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USS ''Akron'' (ZRS-4) was a helium-filled rigid airship of the U.S. Navy which operated between September 1931 and April 1933. She was the world's first purpose-built flying aircraft carrier, carrying F9C Sparrowhawk fighter planes which could be launched and recovered while she was in flight. With an overall length of , the ''Akron'' and her sister airship the were among the largest flying objects ever built. Although the LZ129 ''Hindenburg'' and her sister ship the LZ130 ''Graf Zeppelin II'' were some longer and slightly more voluminous, the two German airships were filled with hydrogen, so the US Navy craft still hold the world record for helium-filled airships. The ''Akron'' was destroyed in a thunderstorm off the coast of New Jersey on the morning of 4 April 1933, killing 73 of the 76 crewmen and passengers. This accident involved the greatest loss of life in any known airship crash. ==Technical description== The airship's skeleton was built of the new lightweight alloy duralumin 17-SRT.〔Smith (1965). p 181〕 The frame introduced several novel features compared with traditional Zeppelin designs. Rather than being single-girder diamond trusses with radial wire bracing, the main rings of the ''Akron'' were self-supporting deep frames: triangular Warren trusses 'curled' round to form a ring. Though much heavier than conventional rings, the deep rings promised to be much stronger, a significant attraction to the navy after the in-flight break up of the earlier conventional airships R38/ZR-2 and ZR-1 ''Shenandoah''.〔Smith (1965). p 187〕 The inherent strength of these frames allowed Chief Designer, Dr Karl Arnstein, to dispense with the internal cruciform structure used by Zeppelin to support the fins of their ships. Instead, the fins of the ''Akron'' were cantilevered: mounted entirely externally to the main structure.〔Smith (1965). pp 161 & 189〕 Zeppelin and other rigid designs used a single keel at the lowest point of the hull circumference but Arnstein gave the ''Akron'' three keels, one running along the top of the hull and one each side, 45 degrees up from the lower centreline. Each keel provided a walkway running almost the entire length of the ship. The strength of the main rings, the lower keels, and the fact that helium, instead of flammable hydrogen, was to be used, also allowed the designer to mount the engines inside the hull, improving streamlining. The main rings were spaced at and between each pair were three intermediate rings of lighter construction. In keeping with conventional practice, 'station numbers' on the airship were measured in meters from zero at the rudder post, positive forward and negative aft. Thus the tip of the tail was at station -23.75 and the nose mooring spindle was at station 210.75. Each ring frame formed a polygon with 36 corners and these (and their associated longitudinal girders) were numbered from 1 (at the bottom centre) to 18 (at the top centre) port and starboard.〔Smith (1965). p 191〕 Thus a position on the hull could be referred to, for example, as "6 port at station 102.5" (the number 1 engine room). While Germany, France and Britain used goldbeater's skin to gas-proof their gasbags, the ''Akron'' used Goodyear Tire and Rubber's rubberised cotton, heavier but much cheaper and more durable. Half the gas cells used an experimental cotton-based fabric impregnated with a gelatin-latex compound. This was more expensive than the rubberised cotton but lighter than goldbeater's skin. It was so successful that all the gasbags of the ''Macon'' were made from it.〔Smith (1965). p 196〕 There were 12 gas cells, numbered 0 to XI, using Roman numerals and starting from the tail.〔Smith (1965). pp 182 & 191〕 While the 'air volume' of the hull was , the total volume of the gas cells at 100 percent fill was . At a normal 95 percent fill with helium of standard purity, the of gas would yield a gross lift of . Given a structure deadweight of ,〔Smith (1965). pp 181 & 183〕 this gives a useful lift of available for fuel, lubricants, ballast, crew, supplies and military load (including the skyhook airplanes). Eight Maybach VL-II gasoline engines were mounted inside the hull.〔Hook 1976. p. 47〕 Each engine turned a two-bladed, diameter, fixed pitch, wooden propeller via a driveshaft and bevel gearing which allowed the propeller to swivel from the vertical plane to the horizontal.〔Summit Memory. (U.S.S. Akron – Propeller ). Retrieved 2008-07-22〕 With the engines' ability to reverse, this allowed thrust to be applied forward, aft, up or down.〔Smith (1965). p 193〕 It appears from photographs that the four propellers on each side were contra-rotating, each one turning the opposite way to the one ahead of it. Thus it would appear that the designers were aware that running the propellers in the air disturbed by the one ahead was not ideal. While the external engine pods of other airships allowed the thrust lines to be staggered, placing all four engine rooms on each side of the ship along the lower keel resulted in the ''Akrons propellers all being in line. This was to prove problematical in service, inducing considerable vibration, especially noticeable in the emergency control position in the lower fin. By 1933, the ''Akron'' had two of her propellers replaced by more advanced, ground-adjustable, three-bladed, metal propellers.〔Smith (1965). p 75〕 These promised a performance increase and were adopted as standard for the ''Macon''. The outer cover was of cotton cloth, treated with four coats of clear and two coats of aluminum pigmented cellulose dope. The total area of the skin was and it weighed, after doping, .〔Smith (1965). p 182〕 The prominent dark vertical bands on the hull were condensers of the system designed to recover water from the engines' exhaust for buoyancy compensation. In-flight fuel consumption continuously reduces an airship's weight and changes in the temperature of the lifting gas can do the same. Normally, expensive helium has to be valved off to compensate and any way of avoiding this is desirable. In theory, a water recovery system such as this can produce 10 lb of ballast water for every lb of fuel burned, though this is unlikely to be achieved in practice.〔 The ''Akron'' could carry up to of gasoline () in 110 separate tanks which were distributed along the lower keels to preserve the ship's trim, giving her a normal range of at cruising speed.〔Smith (1965). pp 180-183〕 Theoretical maximum ballast water capacity was in 44 bags, again distributed along her length, though normal ballast load at unmasting was .〔 The heart of the ship, and her sole reason for existing, was the airplane hangar and trapeze system. Aft of the control car, in bay VII, between frames 125 and 141.25, was a compartment large enough to accommodate up to five F9C Sparrowhawk airplanes. In fact, two structural girders partially obstructed the ''Akrons aftmost hangar bays, limiting her capacity to three airplanes (one in each forward corner of the hangar and one on the trapeze). A modification to remove this limitation was pending at the time of the ship's loss.〔Smith (1965). p 67〕 The F9C was not the ideal choice, being designed as a 'conventional' carrier-borne fighter. It was heavily-built to withstand carrier landings, downward visibility was not very good and it was something of a 'hot ship'. But the primary role of the ''Akrons airplanes was long-range naval scouting. What was actually needed was a stable, fast, lightweight scouting airplane with a long range,〔Smith (1965). pp 27 & 201〕 but none existed capable of fitting between the structural members and into the airship's hangar, as the F9C could. The trapeze was lowered through the T-shaped door in the bottom of the ship and into the slipstream, with an airplane attached to the crossbar by the 'skyhook' above its top wing, its pilot on board and its engine running. The pilot tripped the hook and the airplane fell away from the ship. On his return, he positioned himself beneath the trapeze and climbed up until he could fly his skyhook onto the crossbar, at which point it automatically latched shut. Now, with the engine idling, the trapeze and airplane were raised into the hangar, the pilot cutting his engine as he passed through the door. Once inside, the airplane was transferred from the trapeze to a trolley, running on an overhead 'monorail' system by which it could be shunted into one of the four corners of the hangar to be refueled and re-armed. Having a single trapeze raised two problems: it limited the rate at which airplanes could be launched and recovered and any fault in the trapeze would leave any airborne scouts with nowhere to land. The solution was a second, fixed trapeze permanently rigged further aft along the bottom of the ship at station 102.5 and known as the 'perch'. By 1933 a perch was fitted and in use. Three more perches were planned (at stations 57.5, 80 and 147.5) but these were never fitted.〔 The ''Akron'' revived an idea used, and eventually rejected, by the German Navy zeppelins during World War I: the ''spähkorb'' or 'spy basket'.〔Smith (1965). p 55〕 Intended to allow the airship to remain hidden in a cloud layer while still observing the enemy below, a small car, rather like an airplane fuselage without wings, could be lowered on a cable. The observer on board communicated with the ship by telephone. In theory, the spy basket could also be used for determining whether the air below a cloud layer was clear enough to allow a landing or if the murk reached all the way to the surface. During the design stage, in 1929, the navy requested an alteration to the fins. it was considered desirable for the bottom of the lower fin to be visible from the control car. Charles E. Rosendahl had witnessed the Graf Zeppelin almost snagging her fin on power lines during her heavy take off from Lakehurst to start her round-the-world flight earlier that year. The design change would also allow direct vision between the main control car and the emergency control position in the lower fin. The control car was moved aft and all the fins were shortened and deepened. The leading edge root of the fins no longer coincided with a main (deep) ring and instead the foremost attachment was now to an intermediate ring at frame 28.75. This achieved the required visibility, improved low-speed controllability, due to the increased span of the control surfaces, and simplified stress calculations, by reducing the number of fin attachment points. The designers and the navy's inspectors, led by the very experienced Charles P Burgess, were entirely satisfied with the revised stress calculations. However, this alteration has been the subject of much criticism as an "inherent defect" in the design and is often alleged to have been a major factor in the loss of ''Akrons sister ship, the ''Macon''.〔Smith (1965). p 197〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「USS Akron (ZRS-4)」の詳細全文を読む スポンサード リンク
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