By Tommy H. Thomason

Sunday, March 21, 2010

Missed it by That Much V

The wings on the Grumman XF9F-2 Panther were oriented straight up when folded.
The folded configuration—with the fold joint well inboard—utilized all the hangar deck height to maximize the number of airplanes that could be parked on deck and in the hangar.

Unfortunately, the Panther—like all the early carrier-based jets—proved to not have enough endurance, so permanently mounted tip tanks were added to increase the fuel capacity. Since the hangar height limit had already been reached and there were disadvantages to under-slung tanks and folding the wings past vertical, the wing fold angle had to be reduced to provide the requisite clearance.

Tip tanks were added to the McDonnell F2H-2 Banshee for the same reason. In this case there was still overhead clearance available so the wing fold remained the same. However, the wing fold mechanism and structure hadn't been designed for the weight of wing-tip mounted fuel, so the tip tanks had to be empty when the wings were being folded and spread. If tip tank fuel was needed, which it almost always was, the F2Hs had to be spotted for launch with the wings spread, taking up scarce deck space. Although the F9F now took up more deck space when folded than originally planned, it didn't have this constraint so the tip tanks could be fueled when the wings were folded.

Saturday, March 13, 2010

The Hell It Won't Fit II

As carrier-based airplanes got bigger, folding became more necessary. The basic size limitations were the length and width of the inline elevators and the height and width of the openings from the deck-edge elevators to the hangar deck.

Early on, hangar heights varied somewhat. Beginning with the Essex-class carriers and continued in the Midway-class, the height became standardized at 17' 6", which set the maximum height above deck of the folded wing tips. When the much bigger multi-engine nuclear-strike airplanes were added to the carrier's air groups, the vertical fin had to be folded as well. The first of these was the North American AJ Savage.
In the original layout, the tips of the folded vertical fin, the horizon stabilizers, the folded wing, and the propellers were all about 16 feet above the deck, well within the hangar overhead height limit. (A  clearance of about a foot was desired to accommodate variations in the airplane's "sit" caused by flat tires, shock strut pressure, fuel and bomb load, etc.) However, tip tanks had to be added to the design to address a deficiency in range, resulting in a significant reduction in clearance, as depicted in the drawing above and the picture below.

Sailors are positioned on the wings to insure that the tip tanks will clear the opening to the hangar deck. Also noteworthy in the picture:
- To minimize weight, power folding was not incorporated. In order to fold the wings and vertical fin, hinges with a hydraulic actuator had to be bolted to the wings and a manually operated screw-jack actuator had to bolted to the tail. Note the ladder which was hung on the aft fuselage to provide access to the fin for the latter installation.
- Sailors with chocks are poised to place them athwart the wheels if necessary to stop the aircraft in place.
- Two tow tractors are ganged together to provide the horsepower to pull the big Savage into the hangar bay.

In any event, the AJs were rarely folded, much less taken to the hangar deck. The process of folding the airplane was determined in Board of Inspection and Survey trials to take 16 minutes with a four-man, well-trained crew, on land, with negligible wind. The time increased significantly when it had to be accomplished a pitching, windy deck. As a result, a Savage was only folded and taken below when it suffered an unfixable casualty and needed to be gotten out of the way. (Note that the tip tanks have been removed.)

Friday, March 12, 2010

Snapshot of A Transition II

 Somewhere in the Pacific early in World War II, a Grumman F4F Wildcat is on the elevator of an aircraft carrier with three Douglas TBD Devastators on deck behind it. The big three-man TBD torpedo bomber has power-folding wings to increase the number that can be accommodated aboard, the first U.S. Navy carrier-based monoplane to be so configured. Like almost every other carrier-based airplane up until the start of the war, this model of the Wildcat, the -3, does not have folding wings.  The F4F-4 will, like almost every carrier-based airplane the Navy procures thereafter with one famous exception.

The folding wing meant more than doubling the number of Wildcats that could be stowed aboard. Although it looks like the fold joint must be double hinged, the outboard wing panel actually pivoted on a single axis tilted aft and outboard. The concept was famously developed at Grumman by using a drafting eraser for the inboard wing stub and a bent paper clip simulating the pivot axis and outboard wing panel.

The crank handle did not fold the wing. It pulled the pin that locked the wing in the flight position and extended the wing lock indicator to the unlocked position.

The U.S. carrier Navy was about to be devastated by the TBD losses at Midway. Its withdrawal from combat use in favor of the faster Grumman TBF Avenger torpedo bomber was already a foregone conclusion, however. It was a sad end for an airplane that only five years earlier was the most technically advanced airplane in the Navy's carrier-based arsenal.

The Chief is striding past an arresting cable that has been temporarily repositioned to allow the inline elevator to be used. Note how much smaller in diameter the cable was when airplanes weighed much less than 10,000 pounds on landing and stalled at 70 knots or less.

It is likely that this is the forward elevator on a Yorktown-class carrier (Yorktown/Enterprise/Hornet). Early on, some carriers had a second set of arresting gear for over-the-bow recoveries in the event that an airplane had to be landed with the deck configured for launch, the aft end of the deck had suffered combat damage, or its inline elevator had suffered a casualty while not full up. (Presumably, the Wildcat pilot here is making a message drop.)
The over-the-bow recovery capability, like non-folding wings, would soon disappear from the carriers.

Tuesday, March 9, 2010

Procrustes at BuAer

BuShips and BuAer were independent kingdoms in the Navy. While BuShips cooperated and coordinated with BuAer and the Chief of Naval Operations ruled them both, once the constraints were agreed to and the ships built or altered accordingly, BuAer's aircraft manufacturers had to design their airplanes within those constraints of hangar deck height, elevator dimensions, catapult and arresting gear capability, etc. Although bigger and more capable was usually reflected in a new class of ships, after the Korean War most of the Essex-class carriers received major upgrades to be able to handle larger and heavier airplanes.

As it happened, BuAer almost invariably required new aircraft to operate from the smallest carriers then in the fleet. Because of austerity measures, however, before or shortly after these aircraft arrived in the fleet, the carriers for which the new aircraft were constrained to be compatible had sometimes been decommissioned or were about to be. In a few cases, a new airplane never deployed from the smaller, less capable carriers that it had been designed to operate from even though they remained in service. For example, the McDonnell F4H Phantom specification incorporated the limitations of the Essex-class carrier and one set of carrier trials were accomplished on Intrepid, but the Phantom was never deployed on one.

That led me to revisit the SCB-125 forward elevator extension on the Essex-class carrier described in my prior post. While the A3D wasn't even close to fitting on this elevator and the other carrier-based fighters and attack aircraft didn't need the added length per se, there was one mid-1950s new program that did, the North American A3J Vigilante. Its folded width was 42 feet, providing the requisite one-foot clearance on each side. With the radome and tail folded, it was actually a bit shorter at 65 feet and a few inches than absolutely necessary. (The nose looks a little odd because the radome has been tilted upward into the folded position.)

There's a chicken and egg element to this theory, since I'm not sure of the relative timing of the SCB 125 elevator change and the A3J design freeze. In any event, unlike the A3D which it was intended to replace, the A3J never flew to or from an Essex-class carrier.

10 March 2020 Update: It subsequently occurred to me when I realized that the forward elevator was completely enclosed except for its aft end that a more likely reason for the extension was the desirability of having a tow tractor pull an airplane onto the elevator when it was on the hangar deck and ride along with it. It would also make it much simpler going down, with the tow tractor pushing it off the elevator onto the hangar deck.

Friday, March 5, 2010

What Was That All About?

The forward elevator on the Essex-class carrier was particularly important during the launch portion of its operating cycle. If an airplane experienced a mechanical failure as it approached or was on the catapult, it was probably the least disruptive way of getting the "dud" out of the way of the airplanes behind it waiting to be launched. Maintaining the launch schedule was critical, because much of a delay might impact the timing of the recovery of airplanes in the air from the previous launch. The elevator was originally 44-feet wide and 58-feet long. When down, it opened into the forward end of the hangar bay.

As part of the SCB (Ship Characteristics Board) 125 change to add the angled deck to Essex-class carriers in the mid-1950s, some ships received an enlarged forward elevator, with a triangular section was added to its forward end to make it 70 feet long. It isn't clear why this change was made. The elevator was still too small, both in width and length, for the Douglas A3D Skywarrior, which had to be moved to and from the hangar on the deck edge elevators. The next longest carrier-based airplane was the McDonnell F3H Demon at 59 feet, which was awkward to position on the 58-foot elevator, but did not require a 70-foot one.

My guess is that the extension on the elevator allowed a tractor and tow bar attached to the dud to all be accommodated on it. The dud could then be quickly towed onto the elevator, lowered with the tractor still attached to the hangar deck level, and pushed into the hangar so the elevator could be raised back up to the flight deck as quickly as possible and the launch resumed. This eliminated the time required to detach the tractor before the elevator was lowered and then reattach the dud to another tractor when it got down to the hangar deck so it could be pulled off the elevator.