As fuel prices rose in the late 1970s, the air transport industry became increasingly interested in technology and procedures that could make airline operations more efficient. One area manufacturers focused on was airframe drag reduction, because even a small reduction in drag could have a tremendous impact on operating costs. A one percent drag reduction on a Boeing 727 airliner, for example, was estimated to save 20,000 gallons of fuel per year. [Ref 8-2]

Preliminary experiments in the low turbulence wind tunnel at the Langley Research Center showed that smoothing a wing surface area by applying an adhesive film coating could reduce its drag by as much as 12%, which translated to a 23% reduction in the overall drag of the airplane. [Ref 8-3] In 1977, the Boeing Commercial Airplane Company began investigating the potential of specific coating materials, running a total of 15 liquid coatings and 60 film adhesive through a series of 17 laboratory tests to determine which of the different materials could best withstand the rigors of airline use. The materials were first evaluated for their resistance to erosion from rain or various aircraft fluids such as fuel, deicing solutions, or hydraulic fluid. Out of all the materials, four adhesive films and three liquid coatings proved the most resistant to erosion and were recommended for further testing.

A Langley technician works on the wing of the model Boeing 737 in the 14 by 22 foot subsonic wind tunnel

Since the adhesive films had a lower resistance level than the liquid coatings, however, they were not recommended for high erosion areas of the airplane, such as the leading edges of the wings. The films showed potential for areas like the forward body of the airplane, but installing adhesive film on large, curved surfaces would have been an expensive and timeconsuming effort. As a result, the rest of the evaluations focused primarily on the liquid coating materials.

The liquid coatings, which went by the names CAAPCO, Chemglaze, and Astrocoat, were run through another series of laboratory tests to further evaluate their resistance to erosion and their compatibility with aircraft systems. The CAAPCO and Chemglaze coatings were then applied to two operational airliners for flight service evaluation. [Ref 8-4] Continental Airlines applied the coatings to the wing and horizontal tail leading edges of a Boeing 727 and tested their resistance to erosion in two different environments. The aircraft was flown for 14 months in the Air Micronesia route system, where an annual rainfall of over 90 inches and the fact that some airports used coral runways caused significant erosion problems with paint on aircraft leading edges. Continental then conducted a second, 11month evaluation of the coatings with the same B727 on domestic routes in the United States. Although the coatings suffered significant erosion in both cases, an outboard horizontal tail section with the coatings applied by Boeing Laboratory technicians was then installed on the 727 and flown for an additional 18 months, accumulating 3800 flight hours with almost no erosion of the coatings. The two coatings were also tested by Delta Airlines on a B727 to evaluate the impact of different colors and primers on the effectiveness of the coatings. After 16 months and 4348 flight hours, some coated areas had peeled or eroded, but others were still holding up fairly well.

The next question was whether or not this kind of liquid, elastomeric polyurethane coating could reduce aircraft drag. Since any reductions in drag were expected to be small, results from wind tunnel models or smaller aircraft would not necessarily apply to transport class airplanes. Consequently, drag measuring flight tests had to be conducted on a fullscale, transport class airplane that could record precise data through instrumentation on the surface of the wing. This was obviously beyond the capabilities of operational airliners, but the NASA TSRV 737 was perfectly suited for the task.

The test substances were applied to the upper inboard surface of the left wing and compared against the performance of the same section on the right wing, which was kept as a bare metal surface. The flight tests compared the performance of the CAAPCO substance against the Corogard corrosionprotective paint that was a popular aircraft surface coating at that time, as well as a gritty strip that was installed on the leading edge of the wing section to represent a coating that was somewhat eroded. The results showed that compared to the bare metal surface, the CAAPCO substance reduced the total drag of the airplane by about .2% in cruise conditions, whereas the Corogard coating increased aircraft drag by about the same amount.

The Boeing and NASA experiments showed that the CAAPCO coating could be resistant to erosion and could reduce drag on the wing of an aircraft, but more research was needed to determine if a CAAPCO or Chemglaze substance would provide as much protection against corrosion as the coatings already in use. The coatings also might not be as effective in actual airline operations, because the quality of the application process clearly affected their lifespan and effectiveness. Although the laboratory coated wing tested by Continental held up extremely well, laboratoryquality work would be difficult, if not impossible, to achieve in a busy airline maintenance environment. But while there were issues that might affect the commercial use of the substances, the TSRV 737 flight tests showed Boeing that elastomeric polyurethane coatings at least had the potential for reducing drag on transport aircraft. [Ref 8-5]