An Overview of Flight Simulator Motion System Evaluation

The Vital Role of Flight Simulator Motion Cueing Systems

Motion cueing systems are a fundamental part of advanced flight simulators. These systems provide pilots with essential vestibular feedback, simulating the physical sensations of flight. This feedback is critical because it works in conjunction with the visual and instrument information to give pilots a complete sense of the aircraft's movement.

• The motion system's feedback is especially important in helping pilots control the aircraft's dynamics, particularly when the aircraft is affected by external factors like turbulence or wind gusts. The system provides the physical cues that allow pilots to feel the changes in acceleration, orientation, and position.

• It is crucial that the motion cues perceived by the pilot accurately represent the motions of the specific aeroplane or helicopter being simulated. This includes replicating the timing, intensity, and direction of the forces and accelerations experienced in real flight. For example, the sensations of touchdown during landing should be directly related to the simulator's simulated rate of descent.

• Motion feedback is particularly significant in the simulation of rotary-wing aircraft (helicopters). Helicopter pilots rely on a wide range of motion cues, including high-frequency vibrations produced by the main rotor. These cues provide critical information about the helicopter's dynamic state and help pilots identify both normal and abnormal operating conditions.

Minimum Requirements and Degrees of Freedom

The design and capabilities of a motion system in a flight simulator are defined by specific requirements, particularly concerning the degrees of freedom it can simulate.

• A motion system in an FSTD (if it has one) must have a minimum of 3 degrees of freedom. These three essential degrees of freedom are:

  • Pitch: Rotation of the simulator about its lateral axis (nose up or down).

  • Roll: Rotation of the simulator about its longitudinal axis (wing tip up or down).

  • Heave: Vertical movement of the simulator (up or down).

These three degrees of freedom are the minimum necessary to provide basic but essential motion cues for many flight maneuvers.

• However, higher-level FFSs are often required to produce motion cues at least equivalent to those of a six-degrees-of-freedom (6-DOF) synergistic platform motion system. This more sophisticated system adds three more degrees of freedom:

  • Yaw: Rotation of the simulator about its vertical axis (nose left or right).

  • Sway: Lateral movement of the simulator (side to side).

  • Surge: Longitudinal movement of the simulator (forward and backward).

A 6-DoF system provides a much more comprehensive and realistic representation of aircraft motion, capturing a wider range of accelerations and rotational movements.

• For FTDs and FNPTs, motion systems are generally not specifically required. These devices often focus more on instrument and procedural training, where the visual and cockpit environment are the primary focus.

• However, if an FTD or FNPT is equipped with a motion system, it will be thoroughly assessed to ensure that the motion does not negatively affect the overall qualification of the device. The motion system should not introduce unrealistic or misleading cues that could hinder training.

• For certain levels of FTDs and FNPTs, if a motion system is installed, it should meet at least the motion requirements of a Level A FFS. This ensures a minimum level of motion fidelity if motion is included in these devices.

• It's also worth noting that for Level B FFS for helicopters, a reduced motion performance envelope may be acceptable. This acknowledges the complexity of helicopter motion and the potential limitations of current technology in fully replicating it at this level.

A 6-DoF Hydraulic Motion Base

Functionality and Cueing

The effectiveness of a motion system relies heavily on its functionality and the quality of the motion cues it provides.

Motion systems should provide sufficient cueing to accomplish the specific training tasks. This means that the system must be capable of generating the appropriate sensations of acceleration, rotation, and vibration necessary for the pilot to perceive the aircraft's motion. While some cueing may be of a more general nature, it should still be adequate for the intended training objectives.

What about vibration?

Motion vibrations that result from the operation of the aircraft and can be felt in the cockpit should be present in the FSTD. These vibrations provide valuable sensory information to the pilot.

• Vibrations can indicate specific events, such as engine start-up or touchdown, or they can convey information about the aircraft's state, such as buffeting or stall warning.

• These vibrations should be measurable and comparable to data obtained from the actual aircraft to ensure accuracy.

For helicopters, the accurate reproduction of vibration levels is particularly important. Vibration feedback provides pilots with subjective information about the stresses the helicopter is experiencing during certain maneuvers. This can be crucial for assessing the aircraft's limits and maintaining control. In modern simulators a separate vibration platform is incorporated alongside the traditional 6DoF motion base to ensure accurate and effective vibration cueing.

Special Effects

Special effects programming can enhance the realism of the motion system and provide additional cues. These effects may include:

• Runway rumble: Simulating the feeling of the aircraft rolling on the runway.

• Oleo deflections: Replicating the movement of the aircraft's landing gear struts.

• Effects of groundspeed and uneven surfaces: Simulating the sensations of taxiing and the bumps and jolts of the ground.

• Buffet due to translational lift in helicopters: Simulating the vibrations and turbulence experienced when a helicopter transitions to forward flight.

Cueing Critical Objectives

For maneuvers where motion cueing is most crucial, FSTDs should ideally exhibit:

• High tilt coordination gain: The system should accurately coordinate the tilting of the simulator to represent sustained accelerations.

• High rotational gain: The system should accurately represent rotational accelerations, such as those experienced during turns.

• High correlation with respect to the aircraft simulation model: The motion system's response should closely match the calculated motion of the simulated aircraft.

Objective Testing and Validation

To ensure the motion system's performance and accuracy, a series of objective tests and validations are conducted. These tests assess both the hardware and software aspects of the motion system.

Motion System Checks

These checks are performed to demonstrate the proper functioning of the motion system hardware. They also verify the integrity of the system's setup, including calibration and the absence of excessive wear. Examples of motion system checks include:

• Frequency response tests: Assessing how the system responds to different frequencies of motion inputs.

• Leg balance checks: Ensuring that the weight of the simulator is evenly distributed across the motion platform's legs.

• Turn-around checks: Verifying the smooth and accurate rotation of the system.

It's important to note that these tests primarily focus on the mechanical and electrical/hydraulic/pneumatic performance of the motion platform itself and are largely independent of the specific motion cueing software. They can be considered "robotic tests" in that they evaluate the system's basic mechanical behavior.

Motion system repeatability tests

Motion system repeatability tests are carried out to ensure that the motion system software and hardware maintain consistent performance over time. These tests verify that the system has not degraded or changed its characteristics due to wear and tear or software modifications.

• Repeatability tests are diagnostic in nature and are often employed during recurrent checks of the simulator to identify any potential issues.

Performance Signatures

A motion cueing performance signature is established during the initial qualification of the FSTD. This involves recording the motion system's response to a specific set of automated maneuvers defined in the Qualification Test Guide (QTG).

• This performance signature serves as a baseline for future comparisons. By comparing the system's current performance to this baseline, evaluators can detect any changes in the motion software or hardware that may have occurred over time.

• It is crucial to understand that this performance signature is not intended to be a direct comparison against accelerations recorded during actual flight tests. While flight test data is valuable for developing the simulation, the motion system is designed to provide effective cues to the pilot, not necessarily to perfectly replicate every nuance of flight accelerations.

When evaluating characteristic motion vibrations, the testing process requires recorded test results that allow for a detailed comparison of the relative amplitude of vibrations at different frequencies between the FSTD and the actual aircraft.

• These results are often presented in the form of a power spectral density (PSD) plot, which graphically depicts the distribution of vibration energy across different frequencies.

Subjective Assessment

While objective testing provides crucial quantitative data about the motion system's performance, it is equally important to conduct subjective assessments. These evaluations, performed by qualified and experienced pilots, focus on the overall feel and effectiveness of the motion cues.

• Even with comprehensive objective tests, it is essential to have pilots assess the motion system to ensure that it effectively supports the piloting task. The pilot's experience and judgment are invaluable in determining whether the motion cues are realistic, appropriate, and conducive to effective training.

• A key aspect of the subjective assessment is to identify and eliminate any "negative cueing." Negative cueing occurs when the motion system provides misleading or inappropriate cues that can confuse the pilot or lead to incorrect control inputs. This can be as detrimental to training as a complete lack of motion.

By combining objective measurements with these subjective evaluations, a well-rounded assessment of the motion system's quality and suitability for flight simulation is achieved.

Transport Delay and Latency

A critical aspect of motion system evaluation involves assessing the timing of the motion cues in relation to the pilot's control inputs. This is addressed through the concepts of transport delay and latency.

What is it?

Transport delay is defined as the total amount of time it takes for the entire FSTD system to respond to a pilot's control input, specifically the time until the motion system initiates its response.

• It is critical that this transport delay remains within specified tolerances. Excessive delay can disrupt the pilot's sense of timing and coordination, leading to unrealistic or even detrimental training.

For FTDs that are equipped with a motion system, specific requirements are in place regarding how transport delay or latency is measured and what the acceptable limits are.

• For example, for helicopter FTDs, the transport delay may be limited to a maximum of 150 milliseconds.

• A fundamental principle is that the instrument response within the simulator should never occur before the onset of motion. This ensures that the pilot receives the correct sensory feedback sequence.

Furthermore, the motion response should occur before the visual system completes the display of the first video frame that contains information resulting from the pilot's control input. This coordination between motion and visual cues is essential for a cohesive and immersive simulation.

In essence, both transport delay and latency are measures of the overall timing accuracy of the FSTD. They ensure that the motion system's response is properly synchronized with the pilot's actions and the other simulator systems.

Specific Considerations for Helicopters

The evaluation of motion systems in helicopter simulators presents unique challenges due to the complex and dynamic nature of helicopter flight.

Conventional 6-DOF motion systems, while effective for many aeroplane simulations, often have limitations in their ability to accurately reproduce the wide range of frequencies and amplitudes present in helicopter vibrations. Helicopter flight involves a broader spectrum of vibrations than fixed-wing flight, and these vibrations provide important cues to the pilot.

To address this, a dedicated 3-degree-of-freedom (3-DOF) vibration platform may be used to augment a 6-DOF motion system in helicopter simulators. This approach separates the cueing frequency bandwidth:

• The main 6-DOF motion system focuses on providing the lower-frequency motion cues that represent the helicopter's overall movement (pitch, roll, yaw, etc.).

• The dedicated vibration platform handles the higher-frequency vibrations, which are particularly important for conveying information about rotor dynamics and the helicopter's response to control inputs.

For helicopter FSTDs equipped with a motion system, the testing process must specifically demonstrate that each axis's onset cue is properly phased with the pilot's input and the helicopter's response. This means that the timing and direction of the initial motion sensation must accurately match the pilot's actions and the simulated helicopter's behavior. This is crucial for creating a realistic and believable sense of control.

Ensuring Realistic Motion Feedback

Motion cueing systems are an indispensable component of advanced Flight Simulation Training Devices (FSTDs). They play a critical role in providing pilots with the sensory feedback necessary to accurately perceive and control the aircraft.

The design and evaluation of these systems require a multifaceted approach. This includes:

• Careful engineering to achieve the desired motion characteristics.

• Rigorous objective testing to validate hardware performance and system accuracy.

• Subjective assessment by experienced pilots to ensure the realism and effectiveness of the motion cues.

• Thorough consideration of transport delay to maintain accurate timing between control inputs and motion response.

Both EASA and FAA regulations provide detailed guidance on the requirements and testing methodologies for motion systems. These regulations emphasize the importance of providing pilots with realistic and effective sensory feedback, which is crucial for safe and efficient flight training.

Furthermore, the unique characteristics of helicopter flight necessitate specific considerations in motion system design and evaluation. The need to accurately reproduce helicopter vibrations and ensure properly phased motion cues highlights the complexity of simulating rotary-wing aircraft.

By adhering to these rigorous standards and best practices, the aviation industry strives to create flight simulators that provide pilots with a truly immersive and effective training experience.