& PREVENTING WATER DAMAGE _____IN AIRCRAFT_____ Radiation into a cold sky or a cold earth can lower airframe temperature to produce condensation. Water damage is a nagging concern, especially for aircraft owners in humid parts of the country. Water damage, in
by Vernon Yeich, EAA 86787 2813 Whitehall Rd.
Whitehall, MD21161
the form of metal corrosion and wood and fabric rot, will affect flight safety as it progressively compromises integrity of the airframe. Detecting this damage is often difficult, and repair may be expensive. Owners should look for ways to protect their aircraft from deterioration of this kind. Strategies for minimizing water damage should consider that many light aircraft spend more than 98% of their useful lives idle on the ground. To reduce damage during idle periods, the airframe should be kept as dry as practical. Obviously, a hangar will keep off precipitation. However, hangars cannot assure dry airplanes in regions having high relative humidity and large temperature variations; that is, in climates which produce condensation. Devising ways to keep aircraft dry in these conditions requires understanding the way condensation forms on aircraft. Water vapor in a humid atmosphere will often condense on cool surfaces to provide liquid water, as happens on a warm day when a drinking glass is filled with cold water. In the same way, water will also condense on aircraft. However, as with the water glass, the airframe must be made cooler than the ambient
rair.
A familiar means for cooling an airframe below ambient air temperature involves top-structure cooling by radiation into a night sky. This process produces a skim of water on top surfaces of aircraft kept outdoors on clear summer nights. Hangared aircraft may also be affected, especially in buildings with metal roofs when the roof becomes cold enough. In a simple uninsulated hangar the aircraft's top surfaces will radiate into the hangar roof which will, in turn, radiate into the sky. Cooling due to roof radiation can be avoided by installing a ceiling to serve as a radiation barrier between the hangar roof and top of the aircraft. The ceiling may be made of opaque, low density material such as sheet insulation used in home construction. Less expensive materials might be adapted so long as the material provides a barrier to radiation. However, FBO policies and local building codes may apply. A homebuilder acquaintance of the writer observes that steel parts he fabricates will rust rapidly when left uncovered in his steel-roofed hangar. The suspected cause of this rapid rusting is condensation brought on by radiation
cooling. Covering or wrapping the parts provides a barrier to radiation so that the influence of air temperature will be greater in determining the temperature
of the metal. The closer the metal temperature is to that of the air, the less condensation will occur. Floor cooling may be even more important as a cause of water accumulation on airframes. Floor cooling will affect the aircraft bottom-structure by absorbing airframe heat into an earth or floor much cooler than the ambient air. This will most likely occur when air is warming quickly with earth temperature lagging by a significant amount. Because hangar floors are shielded from direct sun radiation, warm-up will be delayed and condensation will be prolonged on aircraft kept in hangars with floors of earth or concrete. The water may form on inside as well as outside surfaces of the skin, and steel tube fuselages may produce condensation along bottom longerons, a common location for corrosion. Floor cooling of the airframe can be minimized by placing a radiation barrier under the airplane. This can be a lowdensity material such as wood or building board. Carpeting would also be useful, and old rugs could provide an inexpensive solution. The result will be a drier aircraft inside and out, with reduced chance for water damage. ^*
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SPORT AVIATION 63
