Tolefson, H B

English

NASA Technical Reports Server

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NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
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0R1GINALL% ISSUED
July 1947as
AdvanoeRestrictedReportLX27
~ FLUCTUATIONSAS A MEASURE
OF ATMOSPHERIC~
By H. B. Tolefson
LangleyMemorialAeronauticalLaboratory
Lmgley Field,Va.
.,,\
.,
NACA
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NACA WARTIME REPORTS are reprints of papers originally issu~d t; provide ;apid distribution of “
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31176
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NACA
01364 MX26
ARR NO. L5F2Y
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
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ADVANCE RESTRICTED REPORT
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AIRSHIED l~LUC’PUATIOWSAS A MEASUF!!
OF ATMOSEITRIC TURBULENCE
ByH. 3. Tolefson
Because of’the need for a reliable quantitative
method of reporting at~logpheric t~-aUienc8 fr~ @.ir@~es
h fll&ht, the rapid fiuc tuations in tk.epilot ~s
Lndicated-alrspoed read Lws In rou@ air were analyzed
in relation to control difftcultles experienced by the
pilot and to structural loads. The dhta uned for this
purpose were obt~.inedwith tha XC-35 airplare in flight
throu(ylcumulus, cumlus-congsstus, and cuinulo-nimbus
clouds .
We rapid fluctuations in the p~-lot~s ir.dicated-
airsyeed rendings were found to apply with small error
to tl~eestj~atlon of the structural loads resulting from
~li~ht in turbulent air. Data obtained from the airspeed
met;r, ““astn the case of cicceleration data, however, do
not permit accurate estimation of the control difficul-
ties due to atnosphertc turbulence that a?o experienced
by the pilot.
A rellsbls method by which pilots can maks reports
during fll@t of the intensity of atmospheric turbulence
encountered has long been needed in order to provide
quantitative data on the ttibulence that would be of.
comnon signiflc+ce to the meteorologist, the pilot, and
the atiplane destgner. Various methcds for reporting
turbulence have been used, hut &he results” obtained have ,
been unsatisfactory because the reports are not stand-
ardized and cannot be converted to the desired form of
data. For example, scales of turkulenoe based on the
pilotls observations of tha behavior and control response
I ——. -—
. . . . . .
2 NACA ARR No. L5F27
of the atiplane in rough air (references 1 and 2) cannot
be conveniently Interpreted because the reactions of an
airplane to atmospheric gusts, and consequently the
pilot~s opinion of the turbulence, depend upon the air-
plane chane.cterj.stiesand flight speed as well as upon
the gust velccitiee. “1-ttemptstc utilize data obtained
solely from the meaaured accelerations of an al:’plane
have be~ unsuccessful for slmllar ree.eons.
CGnsideratj.on of the difflcultiss which have been
experienced ind~.catee that, in order to obtain the
desired results regardless Qf the type of airplane, the
reports of atiuoapka~l.ctm’lxlence should be Independent
of the airplane ch.azzacteristics,the flight speed, a.m
the judgment of’the pilct. A convenient method of
obtaining such data on the turb-~lence Is tb.euse of the
rapid fluctuations in the pilot~s Ind?-ceted-airspeed
readings, wh.lch are a measure of the horizontal compo-
nents of gusts tk’atare encountered by the ~itot-static
head of the airplane. 13published data have shown that
hm?izontal and vertical cox!?onents of gusts of equal
int9nsity occur witln equal frequency h a @ven region
of the &tmc)Sphere and, although equal horizontal and
vertical intensities me not in general encountered
simultaneously, the fluctuations in the airspeed lndi-
catians ma~ be considered a measure of the vertical as
well cs the horflzmb,nl components of the gusts. Control
diff’lculties experienced by thg pilot and ~mst ~@&ds
Imposed u~on the str-~cture during flt~;ht in tuzzbulent
air, however, result from the action of ~zsts over tb.e
entire airplane v;he~sas fluctuatlcas in the pilot~s
~ndicnted-airspesd ra&d.ln:s are point measurements of’
the gusts. ~9 presect wwk, therefore, wea undertaken
to 5etermine whether tke fluctuations In indicated air-
speed are sf~nif’leant in relation to contnol problems
and to structural loads.
DurIn: tests in which tineXC-35 airplane was flcmm
in tuubulent alr within clouds (reference 3), dats on
the f~uctuatlons in the pilot!s indicated-airspeed
readings .sndon the dlstw-bcd an~wlar r.otiom of’the
al.rplwe as indicated by the peiatinent naviLatlon instru-
ments were obtained. Pecause control difficulties expe-
rienced by the pilot In maintaining the desired flight
path are indicated by the a:lplitudes of the disturbed
an~lar inotlons of the airplane, theso data have been
used to determine the slgnlflcmce of the fluct~ations
---- -. - . . . - . . —...-—- .- . - . . . . . . -. . . --—---- —. - . .-— — . -II
—--- . . - .. ..— —— .— —-- . . . . . .. —--— -
[
NACA Am No. L5F27 . . 3
in the pilot?s Indicated-airspeed readtngs In relation
......,..,
‘to-dlfficultyof cwxbrol..,,.....
....... .,--
.,.. ,,
Increments in structural loads due to atmospheric
turbulence can be accurately expressed”in terms of flight
speed and effect’lve gust velocltlee (??ef’erence4). Data
from which the effecttve gust velocities could be com-
puted were also obtained durhg the flights of the
XC-35 airplane. These data have been used to determine
the significance of the fluctuations in the pllot?s
Indicated-airspsed readings in relation to structural
loads due to atmospherto turbulence.
APPARATUS
The XC-35 airplane is a revised u]odel of the
Lockheed 1O-IJ,and the two airplu-es may be considered
to have the same handling qualities. Pertinent di.~en-
sions of the XC-35 aiiapl~~~ars ~iven in tho following
table:
Gross l.~ei@ltat t-e-off, pounds . . . . . . . . 11,159
Wing area, squarofset. . . . . . . . . . . . . . 4jf.;
Wing loadln~ at take-off, pounds per square foot .
l.~e~naerod~l~mic chopd, feet . . . . . . . . . ~ . 9.;3
Span, feet. . . . . . . . . . . . . . . . . . ,. . 55
‘Lcm@hsfeet. . . . . . . . . . , . . . . . . . . . 59
Slops of lift curve, per radian . . . . . . . . . 3,(25
A sketoh showing the arrangement and location of the
lpllot~s airspeed in-stallation in the airplane is given
as figure 1.
The special Instruments used in the airplane to
obtain the desired data were:
(1) Cir,~-Kodak Special ].6-millimeter motion-picture
camera
(2) l~ACAair-damped recording accelerometer ‘.
(3) NACA airspeed recorder
(4.)NACA tImer (l-see Interval)
4 NACA ARR Noa L5F27
The motion-pic@re camera was US6C?.to obtain records of’
the indications of thg pilot~s artificial horizon>
directional g~o and clock. The camwa wati oys~ated at
a filinspeed cf $ frames per second a.?~was fitK.edwith
a magazine holding sufficient film for approximately
8 min-~tes of reeord. The a!rspeed recorder and accel-
erometer were opereted at a film speed cf 1/8 inch per
second and c.mrlad sufficient film for ~0 mlnutas of
record. The timer was used to synchronize the records
from the all’~paed recorder and acceleromet=p. The switch
controlling t~.e instrument operation was arrunged so tinet
the rnotton-pfl.ctureca.nma would operate concurrently with
the acceleration and airspeed recorders.
TEST Il%iH(X)S
The method o: conductin.q ~ne f’1.ightests consisted
h maklnc successi~e tr~verses tiousjh cumulus, cumulus-
congestus, and c-.mulo-n:mbus clmds with the e.ir:~~rne.
Before the airplane entered a cloud it was trimwd for
steady level flight, ma dw’ing pa~sage thrm+jh ‘Jhecioud
the pilot attempted LO use a mlnhmm oi’control consistent
with safety. Clouds werG traversed at various altitudes
ranging from approximately ~,CJOOto 34.,GOOfeet, and
traverses varied in length from about 1/2 to 15 nlles.
Tho method of obta~nlng data consisted In taking
continuor.smotion pictu??es of the pllot~s airspeed meter,
artificial horizon, directional g~o, nnd clock, and
simultaneously taking records of acceler&tian and air-
sneed during each travcree throu@ clouds.
RES-JIITS
The total amplitu~e (the difference between opposfte
extr~.mes) of the indications of the artificial horizon
and directional gyro were obtained from the motion-
picture records of these instruments for each traverse
throu@ rough air. The maximm aaglltude of the rapid
fluctuations in the pilot~s inMcated-a?-rspeed readings
was also cbtal~ed from the motion-picture records for
each traverse. Typical time histories of tha pilot?s
airspeed indications and of the motions of the airplane
. — —-. . .-—. -. - . - ..—.— —— .. .. -..—- .— -. . - . .. --- — ..- . . .. . . . . - -d
I
. —- -----
..
NACA ARR No. L5F27
in rolls
-....,,.
sh.ovni‘“tn
heading, and pitch for
flgure””2; This figure
a part of a
Illustr.atgs
5
traverse are
the method
of obtainin~ the data from the motion-picture ”rbc-o%k”.
The effective gust velocities eveluated frcm the
airspeed and acceleration records”by means of the sharp-
edgad-gust formula (equation (1) of reference 4) and
summarized in figure 5 of reference 3 were used to obtain
the .maxlmum effective &ust velocity Ue for each
traverse of the present analysis. All effective gust
velocities evaluated for the p~t of the traverse indi-
cated are also shown in figure 2.
The value of Uem= found for each traverse made
during 12 f’li~lts picked at random from the data of
refgrence 3 was plotted aCainst the maximum value of the
rapid fluctuation in the pllot~s indicated-airspeed
readin~s for the corresponding traverse (fig. 3).
Figure 3 Is used to determine the si~ificance of the
pilot~s airspeed fluctuations as a measuro of the effec-
tive .~st velocities. The total anpiitudes of the
motions of the airplane ia roll, heading, and pitch as
obtained from the mot:.on-picture records of the instru-
ment panel v!erealso plotted against the corresponding
maximum ra~fd fluctuations In,tb.epllot~s indicatcd-
airspeed readin~s in figuz=es4., ~,”and IS,respecti-~ely,
to deterimhe the sign~ficance of the airspeed fluctx.m-
tions as a reawre of’the disturbed anfl~larmot~.ons of
the airplane in rou~ air. Table T gives the coeffi-
cients of’correlation (obtained by the methods of refer-
ence 5) for the data of fi~es ~ to 6.
PRZCISION
The accuracy of the effective gust veloclty depends
upon the preciszon with which the indicated airspeed and
the acceieratiml increment of the airplane can be r
measured. FWom n consideration of the precision of these
measurements, the evaluated n-aximum effective gust
velocit~es are estimated to be accurate wlthfm *.4.-feet
per second.
I
The accuracy to which the indications of the various
flight instruments were read from the ~otlon-picture
.
.—— — —————— . . . . —..-. —.—. -...————— ---- .
.6 NACA AR?/No. L5F27
records Is estimated to be es follows:
Airspeed, rnllesperb.our . . . .. s........ *2
Angle ofroll, depees. . . . . . . . . . . . . ..*2
Angle ofheedlng, degree . . . . . . . . . . . . . . *1
Angle ofpltch, degree.. . . . . . . . . . . . ..*1
DISCUSS 101{
F@I.??e > Indicates Khat the maximm rapid .flucin.aa-
tton in the pilot fs Indicated-airspeed readings
‘V%ax
which occurs over a p~rlod of flight in tuz=bulentalr Is
proportional to the .nsxlmum effective ,qcstvelocity Ue=
computed from the acceleration and air~peed measurements
over the same period. This result and the corresponding
(table I) Indicate t??atthe rapid fluctuations in indi-
cated airspeml are sigrlific&nt in ralation to the gust
loads applied to the structure ~f an airplane in spite
of the fact that values of
‘emax
represent an cverage
vertical ~st velocity slong the spen whereas the air-
speed indications are ~cint ~.easurements of the ~wst
velocity,
Whe experimental data plotted in figure 3 apply only
to the XC-35 airplane because the effective gust veloci-
ties depend upon the airplane charactor~stics. For this
reason Q Reneral expre~slon for determinin,q the effective
gust velocities for conventional airplanes fro,mthe ~ust
velocities indicated by the rapid fluctuations in the
pllot~s Indicated-airspeed readings ‘.vouldbe desireble.
Such an expression, which should be reasonably satisfac-
tory for conventional airplanes, may be c-otainedby using
the rol~tlonship (theo~etlcally and experimentally vsri-
fied hy references ~ and 6, respectively) that shows the
acceleration increment of’an airplane in traversing a
constant-gradient gust tO be & linear .~.Ct~On Of the
gust velccity for any @ven set of conditions. 2he ratio
of the actual acceleration incrarnent An of an airplane
in a gust of any gradient to the computed acceleration
Increment Ans obtained by us@g the actual gust velocity
in the sharp-edged-gust formula may therefore be con-
sidered to be the ratio of the effective gust velocity
...-
NACA ARR NO. L5F27 ~
..to-th: actual gust velocity; that is.
..
m. .
...,-.
...!.... .....
.....
...,.....
.-..,.,.
~n_=lJe ‘-
An~ z;
,-“(ii
where
TTe effective ~ast veloclty, feet per ~econd
u~ actual indicated @.st velocity, feet per second
According to unpublished data for con”~entlonal al.rplanes,
the gust veloclty for ths most ~robable g~st increases
from zero to its max:nm value in about ~ chords. Foi~
this wet, the acceleration ratio was found from equa-
tion r)~)of refarenc3 j+to be 0.65 * 0.1 for conventional
airpl.ants. As has baen noted, the ho~izontal gusts lndi-
ce.tedby the raptd airspeed fluctuations Av~ ~ve a
measure of the actual vertical ~ucts UI l ‘:ihenUi in
feet per second is r~placsd by LV1 in miles per hcur,
l?lax
-ue~u Is witten instead of Ue since only maximum
_———
values are involved, aRd 0.65 Is substituted for An,/Ans,
equation (1) becomes
Ue
0.65 = nm.
1947Av%sx
or
Uem= ‘ 0.~5AV1m= (2)
Equation (2) is plotted in figure 3 and the curve EhC)W8 “
good a~eement with the experimental data for the
XC-35 airplane. 1
It is apparent from figures h to 6 and the corre-
sporxiln~correlation coefficients (table.I) that the
correlation between the maximum rap$d fluctuations in
the pllotls indicated-airspeed readin~s In turbulent air
and the disturbed angular motions cf the airplane in
roll, headinE, and pitch are not sufficiently high to
—8 NACA ARR No. L5F27
. permit accurate prediction of these motions of the air-
plme from the airspeed data. Thls result is evidently
due to the fact that the disturbed augular motions of the
afrplane in turbulent air, even WIVQ fixed stability and
pfloting tGch.nlque, are dependent not so much upon the
mhximm intensity of a single gust as upon the sequence,
spacing, and intensity of’all the Susts encountered.
other factors affecting the angular motio.~s of the air-
plane arg the spattal extent and unsyrzmetrlcal components
of the gusts in both the horizontal and vertical direc-
tions. Wasurements @f all these qumtities cannot be
obtained frcm the pilot~s airspeed fluctuations, and for
this reason the airspeed data are not readtly ad&pted to
the study of control problems arising during fl@t in
turbulent air.
!t?heindicated gust velocltf.es ohteingd from the
rapid fluctuations in tW pllotls indicated-airspeed
readinps may be converted to true gust Velocities, the
form of data used by meteorolo~ists, b~ correcting for
altitude.
Additional attention of’tae pilot to the airspeed
meter wculd be required In order to note the fluctuations
h the readings for the turbuleaao reports. If experience
shows that the pilot cmnot note these fluctuatiorls, an
instrument for holding the visual indicatiofis for short
time Intervals or a recording Instrument would be
required. ~ instrument of this type might consist basi-
cally of a total-pressure capsule in a sealed case with
a canillary leak in the capsule that will b~ass S1OZY
chan~es in total pressure but remain sensitive to rapid
chames like those indicated by the rapid ~luctuations
in tfieairspeed readings in fi~e
could be either an indicating or a
CONCLUSIC!VS
mom an analwis of the rapid
2. ‘Such an instrument
recording type.
fluctuations In the
pilot~s lndlcated~alrspeed readln~s in turbulent air in
relation to the disturbed an@w motions of the
XC-35 airplane and to effective gust velocities, the
following conclusions are made:
.—— . . .
NACA ARR NO. L5F27 9
. .... . .,,,,, ., .,-,
““”””””l:-The-maximum nap.ld.fluct~t$onq. Jaat occur in the
pilot~s indicated-airspegd readings are proportional to
the Maximum effective gust velocities.
2. For computing structural loads for conventional
airplanes, the mxhum rapid fluctuatflons in the pllot~s
lndlcated-alrs?eed readings AV
%ax
in miles per hour may
be convertod to maximum ef’fectlve gust velocities
‘e-
in feet pei’second ‘o~the relat~on Ue = 0.95AV1-.
msx
and
Harrison, L. P.: Atmosnlieric Wrbulenc=. Pt. 111.
Soaring, May-Ju??e 19!+2,p. 9.
Moskovltz, A. 1.: SC-55 Oust Resmrch Project
Bulletin ??0.8 - Analysis OF Gust I’easurements.
IV-CAR13No. l@22, 1314..
Rhode, Richard v.: Gust Loaflson Airplaiies. .SkE
Jour., Vol. LO, no. 3, I’arch 1937, pp. 81-98.
Kenny, John l?.: Xdthei~tiCS Of StQti~tlUS. Pt. I.
L).Van Noshmd CG., Inc., 1939, PP. 183-166.
DonelS, Philip: An F#.p5rimental In-~estigation of the
Normal Acceleration of cn Airplane Model in a (lust.
NACA T1:~$0.706, 1939, p. 10 and ~i~s. 11-16.
-.— -— -- .- — — —. . ..-. .— -.
NACA h No, L5F27 10
7, .,. - -.
.-. -. -,, . . . . . . .,, .,,.. .. T~~~ I.,.., .- ~.,- ,-., ,-,-,,,, --------
CORRELATION COEFFICIENTS BETWZEN MAXIMUM “RAPID AIRSPEED
FLUCTULTIONS AND PARAMETERS OF ?IOTION
OF XC-35 AIRPLANE IN TDRBL%ENT AIR
— -— —.. ...— ——-— -——— -.. —
@antitles correlated with
maximum rapid airspeed
fluctuation W=
—— ---——. — —-.. --—-— —— .——
Naxinur effective gust valocity
‘%lx
lfaxinum total a~gle of’roll
Faxinum koto1 angle of heat!lng
l~~ximnm,total angle of pitch
.——-—— .- -—. — .——. —- ------
Coefficimnt
or
correlatIon
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Airspeed fluctuations as a measure of atmospheric turbulence

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