Tuesday, August 23, 2011
An Extra For The High Road
A pressurized, turbine, high-speed cruiser from Extra Aircraft
Though the piston-powered Extra 400 wasn’t a success, it paved the way for the introduction of the current turbine-powered airplane. The Extra 500 employs a 450 shp Allison/Rolls Royce 250-B17F, coupled to a five-bladed, MT, composite propeller. This isn’t the first application of the Rolls Royce 250 to a fixed-wing airplane. O&E Aircraft of Factoryville, Penn., has been converting Cessna 210 Centurions to its RR B17F Silver Eagle configuration for years.
In production ranks, the Rolls Royce is essentially the same engine that has been used for decades on thousands of Bell 206 series helicopters, the Jet Ranger and Long Ranger, and also on the Hughes/MD 500.
In the Extra application, TBO will be 3,500 hours, with a “heavy maintenance inspection” at 1,750 hours. Aircraft Bluebook estimates an overhaul at $110,000 to $150,000.
The engine has a few advantages over the more common Pratt & Whitney PT6A. First, it’s an extremely small, lightweight powerplant, only 45 inches long by 19 inches wide. It weighs only 200 pounds, compared to the piston Extra 400’s over-500-pound Continental mill. The turbine airplane is longer than the piston model for that very reason. The 500’s nose had to be extended a foot forward to help maintain the airplane’s CG within normal tolerances.
The not-so-good news is that power is limited to 450 shp, the lowest power rating in the class and only 100 more than the Continental on the Extra 400 or the Lycoming on the Mirage. Additionally, the Rolls Royce turbine suffers a high lapse rate; i.e., it bleeds off power quickly as the airplane climbs to altitude. In the Extra 500 application, critical altitude (the height above which the engine can no longer deliver its maximum rating) is 16,000 feet.
A low critical altitude isn’t such a major consideration in a helicopter, as rotary-wing aircraft rarely need to operate more than a few thousand feet above ground. Fixed-wing, turbine, cabin-class airplanes need as much altitude as possible to generate maximum speed on minimum fuel burn. Turbines are reminiscent of some normally aspirated piston mills in that they can provide huge power reserves at low altitude and must be carefully managed down low to avoid abuse.
The result is the Rolls Royce 250 engine won’t deliver full power at high altitude. The Extra 500 is approved for a max altitude of 25,000 feet where cruise is promised at about 210 knots, but if speed is the goal, the airplane does its best work at heights slightly below 18,000 feet. At modest altitudes, the Extra 500 is capable of reaching 220 knots, but only if you’re willing to pour 200 pounds of fuel an hour through the engine (30 gph).
In fact, that’s not such bad economy for a turboprop, and the 500 will do even better at lower power settings. Extra pilot Red Berry, who flew a month-long demo tour around the U.S. last year, including a visit to the 2010 Oshkosh AirVenture, commented that he had seen typical cruise speeds of 200 knots at 18,000 feet, burning only 140 lbs./hr. (21 gph). That’s roughly the same performance as a Piper Mirage but with turbine reliability.
On a max-range mission with a full 172 gallons in the tanks, that translates to seven hours’ endurance for 1,400 nm range, depending upon altitude and temperature. Pull back the thrust a little more, and you can realize a range of 1,600 nm.
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