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Vertical Motion Simulator
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One important key to the accuracy of a simulation, and to a realistic experience for the pilot, lies in the quality of the flight control system. This is especially important at the VMS because of the many experiments conducted in handling qualities and controllability.

The hydraulic flight control system at the VMS is called a control loader, or loader. It must accurately measure a pilot's input and provide high-fidelity cues to the pilot; in other words, the pilot's controls must feel as they would in a real aircraft. The control loader must also be extremely versatile to simulate all the controls of any aerospace vehicle.

The VMS has a large collection of pilot's controls that are made specifically for simulation. On the left is a wheel and column, which is typically used in transport aircraft. Next to that is a center stick, as it is called in fighter jets; in helicopters it is usually referred to as a cyclic. On the right is a hand controller, also called a side stick. Pilots use these controls to pitch and roll the aircraft.

Image of a wheel and column
Wheel and Column
Image of a center stick
Center Stick
Image of a side stick
Side Stick

Most aircraft have pedals, which a pilot uses to yaw the aircraft and, in many cases, to brake on the runway. Another common flight control is a collective, which is used in helicopters to regulate power. Power consoles, with levers for throttles, are used in planes. They are not part of the hydraulic control loader, but have a simple mechanical adjustment for friction.

Image of the pedals
Image of the collective
Image of the power console
Power Console

In addition to physically changing the pilot's controls, their response can be modified according to the aircraft being simulated. The following variable characteristics can be programmed and even changed during flight.
  1. Position Trim (Trim) - The position to which a flight control returns when the pilot releases it.
  2. Force Breakout (Return-to-Center Force) - One force that returns a control to Trim. This is a constant force; that is, the force applied toward Trim remains the same despite the displacement (how far the control is moved) and velocity (how fast a control is moved).
  3. Force Gradient (Spring Force) - Another force that returns a control to Trim, but one that varies with displacement – the farther the control is moved, the stronger the force applied toward Trim.
  4. Force Friction - A force that is opposite to the direction of movement. Force friction is a constant force.
  5. Damping - Another force that is opposite to the direction of movement. Damping varies with velocity – the faster a control is moved, the stronger the force.
  6. Hard Stop - A force that simulates a mechanical limit of travel. By varying the Hard Stops, the range of travel can be adjusted.
In the cab, a pilot's control is connected to one or more actuators, which measure the forces a pilot applies to the control. They also regulate the forces applied by the loader to give the control the appropriate feel. Each axis of a control is connected to an actuator. For example, a stick would have two: one each for roll and pitch commands.

Block diagram of control loader axis

A simulation begins for the control loaders when the host computer downloads the characteristics the controls should exhibit to the digital-analog interface, which converts the signal from digital to analog. This signal is then conveyed to an analog computer. Then three factors – the force, velocity, and displacement of the pilot's input – are measured by a transducer in an actuator. The results are conveyed as analog signals to the analog computer, which generates a force command. The electronic controller processes this signal, together with the force input by the pilot, to generate a signal that is sent to the actuator, resulting in the proper feel being imparted to the flight control.

Besides regulating the flight controls, the control loader can generate vibrations in the pilot’s seat that are not produced by the motion base. These high-frequency, low-amplitude (fast and small) motions vary according to the aircraft’s state and reproduce the vibrations characteristic of different aircraft. This vibration generator, or seat shaker, is most often used in the simulation of rotorcraft.

The versatile, high-fidelity control loader at the VMS provides for the accurate measurement of pilot input and the delivery of realistic cues. The ability to customize the pilot's controls for each experiment, and to change their response by programming variable characteristics, contribute to the pilot's experience of realistic, real-time simulation.

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Last Updated: April 5, 2017

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