Canadair CL-227 Sentinel

The CL-227 Sentinel is a remote-controlled unmanned aerial vehicle (UAV) made by Canadair.

It displays a distinctively unusual bulbous hourglass profile. Lift is provided by a set of coaxial rotors emanating from the waist of the system. The engine air is exhausted upwards to minimize the infrared signature. The blades are made of composite materials as well as all the external skin and legs, in an attempt to reduce the radar signature. The main structure is made mostly of aluminium. The prototype of the CL-227 was designed in 1977, and was made available to NATO allied clients in the early 1980s. In the late 1990s it was replaced by an updated version, the CL-327.

While designed purely for intelligence gathering, its strange and menacing physical appearance inspired Science Fiction writer Bruce Sterling to portray it as an assassination device in the novel Islands in the Net.

Flight sensors
A vertical accelerometer and barometer feed the altitude control loop. Gyroscopes and accelerometers feed the Tilt, Roll and Yaw control loops.

Control
Like a conventional helicopter, altitude is controlled by raising or lowering the swashplate assembly. Tilt and Roll is controlled by tilting the swashplate assembly. As each blade crosses the highest point of the swashplate, it generates more lift. Yaw is controlled by slowing down the upper rotor or the lower rotor using frictionless Magnetic Particle Clutches (MPC).

There are 3 linear servos under the swashplate and two Magnetic Particle Clutches. Unlike a conventional helicopter, the UAV center of mass is located approximately 1 inch above the upper rotor when fully fueled. This results in a vehicle that has a tendency to flip over. The pitch and roll control algorithms have to recalculate a new swashplate position every 10 milliseconds.

The CL-227 Sentinel control systems consisted of the Inertial Measurement Unit (IMU), the Airborne Computer (ABC), the Fuel Controller Unit (FCU), the Servo Controller Unit (SCU) and the Payload Interface Unit (PIU).

The IMU is an entirely analog device feeding the ABC. The ABC itself consists of 3 microprocessors. An Inertial Navigation Processor (INP), a Control Laws Processor (CLP) and an Input/Output Processor (IOP). The INP and CLP bi-directional communication is made possible by the use of a dual port random access memory (DPR1). The CLP and IOP bi-directional communication is made possible by the use of DPR2.

The INP primary functions are to calculate pitch, roll and yaw attitudes. As well as pitch, roll and yaw rates, barometric pressure and vertical acceleration. It is also responsible for calculating latitude and longitude. This could not be achieved very precisely. That feature is only used for relatively short times during reversionary modes (less than 30 minutes). A GPS receiver was never integrated in the CL-227 Sentinel.

The CLP primary functions is automated pre-flight checks, pitch, roll, yaw and altitude control laws. It also includes reversionary modes (what to do when the uplink communication from the ground control station is lost). This would typically include climb for 60 seconds while station keeping (or using a corkscrew pattern), and land at a designated recovery site.

The IOP primary functions are to send commands to the FCU, SCU and PIU via RS-485 links. Also to receive uplink commands from the Ground Control Station (GCS) via the PIU. The IOP also has to send and receive commands to the Take Off Interface System (TIS).

Technology
The processors used are an intel 80186 and intel 80187 at 8 MHz for the INP and the CLP, while a Zilog Z80 at 8 MHz was used for the IOP. The INP and CLP used Intel Pascal. Their real-time operating system consisted of an in-house interrupt-driven priority-based operating system. The IOP is programmed in assembler. The INP and CLP were eventually upgraded to an 80286 and 80287.

Bombardier Aerospace CL-327 Guardian
The CL-327 was developed as an ungraded and advance version of the CL-227. The new UAV production line began in 1996 under Bombardier Aerospace, which acquired the assets of Canadair, the original developer of the CL-227.