By Tommy H. Thomason

Monday, February 26, 2018

F-111A vs B - What Drove the Weight?

Another opportunity to set the internet record straight: what drove the F-111's empty weight? Some assume that it was the Navy's carrier-basing requirement. That's not only in question but some of the Air Force requirements penalized the F-111B's empty weight.

It is true that carrier-basing imposes a weight penalty. The FJ-2 Fury weighed approximately 1,000 lbs more than the F-86 on which it was based, resulting it being underpowered with the Sabre's engine. (That was solved by putting an engine with more thrust in the FJ-3.)
However, the major contributors to that penalty are wing folding, high sink-rate landing strength, and tail/catapult hook components and mounting structure. The variable-sweep wing feature required by the Air Force for long-range deployment and high-speed ingress sufficed for wing folding. The Air Force requirement to land on unprepared fields (most runways in Europe were assumed to be cratered in the first day of the war) meant it had a pretty strong landing gear with excellent sink-rate capability. At that time, Air Force airplanes were equipped with tail hooks for emergency landings albeit not to the same strength as a carrier airplane's. Therefore, the F-111A was penalized only by the relatively inconsequential tail hook and nose-tow-launch attach structure (the F-111B's nose landing gear itself was different).

It's not clear which service was responsible for the side-by-side seating arrangement. It is true that the Navy's two-seat jet night fighters, the F3D and the stillborn F6D had side-by-side seating, in part because shortness is a virtue on a carrier and their radar antennas were humongous. However, I've seen no documented evidence that the Navy required side-by-side seating, other than stipulating a maximum length, which the Grumman F-14 accommodated with tandem seating. On the other hand, the Air Force insisted on a heavy and complicated escape capsule that the Navy had no use for. It was best accommodated with side-by-side seating.

The Air Force also insisted on a bomb bay for nuclear stores and its unprepared field requirement (look up California bearing ratio at your leisure) dictated really big, low-pressure tires. Both of those features resulted in big compartments in the airframe that added empty (no pun intended) weight.

However, the biggest Air Force weight penalty was probably imposed by the Mach 1, low-level ingress on a nuclear strike. That results in the need to design for a very high "q" (dynamic pressure) and gust loading of the structure, neither of which were a requirement for a Navy missile-truck loitering on a Combat Air Patrol station at altitude and then dashing off toward an incoming raid. Moreover, given that the Navy's F-111B was not a true fighter, it probably could have been designed for a load factor of 4 rather than 6.5, further saving weight.

It should be noted that one reason for the F-14 being somewhat lighter than the F-111B was that it didn't have any of those Air Force features other than the variable-sweep wing and moreover, the weight of the big Phoenix missiles over and above that of Sparrows was considered an overload from a structural strength standpoint.


Captain Hook said...

The whole episode illustrates the folly of the doesitall aircraft. Even designs with good initial focus tend to get barnacled with bells as whistles by the brass. Even designs that did turn out to be multi purpose were usually designed for simple mission. e.g. B 52, Pages and pages of specs yield pages and pages of fantasy proposals and overly complicated designs. KISS to start because once reality is encountered there will be add ons. Even when you thought of everything, you didn't.

Pat Donahue said...

That was a good read. I would never had thought that the weight problem was driven by Air Force requirements and not Navy. The swing wing is understandable for Navy storage but the escape capsule, high flotation landing gear, non-tandem seating, etc. was an eye opener as far as contributing to the weight situation if used as a carrier A/C.