By Tommy H. Thomason

Monday, June 22, 2009

Making the Most of the Space Available

The wings fold on most U.S. Navy carrier-based airplanes so as many as possible can be crammed aboard. Smaller is better, within reason. My guess is that the requirement before and during World War II was simply that the folded airplane fit on the smallest elevator with at least a foot of clearance on all sides. One notable example was the competition that resulted in the SB2C, in which there was a requirement that not one but two of the proposed scout bombers fit on an elevator. Combined with the mission definition, that resulted in a notoriously short-coupled airplane.

For sure after the war the metric became how many airplanes could be straightforwardly spotted in the first 200 feet of the flight deck of an Essex-class aircraft carrier. In the case of the OS-130 competition, the requirement was 25. I'm sure that it was okay to have parts of the airplane hanging out over the water; I'm not so sure that the Navy's rules permitted the aft fuselage to extend aft of the 200 foot line as shown in the above diagram from the Vought proposal. Note that the elevator was also a constraint which limited the length of the aircraft (folded) to 56 feet (thereby providing the one-foot clearance on each end), when pulled straight on to it. (See for an angled exception that was undesirable.)

This metric wasn't very useful in planning the composition of a deployed air group, which became more and more complicated from a space standpoint in the 1950s with the proliferation of mission types. The result was the definition of the spot factor, which compared the space required by an aircraft type to that of a reference: the single-seat jet attack airplane du jour (first the A4D Skyhawk, then the A-7 Corsair II, and now the F/A-18C Hornet). The ratio was roughly the number of subject airplanes that could be crammed onto the flight deck—leaving the landing area clear—as well as the hangar deck—not precluding access to an elevator— divided by the number of reference airplanes that could be accommodated in that same space. The reference airplane would by definition have a spot factor of 1.0.

For example, here is the spot of the F-111B (I forget which carrier was used) that resulted in a quantity of 85 total airplanes. The spotting factor was then a ratio to the number of reference airplanes that could be stuffed into the same deck and hangar space.

If the spot factor of the fighter was 1.3, 80 of the reference attack airplanes filled the deck space available on the aircraft carrier in question, and there were 36 of the reference attack airplanes in the air group, then roughly speaking 34 fighters (44/1.3) could also be carried for a total of 70 airplanes in the air group. However, adding in 4 AEW airplanes with a spot factor of 2.0 (8 spots) and 12 heavy attack airplanes with a spot factor of 1.5 (18 spots) meant that only 14 fighters (18 spots) could be aboard for a total of 66 airplanes in the air group. Of course, there was only a finite amount of space available for maintenance shops and spares, which was another constraint...

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