David A. Belforte
Laser cutting may contribute to faster restoration of a World War II fighter aircraft
In the late 1970s, when Craig McBurney was growing up in the New Haven, CT, area, his father, a part time flyer, instilled in him a love for flying. The television series “Baa Baa Black Sheep”–about the fictionalized escapades of the real World War II Marine combat air squadron VMF-214 led in 1943 by Major ‘Pappy’ Boyington–drew him to admire the plane flown in the show, the Chance Vought F4U Corsair.
The Corsair was a rugged aircraft that flew a range of missions from fighter escort to bomber to ground attack troop support. Because of aircraft carrier operations it featured a unique design that coupled a powerful Pratt & Whitney 2804-cubic-inch displacement engine to a cylindrically tapered airframe structure that featured a massive spar (see Figure 1). The spar is the backbone of the plane that supports the cantilevered inverted gull wing designed to allow clearance for a giant four-blade Hamilton Standard propeller and to lower the plane for hanger deck storage, all this with a functioning folding wing–again for carrier operations.
What sets the Corsair apart from other WWII planes is that it is the only one designed, developed, and built (with mostly local subcontractors) in the state of Connecticut and it was the only fighter designed before WWII that continued to be built after the end of the war.
In 1940 Vought-Sikorsky, located in Stratford, CT, was awarded a Navy contract to build a new and faster fighter plane. The F4U Corsair was designed around the Pratt & Whitney R-2800 Double Wasp engine, a powerful air-cooled engine. The total production of Corsairs grew to more than 12,000 F4U types built over a 12-year period. Some of the F4Us were outsourced to Goodyear Aircraft Corp. (Ohio) and Brewster Aeronautical Corp. (New York).
In its various models the F4U was flown by the US Navy, the US Marines, the Royal Canadian Air Force, the Royal Air Force, and the New Zealand Air Force during WWII. It also flew later with the US Navy and US Marines in Korea. Afterward, other nations, the French in Indo-China, various South American countries such as Argentina, and most notably in the so-called 1969 Soccer Wars in El Salvador and Honduras, all flew models of the Corsair. Turned over to the US Navy reserve, they were finally parked for scrapping in Arizona in 1957. Today, less than 100 of these famous planes (of which 10 are estimated to be flyable) are thought to exist in the US.
A noted collector, Bob Bean, bought 20 of the best, most flyable planes for a little more than $900 each (their WWII selling price was $250,000) with the intention of selling to South American countries. In 1970 he sold some for $20,000 each to collectors who bought them as restorable aircraft and for parts and spares. Among them was number 97330, bought by the son of the famed aviatrix Florence “Pancho” Barnes, whose character appeared in the 1980s film “The Right Stuff” about the first astronauts. This period was the start of restorations of the famed aircraft.
The F4U is very complex compared to competitive WWII fighters. It has three times more parts than the P-51 Mustang, and takes 10 times the man-hours to restore.
McBurney’s youthful passion for the Corsair resulted in his purchase of 97330 after a 1991 accident damaged it. The plane had been saved once from being scrapped and had been flown as a pleasure craft by a succession of Corsair enthusiasts. Having flown one of only two flyable WWII B-24 bombers with a traveling aircraft museum, McBurney gained experience in aircraft restoration; he was trained as an aviator at Embry-Riddle University and holds an FAA Airframe & Powerplant Mechanic’s certificate with an Inspection Authorization.
His plan, now 15 years in the works, is to fully restore the Corsair to original flying status and to use it as a promotional exhibit for Connecticut industry and as a teaching tool for young students interested in science, engineering, and mathematics. The drive for the latter is that he has converted many of the factory drawings of the plane into CAD files using SolidWorks. Now he can, and has, used the CAD drawings to fabricate many parts that can not be purchased.
In 2000 he set up Connecticut Corsair LLC (www.connecticutcorsair.com) and began to cajole state industries remotely connected to the aircraft industry to support his quixotic mission to get number 97330 back into the skies over the US as a goodwill ambassador. In 2005, the process was boosted by the official recognition of the Corsair as the Connecticut State Aircraft exactly 60 years to the day the original flew. McBurney has already restored the R-2800 Double Wasp engine.
You may well ask, “How does this relate to industrial lasers?” as this writer did. I met McBurney at this year’s SALA (Symposium for Aerospace Laser Applications) meeting organized by the Connecticut Center for Advanced Technology (CCAT). He was introduced to me as someone with an interesting article possibility on laser cutting.
McBurney has been attending the CCAT-sponsored laser conferences for several years, where he met TRUMPF, another Connecticut company, which immediately stepped up to work with him on developing the laser cutting of aluminum components used in the main spar of the plane. TRUMPF also produced an instrument panel (see Figure 2) from a hand-fabricated part, and McBurney became enthusiastic about the ability of laser cutting system software operating from digitized programs to cut that panel and turn out an exact replica of the damaged original in less than five minutes. TRUMPF introduced a local job shop, Chapco Inc. of Chester, CT, which has been able to step in to laser cut and fabricate other parts for the Corsair.
Metal fatigue problem
Herein lies the problem: the metal is aluminum and the FAA has reservations about metal fatigue performance after laser cutting because the edge produced is different from qualified mechanical (routing) techniques.
So much so that Sikorsky Aircraft Corp. (get the irony) is independently looking at this situation for its flying hardware (ILS will publish a feature on this in the September issue). As reported in a paper presented at SALA by Dr. Michael Urban of Sikorsky, the problem is that micro-cracks were evident in some laser cut edges in a Sikorsky test program, but at a reduced severity level than reported in other published studies. Their initial conclusions are that, in their tests, laser cut specimens were not a fatigue factor compared to conventional fastened joint specimens but the recast and HAZ results might reduce part life by 18% compared to routed edges; pre- and post-heat treatment did not affect lifetime; and laser cutting process variations can be a fatigue driver.
McBurney has to take this into consideration when laser cutting the spar components because they are part of a vibrating structure. This can lead to a structural failure with consequential serious results.
Years ago, Boeing, faced with the same problem, specified that any laser cutting performed on their aluminum parts had to be performed by subcontractors using a high beam quality laser, the old Coherent General Arrow. It’s unclear whether this was relegated to only aluminum sheet metal or structural parts but the talk in the job shop industry was that the Arrow produced a minimal heat affected cut and therefore was the laser of choice.
Right now, TRUMPF, Sikorsky, and CCAT are all looking at this phenomenon, and resolution in one form or another is expected within a reasonable time. As to laser cut components for the Connecticut Corsair, when this writer visited the company, located in a hangar at the private Chester Airport, I was told that Connecticut Corsair will produce parts that meet the original specifications as outlined in the Corsair’s manuals and drawings, which McBurney has carefully replicated. He claims he can set up procedures to ensure the integrity of the parts meets or exceeds the original specifications.
A flying 97730 seems to be anywhere up to three years from reality, depending on additional corporate sponsors and the continuing support of dedicated volunteers sharing McBurney’s enthusiasm for the mighty F4U Corsair.