An Extra For The High Road

Single-engine turboprops are a relatively recent development in general aviation. The Cessna 208 Caravan was the first of the type and the most successful, with more than 2,000 built so far, but the 208 was a dedicated utility airplane, more often a cargo container than a luxury people-hauler.

If you’re considering cross-country machines, the first successful turbine single was the TBM-700, a plush six seater introduced in 1991 and reminiscent of the Piper Malibu. The Pilatus PC-12 premiered in 1995, and the Piper Meridian in 2001, bringing the current certified class to three models.

The price range of the three airplanes is as wide as their capabilities. The Meridian is the entry-level machine, a comfortable 260-knot six seater based on the Malibu, and currently base-priced at about $2.1 million. The TBM-700 has now evolved to become the TBM-850, another six-place airplane that clocks along at 300 knots in exchange for $3.3 million. Finally, the PC-12 tops the class in size and weight with up to 11 seats at 280 knots and a typical price tag of $4.4 million.

There have been several other aborted attempts along the way—Beech’s Lightning and the Interceptor 400 to name two—plus a number of conversions that replaced existing piston powerplants with turbine engines. The only one of the latter to meet with major success has been the Piper Malibu JetPROP. (JetPROP LLC of Spokane, Wash., has sold a remarkable 260 P&W PT6A turboprop conversions for the Piper Malibu and Mirage, nearly 25% of total Malibu production.)

Now comes the Extra 500, Walter Extra’s high-wing carbon-fiber take on what a turbine single should be. The airplane has been in development for almost a decade, though it’s regarded as a new design on this side of the Atlantic. Most pilots know Walter as the father of the famous Extra 230/260/300/330 series of sportplanes that have competed so successfully in the aerobatic arena for the last quarter century.

The 500 is a definite departure for Extra Aircraft of Dinslaken, Germany. The airframe and wing are essentially identical to those used on the Extra
400, a short-lived piston product that premiered in 2003. That first nonaerobatic Extra was a preliminary, toe-in-the-water experiment to see if there might be a market for a Malibu-like, high-speed cruiser. The 400 was a carbon-fiber design that used a water-cooled Continental TSIOL-550-C engine rated for 350 hp, a less-than-optimum choice, as it turned out.

I delivered N400EX, the first Extra 400 to be ferried across the Atlantic, from Germany to Arizona in 2004, and the trip was interesting, to say the least. (Other Extra 400s were shipped to the U.S. in pieces and reassembled at Extra’s American distributor, Aero Sport, in St. Augustine, Fla.) No one on the North Atlantic route had seen an Extra 400, so the airplane was a source of instant curiosity wherever I went.

Rolls Royce
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.

Design Decisions
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.

Entry to the Extra’s cabin is through a door below the left wing. Once inside, it’s immediately obvious this would be a very comfortable place to travel, 55 inches across by 49 inches tall, nearly the same as a Twin Commander. The front office is configured hard by the windshield, which has a fast taper on both sides, wrapping up and over the two pilots.

Extra opted for a T-configuration in the tail to lift the elevator up out of the prop wash and thereby minimize pitch excursions associated with power changes. Pitch control in the Extra is right now, regardless of airspeed or power setting.

Normal approaches work well at between 90 and 120 knots. With the benefit of turbine power, the airplane can use any smooth, unobstructed, 2,000-foot runway. The retraction mechanism looks fairly fragile, but Walter Extra insists it has been tested repeatedly on turf runways without problems.

Standard avionics on the Extra 500 will be the Avidyne Entegra R9, a fully integrated flat-panel display controlled by a qwerty keyboard. Like all single-engine turbines these days, the standard airplane will come equipped with everything you could reasonably ask.

Looking Forward
Company CEO Ken Keith is planning to build a production facility in the U.S. to facilitate domestic production. Keep in mind, this isn’t a conventional aluminum machine put together with rivets. In fact, it’s in some respects closer to many modern composite homebuilt designs than to conventional production airplanes. 

The carbon-fiber airplane will demand exotic construction techniques, but Keith is confident the recent emphasis on composites in commercial and military aircraft construction should provide a strong labor base of technicians with reasonable expertise.

For the buyer, a piece of that expertise will cost you $1.75 million a copy plus options. That makes the Extra 500 the least expensive certified jetprop on the market. Walter Extra’s innovative turboprop has already earned EASA certification (in Europe). The airplane we flew had been delivered to its German owner who had leased it back to Extra for the U.S. tour. An American reciprocal production certificate probably won’t be long in coming.

Make no mistake, the Extra 500 is unlike any existing production, single-engine turboprop. The configuration is unique, and the operating cost is the lowest in the class. Ken Keith and Walter Extra are hoping their unusual single-engine prop jet will find favor on the world market.