Aerospace industry trends are pushing for increased aircraft capability through lighter and more flexible drivetrain components and more powerful engines. Upgrading the propulsion system in both fixed wing and rotary aircraft enables operation at higher altitudes and hotter temperatures, increased payload capacity and better fuel efficiency.
To that end, Parker LORD is collaborating with partners to reduce the risks of repowering the Chinook CH-47 helicopter with a more technologically advanced turboshaft engine. The demonstration aircraft is using larger, more powerful engines, additively manufactured load bearing drive system components and an off-engine torque measurement system provided by Parker LORD Corporation. Testing will characterize structural response, engine governing, thermal compatibility and limited handling qualities within the existing aircraft operating envelope.
This is first time in decades that U.S. Army aviation has put a new engine with a different footprint onto a legacy aircraft. This demonstration aircraft also features the first off-engine torque measurement system, which means rotational force will not be measured by a part within the engine itself.
Previously, Parker LORD has integrated an accurate TRL 9 torque monitoring system on a fixed-wing defense aircraft to enable vertical lift capability. However, this is the first time the torque measurement solution is being implemented on a helicopter.
Typically, the torque monitoring system would be placed directly on the engine but doing so with the larger capacity engines being tested on the Chinook would change the center of gravity and detrimentally affect the aircraft flight characteristics.
As a helicopter’s blades rotate, they require torque to generate lift. This torque originates from the engines and must be controlled before flight is possible. In tandem rotor and coaxial helicopter designs, the rotors turn in opposite directions to neutralize or eliminate torque that would cause the aircraft to rotate or yaw. Any change in engine power output brings about a corresponding change in torque. Additionally, power varies with the flight maneuver and results in variable engine torques that must be supplied for flight.
Often torque monitoring is integrated into the front of a turboshaft engine – when space is available. However, with aircraft, weight and location comes at a premium and sometimes isn’t available for a torque sensor. Moving the torque monitoring away from the engine output allows increased flexibility for the engine OEM to integrate their engine into the airframe.
How Our Off-Engine Torque Measurement Works
In this specific case, the engines create power that goes through a central combiner gearbox and is distributed to the forward and aft rotor systems to provide lift to the helicopter. The Parker LORD system measures the torque being generated by both engines before it reaches the combiner gearbox and digitally communicates that information to an engine controller. This controller then makes engine corrections to limit torque and changes power as required.
The Parker LORD Torque Measurement Sensor works by using an array of variable reluctance sensors positioned around the teeth on a coupling or shaft. By measuring the precise timing of the targets as they spin using a “Zero Crossing Detection” circuit (or ZCD for short), drivetrain health measurements can be extracted.
A continuous and precise level of torque accuracy is required because the power generated from these larger engines far exceeds that of the previous engines and approaches the aircraft design limitations. Torque monitoring mitigates the safety risks associated with these more powerful engines. The torque and torsional dynamics are measured by accurately assessing the twist across two locations on a drivetrain. As an example, consider the bellows coupling shown at the right which has torque applied to it – the twisting targets are measured and used to calculate torque.
In addition to measuring torque, the system also monitors the health of the drivetrain for several other drivetrain motions: such as articulation, axial motion, radial whirl, and shaft speed. The figures below show typical drivetrain motions that are secondary measurements of the torque monitoring system.
Parker LORD’s continued work with its partners and successful flight tests will also enable the company to realize methods of reducing component size for future derivatives. The off-engine torque monitoring system is tailorable—so any engine configuration can use this technology where the system can be adapted to any aircraft regardless of the number of or orientation of engines.