GPRA – Case Study

(General Purpose Rotary Actuator)

When OTM successfully met the challenge set by BAE Systems to develop the duplex throttle actuator for the Herti drone, it could be said that we were the victims of our own success. Why? Because one of the major achievements in meeting the brief was tailoring the design and build of the unit to fit the existing cavity within the aircraft. So, when BAE asked if we could also develop a rotary actuator that could successfully operate the flight control surfaces which could also fit within the limited constraints of the skin of the existing wing profile, we knew we had another serious challenge on our hands.

Re-designing the drone, we already knew, was not an option. The actuator had to consistently produce peak torque rates with exceptionally fast response rates and fit into a constrained wing space offering only 1mm of clearance between the unit and the wing skin.

At OTM we love nothing more than a challenge. Not only were our experts able to design and build the unit that would be able to operate successfully within the size constraints of the aircraft but it was also able to produce 40% more power than any available commercial off the shelf (COTS) solutions. From a unit of this size and weight, this is exceptional and was a crucial factor in the success of the project.

Meeting flight safety requirements demanded that the actuator perform to within just one degree of accuracy of rotation. Working to a positional feedback accuracy of 0.2 degrees, the OTM solution exceeded the safety standards by a factor of four. The actuator also qualified to the stringent DO178 and DO254 software and hardware requirements respectively.

There are multi-installations of the actuator on the drone which operate independently or in concert as required. The wing flight control surfaces are split into three sections under a highly intelligently designed system where the units can work in harmony or independently. This gives the aircraft the required redundancy in flight as, if one fails, the others can still operate autonomously.

  • Features of the GPRA:
  • Positional feedback accuracy – 0.2°
  • Operating Torque – 12Nm
  • Peak Torque – 20Nm
  • Response rate – 70° per second
  • Actuator weight – 1.25kg

Although this simplex actuator was developed for use in primary flight control surface actuation, it has also proved its worth in secondary functions such as braking and steering. Which is perhaps the main message to come out of this project. Despite being a tailored unit, specifically engineered to meet the size and weight demands of a very specialised aircraft wing environment, the GPRA has proved itself versatile enough to be exactly that – General Purpose – and flexible enough to be put to equally good use in other equally taxing applications.