revolutions of the propeller a gearing of two to one, three to one, or four to one, as the case may be, must be used. By this means the small number of turns which can be got on a short thick skein of rubber of great power will still give the number of propeller revolutions required to make a good flight, just the same as with a motor of ordinary thickness and of great length. ”

“ Great diversity of practice exists with regard to the construction of model air-screws, some aero-modellists favouring small diameter with long pitch, others long diameter and short pitch, and still others who adhere to either bentwood or carved screws in either of the above forms. Generally speaking, a screw with a large diameter in proportion to short span has a short pitch, say one and a quarter times the diameter, while those having a short diameter in relation to span should have a fairly long pitch, from one and a half to twice the diameter. ”

“ One of the difficulties that beset the juvenile kite-flyer is the inability to get the kite to rise from the ground unless there is a fairly good wind blowing. Even then the services of an assistant are required, whilst numerous trials to adjust the amount of weight on the tail, the position of the carrying thread on the kite, and the distance required to be run to get the artificial wind to enable the kite to rise above the surrounding buildings, are difficulties that damp the enthusiasm of the most inveterate kite-flyer. ”

“ There are advocates for each form of construction, and it is difficult to state definitely which is the best practice, each having equally good results. The wire plane, especially when steel wire of the music or piano variety is used, is much stronger, offers less resistance to the air, and has a neater appearance than the others, but it is slightly the heavier. A wooden plane can also be made extremely neat and light, although it is a little weak. Birch is the best wood to use for this purpose, as it is extremely tough and not too heavy. ”

“ There is one common enemy to aeroplanes—the force of gravity. Were it [Pg 2] not for the existence of this force, which, as Newton put it, “is unseen and unheard and yet dominates the universe,” the problem of the aeroplane would have been solved years ago.
Fig. 1.—Bristol Monoplane and Biplane
Most readers have handled the toy kite, and since the principles governing the flight of a kite are precisely the same as those which apply to the aeroplane, the latter will be the more readily understood if the principles are explained through this medium. ”

“ If a model has 1 sq. ft. of surface for every ounce of its weight, there is a speed at which the main surface will give a maximum of lift for a minimum of power, and a screw must be fitted whose pitch, multiplied by its revolutions per minute, equals the distance per minute the model should fly. If a screw that is too fast is fitted the model will show a tendency to “stall,” or ascend nose first, and if too slow a one is used the model will appear to be under-powered. ”

“ It has been the aim of the writer to bring the model within the constructional capabilities of the amateur; indeed, it is hardly possible to have simplified the construction further. Now, the success of a dirigible, whether full size or model, depends primarily on the observance of the fact that an airship is lighter than air, and thus, unlike the aeroplane, does not rely on speed to obtain lift. Secondarily, the lifting power of hydrogen must be remembered, and although this varies according to temperature and the purity of it, it may be taken as a general rule that hydrogen will lift 80 lb. ”

“ A two-membered fuselage can be adapted from this design by omitting the bottom member and skid. Such a fuselage would be suitable for a light machine.
It is an essential point with tractor models to fit a chassis; the purpose thus being twofold. First, it protects the propeller, and secondly, it obviates the characteristic tendency of tractor machines to ascend “nose first,” by keeping the weight low (in technical language, providing a low centre of gravity) . Hand-launched tractor machines that are unprovided with a landing gear are seldom successful and notoriously troublesome. ”

“ It will be noticed that the air has a tendency to leak [Pg 11] over the end of the square plane, which is obviated by the tapered wing.
It may be thought that such details as these are unimportant; but when it is remembered that an aeroplane, correctly streamlined, will fly for one-half the power required to fly a machine not so designed, the enormous saving in power will be manifest.
The actual thrust required to lift a model aeroplane is roughly equal to a quarter of its total weight. Thus a model weighing 6 oz. will require 11⁄2-oz. thrust. ”

“ Although planes so built are exceedingly strong, it is not possible to make quite so neat a job of them as with spruce or maple.
Another method of building main planes is to use spruce or birch spars with piano-wire ribs, these latter being bound to the former.
For single-spar models the main spar should be tapered fore and aft from a point one-third of the length from the front of the machine. Where it is necessary to pierce the spar of a model aeroplane for the reception of a kingpost or other member, silk tape binding should be used, the joint being soaked with clean, weak glue. ”

“ The weight of a ship must equal the weight of water it displaces in order to float. Similarly an aeroplane, by its motion through the air, must deflect a volume of air equal at least to its own weight. The aeroplane then would just lift itself from the ground; and the more air it deflects the higher does it ascend. ”

“ Now if we start pumping hydrogen in it will make the contrivance heavier instead of lighter, because no outlet is provided for the escape of air. This could easily be remedied by standing the model on its end with the cycle valve at the top, with a valve or tap on the bottom brass cap. Open this bottom tap to allow the air to escape, and allow the hydrogen to fizz in through the cycle valve. Hydrogen will not escape through the bottom tap until all the air has been displaced. If the hydrogen cannot be had at a high enough pressure to operate the valve itself a valve should not be used. ”

“ Again, if the wind were suddenly to stop, and the engine and air-screw were capable of moving the kite forward at the same rate at which the wind was blowing, the kite would fly, and in all important respects would constitute an aeroplane.
The kite, it will be assumed, requires a minimum speed of fifteen miles per hour in order to sustain itself. If the wind be blowing at fifteen miles an hour the operator can remain stationary. If it blows at ten miles an hour he must run at five miles an hour against the wind. ”

“ The use of gears is obvious; they eliminate torque on the spars, or the tendency which a single skein would have to twist the spar. Pieces of tube are passed over the shafts to bear between the gears and the gear bearing. In order to counteract the tendency of the rubber hooks to pull out straight when the rubber is in tension, the hook ends are secured as shown at A. When it becomes necessary to detach the skeins, it is only necessary to slide the tubes along the shaft to open the hook. ”

“ It is a useful rule to make the diameter approximately one-third the span of the machine for either single-screw or twin-screw machines. This relation seems to give a very small effect on lateral stability, whereas when the diameter is made larger, the machine has a tendency to capsize laterally in the opposite direction to which the screw revolves. This force is known as torque. ”

“ Stability.—The principles underlying the design of a successful flying model aeroplane are almost, if not quite, as complex as those involved in the planning of full-size machines. In some respects perhaps this is more so, owing to the fact that models must be practically automatically stable both longitudinally and laterally, since they are not under any sort of control after they have left the hands of the person flying them. ”

“ The rudder of tractor machines must always be placed above the thrust line and also above the centre of gravity, so that should a side gust strike the machine, the latter does not rock laterally in the air, as a couple is set up between the underhung load and the rudder. ”

“ If the model points its nose in the air it is elevated too much. If the model flies too low it is not elevated enough. In each case it requires adjustment until the precise position is arrived at. The elevator should never be at a greater angle than 8° or less than 5°. If the machine still flies too low with the elevator at 5° the main planes will have to be moved forward slightly; but the exact position is only found by experiment. If it flies too high with the elevator at 5° the planes will have to be put back.
Fig. 89.—The Finished Biplane
Much depends on the way a model is launched. ”

“ Piano wire is used for the landing chassis, and of the same gauge as hitherto used on the spindles.
The triangular side struts of the chassis should first be framed up from one continuous length of wire, lugs being bent at the point where they meet the fuselage, to be bound with fine tinned iron wire and soldered to the spar. Figs. 113, 117 and 118 clearly show its construction. Make the axle of such a length that a 12-in. wheel-base may be left after the wheels are placed on. The writer found that 2-in. rubber-tyred disc wheels left nothing to be desired for rising off short grass. ”

“ To a model aeroplane enthusiast a winder is an enormous acquisition. The converted egg-beater type of winder, so much in evidence, leaves much to be desired, the chief fault being that the bearing spindles wear so quickly, apart from the fact that they are awkward to manipulate single-handed. ”

“ Now, if a 1-lb. weight be laid on a table, the table presses against the weight with a force of 1 lb. If the hand is pressed against the wall, the wall presses back with an equal pressure. If a person fires a revolver, the force of explosion tends to force the revolver and the person in the opposite direction to the travel of the bullet. These are merely illustrations of the law that action and reaction are equal and opposite. It is in reality due to this law that the aeroplane can resist gravity.
Fig. 2.—Deflection of Air
Fig. ”

“ It is much neater to sew the fabric along on the leading edge, as when glue is used an unsightly black smear shows through. The fabric should be stretched from end to end first, the fabric overlaps being glued on the bottom face of each end rib. Drawing-pins should be partially pressed into the ribs to secure the fabric until the glue is set.
At B in Fig. 148 is shown the method of securing the bottom plane to the inter-struts. Convenient notches are cut in the struts into which the plane is sprung. ”

“ For straight flights it is most important that all the planes should be in perfect alignment; but to succeed in making a [Pg 68] twin-propelled model fly perfectly straight is largely a matter of perfect construction.
The model can be steered by means of the elevator (looking at the machine from the propeller end) ; if the elevator is lowered on the left side it will fly round to the left, and vice versa. The adjustment necessary to effect this can be accomplished by means of the slot in one of the elevator uprights. ”

“ It will be obvious that the pitch of a screw should be constant along the whole length of [Pg 40] blade, so that the air is deflected or driven back at a constant velocity. An efficient screw will deliver a solid cylinder of air, whereas an inefficient one delivers a tube of air. ”

“ Care should be taken to ensure that the shaft is quite central sectionally and diametrically.
A method of securing carved screw shafts is shown by Fig. 55, and is self-explanatory. When the elastic skein is in tension it has a tendency to pull the hooks out straight, so releasing the skein, with sometimes painful consequences to the hand. The safety hook shown by Fig. 56 has a brass-tube collar which slides over the end. All the hooks should be covered with valve tubing, to prevent the elastic cutting through. ”

“ Fig. 11 shows the Ridley Monoplane, which secured several well-merited rewards in open competition, and is an excellent machine for distance. Birch should be used for the longerons, preferably of channelled section. The main plane is of piano wire, covered with proofed silk, and the elevator is entirely of extremely thin veneer. Bentwood screws are used fairly short in diameter and of long pitch. The machine is capable of flying a quarter of a mile. The Fairey type of model aeroplane typified in Fig. 12 is a most successful type, and has achieved much in open competition. ”

“ Some efficient wing plans are given by Fig. 37.
Stated briefly, there are essentially three kinds of model aeroplanes. First, the scale model, which is a reproduction to scale of a real machine; second, a modified copy of a large machine, which is so designed as to resemble in general form some well-known prototype, while retaining by means of a suitable motor, generally twisted rubber, some ability to fly; third, a machine which does not in any way follow the lines of full-size machines, and is built for flight only. ”

“ The Umbrella-ribbing Plane.—Umbrella ribbing can be utilised, in conjunction with piano wire, for plane construction as shown by Fig. 62. The channel of the ribbing should be thoroughly cleaned with emery cloth, so that the leading ends of the ribs can be soldered therein. Three spars should be used for spans over 30 in.
For the planes of model flying machines steel wire offers exceptional advantages, as it is practically unbreakable and can be bent to any desired shape. Another advantage is that it offers a minimum resistance when travelling in the air. ”

“ If a kite is launched in a wind it speedily attains a certain height or altitude, at which it remains so long as the wind does not drop. The wind is overcoming gravity, which constantly endeavours to bring the kite to earth, and hence, since the kite remains in the air, the forces acting on the kite are said to be in equilibrium—that is, balanced. The forces are shown diagrammatically in Fig. ”