Tuesday, August 23, 2011
An Extra For The High Road
A pressurized, turbine, high-speed cruiser from Extra Aircraft
Entry to the Extra 500’s cabin, which measures 55 inches across by 49 inches tall, is through a door below the left wing.
Walter Extra selected a high-wing configuration for the Extra 400 and 500 despite the ever-present challenge of a high-wing retractable—where to hide the landing gear. As with the retractable Cessnas and the Swearingen SX-300, the wheels sleep in the belly. The main gear of the Extra 500 retracts forward, and the nosewheel folds aft.
The designer felt the trade-off in complexity was worth it to realize the operational and aerodynamic advantages of a high-wing airplane. There’s no wing spar to deal with in the cabin, fuel flow is expedited by gravity in the event of dual fuel-pump failures, and the wing/fuselage intersection drag problem is simpler to overcome in a high-wing design.
The airfoil designer Walter Extra chose for his 400 and 500 is an NLF (natural laminar flow) design that Extra claims retains attached flow over the forward 70% of the chord. In contrast to the Meridian, the Extra features a comparatively small, 153.5-square-foot wing. From the front view, the wing appears to manifest anhedral—a down-slanting profile—when, in fact, it has very slight dihedral. The wing features a cusp at the rear that slopes up and fully encloses the track of the Fowler flaps.
That’s unusual for Fowlers that most often have hinges hanging down. Though the flaps are relatively short chord, they span nearly two-thirds of the wing trailing edge and extend to 30 degrees. Walter Extra selected Frise ailerons for the remainder of the trailing edge. These help counter adverse yaw and improve roll response, if at a slight drag penalty.
The result is a wing that satisfies the FAA’s 61-knot, dirty-stall requirement, and flies very well at approach speeds down to 85 knots. I flew a one-hour photo mission in formation with a Cessna Skyhawk, and it was a total nonevent. One of the Skyhawk’s doors had been removed for photographer Jim Lawrence, and the resulting drag probably cost the Hawk a good 10 knots, so max speed was about 105 knots. The Extra nevertheless hung in with the little 172 at banks to 45 degrees or more with nary a nibble of a high-speed stall.
More normal takeoffs result in the kind of ascent you’d expect from 450 shp driving only 4,700 pounds of airplane. The book spec is 1,335 fpm, but my flights were all at reduced weight, so much bigger numbers were normal. Upward mobility of 1,500 fpm isn’t unusual with two or three aboard and reasonable fuel. Climbs to 20,000 feet require 20 minutes or less, and the airplane’s 5.5 psi pressurization system can maintain an 8,000-foot cabin altitude at the 25,000-foot max certified altitude.
Normal empty weight is 3,091 pounds; useful load works out to 1,605 pounds. While that’s not spectacular, the Rolls Royce turbine’s low fuel burn means you don’t need to carry much Jet A-1 in order to traverse the country. Unlike piston airplanes that typically operate with full tanks most of the time, turbine models more typically fly with fuel appropriate to the mission. In the Extra 500, pump in 650 pounds of jet fuel, and you have three hours’ range plus reserve at the 21 gph burn rate. That still leaves 860 pounds for people and stuff, just over five folks total or four plus baggage. I’d bet that will be a popular loading configuration in this airplane.
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