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Aviation History
1920
1920 - 0620.PDF
It is important to differentiate clearly between equilibriumand stability. Although forces and moments may be balanced up in the equation for equilibrium conditions,statical stability is required to maintain the balanced con- ditions under disturbances such as sudden rise of wind Fig. 5.—Cable angle as resultant of forces. pressure'acting on the nose of the balloon. Lateral stabilityis maintained on the Caquot balloon by the use of a large air-inflated rudder; the same applies to the Drachen balloon,but in addition this balloon is assisted in lateral stability by a tail of parachutes. The condition of stable equilibrium of the Caquot balloonshown in Fig. 3 is maintained in the higher winds by the efficiency of the large horizontal air-inflated stabilisers. The moment M, or wind action tilting the balloon back-wards, is counteracted by the large dynamic force under the horizontal stabilisers or tails, longitudinal stability beingobtained. Both the inflated rudder and inflated stabilisers performthe same duties as planes, but although less efficient than the FIG. 6 Fig. 6.—The virtual point of attachment JUNE IO, 1920 drag force, the weight of balloon, and moment^about C.G.will necessarily differ from the balloon shown in Fig. 3, separate values of the forces on the two models being obtainedfrom the wind channel. The forces and moments on the balloon fitted with rudderonly will be found to balance up in the equation when the pitch angle of the balloon is 200 and wind speed 28 m.p.h. The angle of pitch is excessive, producing such a high tensionin the cable as to be liable to break it. In the stabilised balloon (Fig. 3) the angle of pitch is broughtdown to 13$° in a wind speed, as high as 40 m.p.h. Such a balloon will have sufficient stability to regain balance afterany disturbance, and moreover a much smaller tension will be put on the main cable ensuring a greater factor of safetyand due to the low pitch angle. In deducing the equilibrium conditions of any captiveballoon, the method of procedure is to tabulate at different wind speeds a range of angles together with the drag, dynamichit force and moment about C.G. obtained in the wind channel and corresponding to the angles. The total moments in theequations will be either plus or minus, the point where the sign changes, or where there is no difference in the momentson either side of the equation, being the tialanced condition. It is apparent that the leverages of the various forces aboutthe balloon will affect conditions of balance. The different angles of pitch of the balloon will shorten or lengthen thedistance any one force acts. The leverage a, at which the latter, are more suitable, inasmuch as planes" are liable to be easily damaged in the work called upon from kite balloons. The Drachen balloon relies on sails fo r assistance i r. longitudinal stability. For the matter of interest, Fig. 4~shows the condition of a balloon fitted with rudder only. •<** It is to be noted that the values of the dynamwflift and 620 V-- ',.' Fig. 7.—Action of forces Tt and T, consequent upon virtual point. downward force component T, of the main cable acts, is31 ft. (Fig. 3) at 13 J° pitch of balloon. At 20° pitch (Fig. 4) this distance is 33J ft. from the C.G., making a markeddifference in the foot-pound or moment. In a similar manner should planes or stabilisers be placedas far aft as possible or otherwise increasing the leverage at which they act, the less will be the area required. Thelongitudinal position of the point of attachment on the balloon should be such that the path of the main cable (which willlie as the angle of the resultant of the forces T, and T 5) ifproduced would exactly pass through the centre of the resultant lift of the whole system. See Fig. 5. The kite balloon envelope is of a streamline Shape, designedto give a centre of buoyancy well forward, thus throwing the centre of lift of the whole balloon in such a forward positionas to permit conditions required for stability of the captive aerostat. Further, the point of attachment being placedwell forward gives a greater leverage for the rudder in swinging the balloon back to head to wind position after yawing. In the two examples given of the equilibrium conditionsof the kite balloon, the point of attachment chosen has been what is termed " fixed." If a " running point" is given tothe balloon the conditions of the forces are now somewhat different. The running point will put the point of attach-ment slightly further aft by reason of .the virtual point. Fig. 6 shows the method of locating the virtual point of attachment. In Fig. 7 is given the position of the new point in relationto the CG. of the balloon. The direction of the force T] gives now a negative moment,opposing M, by reason of its position above C.G. The equation for equilibrium M + F/ + T,c - T,a therefore becomes M +F/ — T,c — Tja = o when running wires are used. The forces and moments will be balanced up with due con-sideration of the new action of the forward force T, and consequent alteration in the lengths of the leverages a and c. A running point of attachment on the balloon consideredhere will be found to give a less successful result than the " fixed " balloon. Having now set out some of the points essential for thestability of the captive balloon, it is proposed to! give asja preliminary measure an outline of what would be requiredfor mooring an airship on similar lines.
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