Aeroplane Construction and Operation

by John B. Rathbun

Probably the most interesting of all the spars is the Nieuport, which is a combined truss and girder type. This spar tapers down from the center to both ends, being thickest at the points where the guys are attached. The top flange (J), and the bottom flange (L), are ash, while the side plates and diagonals (H) are spruce. The diagonals resist the shear, and are held in place by the tie bolts (I). At the left, the spruce cover plate is removed, while at the right it is in place with the interior construction shown in dotted lines. The dimensions are in millimeters.

Fig. 14. Plate Connection for Monoplane Stay Wire Connection to Spar. A Compression Member or Drag Strut Is Shown in the Center of the Spar Which Takes Up the Thrust Due to the Angularity of the Stays and Also the Drag Stress.

Location of Spars. There are a number of items that affect the location of the spars in regard to the leading edge. The most important factors in the choice of location are: (1) Shape and depth of wing section, (2) Center of pressure movement, (3) Drag bracing requirements, (4) Width of ailerons, (5) Method of attaching the interplane struts.

CHAPTER XI FUSELAGE (BODY) CONSTRUCTION.

Purpose of Fuselage. The fuselage of a monoplane or tractor biplane is the backbone of the machine. It forms a means of connecting the tail surfaces to the main wing surfaces, carries the motor, fuel and pilot, and transmits the weight of these items to the wings and chassis. With the exception of the wing structure, the fuselage is the most important single item in the construction of the aeroplane. Fig. 1 shows a typical arrangement of a two-place biplane fuselage equipped with a water-cooled motor. The motor E, propeller Y, and radiator R are placed in the front of the fuselage, and considerably in advance of the wings W and D. Immediately behind the motor is the passenger's seat A and the fuel tank F. The pilot's seat B is placed behind the trailing edge of the wings and is behind the passenger's seat. The cockpit openings G and H are cut in the fuselage top for the passenger and pilot respectively. The rear extension of the fuselage carries the control surfaces, L being the vertical fin, M the rudder, O the fixed tail or stabilizer, and P the elevator.

Resistance. To reduce the resistance in flight, the fuselage is of as perfect streamline form as possible, the fuselage being deepest at a point about one-third from the front. From this point it tapers out gradually to the rear. With the motors now in use it is only possible to approximate the ideal streamline form owing to the front area of the radiator and to the size of the motor. Again, the projection of the pilot's head above the fuselage adds considerably to the resistance. The wind shields I disturb the flow of air. The connections to the tail surfaces and to the chassis members K also add to the total resistance. An arched "turtle deck" J is generally provided, of such a shape that the pilot's head is effectively "streamlined," the taper of this deck allowing the disturbed air to close in gradually at the rear. The flat area presented by the radiator R is probably the greatest single source of resistance, and for this reason the radiator is sometimes placed at the side of the fuselage, or in some other position that will allow of a better front end outline. An example of this construction is shown by Fig. 2 in which the radiator R is placed behind and above the motor E. The front fuselage end Z can now be made of a more suitable streamline form.

Figs. 1-2-3. Fuselage and Motor Arrangement of Tractor Biplanes.

Fig. 3 is a view of the front end of a typical fuselage in which an air-cooled rotary type of motor is installed. Since the diameter of this type of motor is seldom much less than three feet, it is necessary to have a very great diameter in the extreme front. The motor housing or "cowl" marked E has a diameter "d" which should smoothly blend into the outline of the fuselage at "b." In the older types of construction there was often a very considerable break in the outline at this point, especially in cases where the circular cowl was abruptly connected to a square fuselage. A break of this sort greatly increases the head resistance. A "spinner" or propeller cap marked Z in Fig. 3 is an aid in reducing the resistance offered by the motor cowl and also reduces the resistance of the inner, and ineffective, portion of the propeller blades. The cap in any case is smaller than the cowl opening in order that cooling air be admitted to the enclosed cylinders.

Distribution of Loads. Returning to Fig. 1, we note that the weight of the fuselage, pilot, passenger, fuel, control surfaces and motor are carried to the upper wing W by the "cabane" strut members C-C and stays, the lower wing being connected directly into the sides of the fuselage. Continuations of the cabane members on the interior of the fuselage inter-connect the upper and lower wings (shown in dotted lines). The interplane stays in connection with the cabane members bind the wings and fuselage into a unit. A vertical line "CP" passes through the center of pressure of both wings, and approximately through the center of gravity of the machine. In other Words, the machine is nearly balanced on the center of pressure line. The turning moments of weights behind the CP must approximately balance the opposite turning moments of the masses located in front of the CP. The exact relation between the center of pressure and the center of gravity will be taken up later.

Variable loads such as the passenger, gasoline and oil, are placed as nearly as possible on the center of pressure line, so that variation in the weight will not affect the balance. In the figure, the passenger's seat A, and the fuel tank F are on the CP line, or nearly so. Thus, a reduction in the weight of the fuel will not affect the "trim" of the machine, nor will a wide variation in the weight of the passenger produce any such effect. As shown, the passenger's seat is placed directly on the top of the fuel tank, an arrangement widely used by European constructors. In the majority of American machines the fuel tank is placed at the top of the fuselage instead of in the position illustrated. As the pilot is considered as a constant weight, his location does not affect the balance.