Arnold leads the way with new test technique on Army projectile

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A recent aerodynamic test con¬ducted on the Army's Mid-Range Munition (MRM) in Arnold Engi¬neering Development Center's (AEDC) four-foot transonic wind tunnel (4T) provided a new and critically needed performance evaluation capability for current and potential test customers.

"What made this test unique was the successful operation of a novel remote control system that efficiently determined the projectile's performance limitations, while saving the Army time and money in the process," said Charlie Smith, project engineer for ATA's inte-grated test and evaluation department.

The purpose of the test was to de¬termine the aerodynamic lift and drag forces and moments the guided projec¬tile will experience in flight.

"A large amount of data was required to compare various configurations of the test article and determine performance, stability, and control effectiveness of the optimum configuration," he explained.

"Instead of just moving the projectile's control surfaces to a fixed position by remote control, we were able to automatically position the canards to maintain the measured projectile pitching, rolling, and yawing moments at a speci¬fied value, usually zero (trim), while simultaneously pitching and rolling the projectile at simulated flight conditions. We are approaching a 'fly the mission' capability with this development."

Canards are moveable wing-like structures located near the front of the projectile used to guide the projectile. Additional fixed-position fins are lo¬cated at the rear of the projectile to provide stability and lift.

Dr. Richard Roberts, Arnold's Air Force project manager on the test in 4T, said the collaboration between ATA, the Air Force and the test community resulted in an improved capability that would have positive, long-term impli¬cations.

"We are excited to have this new test method," he said. "We were able to work closely with the community to help develop and integrate this method into our test process. The auto trim method will help save our customer's money and time, and has the potential to compress weapon and aircraft acquisi¬tion schedules by reducing the amount of time typically spent during wind tunnel testing and analysis. "
Marvin Sellers, a senior ATA engi¬neer on the test who was consulted to head up the successful effort to develop an algorithm to run the model's remote control system, said the task proved challenging.

"This is the first time that we've ever trimmed all three axes with a remote-controlled system - the model is 'flying' in the tunnel and the system trims all three coefficients to zero automatically," he said. "My job was to develop an algorithm and a control capability that would do that in a wind tunnel."
In computing, an algorithm is a like a sequence of instructions, often used for calculation and data processing.

"We programmed control algorithms into a data reduction program," Sellers explained. "The hard part of this process is you have to incorporate the model design, the measurement of the control surfaces, the angular positions of the model, and the control system - all that has to be brought together in a fashion in which you don't introduce noise or distortions into the control system, which can delay movement of the control surfaces and affect the accuracy of the system."

Sellers said a balance is mounted inside the model to measures forces and moments acting upon the projectile and the moments are sent to the control algorithm as feedback for the control system. That system continually re-positions the control surfaces to null or trim out those moments, bringing the projectile back to stable flight.

The new test capability can be used in more than one wind tunnel at AEDC.

"This capability is not specific to 4T - we could use the same technology in Tunnel A, or the 16-foot transonic wind tunnel (16T)," Sellers said. "Due to the way the program works, it will require us to do little bit of tuning at the beginning of every test.

When I say tuning, this means that we have to develop the constants that go into the algorithm that drives the parameters. Since the constants are aerodynamically driven, it's going to be different for every wind tunnel model, but if you can make the physical model perform similar to what wehad on this test, there would be no reason to believe that it can't work on any wind tunnel model as long as you have the full remote control system."

According to officials at Raytheon, the contractor chosen to develop the Mid-Range Munition projectile, the weapon system is being designed to provide the U.S. Army with a lethal, one-shot capability as the service continues its transformation to lighter, more deployable combat forces.