Flying laboratory capabilities.
The Cessna Citation II “PH-LAB” has been converted into a flexible facility for advanced aviation research. Thanks to the installed instrumentation, and modifications to the aircraft, several classes of project are possible with this flying laboratory:
- Testing of new flight displays. The standard EFIS instrumentation for the right pilot station can be replaced by experimental screens, either one large screen in landscape mode, or two smaller screens in portrait mode. The experimental data acquisition system gives a developer access to a wide range of aircraft data that can be used to generate the screen’s new image.
- Testing of flight control laws. The data acquisition system can be extended to a fly-by-wire configuration, in which the aircraft can be controlled in three axes with the autopilot actuators. Optionally pilot inputs can be included from a side stick mounted at the right-hand pilot station.
- Experiments with additional sensors and equipment. These require additional effort, but due to the flexible set-up of the data acquisition system and the availability of a WiFi link on-board for communication with additional displays and devices, as well as the possibility to use wired network links, truly customized experiments are possible.
The backbone of the flying laboratory is an experimental control and data acquisition computer installed in a rack in the back of the airplane. Some of the characteristics of this system are:
- The host computer runs a real-time hardened Linux version (currently SUSE SLED 12.1 with a real-time kernel with the PREEMPT_RT patch), providing real-time performance. Ample processing power is available for additional client code, e.g., for experimental control laws.
- The computer uses DUECA, a middleware layer that is used in several of the facilities of the Control & Simulation division. Through DUECA, the data acquisition and control applications can run real-time, multi-threaded, with a range of update rates. DUECA provides transparent lock-free, loss-free and multithread-safe communication between different parts of the data acquisition, control and visualization processes. The Flight Test Instrumentation application uses 8 threads for data acquisition at different update rates, communication with devices over WiFi, control of the aircraft and logging.
- The computer is equipped with a number of IO devices:
- A syncho-resolver card, measuring the angles of the control surfaces (aileron, elevator and rudder) and optionally signals from a nose boom, that is equipped with angle-of-attack and sideslip vanes. These signals are sampled at 100Hz.
- An ARINC card, simultaneously receiving data from 18 ARINC 429 channels in the aircraft. All ARINC data is recorded and logged with microsecond-level time stamps, and in addition the computer time is logged once per ARINC cycle (100 Hz), to enable precise synchronization between the ARINC signals and other recorded signals. A selection of ARINC data is converted to engineering units, and transmitted through a DUECA communication channel, for use in experimental displays or control laws.
- A stack of EtherCAT modules, providing analog inputs and outputs, and digital inputs and outputs. Measured analog signals include engine temperatures, positions and control current of the autopilot motors, angle of attack vane position, trim tab position, forces and positions from the yoke, inputs from an optionally installed side stick, signals from an accelerometer in the boom, engine fuel flow, engine fan and turbine speeds and flap position. EtherCAT signals are sampled and logged at a rate of 1000 Hz. If the fly-by-wire equipment is installed, control is through an extended EtherCAT stack, also running at 1000Hz.
- All data is logged in a structured hdf5 format datafile. Data is logged as raw inputs, and an xml file detailing how all signals are to be converted is included in the datafile. Using a custom-developed matlab program, appropriately called HEFTIG (Highly Effective Flight Test Instrumentation GUI), selected data can be re-sampled and output in a Matlab file format. The generated hfd5 data file size is approximately 1 GB per hour.
- The aircraft has a private WiFi system, which is connected to the data acquisition and control computer. For our classroom flights, Android tablets are equipped with a custom written app, that receives the flight data over WiFi. Other communication is also possible over the WiFi system.
- The DUECA middleware layer can also transparently connect multiple computers together, synchronizing timing on these computers and making the data from the DUECA communication channels available in real time in all computers that request that data. This configuration is typically used when experimental displays are tested.