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Aviation History
1920
1920 - 1253.PDF
DECEMBER 9, 1920 This error is, however, comparatively small, and is therefore neglected in practice in order to simplify the calculation. A condition occasionally arises when the gas temperature is lower than the air temperature ; the decrease of lift in this case is known as latent lift. Aerodynamics The study of the aerodynamics of airships is primarily a design problem, but it is so inseparably connected with piloting that I cannot neglect the subject in this Paper, especially in connection with future and larger ships. Head Resistance,—The head resistance of airships of the same form for the same velocity varies as the (capacity) 2/8 and the lift varies as the capacity. The per cent, total weight of the airship required for pro- pelling machinery to give a constant speed therefore decreases as the size increases, or, with a constant per cent, of machinery ; weight, the speed of an airship increases as the (capacity) 1ia or as the £/length. At this increased speed the range for the same per cent, weight of petrol carried is also increased, so that for ships of the same form of the same per cent, weight of machinery and the same per cent, weight of petrol carried the speed and range each vary as the (capacity) ' /9. Dynamic Lift.—The dynamic lift of an airship may be defined as the component at right angles to the flight path of the resistance of a ship moving through the air with its centre line inclined to the path of flight. It is the vertical component of this dynamic lift that is employed to maintain an airship at constant altitude, or to drive her up or down, when the ship is not in static equili- brium, that is, when the lift of the gas is greater or less than the weight of the ship and her cargo. With ships of similar form this dynamic lift may be taken as proportional to the resistance for the same angle of flight, or to borrow a term from Hi A, the same "attitude" of flight. Thus if the same per cent, weight of machinery is employed the maximum dynamic lift (expressed as a percentage of the displacement) varies as (capacity) 8/9 so that practically the same degree of superheating can be dealt with. But if a constant speed were maintained and the capacity increased, the percent, weight of machinery would be decreased and therefore the degree of superheating that could be dealt with without loss of gas would decrease. In the design of large ships this fact must not be overlooked, or for large ships with a long range it may be necessary to carry so much water ballast that the advantage gained by increase in size may be nullified. Controllability As there are so many factors that affect the controllability of an airship, I fear I cannot deal summarily with the subject. A stream-lined airship, if travelling through the air at a small angle to the axis of the ship, instead of tending to return to the direction with its centre line parallel to line of motion, tends to increase this angle. This has a marked effect on the controllability when a ship is flown " light " or " heavy." Thus when a ship becomes light she appears to be nose heavy, and conversely when she becomes heavy she appears to be nose light. This effect acts in favour of the pilot at certain speeds, but at high speeds, when a ship is more than a certain per cent, light or heavy, the effect becomes so great that the elevators cannot cope with it, and the ship, if heavy, will continue to climb, or, if light, continue to dive. The correction if such a case should occur is to slow down the engines. In the event of the elevators jamming, this unstable pro- perty of a stream-lined ship can be employed to pilot a ship back to her base. The German airship pilots have experienced this ef ect, and in L.67 the horizontal fins are placed on the hull at a permanent angle down by the tail. This allows the ship to fly very heavy, thus permitting the pilot to take his ship to a great height when raiding, allowing for the release of weight in bombs and petrol used to bring his ship to equilibrium before landing. Weather There is a general idea that an airship is a fine-weather craft. This is only partially correct. An airship up to the .present time has been under the dis- advantage of having to leave and enter its shed at tli& begin- ning and end of each flight,and as there is difficulty inhandling a ship in and out of her shed in strong winds, this has limited the application of the airship in the past to fine or moderateweather. An airship once in the air is not a fine-weather'craft, and alarge ship with a good range and speed need not fear any type of weather. Experiments are at present in hand which will allow of anairship landing or leaving a mooring mast or tower in strong winds, and when these experiments are completed there islittle doubt that an airship will be almost as independent of the weather as sea-going passenger liners. I will divide up weather into groups, and discuss eachseparately. (a) WindStrong winds in this country and in the Atlantic are caused by depressions or cyclones. These -cyclones are approxi-mately circular in form with the wind blowing round them in a counter clockwise direction with a slight bearing towardsthe centre or low pressure zone. They vary very considerably in size, and may cover halfthe Atlantic or be only 200 miles in diameter. The larger the depression the longer warning we have of its approach, andtherefore the easier it is to avoid or utilise. Also, except in rare cases, a very large depression is onlyassociated with very high winds, over a comparatively small area. The strength of the wind is largely dependent upon the distance apart of the isobars. The shorter this distance the stronger the wind. It is obvious, therefore, that where the winds are strong and the isobars close together the area covered must be small. Thus a pilot, on meeting a strong wind, turns broadside on to the wind, and in a very short time he will be through the bad zone and in a light or favourable wind. It will be seen that the time taken for the ship to cross the bad weather zone does not depend upon the strength of the wind but upon the speed of the ship, also that the amount the ship drifts out of its course depends, on the other hand, on the time taken and the speed of the wind. It is therefore necessary that the ship should have a good turn of speed, so as not to be driven too far out of her course. Except in very rare circumstances, such as when the base at which the pilot wishes to land is in the bad weather zone, a pilot should never beat directly into a strong wind, and even in the above case it often pays to lie oft for a few hours, as the movement of the centre of the depression will move the area of strong wind away from the base. It is owing to this movement of the centre of a depression that, except in exceptional circumstances, a very strong wind does not blow for very long in one place. I have pointed out above the general method of dealing with strong winds, and endeavoured to explain that they are not a serious obstacle to the airship and should not interfere much with schedule flights. A much more difficult wind for the airship pilot to deal with is a head or beam wind of from 20 to 30 miles per hour, as this may blow over a comparatively large area and for long periods. In order to deal with this wind, good meteorological reports must be at the pilot's disposal, and he must vary his course sometimes 12 to 24 hours ahead in order to circumvent such a wind. As an example, the pilot is flying from Malta to Norfolk, there is a large depression centred N.W. of Scotland, giving a westerly and south-westerly wind over the South of England and North of France. If the pilot endeavoured to make good a direct course from Malta to England he would be obliged to edge up into a 30-mile broadside wind over several hundred miles, and the time taken for the journey would be greatly increased. If, however, the pilot was supplied with good meteorological information he would set his course so as to pass out into the Bay of Biscay, just north of the Pyrenees, and when he en- countered westerly winds of increasing force he would turn north at right angles to the wind and use the drift to make his base. Thus during no part of his journey would he be heading into a wind, and although the course taken is some- what longer than the direct route, the time taken will be very little in excess of the still air time, and with a reasonable amount of spare engine power the airship could still make its scheduled time on the journey. Necessity 0/ knowing true height above surface.—In order to use the meteorological information to the best advantage, the pilot should be in a position to read the ground barometric pressure at any moment, so as to know whether he is approach- ing the high pressure or low pressure zones, or flying parallel to the isobars, and thus estimate the rate of movement of the depression. This is impossible at present, as there is no known method bv which a pilot can measure with accuracy or regularity his true height above the surface. Thus the barometric pressure 1255
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