Wind-Tunnel and Flight Measurements 2 - The Douglas A-26 Invader

NATIONAL ADVISORY COMMBTEE FOR AERONAUnCS
March 1946 as
Advance Restricted Report L5Hlla
COMPARISOI'? OF WIND-TUNNEL AND FLIGE -S
OF STABILITY AND COrnOL CH,ARAcmISmcS
By Gerald G. Kayten and William Koven
Langley Memorial Aeronautical Laboratory
Lmey Field, Va.
NACA
WASHINGTON
PROPERTY OF JET PROPULSION LABORRWRY L
CAUFORNU iNSTLTUTE QF TECHNOLO61
NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of
advance research results to an authorized group requiring them for the war effort. They were pre-
viously held under a security status but are now unclassified. Some of these reports were not tech-
nically edited. All have been reproduced without change in order to expedite general distribution.

HACA ARR No, L5Hlla
8ATICNAI kD\rISORY COXIIqTTEE FOR AZ8OIUAUTICS
----
c o ~ P A ~ OF ~ S?gIJJ)- O ~ TciETEL A:% FLIGHT J { ~ A S U ~ ~ : ~ ? ? ~ ~
Stability and control ckisracteri~tios determined
from 'ccsts 5-r~ the Lz,lglsg 19-foot pressure tunnel of
a 0.2375-scale mode! 02 t:ie Dou(~,laa XR-26 zirplme sre
compared sith those moaeurea in flight, tcs'cs of c
Dou-glns A-268 ai19piane i.
Agreement raga.rding static 1ongitvZinal stai13-rillity
as izdicoted by t h o el.evaJ~or-fixed neutral points and Sg
the varletion of elevator deflection in both straight and
turning fllght was folnd to be good except at speeds
apppoaching the stall. At these low s~eeds the airglene
possessed noticea3ly lnprovod sta'fillity, which was
attributed to pronou.ncod atalllng at thn root of the
production ring. The procom.ced ~oot stalling did riot
occur on the ssmooth, well-foired mocl~l viing. Elevator
tab efFectivenes~ deterrninccl from modal tests agrzed ~flell
~~ith fllght-test tab effectivenoso, but control-f orce
varisttons with speed and acceleration were not in good
a~rcginent, i?,l.l;hou:,h some discrepancy was introduced by
absence of a seal on th2 model elevator ane 57 small
diffeyences in the detenniaation of elevator deflections,
coprelation in coxtro1,-f orco characteristics Vcas alsa
influenscd by the effec-ts of fabric distortioi~ at high
specs ;nd by small construction dj..ssirniLaritioa such as
differenoes in trallfng-ed.ge angle. >&cspt for the vuavtioff
condition, in ;.aich the tunn3l resuli;s indicated
rudder-force reversal at a higl~r speed t11c.n the flight
testa, agrecxenf in both rudder-fixed and rudder-free
static directional stability was good. li:ociel arid airplane
indioetions of stick-fixed and stick-frse dihedrsl
offoct mere also in good agreement, althongh some dlfferenco
in geometric dihedral may heve existed because of

2 XACA No. L5Rlla s
wing 5ending in flLght. The use of model hinge-monent data
obtained at zero sideslip ap~~eared to be satisfactory for
the determination of aileron forces in sideslip. Fairly
good correlation in tiileron effectiveness and control forces
was obtained; fabrf c distortion 3ay hava been responsible
to some extent for higher flf3ht values of aileron force
at high sne'eds. Estimation of sideslip developed in an ..
abrupt aiieron roll wzs fair, hu.t determination of the
rudd.er deflection reqxiired to maintain zero sideslip in a
rapid aileron roll was not entirely satisfactory,
Althou:nec tion vvi tk the 4sarelo?ment of the ~ougias
twir,-engine attac1.r bomber, a @&pies of investigations has
been coneucted at the Lan4ley Laboratory of the XatFonal
Advlsory Committee for ~ercnautics . These investigations,
the results of which hsve not been nu.blrPshed, included
tests of a 0,2375-scale powered model of the XA-26 airplane
in the Langley 19-foot pressure tunnel and fl-ight tests
of an A-26~ alr~lane. 537 use of the unnulnlished windtunnel
data, calculations have 3een made predicting the
flying qualitiss of the airalane for correlation with the
cliaracteriatics meas~~red in the fli;;k~t tests. The results
of the corre1at;ion are -resented herein; the flying qualities
ars not discussed exce2t for the purpose of comparison,
Thotographs and drawin~s of the A-26~ airplane and
the XA-26 model are shown as fizurus 1 and. 2, respectively.
In table I general dimensions ttnd specifications are shown
for t.he airplane an?,. tbe model, as well as for the model
scaled up to airplane size. Sone discre~ancies of neg-
ligible importance &re noted in this table but it can be
seen that, with respect to genorsl dimensions, the XA-26 4
and the A-26.3 are essentially the same airplane. As shown
e

iil fig- re 1, the model during 'tile stability and control
tests was equipped with a fuselage rlose which was so,?;r;vha.t
different f ro~n thzt of ths airgln~e, 'The s~l-nners s:'?(~YIE
, on the model proyellers were 1i0t 11sed on the aiy?la.ne, and
the airplane oil-cooler tiucts outhoard of the nacelles
were removed fro21 .-the model wing clurinz the stabilLty and
control tesks with the exception of the aileron tests,
Several more si~nificant differences existed between
the :model an$. the cirplane . During most of the tunnel
tests the :nodel n;l.dd.er and the elevator, which were of
the pluin overhaqg -balance type, remained unsealed, Sut
the airplane control surfac?~ :!"ere equipped with r~b'cerized
cai?Lvas seals. 'I'he coiztrol sv.~>l"aces, all of which were
faSric-covered on the airglaie, :sJere of rigid metal con-
-
s tru-ct ton on the mode 1. Yne al.r:?!_ane ailerons we re equipped
witl: balancing taGs arrangsd so that 8' of alleron deflec-
ti on produced ap?roximately 3O of o1;poslte tab deflection.
On th.2 model i;he balancing tab when connsctad moved lo
for a 10 aileron deflection.
Thin m5tal strips ware fastaned to the up25r aiid lower
surfacas of tb.e air~lane el,;? V- ~a-t~r causing srnall rl?.ges
dirsctl;~ in front of ths taS. These ridps were not
re~3resented on the modsl, but their effect on elavator and
tab ~l~.a~acter;.is.'siss is belisved to be negligible.
The wfnd-tunnel program included a fairly sxtensive
series of conventional stability and. control tests. The
mod-el ailsron tests o nadc at a i?e)Tiiolds nU:nber of
approximatel;? 5.h x 10 , The re!:iaining modal tests
were xade at a Iieyfio1d.s aurn3e:r of approximatsly 3.6 x 10 6
except for .the tests at high thru.s.i; coefficients, idvhich
b.2cause of model motor lirii-tations i.vere made nt Reynolds
numSers reciuced to anproxilnately 2.6 x 106. The portion
of the fli?;ht tests devoted to sta.3llitg and control were
of tks t~~l3e u~?.~~lly conductsd by the FZBCA for the Durpose
oi' deteratning tha flying qualities of an air~lane, The
wei,$it of ths airplane, varied from 27,000 to 31,DOG
~ounds in t!~e flight tests < was assi~.med for t3.e analysis
of JChc ttunnel 6ata to 3s 20,000 pov.nGs corresponding to a
wing loa8ing of 51.8 pounds per sq!l;ire foot, The analysis
was based on an altltude of 10,000 fest, which rizpresehted
an ap?roximate mean of the flight-test altftudzs.
Analysis of the tunnsl data h ~ Seen s made for conditions
rsp~?esenting ai rplar'e rated 3ovvar erld 75-perc~ilt
rated power at the approprista airplane weight and eltltvdes

?JACA ARR No, LTBll a
and fo? a gliding fligkt conditinl~. In represen'bation of
the gliding t'ligl-;t co~id.ition, it has Seen assurnsd that s
sngfnss-idling and zero-thrust conditions nay Se considered
identfcsl. Any discrepancy in results introduced by the
dii'ferei-ice between these power conditions probably vv-ill
be -,small.
In comnut ing clavator , aileron, and rudder control
forces f rori model hinge-noment data, cor~esponding
control li,nka:.es ., msasured on t3e air>larie were used.
6c elevator deflection, dedrses
6p flap deflection, dega3s
6t t2b deflection, degraes
Ch hhge-moment coef2'icient
'i
indicated srrspeed, miles per hour
F e elevator control force, pounds
- pb wing-tiphelix angle, radians
2v
whe re
R hinge noment , foot-pounds
S ming span, feet
-
c ~oot-mean-square chord, feet
q d~pt;afc pressure, pounds par square foot

WACA ABR $Joe LjHlla
P nass density of air, slugs per cubic foot
V e.irsyeed, feet per second
T total tnrust (two propellers), 7ou.nds
D propeller diameter, feet
P rollicg veloci tg, radi~ls per second
S ~iiil?g area, square feet
C, angle of attack, degrees
a t.
taail mgle of attack, degrees
Q acceleration of qravfty, feet per second per
s e cbnd
1,ongitv-dina?. Stability end Control
Curves of elevator mgle a;1d elevator control force
required for trim fm straight flight t!~rougliout the speed
range are shown in figure 3. 'Various fla13 and PoTtLyTer
combinztfons are considered at three center-of-qravity
1ocatkor.s. For the f'lsps-retracted conditf ons, the tunnel
control-force cxrves were obtained by ap?lying th.e tab-
effec-tiveness data of figure 1; to the tab-ne~trai curves
es tI,-catec?. From the tu.rae1 hfnga-moment data. The axour;t
o? tab deflectior: required to adjust the tunnel curve for
trix at the flight-test trirr: speed was determfned for each
nower condition snd center-of -g~.avf tj location, and this
ar~ount. of t sb dei'leckion was a.ss~n:ed constant t!iroughou.t
the sveed range. Inasriiuch a.s i~odel trj-m-tab teats were
not ~ade wf th flaps deflected, the -i;rfrmiecl. control-f orce
curves for this condition were obtained by Ilieans of a
constmt adjustment to eack original curve of Ch
against Thf s const ant henge-morneat shzf t is believed
L'
justified because the data of figure !, indicate a negligible
change in tab effectiveness with change in pewer
(f lans retracted) and because analysis of stabilizereffec.~iveness
data indicates that the variation in
average dyne~cic-pressure ratio with speed is small for

the f laps-def lected conditi:>n. The flaps-def lected ,
control-force curves for zero tri:n tab are included in
PS-gure 3.
The sideslip requfred for stra.ight flight at low
s2eeds was considerec! to have a negligible effect on the
long1 tudinal characteristics or this airpl=le; hence, the
characteristics determined fro~c tunnel data are based on
tests at zero sidesli2.
The variation of tab effectiveness with speed has
been calculated from f laps-retracted wind-tunnel tests
~nsde at elevator-tab settings of 3' and -5' with 6, = 0'
and is sl.lo~vn in figure 1,. co~npared with the flight-test
curve.
Elevator def lecttons md control forces in steadjr
turnlng flizht are ~ko~~lr~ in figures j to 7 for vzrious
center-of -gravity locations. The calculated results are
Sased or, tunnel tests at the tlvllst coefficient approximately
corresponding to the appropri ate flight-test
conditions.
Althougli s 0m.e s~all d.if f ersnces exf s t in tile absolute
elevatop engles, the slopes of the curves in figures 3, 5,
and 7 show good agreement between tunnel and flight results
for both straight md turning flight, except at speeds
close to the stall. At these low speeds, the flight data
shov~ pronounced inc.reases in the ?mount of up-elevator
movement required for speed reduction in straight flight.
These marked increases are not apparent. in the tunr-iel data.
Thfs discrepancy in results is believed due largely to the
fac.t that the produ.ctfon airj21ane exhibited a decfdedly
rnore deffnite stall at the 73~4r,g root tlxm did the smooth,
polished nlodel., Althougi~ direct comparison of identical
configura.tions f s not possfble, the ~ifference in stalling
characteristics at the wing root is indicated by the diagrarils
of "c;ln~el arid flight-test tuft studies shown in
figures 8 and 9. The more ?renounced root stalling on
the ai~plane would, in all ..>robabllity, be accomnanied by
a reductfon in dovmwash and rate of downnash a.t the horizontal
tafl as ell as a decrease in wing ~itc'ning noxnent,,
resulting in =-I fu;prove;i;ent in stability and requfring
.~reater up-elevator deflections for trln!. At liigher air-
9 1 1
speeds the agreement betwee;.: Plight and tunnel results is
raeasons.bly consistent wf tn the experimenta.1 accuracy of
both.

NACA ARR No. L5Hlla 7
The tunnel and flight curves of elevator-fixed neutral
poifit plotted against afrspeed fn figure 20 for the
f lam-neutral conditions agree to within approximately
2 percent of the mean aerod~jnamic chord except at low
speeds with idling power. This difference is practically
within the bounds of the experimental accuracy wlth which
the flight and the wind-tunnel neutral points are determined.
The discrepancy increases with reduced airspeed
as the airplane demonstrates co~~~araJcively greater stability.
Because of the ilifficulty in obtaining consist=nt neutralpoilrt
result s, particu.larlg at very high airspeeds, neutral
points were not determined for -tnese speeds. The curves
of figure 3 serv3 as a mnensure of the stabflity in the
high-speed range and are, in fact, believed more reliable
for comparison throughout the speed range than the neutralpoint
curve s. Alt3ough the curves for the f laps-d.ef lectzd
coiidFtions are included for couplo-teness, direct comparison
should not be made inasmuch as ti19 flag settings used in
flight and tunnel tests were not Identical.
Examination of the straight-flight control-force
curves of figure j reveals comparatively poor agreement
be tween tunilel and f 1-2 gkt results . Ths force measurements
shown j.n the tab-effectiveness curves of figure 4, however,
are in excellent agreement. 30th flight and tunnel contrcl-
force measurements are believed to be accurate to within
an:?roximztel;y t3 pounds. tilthou& some discrepancy in
the elevator control-force cupves of figure 3 would be
exnected because of the absence of a seal on the model
elevator, analysis based on brief check tests in whfch the
model elevator was sealed indicated that d.ifferencss of
the magnitude shown in f'i;;ure 3 cannot be attributed to
effects of the elevator seal. In an effort to determine
the cause of the disagreem2nt, the effects of the discrep-
ancies in elevator deflection xere investigated. Hy-po-
thetical control forces were computed from tunnel hinge-
moment data by using the values of elevator deflection
determined from flight rather than thosa determined from
tunnel data, For these computations, the wind-tunnel tab-
effect.ivzness data were used, but the tab deflection was
that employed in the flight tests. The curves obtained
in tkis manner are shown in ffgure 11 compared the
Plight-test data. In general, figreernent in fisure 11
appears considerably improved; for several flight con-
ditions, in fact, agreement is excellent up to speeds
above 200 miles per hour, beyond 2vhich the flight-test
curves become noticeably more stable. This difference
may be explained to some extent by the observations of

elevator-f abric ciistopti on arid internal nressures made
durfng the flight tests. The internal ..?resscre~ .were
found to be only slightly higher than free-streaa static
?ressure, causfns fa.brlc distortion of the ty?e 2ll.u~tratsd
in fFgure 12. Az dsrlzonstrated in reference 1,
eleva-bor-fabric distortion or" this tpe RaTT be e;zpected
to pr~duce increases in the variatioa of force with
airsneed at high speeds. Inasi~.uch as tine f laps-retracted
flight-test triin speeds of figurs 3 =e all in this
high-speed range, tne trim-tab ,

NACA A?? 1:o. L5Klla
T?ie resosctive values of b~~d6.5~ and ysed
in tkes2 coil!v)utatfons ;%ern -0.0037 cnd -0.~313 for che
unsealed elevator an6 -6.0050 and -0.6052 for tkie seelsd
elevator. The resultin2 curves of forcs :-)er ,g a);sinst
center-of-pravity location are shoim in figure 1:. The
exve for the unseaied elevator is ~ractj.call:. ide~.tical
wit? inat greviously determined fnr tzs unsealed
el?vator (fi~. 7) by the met!loci of reference 2. For the
seglea elevator the values 01' Torcs rer g 2re still
vey- auch lower t -Ian tile f liqht- test values, a1 tiiou;h
the vsriation of g with center-of -grsvit~- location
is xors nearly narallel to tkat deternined in flight.
The ccimarison of cqntrol forces in acceleratzd flight
has beel-1 !;lade at a fairly high si?ee&. aeference 1
in9ic8tes chat f aSrLc distcrtion of the type experienced
iF1 the A-26~ f1i:ht tssts Kay be ex>ected to produce
increeses in the vnriatfor, of force ~ ~ g i t iacceleration l
in the nOPi a1 center-of -gravity range and in the
v~irtinn of L'nrce -7er g wit21 censer-of-gravity
location. This com:~arison as well as that for straight
fli~i~t Tvvould alsc be influehcad by any ulf'ferer,ces in
confrol-surf ace cons truccion,
Agree~fient 5n the curves of elevator-free neutral
aoint aga'nst airs-oeed (fig. 10(c) )is rather ?oar mi;
becoiies worse PS the sneed increases. The flight-test
e1svato~-free neutPal .,?oint Roves railidly reareard
with increasing s~eed, 2nd st 1-1.igh speeds tiLe cir9lm.e
a*,i3ears ;i:ore s tr.51~ with elevator free tl~ar. wltb levato tor
fixed. It is believed t32t th;~ lapse reerwz:-d sk4ift
in tke elevator-free neutral point sith increasin~ airsgeed
clay be a result of the faSric distortion.
In general, the present correlation indicates that
su.ccessful ?rediction of elevr.tor control-farce characteristics
from wind-tunnel data can he rna.de only if
ex*cx?e care is used in rerresenting closely the airplane
i.n its constructj.cn fcrm - particularly sith regard
to. tne cont~ol surf aces. Agree~lient ~ 5th flight
rtieasureicerzts aight also be improved considerably if
-. eftects such as fabric -di~.+fidf~ri co~ald be taker, into
- acsalmt,. A rrio-e beneficial solution, b.owever, would
be to r~in.hnize these effects in the &~nstruction of
the al.rpla~e.

La.te~ai 3tabi.lity and Control
Steady .s.ideslip characteristFcs. - Cheracteristics
r
of thG1"rplane in. steady sideslips, which are used
as flight-test r.easures of directional stability, ..
direct1 onal control, dihedral eff sct, side-f orce
chara.cteristi cs, and pitchirlg clonlent due tc sideslip,
are skiotvn in figure lQ.. Although con;?lete hinge-nornent
data for the model ail3rons md elevator v~e-e not
obtained in sideslip, aileron forces in sideslip were
estlmated from the tu.nne1 data by taking into account
the change in effective angle of attack due to sideslip
but a-ssiming no direct chakge in aileron hinge-rr~oment
characteristic.^ with sides1i;s.
For both idling a15 rsted-power flight with flaps
retracted, f igure 1b si=ows excellsnt a.grnement in the
variation of control se';tings, afiglc-: of Sank, and rudder
force with sidesl.ip, elt'nouzh corm driffel.ence eriists
in absolute values. Some of the difference in absolute
values mag be due to the fact that nod el tare tests
were not made in sideslip. It is especTally interesting
to note the close agreemo~t Ln the variation of aileron
crrgle with sideslip, which serves 9.3 ti flight-test
indicatf or; of dihedral effect. ' It xas round in the
flight tests .that the airplan3 ~iing En norrna.1 flight
a??eared to bend u3ward noticeably with respect to its
-position at rest. Despite the wing bending, however, the
anount of effective dihedral determined from flight
tests was also found to be no greater than that which
would ordinarily be exyected for an sir?la.ne of this
t:ype with 4.5' of geonetric dihedral. Analysis of the
elastic properties of .tile model wing under load indicates
that the ~iodel v~irig bending was neglizible. 3n t0.e basis
of the agreement between xxodel and afrplme results,
it a,-ipea.rs that tha observed airplme win^ bending nay
have had very little effect in increasing the dfhedsal
effect beyond the normal a~lount for 4.$O of geometric
dihedral. Fu~ther ir,form;ltion regarding tl3.e ela.stlc
pro2erties of the air2plane wing znd the effects of
these 32roperties would have been desireble but wzs
not ava.$l~.ble. Com~arfson of the flight and tunnel
aileron-force curves e?.pears to indicate that little
error was introduced in deternifnation of the latter by
the assWi3ti on that aileron hinge-moment characteris tics
rernainsd unaffected by sideslip. The sideslip characteristics
vtitii flaL2s deflected do not agree as closely

NACA Al?R No. L5Hlla
as do the flaps-retracted cbaracteri.stics, particularly
in the case of the aileron-deflecti on rudder-Porce
varf etions. The flight-test rudder forces s;iov? a tendgncy
tovard reversal in figur~ 14(c) but do not actually
reverse as in the case of the model forces. At an
airspeed slightly lower t.km that for which! the data
aye presented, however, r~dder-force reversal did appsar
fn the flfght tests in this wzve-off condition.
Dihed-ral effect with flaps def lecbed and rated power
at low speed appears s0mewha.t lower in the tunnel
:ne zsure:r~ents than ir, the f liglit data. The flap def lec ticn.,
nowever, was 5C greater on the model than on the air-
nlane.
In figure 15, rudder hinge-moment characteristics
estinia.ted from flight-test rudder kicks are conlpared
wi th rudder hfnge-moment characteris tics rzeasured in
the tunnel tests wltk flaps retracted. Although the
model. rudder hinge-moiaent 2nd force results are foia an
unsealed rudder aid are also sub jec$ to effects of small
surf ace and trailing-edge irregularities 2s in the case
of the elevator results, a.greement in this respect is
good. As ~reviously shown in figure lk, the rudder
forces lin steedg sideslip are In good aereement for
thls flap condftion. In regard to rudder hinge mol~ients,
the tunnel results which slnoivzd no aositfve values of
the parsrneter zh,?? a for the rudder, indicated that no
rudder snaking would occur in flight. This indication
was confirmed in the flight tests.
AEleron chsracterfstics. - No tu~nsl tests were made
to investigate eileron characteristics for the 3 8 tab
linksge wltli which the air.:?lar~e wa.s tested. If, liowever,
linear tab eFfectiveness is assumed, these ciiaracteri sttcs
for the flaps-retracted condition cm be estimated from
the results of tunnel tests of the plain ailerons and
the ailerons with a l:l balancing-tab ratio. Estimates
of control force and helix angle made in this nmner
are comi3ared with flight measurements ir1 figure 16 for
inufcated airspeeds of 135 and 383 miles per hour.
As recoimilended in reference 2, helix angles vvere
estimated as - pb - 0.8~~
, where CZ is the total aileron
2v
ZP
rolling-moment coefficient and a value of 0.57 was
used aa the damping-moment coefficient C Although
Z~ '
11

12 NACA ARR No. L5Hlla
the mgles of attack selected for these estimates
corres;~ond to rated-power flight at the appro~riate
speeds, the model aileron data wsre obtained in Foweroff
static tests. Inasriiuch as the tunnel measurements
were made for right rolls only, the. tunnel estimates
are exactly syrmetrical fcr right and left rolls,
whereas the fli3ht results are not. Agree~ent in the
curves of !lelix argle is excellent in the range bv1

For the subject alrnlme; l~!hicl~ exkiibited essentla11~
constz.nt 910329, %he fvnree irethods or" corr,:2utatloz baszd
on y~lind- t?jnnel pasults 2:;3-:3e a? to L riij-3 very s il;lilap
results w i t 3 res2ect 'to xa;c4.;,;u.:: sfdeslfi-, a;ri:-le, all 05'
:I?I ;,ich
L~ - a.Fe z,.-,~~oxir!e t3l.y. b0 hig5.e~ t:17,;1.:& the f If pht-tes t
~rai~ne. bong tlie factors ~>ossibl:[ c.o:i-LribmtLn.2 to +' -ie
lee? of ,-erfect agreel3er.t is t:!-Li-ie 2.iTferensa tet:~~irsen tile
n
instault8.zsous eo:~trcll del'lsctio'rl ass~c~ed , o- the ccli;p:lte.tions
azd t,hs actuel control .iiove;~3nt in the flight
test. Another rector infl~enci;i..- t.::~ results xay be the
c!l?ent?.e Pn :lorrr?al ~.ccaler;;t,fan exrerl.snced by trle eii-plry;~
in fts poll out of t?ie turn, Alt3.ou:c;h no flig,ht record
of nornel acceleration v!es o%tai.rled Tor the test in
qu~-estion, sl:nilar. f1i.r;-.t-test
c ..JJ res1:lts iadlcate i;:?.at a
considerable varPst.ion r~a~7 heve occu.rred during the
;Yaceuver. Analysis fnc?.iccwt tix2t trle change in norxal
acce1ers:;ion ~nd, col?seque:riti:;, Iff -t; eoei'ficlent ma.7
intro5uce sonditioas conuideraS1y different from those
considgred i.n the t,%eoretical. calcui?.tFono.
li simqle static estil::ati= 05' the mount of rudder
deflection required to rnpintain zaro sldesi~-~ . in en
aileron roll was ~ade 2s su;;~ested in ~eference 2; that
- .
is, it b~jnjas ar.su;ned thqt tile 2esll-e:I yuddsr deflection
would be thzt required to c,o?.mtarect tk13 combination or'
af leyoli adver s s -yawing ~oicen t a.2.d p:.anrinp: a nioxent due to
o l l n Ths estirriated va1-a~ oSta2ced by this rLet,hod :-;as
;;":;,proxi~r~atelp 8" for fLa~p>s-~etr~cted f lirii1-t -- ?.;rit?l levelf'lfsht
poi:vep s.$ en flid-jce-te,5. eiysunr- ,?: of i!:.'j i7.,iles ;?e$
4 .
;lour. Alt3ou55 no f lP

14
MAC A ARR No. L5Blla
0,2375-scale powered model of the Douglas XA-26 airplane
have been compared wlth results of flight tests of a
Douglas A-26~ airplane,
The significant results of the comparison may be
summarized as ToZlows :
I. Good correlation was obtained regarding elevator-
fixed. neutrai points and the variation of elevator
deflection ir: both stralght and turning flight except at
speeds approaching the stall. At these low speeds
the airplane showed a distinct improvement in stability
not Ind.lcated bg the xiode1 kcsts, The differonce was
attributed to the fact that the pronounced stalling a-t;
the root of the p20duction ail>plane vting did not take
place cn the smooth, well-faired model wing,
2. Thc variatl.ons 3f 8levztor ~olitrol force with
airspeed and acceleration wcro not in good agreement,
Although some discrepancy was introduced by the absence
of a seal on the mods1 elovator and by small diPferences
in absolute values of ele;i..ztor doflcctbon, the correlation
in control-force chzracteristica was also
influenced by the effocts of.' fabric distorticn at high
speeds and by small con.atr.ucStion. c?issimi?.arities such
as differences in trailing-cdge angle.
3. Elevator tab effectiveness as determined from
t~anel. data was in good agresmcnt wfth Plight-test ta.b
sffectfveness.
It.
Agreement in both rudder-fixed and rudder-free
static direckional stab2li.t~~ was good except in the
wave-off conditi-on, in which "iho model tests ind2cated
rudder-force reversal at a higher speed than the flight
tests.
5. Model arid airplam indications of stick-
fixed and stick-free dihedral effect were in good
agreenzent , although son6 slight difference in geometric
dillecral ma;y have exigtad because of wizg bending in
Flight, The use of' model hinge-moment data obtained at
zero sideslip appeared to be satisfactory for the
Getermination of aileron forces in sideslip.
6, Fairly good correlation in aileron effectivenesb
and control forces was obtafned, Fabric distortion was

SelFeved res,)onsible to some extent for higher fl;ght
values of' aileron force at high sneeds.
7. Estfr;ia.tion of sfdesli:?! ~i1eveloy)ed in EZL zbru:?t
aileron roll ws.s fair, but det,er~ninatPon of the ::iaxfii?ur;
rlxiLder deflection required to zaint ain zero sideslip
5.n ac. abrc.?t roll was not entirely satisfactory.
Oil the basis of these f inlings, it aL3?ears that
2cree:nent between stability md control cha.ractei?istics
estiir:ai;ed fi-oi wind-tunnel rezults arid those iiie~.s;~recl
in f'll,yht cannot be comm31eteQr- satisfac-Lory w:iess
e4rZ;airl factors now usually neglec.i;ed in 1v;vfnd-tunnel
t?stlns cp-9 be taxer! fiifvo co~si5eration. These factcrs
invclvs s;~,all differsnces betvieen t.he ~~iodel pad the
air:~lene ~nd Include differences iri elastic :2roperties,
surf ace finish, and construction accuracy. These f'act,2rs
* ,
sbould be considered, i.t po::stbla, in futurs inve gtigations.
Lar~gleg b:einori~l Aerori~utf
cal LaSoratorg
National AdvTsory Coi:,li:f ttee for Aelonautics
Langley Field, Va.

3ACA ARR No. LrjHlla
1. '*:atl,?sws, Charles V.: -An baiytical InvestigatLgn of
the Tffects of Elsv2tor-Fabric Gistortion on the
Longitudinal Stability and Control of an AI~3lane.
FACA ACR lie. .&E~o, lfk.
2. Kzyten, Gerald G. : A~alysis of urind-Ta!nel atability
and Control Tests in Terns of Flying cdalfties of
Full-Scale Airplanes. M,4CA ATE Eo. 3522, 19ic3.
5 Jones, Robert T. : k Slrr,pfif ied Applicstion of the
Yetho2 of Overators to the Calculation of Gis turSe -1
:.'otions of an Air~lane. NA!ZA Re3. Xo. 560, 1556-
4. iJlolowi.c~, Chester H.: Prediction of I,lotiolis of ar-,
Ai r~lane Res:~l-t Png r'roix kbru.7 t ;?iovzlnen t of Lateral
cr Directional Controls. NACA AR2 >To. L5EO2, l?ki5.

Sweepback of L.E., deg
Incidence, root, deg
Incidence, tip, deg
Dihedral, deg
Airfoil section, root
Airfoil section, tip
Wing flaps (double slotted) :
Area (behind hinge line), sq ft
Horizontal tail:
Area, including fuselage, sq ft
Incidence, deg
Dihedral, deg
Elevator area (behind hinge line), eq ft
Balance area, sq ft
Trimming-tab area, total, sq f t
Distance elevator hinge line to
25 percent M.A.C. of wing, ft
Vertical tail:
Area (excluding dorsal), sq ft
Rudder area (behind hinge line), sq ft
Trimming-tab area, sq ft
Height above top of fuselage, ft
TABLE I
GENERAL DIMENSIONS AND SPECIFICATIONS
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS

CA ARR No. L5Hlla Fig. la

NACA ARR No. L5Hlla Fig, lb

NACA ARR No. L5Hlla
Fig. 2a

Fig. 2b NACA ARR No. L5Hlla

NACA ARR No. L5Hlla Fig, 3a

Fig. 3b
NACA ARR N

NACA ARR No. L5Hlla
Fig. 3c

Fig. 3d NACA ARR No. L5Hlla

NACA ARR No. LSHlla
' /n d~ca fed airspeed, K, mph
6qure 4 - Var~ation of ele va for trlm-tab
effectiveness with ampee d .
Fig, 4

Fig. 5 NACA ARR No. L5Hlla

NACA ARR No. L5Hlla Fig, 6

Fig. 7 NACA ARR No. L5Hlla
Cen ter-o f-qrav~ fy /ocaf/on, percent MA. C.
figure 7 - l/arlcut/on of eleva fw con fro/- force cmd def/ecf/on
qf ~ci/ents w/fh cenrer-of-qmw fy /ocot/o/7. =260 rn//es
per hour of 10, 000- foo t a/f/f~de ; df=O; rofed powwj
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS

NACA ARR No. L5Hlla
Turbulent
S talied
V
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS
Fig. 8
Figure 8.- Diagrams of stall progression in the gliding oondition.
Engines idling; flaps and landing gear up; cowl flaps closed3
oil cooler one-half open ; Douglas A-26B airplane.

Fig, 9 NACA ARR No, L5Hlla
n
j cross f/owin
fbe dkct~on sfu#e Cofnp/efe/y d
of or-fo ws
n
NATIONAL ADVISORY
CUMMIiiEt FOR AERONKUTICS
figure 9, - Bwer -offsf@//
d/Ugriin?S forthe 0.2375 -sca/e mode/ of the XA -26 o/rp/ane.
Si'ondafd mode/ confi3ufot/on w/fh a/fp/une 011-coo/er ducts, Reyno/ds nu&, 4.25 x /O ;
Mach number, 0./3/; Sf = 09

NACA ARR No. L5Hlla Fig. 10a

Fig. lob NACA ARR No. L5Hlla

NACA ARR No, L5Hlla Fig. 10c

Fig, lla NACA ARR No. L5Hlla
/nd/ccufed wlrspeed, &; mph
(cl) ficyp~ r-efrcuc7ed; ra fed power.
bqure l'/ - hr~lat/on
of elevufor con71-d force! w/fh
ind~ccrfed mrspeed. Model elevator and tab deflec-
Porn iden flccrl w/th fllqh 7- fes f set t/ncys.

NACA ARR No. L5Hlla Fig, llb
- %
(b)
/OO /40 /80 220 260 300
/nd/cofed o~rspeed, 6, mmph
flaps retracted; 75,oercent rafed poweE
fiqure / 1. - Con flnued.

Fig. llc
/ \ r\
V
,
/
1 - 0
NACA ARR NO. L5Hlla
---
---
A
V
/'/
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS

NACA ARR No. L5Hlla Fig. 12
I
Y = 360 mph
&=370 mph
Gi d 28 percent ML -
No- load h&ic tens~on, 2.7 /b
--
--- -
4 = 270 mph
- Sectlon under no load
--___ Section ln fltghf
Elevator ~ecfm
822 in. from center //m
of mrplune
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS
Figure /'. -€/e vo for- fahc d~forilon at
various indcafed alrsped, Doug/as A26 B
a/rp/me wl f i cen fer of gravity a f 32 perrent
A C excepf where nofed.
r,

Cen~-er-of-qrav/ty /ocaf~on,percenf MA.C.
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS
FTgure 13-Vcrrlaf/on of e/evofor confro/-force gr~d/enf w/th
center-of- grwv/fq /ocat./on esf/m w fe d for sea/ed md unseded
e/evotors.Lf=260m//e~ per hour at /O, 000-foof u/f/tude;
df = 0 7 rofed power; s feady furn/r/y f//yh T:

NACA ARR No. L5Hlla
Fig. 14a
Control Tunne/
Rudder - --
E/evufor
A~kron -
(total)
NATIONAL ADVISORY
COMMITTEE FOR AERONAUTICS
Leff &de//p ang/e?deg t?/ghf
(a) &s re fructedj rafea' ,oo wer; & =/4/m//er per hou~

Fig. 14b NACA ARR No. L5Hlla
20
9
8 o
Q k
j- 8
\I I0
Gf
0" 20
,.h
c P lo
2 Q
figure 14. -Cont~nued.
Control
Turn/
0 Rud&r ---
0 Elevafor---
El A//eron ---
(t.ta/)
(b) naps re frac f ed; T, =O ; @ =/33m//es pw hour.

NACA ARR No., L5Hlla Fig. 14c
Con f ro/
Fl/qh t Tunne/
O Rudder - --
0 Elevator - - -
El A//eron -
(tot 01)
Tunnel
A fl~ghf
NATIONAL ADVISORY
COMMITTEE FOR A€RONAUTICS
(c) flaps deflected; fated power; &=/I/ miles per hour.
fiqure 14.- Concluded.

Lef~ Rlghf Left Rlq-ht
chmqe ~n rudder de flec f.on, Chanqe ln J/~QJ//,o onq/e,
deg ,AT, ADVtSORY
COUUITTEE FOR AERONAUTlCs
Fqur e - l/ar/lof/on of rudder hlnqe-momen f coefmenf w/th rudder de flec f~on
and angle of s/des/lp of 1/;. =/40 m//es per hour. F/ops refrac fed. -

NACA ARR No. L5Hlla Fig, 16
Left Change /n toto/ a~leron anq/e, deq R/gnt
figure /6. - Varlcr7"lon of a~leron
wheel i'orce and be//x
mgle pb/ZV w/fh change ln 70 fa/ wl/ef-on ang/e /r,
ro//s wi fh rudder fixed, flaps re trwc fed, and rcyfed
power. NATIONAL ADvlsoRY
COMMITTEE FOR AERONAUTICS

Fig. 17 NACA ARR No. L5Hlla
-- -
---- Reference 2
Zme, ~ e c NATIONAL ADVISORY
COHHITTEE FOR AERONAUTICS
Flgur e / Z - Rol//nq ve/oc/ tl/ and s/des//p dur~nq. ader on
ro// OUT of 30° banked turn. 6.=0°; 1/;-=/45m//es per
hour uf/O, QOO-fwt a/t/fude ; /eve/ f//ghf poweE