Sunday, August 14, 2011

Flying aircraft carriers

This page concentrates on the airships that could carry small planes.

USS Akron and USS Macon

The United States Navy airships U.S.S. Akron (ZRS-4) and U.S.S. Macon (ZRS-5) were designed for long-range scouting in support of fleet operations. Often referred to as flying aircraft carriers, each ship carried F9C-2 Curtiss Sparrowhawk biplanes which could be launched and recovered in flight, greatly extending the range over which the Akron and Macon could scout the open ocean for enemy vessels.
Both Akron and Macon were designed as airborne aircraft carriers, which could launch and recover heavier-than-air planes for use in both reconnaissance and self-defense.

(Above): N2Y-1 training plane beneath the T-shaped opening of the Akron's hangar deck.
The ships were equipped with hangars, approximately 75′ long x 60′ wide x 16′ high, which could stow and service up to five aircraft in flight. Aircraft were launched and retrieved by means of a trapeze, and could enter and exit the hangar though a large T-shaped opening at the bottom of the hull.The capacity to embark and deploy fixed-wing aircraft was the essential element of Akron and Macon’s ability to serve as naval scouts. Airplanes greatly increased the range and area over which the airship could search for the enemy, but also addressed the airship’s own inherent weakness; its vulnerability to attack. The giant airships made large, slow targets which were highly vulnerable to destruction by an enemy’s planes.
Although the Navy originally envisioned the airships as scouting vessels which carried airplanes for fighter defence, over time the Navy realized that the vulnerable airship itself was best employed in the background, out of sight of the enemy; the airship’s function would be to carry scouting planes within range of the enemy. As naval airship doctrine eventually developed, rather than the airplane extending the scouting range of the airship, it was the airship which extended the scouting range of the airplane.

(Above): USS Macon launching and recovering aircraft.
Development of the Akron and Macon
The Akron and Macon grew out of the Five Year Plan proposed by the U. S. Navy’s Bureau of Aeronautics, which had been approved by the United States Congress in 1926, and which authorized the construction of two large rigid airships.
The Navy contest to design and build the two new ships was won by the Goodyear-Zeppelin Corporation, a joint venture and patent sharing arrangement between the Luftschiffbau Zeppelin and the Goodyear Tire and Rubber Corporation which had been created in 1923. (There was no serious competition for the contract, and it was clear to everyone involved in the process that Goodyear-Zeppelin was the only firm with the ability to design and construct these ships for the Navy.) Goodyear-Zeppelin and the United States Navy signed a contract for the construction of two large rigid airships on October 16, 1928.

(Above): F9C-2 hooked on trapeze (left) and stowed on hangar deck (right).
As part of the Goodyear-Zeppelin arrangement, the Luftshiffbau Zeppelin had sent technical experts to Akron to train Goodyear employees in the design and construction of airships. Goodyear president Paul Litchfield had insisted that the Zeppelin Company’s chief stress engineer, Karl Arnstein, be included in that group, and in November, 1924 Arnstein arrived in Akron along with a team of 12 hand-picked Zeppelin engineers. It was under Arnstein’s leadership that Goodyear-Zeppelin developed the plans which became the USS Akron and USS Macon.

(Above): USS Macon under construction.
Arnstein’s design was radically different from the conventional zeppelin designs he had worked on at Friedrichshafen. No longer under the direction of the conservative Ludwig Durr, the Zeppelin Company’s chief designer since the LZ-2 of 1906, Arnstein was free to develop new designs and techniques for Akron and Macon.
Deep Rings
Traditional zeppelin design featured a series of main rings built of a single braced girder, which were generally spaced 15 meters apart with unbraced rings in between. Arnstein’s design for Akron and Macon utilized a series of “deep rings,” which which were large triangular structures — similar to the keel — spaced 22.5 meters apart.

(Above): Design of main rings of Hindenburg (left) and Akron/Macon (right).
Arnstein’s deep-ring, three-keel structure was considerably heavier than the framework of a traditional German zeppelin, but it was also believed to provide greater structural strength, which was very appealing to a Navy which had just seen the USS Shenandoah crash after suffering in-flight structural failure during a storm.
The deep-ring design also accommodated a Navy requirement that all areas of the structure be accessible during flight; the 8-foot deep rings were large enough for a man to climb their entire circumference.
Three Keel Design

(Above): Triple keel design of Akron/Macon.
Traditional zeppelin design was built around a single structural keel running the length of the ship along the bottom of the hull. Arnstein’s design was radically different, and featured three large triangular keels; one at the top of the ship, and two on either side at a 45 degree angle from the bottom of the hull. The main keel, at the top of the ship, provided access to the valves for the gas cells, and the two lower keels provided support for the engines and crew spaces.
Engines and Propellers
The three-keel arrangement, along wth the use of non-flammable helium, also allowed the engines to be carried internally, along the lower keels, rather than in external power cars; this significantly reduced aerodynamic drag and allowed for easier access and maintenance of the engines.

(Above): The propeller of the USS Akron.
The 560 horsepower Maybach VL-2 engines were connected to outrigger propellers by long shafts with bevel gears which allowed the propellers to be rotated to provide thrust not only forward and reverse, but also vertically downward to assist in take offs and landings.
The mounting of the engines on the two lower keels did create one design element which was accepted only as a compromise; the four engines on either side were mounted in a straight line, and not staggered as the external power cars of earlier zeppelins had been. In earlier zeppelins, the staggering of engines at differing heights along the hull allowed each propeller to operate in clean air, undisturbed by the prop wash from the engine in front of it, whereas the propellers on Akron and Macon operated in the disturbed air created by the engines ahead of them. Placing the engines in a straight line along each of the lower keels, however, allowed for a much simpler and lighter design, and was accepted as a better alternative than the additional weight and complexity of the framework that would have been required to stagger them.

(Above): One of the eight engine rooms aboard the USS Akron.
Non-cruciform Tail
Traditional German zeppelin design included a cruciform tail structure for strength, which Arnstein and his design team eliminated in the Akron and Macon.

(Above): Modification of the Stabilizers

One other design element which would have great significance in light of later events was the shape and position of the stabilizing fins, which were modified from their original design to accommodate a Navy request that the lower fin be visible from the control car. Experience had taught airship commanders that the lower fin was vulnerable to damage in operations near the ground; Charles Rosendahl had been aboard the Graf Zeppelin during its difficult overweight takeoff from Los Angeles during its 1929 Round-the-World flight, when the lower fin, which had not been visible from the control gondola, only narrowly missed hitting power lines at the edge of the field. Both Rosendahl and zeppelin commander Hugo Eckener believed it was important for the officers to have an unobstructed view of the lower fin, and this requirement led to a modification of Arnstein’s original design which would later have tragic consequences in the crash of USS Macon.

(Above): The final, modified stabilizer arrangement of the Akron/Macon, showing the main rings (highlighted in yellow).
In the original design, the fins were to have been attached to the hull at three main rings: Ring 0 at the tail; Ring 17.5 at the center of the fin; and Ring 35 at the leading edge of the fin, which carried heavy loads. In order to make the lower fin visible from the control car, however, the design was changed to shorten the fins, and the modified fins were attached to only two main rings (numbers 0 and 17.5). The leading edge of the fins, which were subject to very heavy aerodynamic loads, were not firmly attached to any main, load-bearing structural element, but merely to weaker, intermediate framing.
Given the in-flight structural failure of the tail section of USS Macon, there was considerable controversy regarding decision to eliminate the cruciform structure of German zeppelins, and even more controversy regarding the decision to move the leading edge of the fin so that it was no longer anchored to a main ring.
Water Recovery Apparatus
One notable feature of Akron and Macon were the water recovery apparatus designed to recover water from engine exhaust to compensate for the weight of fuel burned during flight, to avoid the need to valve helium to maintain equilibrium as fuel was burned.

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