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Aviation History
1950
1950 - 2146.PDF
FLIGHT, 14 December 1950 PILOTING A FREE BALLOON . . . " Ballasting-up " before flight and before landing is most important It is by such means that the pilot retains control over the balloon at two vital moments. The " ground officer " is responsible for the first ballasting- up; the ground party is under his control, and its members must maintain silence and obey his orders instantly. When the balloon is vertical and ready for flight he gives the order "hands off," then "hands on." If the balloon is heavy it remains on the ground. Some ballast bags are taken off and the process repeated until the pilot is satisfied that the lift is correct. Then the neck is untied. Before the final "let go all" is given a note should be made of the remaining ballast and entered in the log. On no account must the ground party give the basket an upward push. This only gives a false Lift and hinders the pilot. Much time is saved, and more accuracy attained, if a rough calculation is made beforehand showing how "light" the balloon must ascend to gain a given altitude. No pilot should ascend unless he is quite ready and satisfied with his lift. Knowing the approximate height at which the balloon will be in equilibrium, the pilot keeps a sharp eye on his rate-of-rise-and-fall indicator. Tissue paper is also very useful: small pieces are thrown out, and give an indication as to whether the balloon is rising, falling or in equilibrium. The statoscope is another guide. The great point is to leave the ground with just sufficient lift to clear all obstacles. Hence an open and clear space, with plenty of room down-wind, is far preferable to, say, a gas works with its high gas containers. If the start has been managed properly, and the balloon correctly ballasted- up, the initial rise can be slow. There is then Little momentum and the first run-down can be checked with a very small expenditure of ballast. The higher the wind the greater must be the initial lift. When the balloon starts to descend it immediately becomes flabby. Then, if air and gas are at the same temperature, it can rise again without losing Lift When it is full, the so-called " pressure height" is reached. If it continues to climb gas is again lost and it again descends. The pressure height increases with each rise, and a carelessly piloted balloon makes a series of rises and falls, until both ballast and gas are exhausted and the flight comes to a premature end. A good pilot, therefore, aims at keeping his balloon as nearly in equilibrium as possible. This, however, is easier said than done. A balloon has only to become a few ounces heavy or light for its equilibrium to be upset, and it is essential to carry plenty of ballast. Altitude and length of flight, indeed, depend on this. If approximately one-thirtieth of the Lift is lost per 1,000ft and the desired maximum altitude is, for example, 10,000 ft, one-third of the lift will be lost. At least one-third of the gross Lift must therefore be allotted to ballast, while more must be carried in reserve. So far we have assumed that air and gas temperatures are always equal. In practice this is seldom the case. The hydrogen in the envelope is warmed directly by the sun, and the surrounding air by radiation from the ground. The gas, however, heats up some seven times faster than the air; and when the gas thus " superheats," and the balloon is full, there is a gain in lift equivalent to the weight 559 of gas lost If the envelope is flabby, there is a gain in lift equivalent to the weight of additional air displaced by the expanding gas. All this must be allowed for, otherwise the balloon may lose so much gas that it becomes very heavy, and a fast downward run then becomes difficult to stop. Hence the need for plenty of ballast. If the balloon is full, and when air and gas are at the same temperature, a rise in temperature means a loss of lift. Gas is then lost through expansion and a fixed volume of gas displaces air, which is lighter through expansion. If the balloon is flabby, Lift does not alter, for the gas expands at the same rate as the weight of air displaced decreases. Besides watching his instruments, therefore, the pilot also keeps an eye on the sun. If, for example, an undesired rise occurs and the sun is about to be clouded over, there will probably be no need to valve gas. In the same way, the sun " emerging from " a cloud may check a run down. The nearer the balloon is to equilibrium the less the ballast and gas that need to be lost in checking vertical movements. The quicker the pilot is, too, the less ballast he need jettison to stop a fall. If he hesitates, and allows the balloon to achieve a momentum the more ballast he must let go; and the same applies to upward runs and the gas valve. There is always a time-lag, however, and the pilot must not expect that a fall will be checked immediately. When near the ground thermal currents must be allowed for. When passing over hot sand, for example, or short grass, rising currents may cause the balloon to rise, espe- cially if it is in equilibrium. When passing over water or trees, the balloon starts to descend. Cloud Effects When a balloon is flying above the clouds with a clear sky overhead the sun causes much loss of gas. When descending, the balloon often " bounces " on the clouds, for heat is radiated upwards from them, and expands the gas. Once in the clouds the gas cools and shrinks, and, to stop a fast downwards run, ballast must be thrown out. Here, again, delay means gathering momentum and a waste of ballast. As a balloon descends it normally meets increasing air temperatures, and this helps the downward run. At night it is much easier to achieve equilibrium. Tem- perature inversions, however, may be encountered; these cause a temporary loss of Lift, for the air is then cooler than the gas. This is known as a "latent Lift," and the pilot must be ready with a little ballast to stop a run-down. If a thermometer is carried the pilot knows better what is happening, and so saves much ballast Landing is an art in itself, and a pilot can be judged by his landings. Jettisoning a large quantity of ballast at the last moment points to bad piloting. Every landing is made in a different place, and under varying conditions, so much experience is necessary. The late Mr. Griffith Brewer, who was one of the most expert British pilots, used to tell his pupils in the R.N.A.S. to ballast-up at about 500ft. If a balloon can be brought into equilibrium at that height it certainly makes landing much easier. If the weather is windy it is best to land in a sheltered spot—behind a wood, for example—and in that case the rip panel should be pulled out before hitting the ground. This prevents an upward bounce, and empties the balloon in a few seconds. In a strong wind the red An unusual type of balloon, with netless envelope, flown by a Swiss competitor in one of the Gordon Bennett races. Ground party handling the basket of a 170,000 cu ft balloon (pilot, the late W/C. C. M. Waterlow) operated by the R.N.A.S. in 1917.
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