Wash-in and Wash-out.-The turning of the propeller produces a tendency to turn the whole airplane around in the opposite direction to that in which the propeller is running. This tendency was very marked in some of the earlier machines, especially the small monoplanes. This is overcome in some machines by increasing the angle of incidence of the plane on the side which would tend to tip down and in some cases to decrease the angle of incidence on the other side. By so doing there is slightly more lift on one side of the machine than on the other, which corrects the tendency to turn around the center line of thrust. Two terms which are used in this connection are "wash-in" and "wash-out." When the angle of incidence increases from the center to the end of the plane it is called "wash-in," and when it decreases from the center to the ends it is called "wash-out." This is clearly shown at Fig. 102 C.

Tail Assembly. The horizontal stabilizer, vertical stabilizer, rudder, and elevators are assembled to form the empennage. As shown at Fig. 106, the horizontal stabilizer is mounted at the rear end of the fuselage with its lower surface resting on the top edge of the upper longerons. A system of struts arranged from the under side of the stabilizer to the lower longerons and tail post anchors the stabilizer to the fuselage in a fore-and-aft direction. The vertical stabilizer is anchored on the upper center line of the horizontal stabilizer by suitable clips and tie-down cables.

The rudder is hung from the end edge of the vertical stabilizer and tail post of the fuselage. The guy lines from the control braces to the trailing edge are so fixed as not to interfere with the elevators during any position of operation. The upper edge of the rudder is in a continuous line with the leading edge of the vertical stabilizer. The elevators are arranged on the trailing edge of the horizontal stabilizer. The inner edges of the elevators are fixed so as to permit of operation of the rudder through an arc of at least 30 degrees each side of the fore-and-aft center line.

Landing Gear. The landing gear is of the "V" type crossbraced construction. It is composed of two trusses, properly

separated and cross-braced. The lower ends of the members of each side truss end in the fittings of the continuous cord shock absorber bridge. The landing gear is connected to the lower longerons with proper fittings. The axle is properly streamlined. The bridge is so aligned vertically as to permit an upward and downward movement of the landing gear axle. The shock absorbing bridge is of the style known as the continuous rubber cord shock absorber.

The shock absorbing unit of the bridge is a continuous built-up rubber cord covered with fabric. This cord is firmly

Fig. 107. Rear View of Fuselage with Horizontal Stabilizer Attached. wound around the axle saddle, which passes through the steel bridge and rests over the axle on both sides of the struts. The bridge itself is a lightened steel member with a slotted arrangement allowing the vertical movement of the axle. This guide for controlling the vertical movement is curved in a transverse direction to accommodate the vertical rotation of the axle about one wheel in case of a side landing.

Horizontal Stabilizer. This member is assembled to the fuselage after the upper longeron is levelled up. Each upper longeron has one U bolt and one special bolt to fasten down the

Vertical Stabilizer

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horizontal stabilizer. This U bolt is just ahead of the tail and passes under the longeron with the legs pointing upward. These bolts extend through the stabilizer and are fastened with nuts. They serve to hold the leading edge of the stabilizer. Two special bolts are arranged at the tail of the machine so that they extend through the horizontal stabilizer, one on each side of the vertical stabilizer. These bolts also extend through a small L-shaped piece on each side, which is fastened to the vertical stabilizer. This fastens both stabilizers

Fig. 108. Rear View of Fuselage Showing Vertical Fin Assembled and Stabilizer Braces in Place.

to the tail of the fuselage. These two bolts are flattened on their lower ends so that they rest against the tail post and are held to it by one bolt running through and by two screws, one on each side. All nuts are castellated and fastened with cotter pins. (See Fig. 107.)

Vertical Stabilizer. Next, the vertical stabilizer is fastened to the horizontal stabilizer with the bolts which pass through the fore-and-aft parts of the horizontal stabilizer and with the hard wire stay lines running to the upper surface of the horizontal stabilizer from the top of the vertical stabilizer. The

forward bolts pass through the clip at the lower front point of the vertical stabilizer. The bolts which are fastened to the tail post of the fuselage, and engage the after end of the horizontal stabilizer, also engage the lugs fastened to the bottom edge of the vertical stabilizer at the rear. The nuts should be drawn up tight and locked with cotter pins. To align the vertical stabilizer hard wire lines and turnbuckles are used. (See Fig. 108.)

Elevators. In assembling the elevators, first put on the control braces which will be found with all necessary bolts,

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nuts, and cotters in the case with the wing panels. The position of the base of the control brace is indicated on Fig. 123. The upper tips of these braces point to the hinge line. Hinges and hinge pins are used to mount the elevators to the horizontal stabilizer. Cotter pins are used to keep the hinges in place, and are inserted through the holes drilled in the bottom of the hinge pins. (See Fig. 110.)