Story ID: 329Stratos
Stratos 714 Arrives at Oshkosh 
By: Mark Huber
Caption: The company could offer a kit version of the aircraft before a certified version.
Word Count: 1088
More than a decade after the light jet was conceived, Stratos Aircraft flew its first proof-of-concept Model 714 into AirVenture this year after logging 70 hours of test flights since its initial flight last November 21. A particularly brutal winter in Redmond, Oregon, substantially slowed the progress of the flight-test program, effectively grounding it for two months, said chief aerodynamicist Gordon Robinson.

To date the aircraft has attained altitudes of 18,000 feet and speeds up to 250 knots indicated, or 330 ktas. The aircraft hasn't been flown pressurized, but it will be, later in the test program. The aircraft that arrived here at AirVenture was fitted with a complete interior that was designed, fitted, and installed by Strato, save for the seat structures that were provided by Millennium Aircraft. In fact 90 percent of the aircraft was fabricated by Stratos including the trailing link landing gear that was fit-prototyped using additive manufacturing (aka 3D printing). The wind tunnel model was also constructed using 3D printing, a method that Robinson credited with yielding more precise results. The 714 that flew into Oshkosh was equipped with Garmin 3X avionics, but the company said the final decision on an avionics suite is still years off.

Although the Redmond, Oregon company is relatively small, with only 16 employees and 10 to 12 consultants, CEO Michael Lemaire told AIN that the company is well-equipped to rapidly prototype aircraft with 3D printers, CNC milling machines, precision composite cutting technology, and is currently building its own autoclave. About 43 parts on the proof-of-concept aircraft were fabricated using 3D printing including the rudder pedals and all the vortex generators. Lemaire said the company continues to focus on raising the bulk of the estimated $200 million required to finish certification and that, while interest in the aircraft has increased substantially since first flight last year, the company is three to four years away from taking deposits. Stratos also has not ruled out offering the aircraft as a kit before it offers it as a certified aircraft. While Lemaire said it is too early to set a price, it should be in the range of or less than those of other high-performance single-turboprop aircraft such as the TBM 930, John Hadlich, Stratos's prototype project manager said. Hadlich said the company currently has sufficient funding in hand to build a second aircraft and continuing ongoing development efforts.

The aircraft at AirVenture is not an exact copy of the final product, Robinson said. It is fitted with an older generation Pratt & Whitney Canada JT15D-5 engine at present. The original design called for the Williams FJ44 and future plans now call for the production aircraft to carry the newer P&WC 535E engine, the same powerplant on the Embraer Phenom 300. While landing speed on the production aircraft is estimated at 71 knots, on the proof of concept it is currently 79 knots due to the changes made to accommodate the JT15 installation. "If you look at the base of the wing, there are still some vortex generators (VGs) there," Robinson explained "because the roots are not as strong as they should be because of the engine change. So we have a pronounced root stall—very benign—that yields a buffett without any tendency to drop a wing." Robinson said on the production aircraft the stall will start approximately 40 percent out, generating a stall that is still safe. "The airfoils on this wing have a trailing edge stall that is very progressive. We've got the configuration right." Robinson added, however, that the aircraft has yet to perform a fully developed stall. 

The other consequence of going with the JT15 engine was the need to add fuel capacity, necessitating a very thick wing. The production aircraft will hold 410 gallons and overall aircraft maximum gross weight has increased to 8,400 pounds.

The need for a 3,000-pound-thrust-class engine to meet the aircraft's mission parameters of Mach 0.7, 1,500 nm of range, and four 200-pound passengers drove its current configuration, Robinson said. "It's also great CG-wise. If you look at the empty CG of this aircraft where passengers get in and out they are not changing the CG. We set out to do carefree loading. You can distribute the 1,300-pound payload  pretty much how you like it throughout the aircraft and you are pulling [the CG] forward and back less."

The aft fuselage positioning of the engine makes servicing comparatively easy, as it requires simply removing waist-high side service panels and lowering it directly onto a trolley for removal, he said. However, the engine positioning did create some special challenges when it came to the engine inlets' geometry. The solution was to create the distinctive scallops that begin aft of the nose and continue to the inlets. "That straightens out the flow pattern to the engine," Robinson said, creating less aggressive bends and carrying most of the water droplets through the engine and out. There's still an anti-ice patch inside the inlet "but it is much smaller" than it would have been otherwise, Robinson said. The scalloping also mitigates blanketing the downwind inlet in high crosswind situations, he said. The scalloping did consume a bit of cockpit space under the armrest and did require more structural reinforcement around the cabin door, but the overall impact to the design was minimal.

The decision to go with cabled sidestick controls presented several special design challenges, Robinson said. "You have to balance the ailerons to get the stick forces down to a level where you can do a sidestick. So we set the aileron hinges aft, and there's an overhang to help balance the forces. And there's also an anti-balance gear tab that acts opposite the aileron and reduces the forces." Robinson said Stratos also added vortex generators to sweep away the boundary layer that inhibited the operation of the tabs in the dead air. He said the process of tailoring the stick forces is continuing during flight test.

The aircraft's other unusual design feature is a set of large "barn door" air brakes that deploy from the aft fuselage at detents of 30 and 60 degrees to enable rapid descent in the event of engine and/or pressurization failure at altitude, up to 41,000 feet. Robinson said they can generate descents in the range of 12,000 fpm without a change in trim speed. "It's all about staying in this envelope of physiological survivability," Robinson said. He added that the speed brakes wouldn't help much during a normal approach because the aircraft has double-slotted flaps that "give us buckets of drag."