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Aviation History
1920
1920 - 0619.PDF
JUNE IO, 1920 .-,;:: •:; THE PRINCIPLE OF THE CAPTIVE BALLOON Whj a Reference to its Application for the Mooring of Airships By CAPTAIN P. H. THE Drachen and Caquot observation balloons are the direct descendants of the old spherical balloon used for spotting in the early days of military aeronautics. The success of the modern observation balloon is due to the more careful study of stability of the aerostat under various wind con- ditions. A knowledge of all the forces acting on a captive aerostat must be considered, and is today comparatively easily HIT* nmrr* Fig. 1.—Fixed and running point of attachment obtained by the use of the wind channel and the application of dynamics. Stability is maintained on the Caquot balloon in winds up to 80 m.p.h. It was not possible to fly the old spherical balloon on a cable in winds of even 20 m.p.h. Flying in the higher wind speeds is accomplished and primarily OTNAMIf. LIFT Z LIFT a* GAS SUMNER, A.M.I.N.A. In order to clearly understand the various forces and their direction the following is given :— M is a moment about the centre of gravity, due to the wind tending to tilt the nose up and so twist the balloon back around the C.G. in the direction of the arrow shown in the diagram. Its value is obtained from the model in the wind channel. F is the value of the gas force lifting the balloon vertically upwards through the centre of buoyancy, the centre of the lifting effort of the gas. T, and T3 are the force components in the main cable. T, being the horizontal force and T, being the downward force, a and c are leverages from the C.G. at which T, and Tj act. /is the leverage from C.G. at which F acts. The drag force on the balloon tending to blow the balloon backwards on a horizontal path is designated X. The value of this force is obtained on the model in the wind channel. The dynamic force Z due to the wind lifting the balloon vertically upward is also obtained from the wind channel experiments on the model. The force component T,, or the forward pull in the cable will be exactly the same value as the drag force X. The force component T., or downward force in the cable will equal the upward forces F plus Z, less the weight of the balloon W. The value of F is easily obtained from the lift of the gas contained in the balloon, and at the desired altitude—each thousand cubic feet of gas being equal to 68 lbs. lift. The loss of lift will be about one-thirtieth of the gross lift every thousand feet increase of altitude. In Fig. 3 are given the values of the various forces on the FIG 3. «*•<• ' Fig. 2.—Forces on the captive balloon due to two things, the position of the cable attachment, on the balloon and the provision of ample stabilising surface in the shape of planes, or their equivalent. The main cable from the balloon winch is attached to the balloon by four lengths of wires placed in pairs at a position well forward. These wires are known as kite wires, or more generally as the " Metallic Vee." All four wires or legs are in the form of a vee and terminate at a point some distance below the balloon. This point is termed the " point of attachment," being the point where the mooring cable is secured. The position of the point, and whether it is a fixed or running point, greatly determines the success of the principle. By a running point is meant the wires are made to run through a pulley. In determining the equilibrium conditions of the balloon, all the forces acting are considered. These forces are the wind forces, lift of gas within the balloon, and gravity. The forces can be resolved to two forces only, acting in opposite directions at any point chosen on the balloon and a moment obtained at the chosen point. It is usual to resolve the forces in a plane about the centre of gravity, but any point may be chosen with the same result for the purpose of equilibrium. Fig. 2 shows the direction of forces on the balloon. The conditions of these forces are to be expressed by the equation M -)- F/ + TiC — T,,a = o, the condition of longitudinal equilibrium for the balloon with fixed kite wires. Fig. 3.—Force values and stable equilibrium balloon, such a balloon being in stable equilibrium at the wind speed of 40 m.p.h. The equation for longitudinal equilibrium is as follows :— M + F/ + T,c — T.a «= o F/ - 1,638 Tic - 660 X M - 27,278 7-3 -".95713,088x 19-83 •52,323 T, = F + Z-W = 1,638+ 1,304 -1,254 and T,« •= 1688 x 31 FIG. 4. Fig. 4T—Condition of balloon fitted with rudder only 619
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