Aeroplane Construction and Operation

by John B. Rathbun

When at rest on the ground, the weight of the rear end of the fuselage is supported by the tail skid N. The length of this skid must be such that the tail surfaces are kept well clear of the ground. The center of the chassis wheel Q is placed in front of the center of gravity so that the weight of the machine will cause the tail skid to bear on the ground when the machine is at rest. If the wheel were behind the center of gravity, the machine would "stand on its nose" when making a landing. The wheels must be located so that the tendency to "nose over" is as small as possible, and yet must not be set so far forward that they will cause an excessive load on the tail skid. With too much load on the skid, the tail will not come up, except after fast and prolonged running, and heavy stresses will be set up in the framework due to the tail bumping over the ground at high speed. The skids should not be dragged further than absolutely necessary, especially on rough ground. With proper weight and wheel adjustment, the tail should come up in a short run. The wheel adjustment will be taken up under the head of "Chassis."

Position in Flight. In normal horizontal flight, the center line of thrust CT is horizontal or nearly so. This line of thrust passes through the center of the motor crankshaft and propeller. In an upward climb, the CT is inclined at the angle of climb, and since the CT indicates the line of flight, the streamline curves of the body should be laid out so that the axis of least body resistance will coincide with the line of thrust. When flying horizontally at the normal speed, the body must present the minimum of resistance and the wings must be at the most efficient angle of incidence. In climbing, or flying at a very low speed, the tail must necessarily be depressed to gain a large angle of wing incidence, and hence the body resistance will be comparatively high owing to the angle of the body with the flight line. It is best to have the least resistance of the fuselage coincide with the normal horizontal flight speed. This condition at once establishes the angle of the wings in regard to the fuselage center line.

Center Line of Resistance. The center line of thrust should pass through the center of total head resistance as nearly as possible. The total resistance referred to is composed of the wing drag, body and chassis resistances. In an ordinary military type of aeroplane this line is located approximately at one-third of the gap from the bottom wing. Owing to variations in the drag of the Wings at different angles, this point varies under different flying conditions, and again, it is affected by the form and size of the fuselage and chassis. The exact location of the center of resistance involves the computation of all of the resistance producing items.

In addition to passing through the center of resistance, the center line of thrust should pass slightly below the center of gravity of the machine. In this position the pull of the motor tends to hold the head up, but in-case of motor failure the machine immediately tends to head-dive and thus to increase its speed. The tendency to dive with a dead motor automatically overcomes the tendency to "stall" or to lose headway. With the centerline of thrust determined, and with given motor dimensions, the fuselage position can be located at once in regard to the wings. This is good enough for a preliminary layout, but must be modified in the final design. As before explained, the centerline of thrust is located at a point between the two wings, approximately one-third of the gap from the lower wing.

In machines having a span of 35 feet and over, it is a trifle less than one-third the gap, while in small speed scouts it is generally a trifle over one-third. This rule checks very closely with the data obtained from 22 standard machines. Thus, in a machine having a 6-foot gap, the propeller centerline will be located about 2 feet above the lower wing. The top of the fuselage (measured under the stabilizer surface) is from 5 to 8 inches above the center line of thrust. At the motor end, the height of the fuselage above the CT is controlled by a number of factors, either by the type of motor, or by the arrangements made for access to the motor parts. In a number of European machines, the motor sits well above the top of the fuselage, this always being the case when a six-cylinder, vertical water-cooled motor is used. With an air-cooled type, the top is governed by the cowl diameter.

Motor Compartment. The space occupied by the motor and its accessories is known as the "motor compartment," and in monoplane and tractor biplane fuselage it is located in the extreme front of the body. The interior arrangement varies with different types of motors and makes of machines. With rotary cylinder motors, the "compartment" is often nothing but a metal cowl, while with large water-cooled motors it occupies a considerable portion of the body. Water-cooled motors are generally covered with automobile type hoods, these usually being provided with "gilled" openings for ventilation. Owing to the heat generated in the motor, some sort of ventilation is imperative at this point. Whatever the type, the compartment is always cut off from the rest of the fuselage by a fireproof metal partition to guard against fire reaching the passenger or fuel tanks. The fuel and oil should always be separated from the motor by substantial partitions since a single carbureter "pop" may cause serious trouble.

Fig. 4. Mounting and Cowls for Rotary Cylinder Motors. Courtesy "Flight."

Accessibility is a most important feature in the design of the motor end, and hence the hood should be of the hinged automobile type so that it can be easily raised for inspection or repairs. In the Curtis JN4-B Military Training Tractor, the cylinder heads and valve mechanism project slightly above the top of the hood so that these parts are amply cooled and are entirely accessible. Access to the carbureter can be had through a small hand-hole in the side of the hood. The radiator in this Curtiss model is located in the extreme front end of the fuselage—automobile fashion. The propeller shaft passes through a central opening in the radiator. In Fig. 6 the vertical motor E is set down low in the frame, the upper part of the fuselage F ending at H. The engine bearer B, which carries the motor, forms the top part of the fuselage at this point. The engine is thus in the clear and access can easily be had to every part of the motor. The radiator is in front of the motor at R. When in flight the motor is covered by a sheet metal hood similar to the folding hood used on automobiles. This type is used in the Martin, Sturtevant, and several European machines.

Fig. 5. Motor Compartment of a Curtiss Tractor Biplane Using a Front Type Radiator. Note the Two Exhaust Pipes Which Carry the Gases Over the Top of the Wings.

Fig. 7 is a very common front end arrangement used with side radiators. The top fuselage member F is brought down in a very low curve, leaving the greater part of the motor projecting above the fuselage. At the extreme front, the upper fuselage member F joins the engine bearer B, the connection being made with a pressed steel plate. The radiator R is shown at the side of the fuselage. The cylinders are not usually covered when in flight. In the front view it will be noted that the radiators are arranged on either side of the fuselage. A side view of the H. and M. Farman Fighter is shown by Fig. 10. This is a very efficient French machine which has seen much active service in the war. The front end is much like that shown in Fig. 7 except that a spinner cap is fitted to the propeller boss. A "V" type motor allows of the radiator being mounted between the two rows of cylinders, and in a position where it will cause the least possible head resistance.