By Tommy H. Thomason

Tuesday, June 30, 2009

Missed It by That Much III



Further to the maximum folded span discussion, the Navy's experience with the XF3H in its carrier trials may be an indication that angling aircraft to fit them on the elevator was undesirable. The McDonnell XF3H Demon was 59' 4" long. In order to fit on the Essex-class elevator, it had to be angled. In an unpublished anecdotal report, McDonnell engineer William E. Elmore wrote "It took an excessive amount of jockeying for position...to place the airplane (on the elevator). It was felt that if the 59-foot long F3H-1 had been just three feet shorter, (it) would have not required excessive, if any, jockeying." (Note that there was about two feet of clearance when properly angled, twice the minimum.) In any event, the Demon's successor at McDonnell, the F4H Phantom, was originally 56 feet long...

Sunday, June 28, 2009

Why 27 feet six inches?

The A4D Skyhawk's wings did not fold, with the wing span limited to the maximum folded dimension, 27' 6". The question is, what established that number? It seems to have been a BuAer-issued limit, since at least four of the J65-powered single seat tactical jets proposed at that time had folded wing spans within a few inches of that dimension.

My first guess was that it was established by the then-standard forward Essex-class elevator, which was 58' by 44'. Two Skyhawk loaded side-by-side athwartships with one foot between them and one-foot clearance on each side (yes, that is the standard minimum clearance) equals 58'. Length was not a problem. However, that turned out to be incorrect, since that elevator was only open to the hangar deck on one of its narrow sides, so two Skyhawks could only be loaded fore and aft.


Grumman and Vought proposal brochures depict a similar orientation. Two F11Fs fit with one-foot clearance with the wing tips manually folded down. The V-384 had to be angled.





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 http://thanlont.blogspot.com/2009/06/missed-it-by-that-much-iii.html 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...

Friday, June 19, 2009

Minimizing Spot Factor

Like Vought, Grumman also included a folding horizontal tail in their single-seat A-6 proposal for the Navy's VAL program.* The benefit was demonstrated by pulling three A-6s off the production line, fitting two of them with mockups of the proposed tail, and packing them together as closely as possible. Grumman claimed that the resulting spot factor of a folded Intruder was only 25% greater than that of the smaller (albeit nonfolding) A-4E Skyhawk used as a baseline. However, the winning Vought proposal that became the A-7 Corsair II had a spot factor that was 5% less without the optional horizontal tail fold and almost 13% less with it.

* See http://thanlont.blogspot.com/2008/08/single-seat-6.html

Thursday, June 18, 2009

It takes all the running you can do to stay in place II...

Perhaps you were wondering why the trailing edge of the Vought A-7 Corsair II changed direction above the rudder? Probably not. However, the reason for that and its blunt nose is the critical requirement for minimum usage of the scarce real estate on an aircraft carrier. At the last moment before proposal submittal, a Vought executive took a drafting triangle and drew a line upward from the deck through the vertical fin above the rudder to minimize the overall length. (Whoever subsequently revised the side view in the proposal brochure forgot to change the location of the right arrow/line defining the length. Or it was too much trouble—no computer graphic programs then.)













Note that the proposal has a more pointed nose than the actual aircraft. During detail design, the fuselage had to be lengthened and blunting the nose minimized the impact. Note also that the proposal offered a folding horizontal tail as an option to further reduce the spot factor, which is the measure of the space used when the airplane was folded. The Navy passed on that idea.

Sunday, June 14, 2009

It Seemed Like a Good Idea at the Time IV

This was one of the last illustrations cut from Strike from the Sea to make the limit. Provided by the Grumman Historical Center, it depicts a single-seat A-6 providing close air support to the Marines. The proposal was in response to a 1962 Navy requirement to replace the single-seat Douglas A-1 (AD) Skyraider. (The two-seat A-6 had replaced the A-1G (AD-5N) night attack variant.) Also see http://thanlont.blogspot.com/2008/08/single-seat-6.html

Unfortunately, the Navy had in mind not only a variant of an existing aircraft, but one powered by a single TF30 turbofan engine, which not coincidentally at the time was also the basic powerplant for the nascent Grumman/General Dynamics F-111B. Grumman management was forced to choose between full compliance, a TF30-powered F-11 (F11F) Tiger—don't scoff, the winner of the competition was Vought's proposal, which closely resembled the Vought F-8 (F8U) Crusader)—and a proposal that they hoped the Navy would realize was a better deal than an all-but-new design powered by a new engine. They were wrong, although their Attack Tiger would probably have lost to Vought's excellent proposal anyway, as did Douglas' proposal of an A-4 (A4D) Skyhawk on steroids. For more, see chapter nine in my book on U.S. Navy attack aircraft, Strike from the Sea.

Friday, June 12, 2009

It Seemed Like a Good Idea at the Time III

In the late 1940s, it had become clear to the Navy that the then-standard 20mm cannon armament was unlikely to be effective against jet bombers. It would take a lot of bullets to bring one down and the geometry of the attack meant there was not enough time to do that with deflection shooting. A classic tail-on attack, which might provide the requisite firing time, meant dueling with the bomber's tail gunner, not a good option.

The solution was thought to be a salvo of 2.75-inch folding-fin rockets. They weren't very accurate, but only one had to hit the bomber to have a high probability of bringing it down. Two interceptor programs were initiated to provide this capability, the bat-winged Douglas XF4D with externally carried rocket pods, and the sleek McDonnell XF3H with an internally mounted pair of launchers loaded with a total of 26 rockets.

As it turned out, the combination of fire control system and rockets was much less accurate than expected, with even one hit being unlikely. Fortunately, air-to-air guided missiles were in development that proved adequate to the task.

The illustrations are from a 1949 Navy Confidential Bulletin.

Thursday, June 11, 2009

On The Other Hand

At least two Skyhawk pilots scored air-to-air kills with unguided rockets: VA-76's LtCdr Ted Swartz, flying off Bon Homme Richard, knocked down a MiG-17 with a five-inch Zuni rocket on 1 May 1967; and 13 years later, Col Ezra Dotan reportedly bagged another one over south Lebanon. It must be noted that the A-4's gunsight was all but useless for this kind of engagement.

The picture is a doctored one, implying the A-4 was in level flight when it was really in a dive, but illustrates what the marketer of a folding-fin rocket would claim is a feature: most of the rockets flying straight out but at least a couple starting to diverge to create somewhat less than a full choke pattern. However, one hit with a five-inch Zuni would be even more devastating than one with a 2.75 inch Mighty Mouse like the ones shown here.

Wednesday, June 3, 2009

A-12: The Gift That Keeps on Giving

A gift to lawyers, in this case. I was surprised when writing the chapter on the A-12 Avenger II program in Strike from the Sea to discover that its termination was still in dispute. Given that it had occurred in January 1991, almost two decades before, that seemed unlikely until I realized that it involved a $2.3 billion claim by the Navy against GD and Boeing (which now owns McDonnell Douglas, GD's original partner in the ill-fated program). That sum consisted of a demand for the repayment of $1.3 billion in progress payments and $1 billion (and apparently still counting) in interest. Avoidance of the payment of that kind of money justifies the efforts of legions of lawyers.

There were eventually two trials. The contractors won the first in 1995, which the government successfully appealed, and the government won the second in 2001, which the contractors successfully appealed. (A 2002 settlement floundered because the contractors and the government couldn't agree on the value of the "in-kind payments", i.e. goods and services as opposed to cash, to be made by the contractors.) According to today's Wall Street Journal, the U.S. Court of Appeals for the Federal Circuit has finally ruled on the 2007 Claims Court ruling against the contractors, upholding it.

It's probably finally over but I wouldn't bet on it.

Tuesday, June 2, 2009

Self Boarding

For decades, one of the requirements for Navy carrier-based aircraft was self-boarding. Ladders were anathema on a crowded, windy carrier deck full of whirling propellers and jet intakes. This became more and more difficult to accommodate with jets, particularly those with long nose gears to provide a high angle of attack for takeoff and landing. In the case of the XF3H Demon, for example, McDonnell expected the pilot to scale the nose landing gear until he could reach the hand/foot holds at the bottom of the black stripes and put his right foot on the black non-slip area on the nose gear door. After that, the ascent was fairly straight forward.

This concept was arguably more challenging than the production version shown in a prior post (http://thanlont.blogspot.com/2008/05/i-had-hoped-to-find-picture-like-this.html), which required the pilot to clamber up onto the wing from its trailing edge, walk up to the leading edge, and then traverse the fuselage from there to the cockpit by stepping on small pegs that extended out when the canopy was opened. Detachable ladders soon became acceptable and self-boarding was waived for the A4D Skyhawk and the production F4D Skyray.

Mark Nankivil provided the illustration from the XF3H Flight Manual.

For more on self-boarding, see http://thanlont.blogspot.com/2015/10/carrier-based-airplane-self-boarding.html

The General Purpose Fighter

One of the mission specifications during the early years of carrier-based jets was general purpose fighter. This was essentially a fighter being used as an attack aircraft. The McDonnell F3H Demon, originally intended to be a dedicated interceptor as an alternate to the Douglas F4D Skyray, was redirected to be also be an alternate to the Grumman F10F Jaguar with a general purpose fighter capability. This F3H-2N, BuNo 137029, is loaded for display purposes as a general purpose fighter with six small, box-tail, general-purpose bombs on the wings and two 1,000-lb low drag bombs on the belly. The picture was taken in May 1957 at Lambert Field in St. Louis by Aviation Week's Pete Bulban. (The aircraft in the background is McDonnell's XV-1 compound helicopter.)