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Faux Fighter Inspires and Impresses
By Frederick A. Johnsen
July 20, 2015 - It's a wooden warbird; a faux fighter. This special Spitfire inspires those who see it at EAA AirVenture 2015.
Arizonan Bob DeFord liked the idea of building a replica fighter, but sub-scale projects weren't for him. He got with French homebuilder Marcel Jurca, who made plans for an all-wood 1-to-1 scale replica Spitfire powered by a 700-hp V12. Bob wondered why a full-size airframe couldn't accommodate a beefier wartime V12 like an American Allison engine, and the idea snowballed.
Bob said the changed a lot on the original Jurca plans. French engineering studies yielded a welded steel tube fuselage structure that could support an Allison V-1710 engine capable of generating 1,400 hp on takeoff.
That steel tube internal fuselage structure is covered with a non-load-bearing aluminum skin riveted in place. Bob credits Vern Goodsell of Sisters, Oregon with much of the innovative construction on the replica. Bob and Vern became good friends over the eight years it took them to complete the Spitfire stand-in.
Another wooden Spitfire project that never flew contributed some airframe parts like landing gear doors, flaps, and the characteristic bulging canopy, which is one of the few original Spitfire components on this machine.
The one-piece wing on DeFord's airplane spans 36 feet, 10 inches. The wing spar is made of laminated Douglas fir planks hosting wooden ribs. The whole skeletal structure of the wing is sheathed in 3/16-inch aircraft plywood.
Bob is proud of the way he learned to cover the wing. Using a rotating wing fixture first constructed for the replicating of a Hughes racer, Bob mounted the spar and ribs, turning the wing to the vertical. In this way, gravity would not deform the structure during application of the plywood skins.
The unattached skins were soaked from both sides, and then placed flat in the sun just long enough to dry the surfaces while keeping the interior moist. The inside surface of the sheet next received a rolled-on coat of epoxy before being aligned on the wing and hit with a staple gun. The plywood dried, conforming to the airfoil shape of the wing. Bob's face crinkles into an expression of great satisfaction as he recalls the success of the wing fabrication operation. Fiberglass topped with epoxy finished the job, giving the wing protection and smoothness.
Bob and Vern painted the Spitfire replica when they finished it in 2003. About three months ago, they added replica 20-mm cannons to the wing, and faired the paint flawlessly.
To look at it, the airframe screams Spitfire, from its elegantly elliptical wingtips to its snugly cowled engine. But to execute that aesthetic design, some innovative design features were added.
Atop the windscreen is a streamlined rearview mirror—a lifesaving device for fighter pilots. Originals sometimes surface for about $300, Bob said. But a chance trip to a kitchen store yielded a soup ladle that replicated the rounded shape of the original. Bob said a convex automobile mirror from an auto parts store "just fell in there like it was meant for it," and the whole contraption cost only $12.
The main landing gear uses T-6 Texan hydraulics and T-28 wheels and brakes support tires normally found on a business twin. The main gear struts are newly made. Instead of using oleos for shock absorption, Bob worked with a spring vendor to come up with heavy springs that cushion the load of landing without the complexity of oil shocks, giving the gear "no maintenance—it's perfect," he explained. He said the result is a ground ride that "hops like a real Spitfire," based on his review of Spitfire ground handling films.
The Allison engine started life as a P-39 Airacobra powerplant configured for the Airacobra's long drive shaft. It now uses a standard shaft, plus an oil system for changing pitch on the Hamilton Standard propeller Bob got from a DC-3. The exhaust stacks are reproductions like those constructed in Russia for replica Yak fighter projects.
The Allison, unlike the Spitfire's original Merlin engine, uses a downdraft carburetor inlet atop the engine. That's why typical Allison-engined fighters have a noticeable air scoop on top of the cowling. Bob is proud of his design efforts that yielded a duct running from the traditional Spitfire chin scoop back to the firewall and up to the top of the cowled Allison.
That's when innovative engineering solutions quickly came into play. Uncowled, the Allison ran authoritatively on an early ramp run-up. But once the snug cowling was buttoned up, the engine backfired badly, Bob explained.
He devised a gap in the ducting feeding the carburetor that allowed greater airflow into the carburetor—especially needed on takeoff. That, plus the addition of foil insulation around some heat sources, yielded a smooth-running powerplant.
Still the prospect of a major backfire troubled Bob; he could imagine the pressure blowing the aluminum cowling off the airframe. The answer came from an air racing friend in Boise who devised spring-loaded doors in the top of the cowling that will blow open in the event of a carburetor backfire, rather than allow aluminum skins to take flight.
Bob says he has hit 300 mph in a shallow dive at 8,000 feet. His fuel load of just over 100 gallons in two fuselage tanks gives an endurance of about an hour and a half. The trip from Prescott, Arizona to Oshkosh included five legs.
He finds the fabricated fighter to be a comfortable cross-country platform. "You can't take your hand off it, though," or the airplane will begin to roll. "It will not fly straight and level hands off."
And aerobatics are a joy. The wooden wing and steel tube fuselage are stressed for plus eight and minus five gs. Looping the Spitfire replica is a joy: "It makes a round loop," Bob said. "You come out of the loop and hit your prop wash, which is the mark of a good loop."
Spending any amount of time with Bob at Oshkosh, one realizes his homebuilt Spitfire is a source of inspiration to others. It's as if he has blazed a trail with his audacious effort at building such a machine. Fellow homebuilders approach him and say things like: "Thanks for building this," to which Bob smiles and replies: "You can do it too."It's a reassuring message; Bob does not set himself apart as some magician who conjured an airplane beyond the capabilities of mortals. Instead, he uses his experience as an example of what can be accomplished by others too. He is willing to share tips and techniques, just as people shared them with him during construction of his replica.