Computer Controls Boost Helicopters

A Boeing H-6 helicopter cruises over the desert landscape of Mesa, Arizona, equipped with the Adaptive Vehicle Management System, or AVMS. The experimental system is designed to give rotorcraft digitized controls similar to the ones common on fixed-wing aircraft.

Rotorcraft are following the flight path of their fixed-wing counterparts as the U.S. Army advances a program to digitize controls. The system is still in design and unlikely to integrate in full form to the current fleet, but it points the direction for future vehicles. Results would enable platforms to adjust not only for environmental factors but also for pilot intent.

The Adaptive Vehicle Management System (AVMS) is a project under the Army’s Aviation Applied Technology Directorate (AATD) that attempts to reduce the heavy monitoring demands currently placed on rotorcraft aviators. “Basically what the program is trying to do is to integrate a lot of existing information that’s already on the aircraft as well as the new technology everyone is trying to get on the aircraft,” explains David Segner, leader of the AATD’s vehicle management team. The system will maximize performance by incorporating the data from an aircraft’s subsystems and reacting to it to maintain performance even under anomalously degraded conditions.

“A lot of accidents are because workload on [the pilot] is so high,” Segner explains. “[The pilot is] only monitoring partially where the aircraft is, and  doesn’t realize the aircraft is moving. Or, in some cases, he can see something, he even has marked it out loud to the copilot or the crew, and he starts doing something else. They all forget it’s there, and he hits it because the aircraft is not stable and wandered off. And he didn’t notice because he was busy doing half a dozen other things, which is what we make [pilots] do.”

Today’s military helicopters, which largely have had the same design for decades, rely on mechanical controls, placing the burden of monitoring and flying completely on aviators. Rotorcraft are more difficult to fly than fixed-wing aircraft, which forces pilots to fight with their machines in an attempt to remain airborne. Currently, pilots have to monitor all their instruments manually while simultaneously keeping an eye on external obstacles to avoid catastrophe.

Directorate personnel now are reviewing the reports from phase one of the two-phase, six-year AVMS effort. The Boeing Company and Sikorsky Aircraft Corporation had contracts in the initial part of the process, working separately from each another. One of the biggest advancements came from Boeing’s work with tactile cueing and an active stick. Tactile cueing is a method through which crew members interface with a vehicle and, in the case of the active stick, receive tactile feedback from the vehicle.

The passive stick used in rotorcraft now basically allows input only. Pilots have to wait to see how vehicles respond through movement and control displays. With an active stick, the input of movement and forces on the control stick generates commands to the vehicle, and a change in forces on the stick or actual movement—if the pilot were to release the stick—relate directly to the vehicle responses. Segner says the tactile cues should improve safety by reducing the monitoring of most limits on the cockpit displays and by indicating in a more timely manner any possible limits exceeded. The cues also allow the pilot’s attention to remain outside the platform. Additionally, adjusting the force required to move the stick can indicate a pilot command input that may cause a problem for the rotorcraft.

The technologies were demonstrated successfully during tests in a modified H-6 rotorcraft. “That had not been done to any extent in a helicopter before,” Segner says. “They flew it, and pilots were able to follow the cues.” Tactile cueing, specifically the active stick, helps enable a more complete carefree maneuvering control system. Carefree maneuvering enables the pilot to operate the vehicle without fear of inadvertently exceeding limits.

That maneuvering is one part of what Segner calls a three-legged stool that also includes task-tailored control laws and regime recognition. Task-tailored control laws are optimized for the task being performed. They define how the vehicle responds to pilot input. The control inputs to the vehicle can be related directly to the position of the stick, the speed at which the stick is moved, the force applied to the stick or a combination of all three. Vehicle response is determined by the control laws.

The third element, regime recognition, refers to knowing an aircraft’s state relative to its capabilities. These three interrelated areas enable the system to interpret pilot intent. For example, if pilots input a large amplitude—a large shift of the stick in any direction—with a fast movement when hovering, they probably want the vehicle to move quickly. A large amplitude with a slow movement indicates that they want to hover the vehicle over another spot a distance away but still maintain the vehicle’s level attitude.

Boeing also looked at a methodology for evaluating the importance of certain technologies in cases of limited resources. The research helps answer questions such as what happens if certain pieces of systems are left out because of cost or if a sensor is not as effective as the one originally assumed in plans. Additionally, company researchers examined how to improve flying with sling loads. These external masses cause changes to flight controls if they move in a divergent fashion from the rest of the aircraft. Developers looked at how control laws could help helicopters fly faster or with heavier loads.

Another problem for rotorcraft is passing power and information across the vehicle, because they have to move through the rotating planes. Traditional construction uses slip rings to solve the problem; but the rings are unreliable, high-maintenance items. Both Boeing and Sikorsky worked on addressing the issue—Boeing used a wireless technique, and Sikorsky employed liquid metal slip rings. Both companies evaluated other technologies as well, such as various transformers, to enable the passage of data and power across rotorcraft.

Sikorsky also looked at load-limiting control laws that could help guarantee certain areas of flight control are not overloaded, resulting in new aircraft designs that save weight and are more efficient. The work involved a start-from-scratch architecture design on a new vehicle management system that incorporates multiple electronic subsystems.

One of the other areas examined by Sikorsky involved how controls recognize if a rotorcraft is in the air or on the ground. “It’s always amazing, even in the helicopter world, to remember this—when a helicopter is flying, it has different physics than when it’s touching the ground,” Segner states. The physics also change depending on how many contacts rest on land. Current rotorcraft use squat switchers, which are mechanical. However, Segner says, they can fail either while open or closed, so computers largely ignore them.

Sikorsky researched how to develop a system that provides reliable ground contact information through a device such as a virtual switch. Company personnel also studied how to derive trusted information about what each rotor actually is doing, not only the combined effect, and how to possibly control each rotor independently. The liquid metal slip rings come into play here as developers try to determine the best method to pass the necessary information. Finally, Sikorsky looked into accurately measuring load weights on rotorcraft. With that knowledge, aircraft personnel can improve engine efficiency.

All the research pieces are intended to come together in the AVMS to make rotorcraft safer and easier to fly. “One thing we’re trying to do is give the pilot a more controllable platform so if he lets go of the stick, it’s going to sit there and wait for him to put an input in,” Segner shares. Currently, aircraft are moving constantly, and even the best pilots must have a wide area of space in which to maneuver while they hold their helicopters steady. Pilots generally need to use large visuals to track their movement. Through digital controls, researchers can constrict the space necessary for rotorcraft to operate and allow a pilot to safely turn attention from visual cues to perform other tasks. “Today, he really can’t do that,” Segner explains.

The same idea also applies to situations with decreased visibility. With human eyes as the premier sensor for controlling rotorcraft, environments with smoke, fog or other sight impediments quickly can become harmful, if not fatal. If the AVMS can provide a stable platform, it will help mitigate such risks. Segner uses a hover scenario as an example, saying that if pilots know they are 10 feet off the ground and can remain within a three-foot box, they can safely lower the vehicle even without seeing the landing point.

By the end of the AVMS effort, the AATD hopes to have a solid base for the future of vehicle management systems. Army officials want to prove through simulations and limited flight tests that the ideas function. In addition, pieces of the system could find their way onto current vehicles to give some of the benefits to pilots now. Because of the cost and complexity of ripping out the current mechanical control system and replacing it with digital controls, it is unlikely that engineers would place the entire AVMS into a platform already in use. Another barrier to full replacement is that flight control systems are critical safety systems, “so there’s a lot of angst in making significant changes to that,” Segner explains. Putting in the new control is analogous to replacing a car’s pedals and stick controls with buttons.

He shares that a previous program did some flight control work with CH-47F rotorcraft personnel, showcasing the ability to stabilize and control a vehicle digitally. “It was a vast improvement, so they have a taste of what this new philosophy of controlling the vehicle can do for them,” he says. “They are very interested to see what we can do without putting a full-authority digital control system into this old vehicle.”

AATD personnel are pitching the active stick to rotorcraft program managers. Some managers have expressed interest in using it to help pilots stay within the various limits of the aircraft because when they are exceeded they can cause maintenance issues, which drives up costs. Segner hopes that before the AVMS program ends, officials will add it to their list of items to incorporate into vehicles during their next upgrades. Though building only some of the AVMS features into an aircraft will result in only a partial gain of benefits, the technology still should improve operations.

During phase two of the AVMS, the Army aims to demonstrate the benefits and the readiness of the technologies required for the next-generation vehicle management system, reducing implementation risks.

WEB RESOURCES
AATD: www.aatd.eustis.army.mil
Boeing AVMS Story and Video: www.boeing.com/Features/2012/03/bds_avms_03_12_12.html
Sikorsky: www.sikorsky.com