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VMS RESEARCH
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The VMS provides opportunities to advance a wide range of aeronautical research using accurate, piloted simulation. Many of the experiments conducted fall into one of the following areas.

Controls

One area where the VMS excels is flight control research. The controls in the simulated cockpit accurately measure a pilot's inputs and feel as they would in a real aircraft. Experiments at the VMS have investigated the pilot's controls, the behavior of the control surfaces, the laws that describe how controls operate, and the implementation of flight control software.

A series of simulations of the Black Hawk helicopter has examined the communication of envelope limits to the pilot. These are the collection of operational limits, such as main rotor RPM, transmission torque, and retreating blade stall, that the pilot of a helicopter should not exceed for safety reasons.

A recent simulation in the series examined communicating limits to the pilot through a side stick. A conventional helicopter uses a center stick, which is connected to the rotor blades mechanically; through it, a pilot feels forces acting on the rotor blades. In contrast, a side stick is connected electronically and provides no force feel. By programming a side stick to move – to be active – much of the feel of a conventional helicopter control could be reproduced. In addition, envelope limits could be communicated through the control.

In this experiment, the researcher wished to evaluate an active side stick. The VMS incorporated the customer’s system for providing force feel – a side stick with electric motors and flight control software. With these modifications, the VMS provided high-fidelity simulation that enabled the researcher to evaluate and refine the delivery of cues that may become commonplace in helicopters.

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Displays

Modern aircraft design calls for communicating increasing amounts of information about highly complex systems to the pilot. Researchers strive to present the information effectively, ensuring that the most important data get the attention of the pilot with minimal distraction. Simulations at the VMS often involve the research of display technology.

A series of simulations helped develop technologies to be used in a supersonic high-speed civil transport (HSCT). One simulation, called HSCT Design and Integration, was devoted exclusively to displays that are unique to the HSCT.

One of the displays was designed to keep the pilot informed about the state of the flaps, which would generate unusually large amounts of lift in low-speed flight. Control of the flaps would be automated to optimize their performance and to reduce pilot workload. This simulation determined the requirements for effectively presenting information about the flaps to the pilot and for alerting the pilot to problems that would require switching to manual operation.

A second display conveyed information about the autopilot and its use of a unique control law based on the flight path vector, or the angle of ascent or descent. To determine an aircraft's vertical flight path, a conventional autopilot monitors the pitch of the airplane. In contrast, HSCT design requires monitoring the vertical flight path itself and changing the aircraft's pitch and throttles accordingly. A new display was developed to inform the pilot about the state of the aircraft and to allow the pilot to interact effectively with the autopilot. With the piloted simulation provided by the VMS, researchers could test and modify their display designs in realistic conditions before the aircraft is built.

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Guidance

Another area of research that brings researchers to the VMS is guidance – the systems that help a pilot accurately reach a desired location. Modern guidance systems enable a pilot to negotiate crowded airspace, line up safely with a runway, and even taxi on the ground in poor visibility. One experiment at the VMS investigated guidance issues for civil tiltrotor aircraft.

Ames Research Center took the lead in the development of tiltrotor, an aircraft that can take off and land like a helicopter, yet fly like an airplane. The VMS has conducted a series of simulations investigating many aspects of tiltrotor operation. One experiment, Civil Tiltrotor 6, investigated guidance issues in landing, including conversion from airplane mode to helicopter mode during a descent and double segmented approaches.

One part of the study evaluated whether a tiltrotor aircraft could perform all or some of the conversion from airplane mode to helicopter mode during a descent, rather than entirely in level flight. Simulation demonstrated that beginning a three-degree glide slope at 3000 feet provided adequate time and space to safely perform a full conversion. Beginning the descent at lower elevations resulted in the pilot having too many tasks to handle if also required to maneuver to minimize noise, avoid obstacles, or stay clear of the airspace reserved for conventional aircraft.

A double segmented approach involves approaching a landing site at two successive angles: first, a three-degree glide slope typical of fixed-wing aircraft, then a nine-degree glide slope close to the airport. Using the VMS, researchers determined that the best height for changing from a three-degree to a nine-degree slope is above 500 feet. Transitioning above that level means that at 200 feet the pilot is no longer busy with the transition and the aircraft is stable. This is critical because 200 feet is the landing decision point, the last chance the pilot has to abort the landing. This information, learned in the safe environment of the VMS, helped in the selection of approaches for later flight testing.

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Handling Qualities

Another area of research where the VMS excels is aircraft handling qualities. Whether examining a system modification or an entirely new aircraft, researchers must understand how an aircraft will respond to pilot commands.

A series of simulations that focuses on handling qualities is called Slung Load. The research supports the development of the Army's Aeronautical Design Standard-33. This standard incorporates criteria for helicopter handling qualities but does not address slung-load operations. With these experiments, researchers are developing criteria to fill the gap.

Researchers were able to see quickly the effects of changing many variables. These included the length of the sling between the load and the hook, the distance between the hook and the center of gravity, and the response characteristics of the aircraft.

The VMS has been critical in providing effective cues of the effects of the external load to the pilot. With its large motion capabilities and accurate modeling of the complex interaction between the helicopter and its swinging load, the VMS delivers realistic simulation that is advancing helicopter design standards.

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Combinations

Very few experiments at the VMS are actually confined to one type of research; in fact, most experiments overlap considerably. Although the tiltrotor experiment focused on guidance, the approaches were evaluated by their handling qualities. The displays for the supersonic transport depended on a radically new control system. The Slung Load experiments, designed to address handling qualities standards, have led to control laws that are being considered to lengthen the life of the Army’s Chinook helicopter.

One project that involved many areas of research conducted several simulations of a short-takeoff/vertical-landing (STOVL) supersonic fighter. The VMS provided the realistic piloted simulation that helped take the design of this aircraft to a new level.

STOVL aircraft, like the Harrier jet at left, typically handle with the least stability at low airspeeds, such as when landing, taking off, or transitioning between a hover and wing-borne flight. In addition, their role often requires them to land in small areas, either in confined spaces on land or aboard small aircraft carriers. Finally, STOVL aircraft often operate in adverse conditions, with wind turbulence, rough seas, or low visibility.

In order to improve the aircraft's performance in these demanding conditions, the Advanced STOVL project developed and tested integrated control, guidance, and display systems. New control laws were written and implemented to provide more precise handling of the aircraft at low speed. In addition, guidance and displays were developed that, for the first time, enabled a STOVL aircraft to use pursuit tracking.

Pursuit tracking is presented to the pilot on a head-up display. It has two symbols: one shows where the aircraft is headed, and the other indicates the ideal flight path. By aligning the two symbols, a pilot can fly to the intended landing area in poor visibility and come to a hover above a designated landing spot with precision.

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Simulation Fidelity

Experiments at the VMS include investigation into flight simulation itself. Despite the widespread use of motion in research and training simulators, many aspects of motion cues have not been objectively determined. The series called Simulation Fidelity Requirements (SimFR) asks questions such as: Which maneuvers and studies benefit most from using motion? How can motion cues be made still more realistic for simulation pilots? What are the requirements for motion fidelity, the accuracy with which motion cues are reproduced?

These simple questions yield complex answers. The perception of the effectiveness of cues is to some extent subjective, varying from pilot to pilot. Simulating large motion with the limited travel of the motion base adds complexity. Finally, the effect of the interaction of cues, for example, the motion and visual cues, is more complicated than the effects of both cues considered individually.

To limit the factors involved, initial experiments used only the roll and lateral degrees of freedom. For realism, they simulated small maneuvers that could be reproduced with one-to-one motion, in which the motion base moves one foot for each foot of flying simulated. Gradually, the results of previous experiments are being applied to the other degrees of freedom and to large maneuvers that are simulated with less than one-to-one motion.

SimFR 7 finalized the fidelity criteria for roll-lateral motion and began taking data to investigate pitch-longitudinal motion. Efforts such as these improve the accuracy of motion cues at the VMS and contribute to flight simulation standards in general.

The VMS is a highly effective laboratory for advancing aeronautical design. Researchers gain valuable insight in the areas of controls, displays, guidance, handling qualities, and simulation fidelity. Such advances are made possible by the high-fidelity simulation and flexible operation of the VMS.

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Last Updated: April 5, 2017

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