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Ride the air wake.

In the pilot community, landing a helicopter on a seaborne vessel is considered one of the most dangerous maneuvers to execute; landing a helicopter is a difficult enough task on solid land. In the ocean, not only is the ship moving ahead, but it's pitching and rolling with the waves and disturbing the air behind it as it ripples the water below. To prepare for such conditions, pilots execute the maneuvers in real-time trainers with simulations that have traditionally lacked the information to properly mimic the unsteady turbulence they experience when landing on the deck of a ship.

Continuum-Dynamics (CDI), a small Ewing, NJ-based engineering service bureau, is using its 30 years of aerospace experience to outfit training equipment with the data required to simulate the air wake--the turbulent disturbance to the air that is caused by the ship. Think of the ripples behind a boat in the water, or the shape of the contrails that follow a plane--the same action happens in an air wake.

CDI did much of its early work on aircraft wake turbulence and wake vortices. Eventually, the research moved the company into work tied to helicopter aerodynamic prediction and loading. Most of the company's recent work on its aerospace side (CDI also performs a lot of service work for the nuclear power industry) has been focused on applying the design tools that it developed over the years and accelerating the numerical algorithms for real-time pilot training.

CDI's most widely used tool is CHARM (Comprehensive Hierarchical Aeromechanics Rotorcraft Model), which has been used by just about every rotorcraft manufacturer in the world-Westland used it for upgrades on the EH101 (now the AgustaWestland AW101), and Boeing/Sikorsky also used it as a design tool. CDI has been able to take components of the CHARM software and make it run fast enough to be put directly into flight simulations. The company also used Tecplot 360, a computational fluid dynamics (CFD) visualization tool, to develop more traditional, grid-based CFD tools to put together a database of the turbulent flow field behind the ships.

By using this information, the MH-60R trainer, developed by CAE USA for the U.S. Navy in Orlando, FL, enables Navy pilots to simulate landing on the deck of a ship with the air wake that they would feel in an actual flight.


Before CDI, trainers used simple empirical models that would arbitrarily add unsteadiness to each simulation. The unsteadiness had originally been applied uniformly across the entire helicopter whereas, in reality, it varies from the left side to the right and from the front to the back.

"We came along and did CFD calculations for basically every ship in the U.S. Navy and some of the Royal Navy ships for joint operation," says Dr. Glen Whitehouse, a CDI researcher. "We calculated the flow field downwind of the ship in a variety of different conditions, and we packaged that information into a database that could be used in the real-time environment to improve the unsteadiness."

One Giant Problem

CDI's initial challenge was the sheer magnitude of the problem. First, the company had to predict the flow field four-to-five ship lengths downstream of the craft. According to Whitehouse, this is an insanely long distance for a regular CFD calculation.

"Predicting the flow field played into a lot of strengths that we've had in developing software for the helicopter community," says Whitehouse. "We had some unique numerical methods that we could use."

After predicting the flow field, the company was faced with performing 192 calculations--in a normal design case, CDI would map a dozen. "Suddenly, we were doing a lot of production calculations," says Whitehouse. "That is a lot of data to manage. Then we actually had to come up with the algorithms to take that data (many terabytes of CFD data) and package it into a format that's suitable for something that has to run in real-time."

Normally, for CFD, Whitehouse would run 60 seconds worth of calculation time, load it on a computer, and come back in a few hours (sometimes days) when it was done. He now had to simulate 60 seconds of helicopter flight time in 60 real-time seconds in order for the pilot to be able to interact with the trainer. He took the terabytes of ship and wind data, and had to figure out a way to package the information in a realtime environment. For this, CDI turned to Tecplot 360.

"One of the problems with CFD is that you end up generating a very large amount of data and there are only so many commercial tools that are able to effectively manage and post-process the data," says Whitehouse. "Tecplot 360 let's me plot all of the data in 3D, and it let's me plot in slices. I can actually probe the data for direct comparisons. You could use [Microsoft] Excel, but Tecplot is a unique tool for technical computing visualization."

"Whitehouse is using CFD in ways that it hasn't been utilized in the past," says Mike Peery, president and CEO of Tecplot. "He is creating a huge database with all of these variations in flow field, flow conditions, and orientation variations. He has all of these parameters that he's varying and running these scenarios to build a database to be used as a pilot simulator. It is something that has been emerging over the last five years: people running hundreds and thousands of cases as opposed to one."

According to Peery, 360 helps CDI automate all of this information that would otherwise be too time-consuming to run by hand. Though, according to Peery, 360 is "Excel on steroids."

The Grind

Thus far CDI has been crunching numbers for CAE for about two years, and the process is ongoing. Even now, more ships come to the company with requests for upgrades.

CDI started with research in an effort to prove that its design tools could do the job. The company had to validate its tools against wind tunnel measurements to make sure the right answers were being reached. After that, CDI had to generate the results and compile the database, which it took to CAE. In Orlando, CDI worked onsite to integrate the database into the trainer with a long testing process. The final part, and perhaps most rewarding, was receiving the test pilots' blessing.

"The test pilots fly all the scenarios they want and then they say whether or not it represents the actual aircraft," Whitehouse describes. "It's an iterative process."

Whitehouse found it unusual that a company as small as CDI--currently hovering around 23 employees--was able to be as responsive as it was. "We don't have the resources. We don't have access for commercial work for government supercomputers," he laments. "Within the modest capabilities that a small business can have, it was the uniqueness of the tools that we put together that enabled us to be so responsive [to pilot feedback]."

CDI's preparedness warranted little feedback requiring tweaks or changes. Because of the level of physics that the company brought to the problem (full three-dimensional, grid-based CFD), it didn't suffer many iteration loops while other aspects of the trainer required immediate attention.



Know Your Role

Coordinating with multiple companies with a singular vision can be a daunting undertaking, especially for a company used to operating as a single entity. It's a unique scenario in which CDI had to fit into the prime contract with CAE and remain responsive to the Navy.

CDI tended to work remotely, but Whitehouse and company were on site with CAE's engineers for the final integration to make sure that everything was running as planned. Representatives were also sent to critical design review presentations that were sometimes days long.

"Though you may be an important part from a requirement standpoint, suddenly realizing how much has to go into designing one of these moving-based trainers is just insane," Whitehouse recalls. "It's a totally different level of management."

Great Debate

A debate remains in the simulation world: Is virtual simulated training good enough to replace actual flight hours? Whitehouse likes to hope that the higher the fidelity of the trainer, the closer that it is to the actual flight. He hopes that the new generation of trainers is going to help pilots get a better handle on what it feels like to land on a ship before they have to do it for the first time, in the field. Hopes remain; the jury is still out.

"Given the computational resources available, the level of physics that we can put into these models is far superior to [that of the past]," says Whitehouse. "There are always roadblocks, but now we have the opportunity for some real breakthroughs given the nature of computer hardware now, as opposed to five years ago. We can now move a lot of things over from the analysis side of the scenario and start applying that to the training side."

by David Mantey, Editor, PD&D
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Title Annotation:EngineeringAnswers
Author:Mantey, David
Publication:Product Design & Development
Date:May 1, 2011
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