CHAPTER III. Trussing.
The strength of the glider lies in its proper trussing with piano wires which when tightened up should so brace the framework that it will support without appreciable sag or strain, a heavy man hanging from the arm pieces and the ends of the planes resting on a pair of carpenters’ horses.
Two methods of trussing the planes are illustrated in Fig. 22. The machine is divided into five cells the vertical boundaries of which are formed by the stanchions. .
The first method illustrated is the one used in this case for the glider. It is somewhat simpler than the second and does not require the use of any turnbuckles.
Each wire is fastened to one of the eyebolts on the horizontal beams and then run diagonally across to the socket on the opposite beam in the other plane, considering front and rear to be opposed.
Four of these diagonal wires, represented by J in Fig. 23 brace each of the four large cells. The middle cell cannot be trussed up in this manner because the wires would interfere with the body of the operator. So the rectangles formed by the two centre struts with the upper horizontal beams and the two centre rear stanchions with the rear horizontal beams of the upper and lower planes, are braced by means of wires running across their diagonals.
Fig. 22.—Trussing Of Cells.
The rudder is stiffened and trussed to the planes by sixteen wires. Two of these F and H run from the top of the vertical rudder plane to the lower sockets in the rear, 4 1/2 feet from the ends of the planes. The corresponding pair E and G run from the bottom of the rudder to the top sockets of the same stanchions. Four wires A, B, C, D steady the horizontal plane and run from its corners to the sockets in which the rudder beams are stepped on the frame of the glider itself. The remaining eight, indicated by I in the illustration brace the horizontal and vertical planes of the rudder to each other.
Fig. 24 illustrates the method of anchoring piano wires.
The wire is first passed through a short piece of 1/8 inch copper tubing about 3/8 of an inch long, then through the eyebolt. The end is doubled back passed through the tube again but now in a reverse direction. By bending the extreme end of the wire over in the form of a hook and shoving the tube down close to the eye bolt, the wire is secured and cannot pull out. The other end of the wire is fastened in the same manner but before the end is bent over into a hook, the wire must be first pulled tight.
Fig. 23.—Plan and Elevation Views of Piano Wire Bracing.
After fastening all of the wires their tension may be regulated by turning the nuts on the lower ends of the eye bolts. It is very necessary that the frame should be perfectly true and not warped or twisted. Otherwise the machine will be very hard to balance and manage when making a glide. Especially must the rudder be true with the rest of the machine.
Fig. 24.—Method of anchoring wires
Since there are no eyebolts about the rudder which could be used to tighten or loosen the truss wires, a turnbuckle must be included in each wire for that purpose.
Turnbuckles. The construction of these turnbuckles which are very simple and inexpensive is illustrated in Fig. 25. They are made of a bicycle spoke and nipple by cutting off one end of the spoke and using the part which is threaded. The end of this piece is bent back and twisted into an eye. A piece of 1/16 inch sheet brass 1/2 x 3/8 inch has a hole bored in its centre, the diameter of which is such that it will just admit the spoke nipple. The nipple is prevented from passing all the way through by the shoulder on one end. A piece of sheet iron 1/2 inch wide and 3 inches long has a similar hole bored in its centre. The ends of this strap are rounded and bored so that the piano wire may be passed through. The turnbuckle is then assembled and connected as shown in the illustration. The tension of the wire is regulated by turning the spoke nipple while the spoke itself is held rigid.
Fig. 25.—Bicycle spoke turnbuckle.
The second method of bracing illustrated in Fig. 22 requires that a turnbuckle be included in the diagonals of every rectangle, except those formed by the stanchions with the horizontal beams. This method is used on almost all aeroplanes and is considered the strongest but the first method is plenty strong enough for an ordinary glider. If after trussing, the machine is found to be warped or twisted, it must be trued up. By sighting along the horizontal beams and tightening or loosening the necessary wire any curvature may be easily corrected.
The second method of trussing is considerably harder to true up than the first, since when one diagonal of a rectangle is tightened, the other must be loosened. But since it makes an exceedingly firm and rigid structure, it may be well recommended to those who care to undergo the added expense and labor involved by the extra turnbuckles and wires.
To take the glider apart, first remove the bolts holding the rudder beams in the sockets on the machine. Then unfasten the wires which brace the rudder to the machine by loosening the turnbuckles until the spokes and nipples unscrew and come apart. The rudder may now be removed from the machine.
Next take off all the nuts on the eye bolts in the lower plane and pull the eyebolts out of the sockets. The two planes will then come apart. Remove the stanchions by pulling them out of the sockets. The two planes are then laid one on top of the other and will occupy very little room.