AEROSPACETESTINGINTERNATIONAL.COM // SEPTEMBER 2019 31
RELENTLESS BY DESIGN
The Bell 525 flight test program was halted in July 2016 by
the fatal crash of Flight Test Vehicle (FTV) 1 in Texas during
high-speed, One Engine Inoperative emergency testing.
Investigators studying telemetry and recorded digital data
concluded high-speed vibration exacerbated pilot inputs
to the flight control system, and low main rotor speed
brought flapping blades in contact with the tail boom.
Bell introduced a software filter in the collective
control law to dampen biomechanical feedback. Flight
testing resumed in July 2017, and most other changes in
the helicopter have been largely software. “In rotorcraft
engineering, that is pretty remarkable,” says Bell manager
of technology and evaluation Josh O’Neil. “Typically, you
see visible changes from the time they first fly to the time
they certify. When you see pictures of the 525, they look
the same.”
The Relentless uses a 50-50 hybrid construction with
high-speed machined alloy primary structure covered by
fiber-reinforced composites. The big fuselage side bodies,
doors, and fairings, most of the lift-assisting tail boom,
and internal bulkheads and keel beams are composite.
The frames that support the cabin and transmission, roof
beam, nose landing gear, and main landing gear, and the
nose center post between pilot and copilot windshields
are aluminum.
From the start of the Relentless program, Bell and its
suppliers standardized on CATIA V6 design software that
took into account thermal expansion of all the parts in
different materials at different temperatures. According
to O’Neil, “You would see if things didn’t fit together as
expected because of the thermal distances opening up,
gaps, thermal stresses. You could induce displacement
and thermal stresses you didn’t catch in your initial
engineering. . .. You’re able to apply conditions to the
entire aircraft digitally.” He concludes, “That enables you
to go to cold weather and do what we did and get several
months of checks-in-boxes, from cold-soak at minus 40
and fly. That’s what you want.”
flight testing
takeaway – to go out there and really fly the aircraft
through all kinds of maneuvers and generate data that
really is in line with expectations you’ve had for months
is really a testament to the design.”
YELLOWKNIFE SUPPORT
The Bell 525 Relentless aims to be the world’s first
commercial helicopter certified with fly-by-wire flight
controls. Its smooth cold weather test was also the first
supported by the new Yellowknife Airport Cold Weather
Testing (CWT) Group formed in 2018. The Group
coordinated support for Bell’s cold weather test plan,
providing a single point of contact to help with hangar
space rental, runway support, logistics, hotel rooms, and
media access. “They’re very good to work with,”
acknowledges O’Neil. “The staff there was very helpful.”
Yellowknife has hosted test programs such as the
Airbus Helicopters EC175 in 2014 and Airbus H160 and
Sikorsky CH148 in 2017. The Relentless cold weather test
effort ultimately cycled 109 Bell employees through
Yellowknife at different times, most of them deployed
from Fort Worth, Texas. The team included pilots,
maintainers, and engineers from flight test and all
technical disciplines. Northwest Territories regional
airline FirstAir provided the testers a hangar at
Yellowknife Airport. “They have space to accommodate
two large 525 aircraft available for rent,” says O’Neil.
“We’ve had off-sites in the past that have been
challenging in that respect.” In addition to hangars up to
22,960 sq ft (2133m³), the airport has 7,500 and 5,000 ft
(2.29 and 1.52km) runways for fixed-wing aircraft.
The Yellowknife Cold Weather Testing Group now
markets its generally cold, clear, dry weather and
accommodating airport facilities as a competitive
business, and it aims to market the city as the
preeminent northern cold weather test site. According to
Group spokesperson and Yellowknife Airport manager
Randy Straker, “A network of contacts is being created in
order to provide testers access to local representatives
-50°C
Lowest temperature
in Yellowstone
1 // Cold soak testing
required the Bell 525
engines to start after
remaining outdoors
overnight in Yellowknife
cold
2 // The Bell 525 Relentless
underwent two-and-a-half
months of cold weather
testing at Yellowknife in
Canada’s Northwest
Territories
3 // Flight testing at
Yellowknife took the
Relentless to 175 kt Vne and
35 kt sideward and
rearward flight in intense
cold (Photos: Bell)
3
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