Stratos Is Serious About Single-engine 714

 - May 15, 2017, 2:07 AM

In 2008, the market hype about single- and twin-engine very light jets was at a peak, with what seemed like dozens of projects under way, and talk of VLJs darkening the skies prompting concerns among aviation authorities about how all this new traffic could be accommodated. In the midst of this frenzy, a small Oregon company called Stratos Aircraft unveiled the model 714, a single-engine, $2 million, all-composite jet that promised far better performance than its in-development competitors. 

Fast forward a few years, and while Stratos Aircraft made some headway and even exhibited at the annual NBAA conference, the company quietly refined the design in a wind tunnel and began putting an airplane together. Then, on November 30 last year, test pilot Dave Morss lifted off in the prototype 714 from Roberts Field in Redmond, Ore., the airport closest to the company’s off-airport Redmond headquarters and manufacturing facility.

Since then Morss has flown the 714 thirty times for 40 hours, up to 250 kias (300 ktas) and nearly 15,000 feet. Plans are for him to fly the prototype 714 to this year’s EAA AirVenture show in Oshkosh, Wis., although he will have to stay below 18,000 feet because the 714 isn't IFR approved. The prototype also isn’t pressurized, although it has been pressure tested and can be pressurized, but the equipment to do so isn’t installed. The prototype will be fitted with a fresh interior and be painted before flying to Oshkosh.

The moniker 714 has a specific meaning, and it is spelled out in Stratos’s logo, the design of which illustrates the following attributes: “7” for Mach 0.7 top speed, “1” for one engine and “4” for one pilot and three passengers. From the beginning, Michael Lemaire (president and CEO), Carsten Sundin (chief technology officer), Kevin Jordan (chief sales officer) and Gordon Robinson (chief aerodynamicist) targeted ambitious performance goals: a 41,000-foot maximum altitude, max cruise of more than 400 ktas (now it is 415 ktas) and NBAA IFR range (100 nm alternate) with four occupants of 1,500 nm (at 402 ktas cruise speed). All this performance doesn’t detract from airport capabilities; at maximum weight, sea-level takeoff distance to 50 feet on a standard day is just 1,970 feet, and landing requires 2,240 feet. Time to climb to FL370 is 17 minutes. Total fuel capacity is 410 gallons. A baggage compartment is fitted above the engine and is accessible from outside the cabin.

Manufacturing and Production

Construction of the prototype began after Stratos secured fresh funding in 2012 and moved into the new facilities. This is also where manufacturing of the production aircraft will take place; composites will be manufactured in a new building that is under construction. Stratos engineers have designed and manufactured many of the airplane’s components, among them the landing gear, rudder pedals and other parts. Stratos is also incorporating 3-D printing into its design and production process. A Stratasys 3-D printer was used to print fuel system components, which could be checked for fit into the airframe well before the fuel systems supplier was able to make the parts. Then when the real parts arrived from that supplier, Stratos technicians simply exchanged them with the 3-D printed parts installed in the airframe. 

There is one fairly significant change in the design, but this doesn’t affect the 714’s performance, and that is a change to a different engine. The original design was to have been powered by a 3,030-pound-thrust Williams International FJ44-3AP, but according to Sundin, “We weren’t able to obtain that engine.” So Stratos switched to the 2,950-pound-thrust Pratt & Whitney Canada JT15D. “We can meet all of our goals [with that engine],” he said.

The next Stratos 714 will incorporate lessons learned from the prototype and will be much lighter, because some systems tested in the prototype were deemed unnecessary. One example is accumulators used in the hydraulic system, according to Lemaire. The hydraulic system drives the landing-gear actuators, speed brakes and the engine’s thrust attenuator. Another component under evaluation is speed brakes, which are large paddles mounted on the fuselage sides, but they might be unnecessary. 

Morss told AIN that one of the biggest challenges in the flight control design is the 714’s sidestick. The ergonomics have to be just right for good control harmony and feel, because a side-mounted stick doesn’t have as much throw as a floor-mounted stick or a yoke. This is a matter of adjusting stick gear ratios and movement of control surfaces and servo tabs. The prototype’s left sidestick was also lengthened by two inches to give more leverage, which helped improve the control feel and authority. “This is the first jet I’ve flown with a sidestick,” he said. Rudder pedals will be adjustable in the final design.

While he hadn’t flown the full stall series before AIN’s visit in April, Morss had done a high-speed stall during envelope expansion. The full stall series will be completed before the trip to Oshkosh. During one test flight on a cold day, he took off at mtow and used less than 2,000 feet of runway, at Redmond’s 3,080-foot elevation. “This airplane is screaming to go out of a 2,500-foot strip,” he said. “Landing the 714 is easy. The trailing-link landing gear is just a delight.”

There is very little pitch change with thrust changes, because the engine is mounted so close to the center of gravity. But the arm from the tail to the wing is also long, which improves stability, according to Sundin.

After completing flight-testing on the first prototype, Stratos will run ground tests on a load-test airframe then test fly the second prototype. The company isn’t revealing projections for a certification timeline, but the second prototype will conform to the type design. “It’s looking really good,” Sundin concluded.

May 2017
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