WEST PALM BEACH, Fla. – After three test flights, Sikorsky and Boeing are gathering more data from the ground-based propulsion system test bed (PSTB) before it returns to the air, according to Sikorsky Vice President of Innovation Chris Van Buiten.
"We're thrilled with the data we've seen out of ground effect," Van Buiten said June 24 during a media visit to Sikorsky's test flight facility here. "Hover looks good."
One Sikorsky and one Boeing pilot have sat side-by-side in Defiant's cockpit during the three flights. The 33,000-pound aircraft is aimed at fulfilling the Army's need for a Future Long-Range Assault Aircraft, or FLRAA.
Because Sikorsky's partners at Boeing were not present, Van Buiten would not provide much detail on the Defiant flight test program. But the "context" he laid out for Defiant is a cautious flight test program eager to start widening the flight envelope.
"We're going through a tweak right now and the [propulsion system test bed] PSTB is running, with some modifications and we're about to restart the flight tests and you know, we hope to march right through the speed points and you know, you've seen it in the past. We moved the speed out 20, 40-knot increments."
A Sikorsky spokesperson clarified that there is nothing wrong with the aircraft and the period between flights was planned so engineers can gather PSTB data before launching the campaign to quickly expand the speed envelope.
"DEFIANT has successfully flown three sorties," company Spokesman Jeff Brown wrote in an email. " In advance of envelope expansion, as planned, we are first increasing the power levels on our PSTB. We remain consistent with our development process, testing out these higher power loads on our PSTB before conducting additional maneuvers in flight. Upon successful PSTB endurance envelop expansion, we expect to be positioned to quickly expand the aircraft flight envelope later this summer."
Defiant's smaller brother, the S-97 Raider has flown at 207 knots during level cruise and saw that sort of dramatic increase in flight speed during its test campaign. It now flies weekly, often at or above 180 knots, according to Senior Test Pilot Bill Fell.
Where Raider, an attack variant of the same basic compound-helicopter design, is about 16,000 pounds and designed to fly at up to 220 knots, Defiant is considerably larger and faster. Weighing in at around 33,000 pounds, its two engines – powering a dual-coaxial rigid main rotor system slightly larger than the Black Hawk's and a pusher propeller – should propel Defiant up to 250-plus knots in cruise.
"The exciting thing for us is it's our intent to fly, in representative scale, all of the required [Future Vertical Lift] elements and … Defiant is ramping up to fly those for FLRAA," Van Buiten said.
Defiant made its maiden flight March 21 and flew for a second time on April 5.
During those two initial flights, Fell and Boeing pilot Frank Conway "hit every test point and completed each data point we had planned to complete," Sikorsky and Boeing said in a previous statement. "The team proved out the expected speed for these tests: 10 kts forward, left, right and rearward. It also proved out hover performance maneuverability, pedal turns, and a hover ladder from ground level up to 30 feet altitude. These tests are a first step in getting to the flight envelope we hope to achieve with this groundbreaking aircraft. Over the summer of 2019 we expect to expand the envelope in flight tests with additional speed and maneuverability and provide the U.S. Army with the necessary, robust, and relevant data in a timely manner that will enable them to make fully informed decisions as they pursue the Future Vertical Lift program."
Update: This version of the article adds details from Sikorsky on the Defiant flight test program and plans to return to flight in order to expand the flight envelope.