By Tommy H. Thomason

Tuesday, January 31, 2012

Carrier-Based Airplanes Weigh More

Carrier landings are one reason. Not only is the airplane carrying a tailhook attached with structure to allow it to pull on the airplane with two to three times its landing weight,  a nose gear that can pull the whole airplane down the catapult track, a wing folding arrangement, extra wing area, etc, the landing gear has to withstand twice the sink rate of a land-based airplane and not always symmetrically or straight ahead.

The touchdown shown above puts quite a bit of side load on the landing gear. In order to minimize the weight of a landing gear that could withstand it, the engineers who designed the F-14 incorporated a unique load link between the main landing gear strut and the fuselage that was established after the gear was extended.

After the landing gear strut extended into position, a T-fitting on the side of the strut swung up to engage a receptacle located inside an opening in the side of the fuselage.

Why the Rube Goldberg (Heath Robinson to the Brits) arrangement, you might well ask? That's illustrated by a comparison of the Grumman A-6 and F-14 main landing gear struts. They are very similar in overall length, diameter, and attachment to the fuselage even though the F-14 landing weight was 50% greater. If anything, the F-14's side brace at the top of the strut looks less sturdy (i.e. heavy) than that of the A-6.
As often happens with innovative designs, there was an unintended consequence that wasn't apparent in development, qualification, or service test or even in initial operational use. Presumably engineering took into account the splaying loads generated on touchdown that are at least partially mitigated by the lateral friction of the tire against the landing surface. They doubtless took into account that some landings would occur where the tire's resistance to sliding sideways was minimal, such as a touchdown zone coated with rubber deposits and wet from rain.

They might have been aware of the anti-skid coating applied to metal aircraft carrier decks that would have been even more resistant to splaying than a concrete runway.. But what they apparently didn't know, according to Craig Kaston, was that the coating wore down during a six-month deployment and was not necessarily renewed, particularly in the landing area. The result was much more frequent and severe instances of side loads being reacted by the fuselage fitting, with damage eventually becoming apparent in the bulkhead located at fuselage station 569 to which it was attached.

Craig Kaston: "The remedial action was two-fold: ensure that the anti-skid was not allowed to wear down to the deck plates; and inspect and repair the damaged Tomcats. Some aircraft had to have the cracked aluminum milled out of the bulkhead and reinforcing plates added; all had the bulkhead modified in increase corner radiuses and shot-peened (cold worked) to reduce residual stresses and toughen the material. These changes were made to aircraft in production. About ten years later, the material of the 569 bulkhead was changed from high-strength aluminum to titanium in the new-build F-14Bs and Ds delivered from Grumman."

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