CHAPTER I. The Framework.

*A gliding machine*, more often popularly termed a glider, is simply a motorless aeroplane, operating by force of gravity to carry its passenger sailing through the air from the top to the foot of a slope.

*The glider* described herein is the type developed by Octave Chanute and may be considered as the parent of the biplane machines with which the world has lately become so familiar. The machine is known as a biplane since its supporting surface is in the form of two superimposed trussed planes vertically above each other and having a tail in the rear for the control of direction.

There is always a tendency among experimenters to depart from the design and dimensions of any machine or apparatus offered for construction. This, since it develops originality is a good indication, but most of those who will undertake to build a glider are attempting something altogether new and so any radical change from the instructions in this little booklet are unadvisable.

It is better at first to benefit by the experience of others. The glider here described is considered as the "standard" of the biplane type. It has an active supporting surface of 152 square feet which is sufficient to carry the weight of an ordinary man. A machine having a larger surface will support the same weight when moving through the air at a slower speed, but larger surface means an increase in some of the general dimensions. An increase in surface by lengthening the planes will make the machine much harder to keep on an even keel, while increasing their depth in the direction of flight will require greater agility on the part of the operator to keep the centre of gravity in the proper position. A larger machine also means more weight and a heavy machine is hard to make a landing with.

On the other hand a light glider is dangerous and will not stand any rough usage.

*The cost of the glider*, provided the construction is accomplished by the intending owner is so low as to place it within the reach of any person of ordinary means. The expenditure for raw materials varies greatly. It is usually a little less than $20.00 and should not exceed $35.00. A finished glider is worth from $50.00 to $100.00 depending whether or not more than one is made at a time.

*Housing.* One of the first considerations is usually the housing and storing of the glider, but the machine under consideration is so designed that it may be quickly taken apart or "knocked down" and be put away in the cellar, under the porch or in some other out of the way place.

*The framework* is composed entirely of selected spruce, straight grained and free from knots. Spruce is very dense and tough but yet one of the lightest of woods.

*The dimensions* given are for the finished pieces after they have been planed up. The usual method of finishing wood for aeronautical work, so that it has a hard glassy surface and offers little resistance to the air is first to give it a thorough brushing over with hot glue and water. It is rubbed down after drying, using fine sand paper. The wood is then given a coat of thin shellac.

This is rather a tedious operation and instead some may prefer to first smooth up the wood by sand papering and giving it a coat of spar varnish.

The corners of all the woodwork are rounded off so as to reduce the resistance offered to the air.

*Horizontal beams*. The principal members of the planes when smoothed up should measure 20 feet long, 1 1/2 inches wide and 3/4 inches thick. Four of these beams are required. In some lumber yards, twenty foot spruce free from knots is very hard to secure and so instead, two 10 foot pieces may be spliced together at the centre as shown in Fig. 1.

The splicing strip is 5 feet long and has the same cross section as the beams, save for a distance of one foot from each end where it begins to taper down to 1/4 inch thick. Six holes are bored through the splicing strip and the beams so that they may be fastened together by means of six 3/16 inch round headed stove bolts. The holes are located so that the space between the two centre bolts is six inches while the others are located one foot apart.

A large washer having a small hole in the centre is placed under the head of each bolt as well as the nut.

*Struts.* Each pair of horizontal beams are held parallel to each other and three feet apart by six horizontal struts. The form of these struts is illustrated in Fig. 2.

They are three feet long and 1/2 x 1 1/4 inches in cross section. A notch 1 1/2 x 3/4 inches is cut in each end so as, to form a projection 1 1/2 x 1/2 x 1/2 inches.

The location of the struts in the plane is illustrated in Fig. 3. The two in the centre are two feet apart and the others respectively 4 feet 6 inches and 9 feet on either side. The struts on the upper plane are placed so that the projections come above the horizontal members. Those on the lower plane are placed just the opposite, that is so that they come on the under side.

They are fastened with one or two small wire nails and then secured by means of a clamp. Two dozen clamps are required. They are bent out of a strip of sheet brass one sixteenth of an inch thick, 3 7/8 inches long and 1 inch wide. The ends are rounded and a 1/4 inch hole located and bored in each as in Fig. 4.

The clamp also serves to protect the under side of the beam from the action of the nuts on the ends of the eyebolts. The method of fastening the clamp is detailed a little later.

*Stanchions*. The planes are separated by twelve stanchions, four feet long and 7/8 of an inch in diameter.

They are rounded and smoothed up so that the ends will fit snugly into the socket illustrated in Fig. 6. These sockets may be purchased¹ already bored and finished or can be procured at a foundry. They are preferably made of aluminum which metal is at once light and strong but brass or even iron may be used if it is necessary to avoid expense.

There are other methods of joining the stanchions to the beams but the use of the socket is recommended because it is the strongest method and also permits the glider to be readily taken apart.

The base of the socket is 3 1/4 inches long, 1 1/4 inches wide and 1/4 of an inch thick. The cup has an internal diameter of seven eighths of an inch and an outside diameter of one inch and one quarter. It is one inch high above the base. Two 1/4 inch holes are bored 1 7/8 inches apart in the base. Two smaller holes 1/8 inch in diameter are bored 7/16 inch nearer the ends of the base than the larger holes.

*The wooden pattern* is made from the dimensions indicated in Fig. 6. It is thoroughly smoothed up by rubbing with sand paper and then given a coat of shellac. All parts should have a very slight taper towards the top so that the pattern may be withdrawn easily from the sand mould.

If the interior of the mould is coated with lamp-black, the castings will require no other finishing than boring the holes.

Two dozen of these sockets are required. Six are fastened to each of the four horizontal members by means of round headed wood screws which pass through the smaller holes in the base. The sockets are located exactly opposite the ends of each strut so that when the stanchions are in place, they will be separated by the same distances but all lie in a plane at right angles to that in which the struts are.

A 1/4 inch hole is bored through the horizontal beam directly under each one of the 1/4 inch holes in the base of the socket. These holes permit an eyebolt to pass through. The eye bolt is illustrated in Fig. 7. The stock is 1/4 inch in diameter and should be at least two inches long under the eye.

The diameter of the eye is one half an inch. These eye bolts are obtainable already threaded and ready for use with a nut and washers, but can be procured somewhat cheaper in blank form and threaded by the purchaser. Four dozen are necessary, two for each socket. The eye bolts pass through the socket and beam, coming out on the under side directly opposite the holes in the strut clamp. A nut placed on the under side as in Fig. 8 will then hold the clamp tightly against the under side of the beam and secure the position of the strut.

*Ribs.* Forty one ribs support the cloth forming the surfaces. They are each one half an inch square in cross section and four feet long.

They are fastened to the horizontal members one foot apart, flush with the front and projecting one foot in the rear. One or two small wire nails are used to fasten the front ends and then a clamp placed over them and screwed down with two No. 5 round headed wood screws, one half an inch long. A small brad awl should be used to make a hole before starting the screw and so avoid any possibility of starting a split in the wood.

The clamps are bent out of sheet copper strips, 2 1/4 inches long and 5/8 of an inch wide. The ends are rounded and a hole bored through which the screws may pass.

The surfaces of the planes are curved to give them an increased carrying capacity and add to the gliding power.

The best method is to steam the ribs and then bend them so that when they dry they will retain their curve and not tend to push the horizontal beams apart. Only a very slight curve should be given and the amount of curvature should be the same for all the ribs.

Some designers construct gliders having flat planes, intending that the pressure of the air underneath the fabric shall produce a natural curve but such a method is exceedingly poor practice and results in a very inefficient machine.

The ribs must be perfectly rigid and the frame of the whole machine strongly trussed so that it cannot possibly be distorted by the air pressure. The following extract from the report of the Smithsonian Institute well illustrates this point.

"This new launching piece did its work effectively and subsequent disaster was, at any rate, not due to it. But now a new series of failures took place, which could not be attributed to any defect of the launching apparatus, but to a cause which was at first obscure; for sometimes the aerodrome, when successfully launched would dash down forward and into the water, and sometimes (under apparently identical conditions) would sweep almost vertically upward into the air, and fall back although the circumstances of flight seemed to be the same. The cause of this class of failures was finally found in the fact that as soon as the whole machine was up-borne by the air, the wings yielded under the pressure which supported them, and were momentarily distorted from the form designed and which they appeared to possess. "Momentarily," but enough to cause the wind to catch the top, directing the flight downward, or under them, directing the flight upward, and to wreck the experiment. When the cause of the difficulty was found the cure was not easy, for it was necessary to make this great _sustaining surfaces rigid_, so that they could not bend.”

The report in question refers to the experiments conducted with Professor Langely’s model aerodrome.

Some experimenters claim that the parabolic curve gives the greatest lift with the least power required for propulsion but it can be safely doubted. The Wright machine is probably the most efficient in existence. Their curve is very nearly the arc of a circle and is not of the parabolic form.

Four per cent is about the proper curve to give the planes of a glider. This is about two inches for ribs four feet long. After fastening the front end of the ribs, curve them up in the centre by pressing down on the loose and at the rear. Then nail the rib to the rear beam with a small wire brad and screw on the clamp. The nails prevent the ribs from slipping longitudinally while the clamps serve to prevent them from moving sideways or pulling off when the fabric is under the pressure of the air.

Fig. 11 is a plan view of the top and bottom planes. Twenty one ribs, each one foot apart are used on the upper plane. Only twenty ribs are required on the bottom surface because an opening two feet wide must be left in the centre for the body of the operator.

*Arm pieces.* The operator is supported in the machine by two strips of wood passing under his armpits. These armpieces are 3 feet long, 1 inch wide and 1 3/4 inches deep.

They are fastened to the horizontal beams by means of a 3/16 inch round headed stove bolt. The distance between should be just wide enough to be comfortable and is variable with the breadth of the operator between his shoulders. Thirteen inches is about the proper distance for the average person. The upper side of the arm pieces is rounded so that they will not be quite so uncomfortable as they would be if left square. It is not a good plan to pad these pieces by wrapping them with cloth for it will impede the movements of the body in balancing.

*Rudder.* The rudder is composed of two planes at right angles to each other and in the rear of the main surfaces. The vertical portion keeps the machine headed into the wind and causes it to glide in the direction in which it is started or head on into the wind. The horizontal rudder steadies the machine longitudinally and prevents the machine from suddenly diving or pitching. Neither of the rudder planes are movable.

The separate parts composing the framework are illustrated in Fig. 13.

The cross section of all the sticks is the same, namely one inch square. The two long beams, _A_, are 8 feet 11 inches long. The two uprights, _B_, each 3 feet 10 inches long from the vertical members of the directional plane. The horizontal plane is made up of six horizontal strips, two of them, _C_, six feet long and four, _D_, two feet in length.

*The horizontal plane* is fitted together with half and half lap joints. It is first fastened with nails and then reinforced with brass corner braces.

Corner braces are also used to strengthen the vertical plane.

*The rudder beams* are stepped into sockets on the body of the machine so that the rudder is detachable.

*A short cross bar* 2 feet inches long and 1 1/4 x 3/4 inches in cross section, is fastened between the two centre struts of both planes at a point eight inches forward of the rear beams.

These cross bars carry one of the sockets mentioned above as also do the rear horizontal beams. The cross bar and sockets in the upper plane should be directly over those in the lower plane but in an inverted position.

*The construction of the sockets* is illustrated in Fig. 17. The smaller one is fastened to the cross bar and is bent out of a strip of 1/16 inch sheet brass 4 1/2 inches long and 3/4 of an inch wide. The larger socket is the same length and thickness but is 1 1/4 inches wide and is fastened to the horizontal beam. Two of each size are required. The ends are rounded and a 3/16 inch hole bored in each so that a 3/16 inch round headed stove bolt may be used to fasten the sockets to the framework.

A hole is bored in the centre of the top of the smaller sockets so that a bolt may be passed through the rudder beam and cross bar to prevent the former from pulling out.

The two sockets in each plane must be in perfect alignment and lie on a line drawn at right angles to the horizontal members through the centre of the planes.

In Fig. 15 it will be noticed that four bolts pass through each plane near the corners. The bolts are 3/16 inches in diameter and serve to fasten the piano wires which brace the vertical and horizontal plane to each other.

The complete framework of the glider without the tie wires and the ribs on the lower plane will appear as in Fig. 19.

¹ From Spon & Chamberlain