On careful examination it will be found that the fuselage assembly (bare) amounts to a trifle less than the wing group for biplanes having a total weight of from 1,900 to 2,500 pounds. The relation between the wing weight and the fuselage weight seems to bear a closer relation than between the fuselage and total weights. We will set these different relations forth in the following table:
Name of Plane orInvestigator | Fuselage Weight as Percentage of the Total Load | Wing Weight As Percentage of Total Weight | ||
|---|---|---|---|---|
Body AssemblyBare | Body Assemblyand Equipment | Body Assemblyand Power Plant | ||
Curtiss JN4-B | 15.50% | 17.86% | 43.96% | 14.15% |
Standard H-3 | 13.60% | 17.70% | 45.40% | 14.52% |
J.C. Hunsaker | 8.20% | 11.50% | 34.30% | 16.50% |
Author's Experience | 14.96% | 17.62% | 46.66% | 14.60% |
Average of Above | 13.06% | 16.17% | 42.58% | 14.94% |
In the above table, the column headed "Body Assembly and Equipment" includes the body frame, controls, tanks and piping. In the fourth column, the radiator, motor, propeller, water, and exhaust pipe have been added. For the average value it will be seen that the bare fuselage is about 1.88 per cent lower than the weight of the wings. It should be noted that the wing weight given is the weight of the surfaces alone, and does not include the weight of the interplane struts, wires and fittings. The weight of the wing surfaces as above will average about 0.75 pounds per square foot.
Based on the above figures, we can obtain a rough rule for obtaining the approximate weight of the fuselage, at least accurate enough for a preliminary estimate. If A = the total area of the wings, then the total weight of the wings will be expressed by w = 0.75A. The weight (f) of the fuselage can be shown as f = 13.06/1494 x w = 0.65A.
Example. The area of the Standard H-4 is 542 square feet total. Find the approximate weight of the fuselage. By the formula, f = 0.65A = 0.65 x 542 = 352 pounds. The actual bare weight is 302.0 pounds. For several other machines, the actual weight is greater than the weight calculated by the formula, so that the rule can be taken as a fair average, especially for a new type that is not as refined in detail as the H-3.
SIZE OF LONGERONS
The size of the longerons, that is, the section, is influenced by many factors. As these members must resist flying loads, the leverage of elevator flaps, stresses due to control wires, landing stresses and the weight of the motor and personnel it is always advisable to itemize the loading and then prepare a diagram to obtain the stresses in the different members. This latter method is a method for a trained engineer, but an exhaustive description of the method of procedure will be found in books on the subject of "Strength of Materials." For the practical man, I give the following list of longeron dimensions so that he will have at least a guide in the selection of his material.
The length of the fuselage and power of motor are given so that the reader can obtain sizes by comparison, although this is a crude and inaccurate method. As the longerons taper from front to back, the sizes of the section are given at the motor end, and also at the tail. The size of the front members depends principally upon the weight of the motor and the passenger load, while the rear longerons carry the elevator loads and the tail skid shock. If the rudder is high above the fuselage it introduces a twisting movement that may be of considerable importance. The loads on the stabilizer, elevators and the vertical rudder are very severe when straightening out after a steep dive or in looping, and the pull on the control wires exerted by the aviator at this time greatly adds to the total stress. In the front of the fuselage, the motor exerts a steady torque (twist) in addition to the stress due to its weight, and to this must be added the gyroscopic force caused by the propeller when the machine is suddenly changed in the direction of flight. The combination of these forces acting at different times makes the calculation very difficult.
In the case of the Curtiss R-4, the front longerons taper down from the motor 1.63" x 1.25" to a point directly behind the pilot's seat, the section at the latter point being 1.25" x 1.25". From this point the rear longerons taper down to 1" x 1" at the tail. At the motor, the section is 1.63" x 1.25". The longitudinals of the Bleriot monoplane are laminated and are built up of alternate layers of spruce and ash. This is an old type of machine and this practice has since been discontinued. It will be noted that as the power is increased, the size of the front longerons is generally increased, although this is not always the case in speed machines. The Chicago Aero Works’ "Star" fuselage could easily carry a 90 horsepower motor, although this size is not regularly installed.