Handle This RAT Carefully -- Ram Air Turbines, that is

DN Staff

September 28, 2004

4 Min Read
Handle This RAT Carefully -- Ram Air Turbines, that is

Life saver: A Hamilton Sundstrand ram air turbine (RAT), designed for the Airbus 380, gets ready for testing at the Rockford, IL wind tunnel. A new positioning system, featuring vertical, ball-bushing slides on either side of the mounting fixture, allows operators to easily move the heavy unit into the center of the air stream.

Ram Air Turbines (RATs) deliver emergency power to aircraft, allowing
pilots to manipulate flight surfaces for safe landings. Stowed in the wing or fuselage and deployed in the event that an airplane loses normal power, these devices have saved hundreds of lives. Yet how do you make sure that the RAT itself is reliable?

The answer, of course, is careful wind-tunnel testing, which RAT manufacturer Hamilton Sundstrand (www.HamiltonSundstrand.com), a division of United Technologies, has performed for many years in Rockford, IL. But doing wind-tunnel testing on the RAT targeted for the giant new Airbus 380 posed bigger challenges than every before.

"The A380 RAT weighs approximately 400 lbs. and has a diameter of nearly 5 feet," notes Steve Penticoff, the engineer in charge of the wind tunnel facility. "Previously, the biggest ram air turbine we produced weighed less than half that, with blades reaching only 40 inches."

Safety worries

As a result, the new jumbo-sized RATs posed safety challenges for wind tunnel operators, who typically can physically lift smaller RATs onto fixed-position mounting structures in the wind tunnel. In addition, under the previous wind tunnel setup, the larger A380 RAT would have to be placed so far back in the test section that existing structures in the wind tunnel would interfere with the turbine.

To solve these problems, Hamilton Sundstrand turned to Thomson Danaher (www.danahermotion.com), which provided a "near turnkey solution" for a new positioning system, says Penticoff. The key components:

* Two ball-bushing slide systems mounted vertically on either side of the RAT's mounting fixture.
* A two-axis controller already programmed with supporting software.
* Two servo motors, three gearboxes, and cabling for the installation.

"The fact that we could go to one supplier and get all these components in a single system kept me from having to reengineer existing solutions," says Penticoff. "And it only required minimal software tweaking."

Making adjustments

Each side of the positioning system features a Thomson 2DB slide system, driven by an acme screw, plus a second slave carriage not tied to the screw. The designers chose acme screws over ball screws to eliminate back drive.

In addition, the engineers mounted a spherical bearing pillow block to each carriage to remove any bending moments on the carriage. Since only the two upper carriages are attached to the drive screws, the design allows the lower carriages to move independently. A drive system using a splitter gearbox with a common motor guarantees that the two sides of the system stay synchronized.

Hamilton Sundstrand engineers designed the aluminum mounting system that connects both to the RAT and to the positioning system. Comet Fabrication, also located in Rockford, used an NC plasma machining center to manufacture the mounting structure from AutoCAD drawings supplied by Hamilton Sundstrand. Wind tunnel electrical engineer Richard Fisher handled the wiring for the power supplies and safety systems.

Penticoff says that the best feature of the design is its ergonomics. Wind tunnel operators simply lower the positioning system to shipping-level, where they connect the RAT to the mounting fixture. They then rotate the unit to its proper angle and raise it to its operating position approximately 11 feet in the air. During testing, operators subject the RAT to wind speeds similar to conditions encountered during landing-the most critical period for ram air turbine operation.

Emergencies only: Ram air turbines, stowed in an airplane's wing or fuselage, are deployed when planes lose power from lack of fuel.

A distinguished record

"We've developed procedures to meet our goal of 100% reliability for those rare events when the RAT is required," says Penticoff, who adds that RATs manufactured by Hamilton Sundstrand have helped save the lives of more than 1,200 airplane passengers.

For Thomson Danaher, the Hamilton Sundstrand application represents demonstrates its increasing ability to combine products from its growing number of companies. "Our challenge was the pieces together-Thomson slide assemblies, Micron gearboxes, and servo controller and motors from the former Industrial Devices operation," says Chris Hermsen, a key account manager for Thomson Danaher in Roscoe, IL. "Hopefully, this experience will lead to similar industrial applications where you need to hoist heavy loads up and down, and side to side."

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