Model Flying Machines

by Alfred Powell Morgan

CHAPTER III. PLANES AND RUDDERS. ELEVATORS AND TAILS.

In Chapter I it was explained that an aeroplane is fundamentally composed of a supporting surface, divided into one or two parts, usually the planes or wings, which cut the air in an oblique manner, driven by a propeller and motor. Before going further it is perhaps best to understand more exactly how the planes operate and support the machine in the air than it was possible to explain in the first chapter without confusion. A theoretical aeroplane consists of a flat surface or plane. When the propeller is set into motion the plane is driven through the air in an oblique manner and compels the gaseous molecules to glide under its surface. Since the plane is at an angle, the front edge being higher than the back, the air must necessarily leave at the rear in a downward direction. The air molecules in traveling under the surface exercise a resistance upon it which is really a pressure against the plane. When this pressure is resolved into its components, it is found to be made up of two forces, one horizontal, tending to retard the forward motion, and called the drift; the other, vertical and tending to lift the plane.

The centre of these forces is not as might be supposed at the centre of the plane, but at a point between the centre and the front edge called the "centre of pressure." The centre of pressure approaches the front edge as the angle of the plane with the horizontal becomes less.

In order to render a better idea of how it is possible for an aeroplane to gain support in the air consider a skater moving swiftly over very thin ice which would not bear his weight, but since he is moving so rapidly that any one portion of the ice does not have time to bend to the breaking point, he is supported. In somewhat the same manner, the planes pass so rapidly on to new and undisturbed bodies of air, and stay over one body for so brief an instant that there is no time to completely overcome the inertia of the air and force it downwards.

FIG. 12. The action of the air upon a curved and a flat plane. We have seen that by the effects of the resistance of the air, an aeroplane may be sustained in the atmosphere. We must now see in what manner we can use these effects to the greatest advantage.

First of all, we have been continually speaking of a "plane" as the supporting surface, which from the definition of the word would lead one to believe that they were flat. If the wings of a bird are examined, it will soon be noticed that they are concave underneath. Since the first attempts at aviation, therefore, machines have been built with planes or wings concave on the underside. The reason for this is very apparent from Fig. 12. The first illustration shows the action of a flat surface moving through the air. The air streams, as represented by the lines do not follow the surface of the plane, but leave a considerable region of dead air. This is the reason that a flat plane is very inefficient and not capable of giving so great a lift as the curved plane in the next figure where the lines follow the outline of the plane. The less disturbance a plane causes in the surrounding air, the closer it is said to approach to "stream line form." A correctly curved plane is considerably more effectual than a flat one, giving at the same time greater "lift" and less "drift."

Built-up Planes, that is, planes having a double curve approaching true stream line form, come nearer being the ideal plane than any other from some standpoints, but do not possess any advantages when used on models of less than four feet spread.

FIG. 13. Section of a built-up plane showing how a rib is made. When made small, they offer greater "drift" or head resistance than a single curved surface plane and cannot because of the delicate structure necessary to make them light, withstand hard knocks. They have the further disadvantage of being from a constructional standpoint very hard to make smooth and rigid.

There are innumerable substances which would at first seem to recommend themselves as material for planes, but we may immediately thrust the greater portion aside. By all means avoid tracing cloth or linen, not only because its heavy weight forever precludes it from this use, but because it wrinkles and cockles so as to be absolutely useless when slightly damp or wet.

Tissue paper wrinkles easily and is not strong enough.

Jap silk is an excellent material for fabric covered planes, being at once light and strong. However, by far the most satisfactory plane of this kind is formed by silk bolting cloth which has been coated with collodion. The collodion is brushed on with a fine camel's hair brush after the fabric is in place and it is thereby rendered both waterproof and air-tight.

Fabrics should always be stretched over the planes from end to end and not front to back or vice versa. Make the lap joints or pockets around the end spars as long as possible so that they will not draw "dead air" and impede the forward motion of the machine.

Bamboo Paper is one of the best materials for covering the planes of a model aeroplane and is to be highly recommended. It is made in Japan from bamboo fibre and is very strong. It is usually stretched tightly over the framework and then given two coats of collodion or, what is much better, bamboo varnish.

The framework of the planes may be made of rattan, split bamboo, spruce, or steel piano wire. Piano wire is excellent for small machines since it is springy and light and able to withstand shocks. It is easily bent to any shape and offers considerably less head resistance than rattan because of its small diameter. Rattan can be bent into almost any shape by wetting.

Nothing is better for the cross pieces, ribs, etc., of the planes or framework than split bamboo. Bulk for bulk it is heavier but infinitely stronger than other woods. It is easily worked and can be bent into all kinds of shapes. Bamboo must always be bent while hot. The best source of heat is a spirit lamp or a bunsen burner. Always bend toward the hottest side. When bent apply a cold wet rag to cool quickly. If bent more than necessary, it may be straightened by applying heat again and allowing it to straighten itself.

In order to make long bends, such as the ends of planes, alighting skids, etc., first wind a strip of wet rag around, the bamboo and allow it to remain on for ten or fifteen minutes. Then remove the rag, heat the bamboo in a flame and bend slowly.

PLATE III.

With a little care, strips several feet long may be easily split from bamboo rods. The best method of accomplishing this is to use a fine saw, but a sharp knife will often be successful.

FIG. 14. How ribs may be joined to the long members.

Planes of any considerable size require ribs to support and hold the fabric in shape. Split bamboo is one of the best materials for this purpose. Two very good methods of joining the ribs to the long members of the planes are illustrated in Fig. 14. In the first, a strip of thin sheet aluminum is bent around the rib and spar and fastened by lashing with silk thread. Care must be taken to file off all sharp edges on the aluminum which might otherwise cut the thread. The second method is the neatest and probably the best, since the rib cannot so easily twist or slip out of place.

Wood Planes. In spite of the many advantages of fabric planes they cannot approach wooden planes for efficiency on a small machine. Wood is strong, light and does not change its adjustment.

Whitewood and spruce are the best materials for the purpose. Do not endeavor to saw out the wood. Use a carpenter's plane as much as possible in the work. A saw tears the fibres of the wood and will make the finished plane full of tiny splits.

The wood, however, may be sawed down to a thickness of 5/32 of an inch and then planed down from that. The finished plane should be about 1/16 of an inch thick.

When planing down the wood do not butt one end against a bench stop, because as the wood becomes thin, the pressure exerted by the plane against the wood will cause it to rise in the middle and thereby become thinner at that part. Instead, use a clamp to fasten the wood at one end to the bench and plane away from the clamp—Plane down to a smooth surface and avoid the use of sand-paper.

FIG. 15. Form for bending the planes.

Forming the Curve by steaming and bending the wood is a very poor method. It soon becomes distorted and warped.

FIG. 16. A good method of building a wooden plane.

The best method is illustrated in Fig 16. A piece of wood of the same length as the completed plane and having a cross section like that at A is glued to the forward under edge of a flat plane B. After the glue has hardened, the plane is worked down to the shape shown at D which is very close to the stream line form. The plane is then varnished to prevent it from absorbing moisture and losing its shape. The ends may be covered with thin Jap silk, carefully glued on to prevent splitting. The Wright brothers cover the blades of the propellers on their aeroplanes with silk for the same purpose.

Air does not flow smoothly when changing from an interrupted flow to an uninterrupted flow around a square corner and so by rounding the ends of the planes, the disturbance at that point is somewhat eliminated.

Planes having rattan or piano wire edges cannot very well be of any other shape than those which are illustrated in Fig. 17.

FIG. 17. Various shapes a plane may take.

It is a good plan to give wooden planes the shape shown by 3 and 4 in Fig. 17, as the disturbances mentioned above are not so marked.

FIG. 18. An edgewise view of several planes showing the different ways they may be bent to secure stability.

The planes of large man-carrying machines possess the same characteristics, but not to such an alarming extent as in a model. The Voisin aeroplanes overcome the objection by the use of vertical panels set between the planes.

The angles at which the planes are set may vary from 1 in 6 to 1 in 20. One in ten might be called the "happy medium." If the planes are given too great an angle, the drift becomes so great that the propeller thrust is severely taxed. The smaller the angle, the less will be the drift and consequently the greater the speed. However, if the surface is curved the angle must not be made too small or not much lift will result.

FIG. 19. The various ways two planes may be combined to secure stability or form a biplane.

The angle of the tail planes should be adjustable. If too great, the machine will slow down and the tail will drop, destroying the equilibrium of the machine and consequently the flight. If the lift of the tail is too great, however, it will cause that part to rise and the machine will dive downwards.

Elevators and Tails are usually made of thin wood or fabric stretched over a rattan or wire framework. They are usually rectangular or elliptical in shape.

In case they are made of wood one of the best methods of attachment is to fasten the plane to a small stick by means of two or three small rivets. The stick is secured to the framework of the machine by two small rubber bands. Then in case the machine strikes head on in alighting, the band will absorb the shock and permit the elevator to move so that it is not damaged by the fall.

Vertical Fins. It is a much mooted question whether or not a vertical fin is of any value on a model aeroplane since a good model should be so designed that it will fly in a straight line without the use of a rudder. It has been the author's experience that it is often of decided advantage in correcting the flight of an "erratic machine" or in compensating any little difference that there may result in the drift of the two halves of the planes.

FIG. 20. Fins.

The fin should be placed well toward the rear of the machine and, whenever possible, stretched both above and below the centre line of the machine, so that the pressure due to cross winds will be equal both above and below and there will be no tendency for the machine to twist about its longitudinal axis.

When it is not possible to place the fin both above and below the centre line it should be placed above rather than below.

Fins may be made out of thin wood, sheet aluminum or fabric stretched over a wire or rattan framework.