Pearson, H A

English

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%NATIONAL ADVISORY CO “.idTTEE FOR .AERONAUTICS
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W’Alrmm IUUPOIV”
ORIGINALLY ISSUED
September 194.2as
Confidential Bulletin
C(lJDENSATIONTRAILS
WHERE THEY OCCUR AND WHAT CAN BE
By Richard V. Rhode and H.
Langley Memorial Aeronautical
Langley Field, Va.
DONE ABOUT THEM
A. Pearson
Lab&atory
‘. NA6A.”’”
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NACA WARTIME REPORTS are reprints.ofpapers originallyissued to provide rapid distributionof
advance research resultsto an authorized group requiringthem for the war effort. They were pre-
viouslyheld under a security statusbut are now unclassified. Some of these reports were not tech-
nicallyedited. All have been reproduced without change in order to expedite general distribution.
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NATIONAL ADVISORY C/O~HI!CTXZlPOE AEROl!TAUTIOS
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0.01Ulti19SAT10iJTRAILS ~ “
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WHERE THEY OOOUR AIW WHAT CAH Em DOHH A230UT T~M
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By Bichard ?. Rhoda and H..A. Pearecn
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ii’OHEWORD
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A brief , nontechnloal di.ecussion of con”deneatlon
trails Is presented for flying permoncel. World mape
showing trail-forming sones at different altitu&es and
eeaeone are presented. Meane for supFreseing trails are
given. ..
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NATIJRM (N’ CONl)XYSATIGIV TRAILS
Gondensatlon tralle are of three types:
1. Exhaust trai16 - Formed by condensation of moie-
ture from the engine exhaust.
2. Gonveztt9n trailn - Formed under oertain atmospheric
condltlone Re a reeult of rising of air warmed
by FaOeage of the airplane.
3. Aerodynamic trallta - Formed by precipitation of
atmospheric moisture ae a result of adiabatic
temperature drop ae~ociated with” air flow pact
the airplane.
The enumeration of three dletlnct ty~eg of condensation
tralle mhould not, of ~ouree, be construed to mean that
comblnatione, of these types are not poeslble.
Exhaust tralls.- The exhautit trail is the most impor-
—-— -— .-
?ant from military ooneideratione, ae It may be rather
ooneietently encountered at Borne altltudee and latitudea.
It ie peculiar to high-altltude operatlone and is explained
ae follows:
.:
The hydrogen of the fuel used eom%ines with oxygen
from the air and forms vater. When normal aviation
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2.
gasollne 1s burned In an engine , about 1.25 pounds of
water ia formed an vqpor and 5EI dleoharged with the ex-
haust for each pound of fuel Burned.
Behind the engine-oarryiqg body (fuselage or nacelle)
a turbulent region or wake is formed as- the airplane
flies. The exhaust moisture and some of the engine heat
are discharged into this wake and beoome diffused through-
out the wake as a result of the ~lxlng act~on of the tur-
bulence. The moisture and heat do not, however, mix with
the air outside the wake beoauea there the air Is ‘emooth.n
The vortices In the wake grow and rotate more slowly
as they pass downstream from the airplane. Thus the wake
expands and decay?. During this proce6s the energy of the
tur~~lence iS dissipated as heat aa a result of viscosity
or friction, and finally sc much energy has been dltisipated
that the wake can no longer continua to grow. This point
is reached at a mile or more behind the airplane, the ‘ex-
act distance being somewhat indefinite and dependent upon
the speed and power ~f the airplane. By this time, becauee
cf the act!on of. wing-tip vortices, the wake has changed
in form from its origlral compact cro~e section to a more
or letis flat rlbbonllke form with curled-up edgest but this
change In forin does not I=volve any further mixing of the
water vapor with the air.
It is eaey to eee that, “If the kll’-lti.SO oold that it
cannot hold much water as. vapor, the water in the exhaust
may be sufficient, when added to the moisture already in
the atmosphere, to raise the humidity In the turbulent
wake to or beyon?! the saturation value. If this condition
exists , some of the wcter vapor will condense and a visible
trail will form.
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Since the turbulent wake “ie narrow near the airplane,
the density of moistur6 will be greatest at this location.
Farther away, where the wake is larger and the exhaust
moisture Ie more widely diffused, there will be less mots-
ture density; Thus , under some conditions, a short trail
may form that evaporate where the wake” cross se”ctlon be-
comes too large to ma.intaln l.00-peroent humidity. If tbe
amount of uoisutre ‘is great enough to more than satusate
the” wake at its final and greatest arose oection, however,
the trail will be pereietent aid will not disappear unttl
It is finally blown away by the wind or dleeipated by at-
mospheric turbulence.
From the foregoing explanation It is clear that ex-
haust trails are favored by
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(a) Low temperatures,. -
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‘“(b”)Hl&h”atrn6spherlc htimidlty (hut at very low tem-
peratures, pereletent tralle may form with
very low humi~ity) .
(o) High fuel aoneumptlon (hi h power and high spe-
olfio fuel ooneumption 7
(d) LOW drag (lees turbulence and narrow wake) “
(e) Low speed (smaller final diameter of wake because.”
of lower turbulent energy)
ConvectIon trfiila.- /.11 the heat from combustion of
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the f;~~scharged into the air behind the airplane “
except the heat lost by radiation- The air behind the
airplane :8 thus warmed and expanded aligbtly, and ~t will
therefore rlee 61ow1Y, ptip,hingup the unwarmed air above
It. If the ntmosmheric lapee rate at flight level Is not
more than sli.~htly less than the moist adiabatic lapme
rate and, if the humidity is rather high, a continuous
“naturaln cloud will. form behind and somewhat above the
airplane . This type of trail ia very wide .and, i.n general,
dcee not connect with the mlrplane.
Convection tr~ile nre favored by
(a) High power and fuel consumption
(b) Lapse rates. “conditional lneta-on the verge of
blllty”
(c) High humidity
Aerodynamic tralls.- When the air flow8 past the
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wings~he propeller, and other parts of the airplane,
there are reductlone of pressure that cauoe “adiabatlo
cooling” of the air. Thie aoollng may be great enough to .
raiee the humidity relabive in the affected regions to
100 percent or more and, in such caees, “oondeneation will
take place. For the most part, after passing the airplane,
the air comes back to atmoephei’lo preeeure and to substan-
tially atmospherlo temperature, and the.condensate will
evaporate immed~ately and leave no trail.” There are,
however, regions within which the pressure and the temper-
ature remain lese than atmospherlo for a oonelderable dis-
tance downstream, such as, for example, the ooree of the
wing-tlp and propeller-tip vortices. Condensate mar per-
sist in suoh regions uptiz the vortloes deoay.
— -
4When the humidity IS very high, evaporation aan be
so. slow that a persistent or taemiperslstent trail will
form even in the regions where the pressure and the tem-
perature have returned to atmoepberio values. If t30ndi-
tions are favorable for suoh trails to form at subfreez-
ing temperatures, furthermore, the condensate may freeze
and may remain more persistent than at high temperatures,
T
In general, under conditions of very high humidity ~
(both specific and relative) ‘wingfi trails or ‘wing-tipn e
trails w!ll form. AU a rule, but not withGut emeption,
wing-tip trails will perslsi where ordinary wing trails
will immediately evaporate.
Wing trails are fav>red by
(a) ?igh speed and wing loading
(b) Very high humidity
(c) High temperatures (the traile may persist longer m
if the temperature is not above freezing)
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Wing-tip trall~ are not appreciably affected by
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epeea ohangeu. They are favorod by
(a) High wing loading
(b) High humidity
(c) High temperature (the trails may pereiet longer
if the temperature Is not above freezing)
In general, both the aerodynamic trail and the con-
vection trail are eporadic and rather rare In occurrence
because they require relatively unusual ooiabinatione of
conditions. At present they cannot be predicted with
certainty or with oaee. The exhaust trail, on the other
hand, will nearly always ccour at oertain high altitudes
and latltude8 and ie moat Important from the m~litary
etandpolnt.
ZONES oF” 0CCUiiRE19CE OR’EXHAUST TRAILS
World maps are included herein to show the zones of
occurrence of exhaust trail.e at the .diffe.rent 8ea80nm of
the year. These maps are based on available data on monthly
5..-.
mean values of temperattie and humldlty and apply speolf-
ieally.to. the. Bl%3..alrplan~ in..aormal heavy. orulsing
condition. The maps will apply reasonably well, however,
to any modern airplane, provided the power output per eng-
ine at the upper ’altitudes is about bhe same as that of
the 317-EC
At the time of preparation of these maps, 8uffislent
data were available to permit reasonably exaot determina-
tion of the trail zonee only over the North American eon-
tlnent , the western half of the Atlantia Ooean, and the
eastern third of the Paoiflc Ooean In the Northern Hemi-
sphere. The Bone boundarlee shown eleewhere are Pure
estimates, and they are presented as such, pending com-
pilation of the data required ,to oonfitruat more exaat
maps .
No boundary llnee are shown in the Southern Hemi-
sphere beaause of laok of data. Trail zones In this hemi-
sphere may be roughly estimated from the following rulees
1. The mean. south latitudee of the sone boundaries ‘“
in the Southern Hemisphere for the month of
July will be about the same as the mean north
latitudes of the zone boundaries in the Northern
Hemisphere for. the month of January. The name
rule applies for the--other oompllmentary months.
2. Zone boundaries will have a tendenoy to bulge
toward the poles over the Southeast Pacific and
the SouthweOt Atlantic and to bulge toward the
equator of the western ooaete of the continence
of South Amerloa and Afrlaa.
It should be borne in mind that such maps are merely
representative of average conditions, because the maps
are based on mean atmospheric temperature and humlditieB.
It should also be borne In mind that the maps apply to
epecified operating conditions of a single airplane, In
general, the boundary linee on any given map wI1l come
toward the equator for allmblng conditlone, reaede tiovard
the poles for diving or gliding conditions, approaoh the
equator for airplanes having greater single-engine power
than the B17-E, and reoede toward the poles for alr.planes
having less power than the B17-E,
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SUPPR~SS POH W TRAILS . .
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.If a .tri~l 16 formed, there tire-cer”~tilrimo.ae.dre”e
&vatlable to the. pilot for suppressing or eliminating them.
An enumeration of theaa measures follows: . “ ,
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~~~gt +re~ .- In principle, the only completely r
effective method of ‘preventing exhaust trails is to remove $
the water from the exhauet by means of water-recoveky
apparatus . Becauae ~uch equipment is not available and
apparently Is Eot practicable at the moment,. other means
of euppresslon that can .he only par-tly effective must be
resorted to. .These other means will,. of courseb~be more
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.. effea$ive in regions. or at alt~tudee where the tralln are
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not” of great denelty and vice versa,
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.1. If reduction of altitude Is permlesible, throttle
. . engines and glide at high speed to a lower
. . level.
2...Inetnet l.oes of altitude ie not permissible, go
into a shall~w power dive at substantial in-
., orbase in speed, Regain altitude by zooming.
(Short lengths of.porsletent t?ail may be
formed during lattsr part of zcom.) Mterna-;.”
tfve! . J’ly at reduced power. ..
3. If cone retiuctlon In speed 18 permissible with
same powsr output and fuel oonsumption~ ae
during climb, open engine Gowl flaps as wfde. .
ae posei%le. (Airplaneo without cowl f~aps
could be equipped .with similar drag-producing
devices.)
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; ... . . -’ .T&.effect ‘of these .var.$oue expedients will, if .con-
ditiona are no+ -*OO difficult., be to traneform a long,
pers.iateqt trail into a short diselpatfng one or.to” ehorten
stiJl fur$her a dleeipating trail, The trails wll.ljprob-
ably not disappear entirely in.all cases except when rule
1 is a~plied.
cJmz.!2dlQxaAraua l -
1. Climb or descent to new level. (A change cf alt i-
tude cf 1000 to 2000 feet should be ample In
moat oauee to eliminate trail completely.)
2. Stay away from level of stratue and strato-cumulous
cloud formations.
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AerodXnamie tratls - Owing to the faot that. wing-tip
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trails are llttle affected by speed changes , there is no
,— ... . . .. way in.which,,..the,pil~: can ngt:nfpu%ate the airpl,ane to
cause immediate and complete i?e”&sa$fon o~-al.l--a-ero dyns.xnic
=F tra, ils s In general, however, the fallowing proced~:es
sheuld help if tb.ey can be ap:p lied:
~
1. ‘Throttle engine and Ye $-ace Spee’d,
2. Reduce altitude belcw freezing le”rel=)
3. Cl!mh or descend tc new altitude to get out of
humid layers.
4. Avoid, flying near the elevation of stratu~-type
cloud formations,
FLIGHT IN T;31?)STRATOSPHERE
The British in preliminary investigations ha.~e con-
cluded that persistent exhaust trails vill cease a short
distance above the tropopaus~,. They lay great stress’,
therefore, os~the desirability of flying in the stratosphere
on bombing missions. It should %e pointed out here that
the NACA investig?.tion does not bear out this conclusion.
‘I’hetrue explanation of the British findings is believed
to be that in their tests the tropopause was close to the
ceilings of the airplanes used and considerably above the
critical altitudes, The engine powers, therefore~ were at
low levels and the trails ceased because of reduced power
and amount of moisture dischar~ed per unit volune of trail.
Their trails -probably could have been aade to cease at an$
elevation below the tropopause by throttling the engines
in accordance with rule 1 or 2 governing suppression of
exhaust trails. An exhaust trail from any airplane should
be greatly suppressed, if not entirely eliminated when
the” airplane is flying nell above its critical altitude
and near its ceiling, wherever these altitudes uay be and
regardless of the elevat~on of the tropopause.
It should be clearly understood that flying into the
stratosphere will n~t, of itselfg cause cessation of
trails as far as is shown from the fundamental nature of
trail phenomena.
Langley Meaorial Aeronautical Laboratory,
National Advisory (!lonnnitteefor Aeronautics,
Langley Field$ Va.

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Condensation trails : where they occur and what can be done about them

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