As the Army's primary attack helicopter, the Boeing (Seattle, WA) AH-64A Apache uses a 30-mm M230 chain gun, Hydra 70 rockets, and Hellfire missiles. Capable of withstanding hits from 23-mm rounds in critical areas, the helicopter is a twin-engine, four bladed design with a tandem-seated crew consisting of the pilot, located in the rear cockpit position, and the copilot gunner, located in the front position.
In the design of the aircraft, individual rotor blades are attached to the helicopter's drive shaft by a stack of 23 flexible straps known as hinges. By stacking individual straps, rather than using one homogeneous design, the risk of failure due to cracking is reduced.
To build these hinges, arrays of 23 straps are stacked before they are mounted to the helicopter drive shaft and the rotor blades. One 0.018-in.-thick, 1.5-in.-diameter mount of the strap attaches to the main rotor, and two others are fixed to the rotor blades.
In building a custom-designed imaging system to inspect helicopter blade straps, Instrument Technology used three custom borescopes coupled to three off-the-shelf NTSC video cameras and a video micrometer.
To ensure the integrity of each composite flexible hinge, each beveled mount must be accurately inspected. Performing this task, however, requires the imaging system to be placed inside all three strap-mounting fixtures. In this approach, the bevel's accuracy can measure to a resolution of 0.002 in. During the system design, Doric Dodds, custom project manager with Instrument Technology Inc. (Westfield, MA), developed an imaging system based around a borescope, off-the shelf CCD cameras, a video micrometer, and a monitor (see figure on p. 10).
"To image the three 0.018-in. strap areas," explains Dodds, "three custom-built borescopes with C-mount lens attachments are placed above each mounting plate." These borescopes are attached to three LCL211 NTSC CCD cameras from Watec (Las Vegas, NV). Because each mount must be measured individually, the three video feeds from the three cameras are fed first to a video multiplexer and then to a VIA video micrometer. This allows the system technician to manually switch among the three video cameras and to measure the uniformity of each bezel by positioning measurement lines and pointers over the displayed video images.
Interestingly, no computer-controlled equipment was used in the design of the inspection system. "As a custom-built system," reports Dodds, "the customer only required simple measurements to be made on the images. Accordingly, it was more cost-effective to use a video micrometer than to develop custom image-processing routines to measure the uniformity of the strap mounts."