As a result of a joint research program between the National Bureau of Standards and the Civil Aeronautics Administration, coatings have been developed for fabric-covered aircraft which approximately double the time interval between ignition of the fabric and its destruction by fire. These findings, based on both laboratory and wind-tunnel burning tests, provide a basis for more effective extinguishment of fires in flight, particularly those arising from power plants in small craft.
No fire-retardant film-forming materials are known which tauten airplane fabrics as effectively and as permanently as do the dopes based on the cellulose derivatives. These dopes, however, are highly inflammable, and the addition of fire-retardant substances lessens their capacity to tauten. Efforts were therefore directed toward the development of highly efficient fire-retardant coatings to be applied to the surface after it is already doped. In designing the coatings, the requirements of aircraft use made it necessary to consider also aerodynamic smoothness, adhesion, flexibility, and other features.
Fifteen of the more promising coating systems were selected for fullscale wind-tunnel tests. Test panels, 12 inches square, prepared by applying from one to four coats of fire-retardant lacquer to fabric doped with the usual six coats of cellulose acetate butyrate, were placed in the center of the lower surface of a steel wing suspended in a 70-mile-perhour air stream. A spray gun then introduced a uniform spray of aviation gasoline into the air stream, and the spray was ignited by a spark from an air-craft spark plug. This enveloped the wing section in burning gasoline, insuring continuous contact of the specimen with flame during the entire test. The time for destruction of the outer coating and the time of fabric failure were noted, while temperatures were recorded by means of thermocouples and rapid-response recording pyrometers.
The evidence presented by the wind-tunnel tests showed clearly that it is possible to increase the time for fabric destruction from the six seconds characteristic of cellulose acetate butyrate to about 12 seconds. Although fire-retardancy is increased for higher pigment contents, such over-pigmented coatings are not sufficiently durable. Some additional improvement can be achieved by designing the outer coating to have greater mechanical strength after burning so that it will not be blown away so easily by the wind stream. * Communicated by the Director.