Condensation trails: Where they occur and what can be done about them Page: 4 of 13
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(a) Low temperatures.
b') igh h atmospheric humidity (but at very low tem-
peratures, persistent trails may form with
very low humidity) .
(a) High fuel consumption (high power and high epe-
cific fuel consumption)
(d) Low drag (less turbulence and narrow wake)
(e) Low speed (smaller final diameter of wake because.'
of lover turbulent energy)
Convection trails,- All the heat from combustion of
the fuel is discharged into the air behind the airplane
except the heat lost by radiation. The air behind the
airplane 1s thus warmed and expanded slightly, and it will
therefore rise slowly, purhiug up the unwarmed air above
it. If the .tmospherico lapse rate at flight level is not
more than eli.;htly less than the moist adiabatic lapse
rate and, if the humidity is rather high, a continuous
"natural" cloud will form behind and somewhat above the
airplane. This type of trail is very wide.and, in general,
dces not connect with the airplane.
Convection trails are favored by
(a) High power and fuel consumption
(b) Lapse rates on the verge of "conditional insta-
(c) High humidity
Aerodynamic trails.- When the air flows past the
wings, the propeller, andl other parts of the airplane,
there are reductions of pressure that cause "adiabatio
cooling" of the air. This cooling may be great enough to
raise the humidity relative in the affected regions to
100 percent or more and, in such cases, condensation will
take place. For the most part, after passing the airplane,
the air comes back to atmospheric pressure and to subetan-
tially atmospheric temperature, and the-condensate will
evaporate immediately and leave no trail.. There are,
however, regions within which the pressure and the temper-
ature remain less than atmospheric for a considerable dis-
tance downstream, such as, for example, the cores of the
wing-tip and propeller-tip vortices. Condensate may per-
sist in such regions util the vortices deoay.
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Rhode, Richard V. & Pearson, H. A. Condensation trails: Where they occur and what can be done about them, report, September 1942; (digital.library.unt.edu/ark:/67531/metadc61828/m1/4/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.