Arnold Engineering Development Complex is of the first flight simulation testing sites to use composite materials for the rotor blades and spacers in its larger wind tunnels, and the Complex established an onsite facility to maintain these parts.
For the past 50 years, a two-man Blade Shop has been repairing the rotor blades and spacers for the axial flow compressors in the AEDC Propulsion Wind Tunnel facility.
“These tradesmen are the only composite repair technicians on base and are able to complete most repairs necessary to keep the wind tunnel compressors operating,” Joshua Blair, PWT mechanical systems engineer, said.
PWT has several compressors, each with a one-of-a-kind blade and spacer combination designed for the operating characteristics of that compressor. Because unique materials are used in the compressor components, each must be repaired in a different way.
In addition to making the repairs for these compressors, the Blade Shop tradesmen assist in ensuring the longevity of AEDC equipment. They serve as experts on compressor components and are heavily involved in the daily and weekly inspections and major preventative maintenance, in which a portion of one of the compressors must be removed and inspected for damage.
Blair explained that when a foreign object in the airstream strikes a composite component in the PWT wind tunnels, it causes broken fabric layers that must be repaired.
“Generally, scarf repairs are the method of choice,” he said. “First, the damage must be ground out. This means that layers of the composite material must be removed around the damage in a conical shape; the larger the damage, the larger the area that will need to be ground down to make an adequate repair.”
Blair added that new layers of composite material are cut from a roll of dry material or a roll of pre-impregnated or pre-preg material, which is fabric that comes pre-impregnated with resin.
“If dry material is chosen, the Blade Shop personnel choose an appropriate epoxy resin to wet the dry repair fabric. They place the composite fabric in the proper orientation, brush on the resin, and then squeeze out the excess resin. Using pre-preg material, Blade Shop personnel only have to set the material in the proper orientation and avoid squeezing out excess resin.”
Once the composite material has been satisfactorily applied, an air-tight plastic bag is attached to the surface around the repair. A line from a vacuum pump is attached to the bag to pull a vacuum inside the bag so that full atmospheric pressure will be evenly distributed across the repair. After that, the composite component is placed in a large electric oven and baked at a predetermined temperature for a predetermined amount of time. This allows the resin to cure, creating a permanent repair.”
Once this process is complete, the component can be removed from the oven and placed back into service after minor clean up.
The Blade Shop has also played a role in composite repair efforts across Arnold Air Force Base. Other work includes repairing composite rotor blades located in the cooling towers and repairing fiberglass components of the motors and pumps located at the Complex’s secondary pumping station.
“The PWT Blade Shop is one of the many assets at AEDC that make the complex unique and the immediate access to the composite repair technicians reduces lost test time, saves money and ensures AEDC’s reputation as the premier flight simulation facility in the world,” Blair said.