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.

Friday, February 16, 2018

One More Time, The Grumman F12F

This is not the Grumman F12F:

It is the Grumman Design 118, proposed to the Navy in December 1955. The Navy rejected it because they didn't want a second development program of a fighter powered by two J79s (the McDonnell F4H was already under contract) but suggested that Grumman go back to the drawing board and propose a single-engine, Sparrow-missile armed fighter to compete with Vought's proposal for a Sparrow-armed "Super" F8U powered by the P&W J75. Grumman did on 4 May 1956.  The Navy rejected it as well in favor of what became the F8U-3 in a letter to Grumman from the Chief of the Bureau of Aeronautics, RADM James S. Russell, dated 16 July 1956, Subject: Grumman Aircraft Engineering Corporation Model 118A Airplane; proposal for: "The recent receipt of more up-to-date engine data does not alter the relative standings of your design with others already programmed in the fighter field. The Chief of the Bureau of Aeronautics had therefore determined that the introduction of another design using the same engines and comforming to the same general operating requirements cannot be justified or undertaken." Neither Grumman or the Navy ever referred to either of these two proposals as the F12F.

There was a Grumman F12F but the designation was assigned to a production variant of the F11F powered by GE's J79. Grumman had proposed its J79-powered Design 98J to the Navy in January 1955 and then its Design 98L, basically the J with increased wing area, in February. The latter appears to be the basis for the purchase order and contract in August 1955 that the Navy created for two F12F prototypes, which were to be assigned BuNos 143401 and 143402. The following artists concept appeared in the Design 98L report dated 15 February 1955.

This is the US Navy's F12F Characteristics Summary dated 15 August 1955. Although there is no drawing, the dimensions and performance data match the Grumman 98L's including the wing area of 350 square feet and the single J79 engine. (The Model 118 and 118A had a wing area of 595 square feet; the former was to be powered by two J79s and the latter, by one J75.)
It is not clear that the contract was ever issued. It probably wasn't. In any event, it was canceled or terminated in January 1956, probably due to the demonstrated performance of the Vought F8U-1 that first flew in March 1955 and the need to fund the development of a competitor to the F4H. However, the Navy had contracted with Grumman in August 1955 to put the J79 in the last two F11Fs in the first production lot in parallel with their plan to buy the F12F. These were designated F11F-1F and were not canceled, since they would provide the Navy with J79 flight experience desired prior to the beginning of the F4H flight test program.

One or more aviation historians have inadvertently conflated the two programs and incorrectly concluded that the Navy contracted with Grumman for its twin-J79-powered Model 118 and designated it F12F. Another enthusiast subsequently speculated that it was given the popular name Lion, which some have accepted as fact as well. It was not.

Saturday, February 10, 2018

The A-12 Avenger II Program - The Fat Lady Finally Sang

I'm embarrassed to say that I neglected to cap this story off when she did back on 24 January 2014. The settlement, according to a Reuters article, was:

"(T)he Navy will receive three EA-18G electronic attack aircraft from Boeing, and a $200 million credit from General Dynamics toward its work on a new DDG-1000 destroyer."

For the Aviation Week report (it may have still been a weekly back then), click here.

For my penultimate post, which has links to prior ones: