Flight Mechanics, Flight Control and Aeroelasticity

Hardware-in-the-Loop Simulator

A redundant flight control system for the motor glider STEMME S15 was developed for the high-precision automatic control of an aerial work plane within the scope of the LAPAZ project in the national aviation research programme (LUFO IV). To test this system a ground test facility in the form of a hardware-in-the-loop (HIL) simulator was set up. With it the correct integration of the flight control system into the aircraft (STEMME S15 prototype) can be verified on ground and the correct functioning of the flight control laws can be checked. The HIL-simulation is part of a cost-effective development process for safety-critical systems, which was established at the department.


  • The simulation unit is designed to be mobile and robust.
  • All hardware is integrated in two mobile transport containers and can be temporarily connected to the STEMME S15.
  • The workstation of the test pilot is located in the cockpit. From there he controls the aircraft with the help of the autopilot.
  • Manual flight is also possible via the control units of the S15.
  • For orientation the pilot uses the visual display via the projector.


The task of the HIL simulator is to perform tests of the automatic flight control system in connection with the STEMME S15 prototype on ground.

Due to the lack of aircraft movement, the entire sensor system of the flight control system is out of operation and must be simulated. This concerns:

  • the air data systems (AD),
  • the inertial platforms (AHRS),
  • the navigation system (GPS),
  • the Laser Altimeter (LA) and
  • the ground contact sensors (WOW).

Furthermore, the operation of the engine is out of the question for safety, ecological and economic reasons. Furthermore, a realistic function of the drive unit would not be guaranteed due to the lack of flow.
Sensor and motor data serve as input variables for flight control. To ensure the correct operation of the flight control system, the measured values must be generated in a flight simulation. This is done in the simulation unit. For the simulation a mathematical aircraft model of the STEMME S15 is used. In this model the drive unit is also modelled. As output values the actual speed and the exhaust gas temperature of the engine are available. The flight mechanical model of the S15 consists of a non-linear six degrees of freedom simulation of the rigid aircraft, which was extended by the elastic degrees of freedom important for the controller testing. The simulation also includes a sensor model. This consists of a dynamic model (time delay etc.) and an error model (white noise, bias, drift etc.). The simulated sensor and engine data are transmitted to the flight control computers as input variables.
The flight simulation requires as input the information of the control units. The deflection angles of the control surfaces are measured by means of potentiometers. This eliminates the need to model the overall transfer function of the rudder control (actuators, linkage kinematics etc.) in the simulation. The drive unit requires the throttle position and the propeller speed command as input variables. The position of the throttle valve is provided by the TCU by internal measurement using a potentiometer. The rotational speed command is given via the control unit for rotational speed control of the propeller.


The HIL simulator has a similar structure to the research simulator SEPHIR operated by the FMRA. It uses hardware components that have proven themselves in practice. This offers the additional advantage that existing software modules can be adapted for the HIL simulator with little programming effort. The simulators are modular and mainly built from standard individual components. Compared to a complete turnkey system, the procurement and maintenance costs are significantly lower. Furthermore, the system is flexibly expandable.

The actual simulation process is executed on the simulation computer. A high-performance computer is available, on which a UNIX operating system is installed. This has been extended with real-time functionalities and optimized to execute time-critical applications.
The simulation interface (SIMIF) enables data transfer between the S15 and the simulation computer. It consists of a conventional PC and a PCI expansion system to extend the plug-in card capacity. The interface cards include the transmission standards ARINC 429, RS-232, CAN and enable analog / digital (A/D) conversion. The outputs of the cards are connected to the corresponding components of the flight control system of the S15 via a cable harness.
The entire communication between the individual computers of the simulation unit takes place via UDP (User Datagram Protocol). The simulation is controlled by means of a notebook. Various simulation parameters can be changed in real-time. The information about position and movement of the aircraft is processed by the visual computer and displayed with the help of a projector. The display instruments in the cockpit and an aircraft exterior view are generated by a separate computer (monitoring computer) and displayed on the monitors.