Requirements for satisfactory flying qualities of airplanes

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NATIONAL ADVISORY COMMIPEE FOR AERONA$971CS
ORIGINALLY ISSUED
April 1941 as
Advance Confidential Report
REQ,mmTSF OR SATISFPCTORY FLYISG QUALITIES
OF P-rnLANES
By R. R. G1lrut.h
Langley Menorial Aeronautical ktboratorlg
Langley Field, Va.
EXQUIXE'riJZTS FOX SAT ISE'ACTOZT FL'YIBTG QU.iLIT IES
OF A1 B3LB:YES
3- 3. 2. Cilruth
Tke need for quaxtitati-;.e desi.3~c riterions for de-
s c r i b i ~ ytb ose qualitiss of 2.3. a?--glane t h a h a k e up' 33%-
i sfe.ctorp costrolla%i'i9tg, stabilit:?, aEd Lazdlins; charac-
t e r i s t i c s , has been rezlize3 for several sezrs. S o ~ e t i n e
a<o, preliminrry stud.ies s'.lor.reC that adequste data Ccr tLe
formulation 05 these criterio~lsn ere so'; ar;s.ilzble ?.~d
that a large ar?o.unt of pyelin i;..,arg .~or!l: TOU-ldh ave to be
done i n order to obtain the i.cfor:zz-.;ion necessary. It r:ras
apsnrent that f l i ~ h tte sts of tkn flyiny qualities of nu-
merous airpla-c.es :.rere rot;u.irec?, i~ ord.er t o ~ r 0 ~' 5 d?. ef uz~d
of qusatitstive data for correlstf -r,-5th pilots opinioas.
Accordinql:~, n pro.<raz :-ins iasJ;itutcd mL:=ich covered
the vnrious phcses of ;:o~;: recnirei. Tho f i r s t ste? in-
v o l ~ e dt 2e developnoct n : t e s t ?rcce&re a::& test equip-
nenS ahich mould csayura the ckz,raztcristrcs or e?lic% fl:*-
in? qualltie s depend. This phase of the 7:;oril: 5s re3ortsC
i n refeereace 1 , altho~.?S.s izce tkat tize the test nroce-
dure has Sees expan&e&s ad no6-ffie,?Lo c tke basis o: eddi-
tionaZ ex~2erie;?cea :ll,d se-reral cl;aa";es have beea made i n
the equig~~enuts ed.
Another phase 05 the i ~ v e s tyfa tion has involved the
measnrement of the flying qualities of a number of air-
plaees. The procedure used. bas 'jeen i n %ej._eralI n accord
~ i t ht5a t described i-. referent.: 1, kt the present t.iae,
com2lete t e s t s of t 3 i s nat-jre >ave bsen nade of 16 sir-
planes of varied types. T3ese a,irplanes nere :?ade avail-
able larqeiy by tbln e Arng and =ore recectly by ?rivets COX-
panies at the request of the @iv:l Aeroxiautics 3oard. In
ad-dition, flgiiiq-oualities d ~ t aof moye linited scope hare
bsec obtaIne6 fron t i z e to tine oc a cunber of other aSr-
?la.nes, ths t e s t s of ~ L i c hc overed 0x1:: garticular itens
of s t a j i l i t g acd control 5ut ch, ne-iertkelcss, augnent
the fund 02' data nay al;a4l,able.
a
third p'il_ase of the fnves.fi<atiom, one ~ k i c l la lso
3as bsC?n purzusd tkrougLou$ the duration of tke project,
Las i n v o l v ~ dt he ei:al:,rsis of avaI'able data to deterxine
n h ~ t~ czsv-redc haracteristics ~.vere s i ~ z ~ i f i c r nItn G.efiain4
a
sztisfactor:~f lyin? qu-a]-ities, xThat characteristics it aas
reasonable to recuise of ac aiiqlanc, and chat influence
tk.3 variozs dasign features had on zhe observed. flyin%
qualities.
In ozder to cowr t3i.s ~ o r kad equatel:~, n ounber o?
Dagcrr dealinq sepsrctel?~a i t h the -7arious itcns of stabiL-
i t3i- ?nd coctrol are uccessnry. Several sucb papers have
been prep7,rcd or arc i n ?reparation zt t i e present tine.
Deteiled studies of all items 1~111re quire considerable
tine for completion, but it i s jelieved that the coaclu-
siozs reac2ed to date ere conplete en~I'L$h to aarrant a
rovis53n of t i e tentative spezilications set forth i n ref-
erence 1. As opportunit;r 3oi: sdditionzl n ~ a 1 : ~ ~oicsc urs,
i t mould be desirable to cower the in&i*ric%ualr. equire~ents
xt nore lcn<th than is ~ ~ o s s i b lpaet this tfne. As a re-
sult of furtLor studies, it nap .slso be desirable to re-
7rLse ?,$c.La the flyin$-quclities syecificc$ions 9;iven here,
In adaition t o the ~ c t u a ls pecificsfions, the chief
reasors -~ehi-ll_ctlk e q-r~scific:~i,t.lons? re c:,Pocusse?. T-xere~er
possible, iaterpretntion of tke specifics-;fox i s nade i n
terns of the desi~nf eztures owthe cirglzce unless the
subject Ts corered ir repcrts 02 reference. +
In lorzmlatinq the s ~ e c iifc atf oos, every atLenrt h ~ s
been nade to define the requirei! ckaractcrisfics i n ensilp
3ensu.rcbie, yet fun3.nnen$p,l tcrcs. It TTF.~n ecsssarg 50
consiler rulL stability ~ n d .c ontrol requirczesf;~i, r arriv-
1x1.: at each ia&ivi&-c?nCl tem bccnuse of t i e varied. Cunc-
tions of the ic&i~?iduzclc ntsois snd the conflictin9 na-
ture of mn:T of these functions.
The r2ecificetions require chsrnctcristics t3nt have
bein c7uoxonstratcd to bc cssenticl ?or re~~sonabl-sra fe an&
cffZcient oporatiozl of ni? airplane. They 50 ns f a r toward
requiring ide?*l chzracteristics 2 s ~ r e s o n td esi$n nethods
r i L i pernit. Conpiizoce mf th the specifications sLould
insure satisfactory flgir5 qu-lities on the basis of pres-
cnt stznd~~rdszl, thou$h ~ , asd ditioa,?l knovledqe i s obtained
it nap be possible t o denrnd c closer qpgro2ch t o i d c ~ l
characteristics without f n zny Fay pens22zin$ the essen-
ti%l itens of perzornnnce,
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These requirements pertain t o all f l i g h t conditions
i n which the airplane may be flown i n normal or emergency
oparation, w i t h the center of gsavity at any point within
the placarded limits, Some of the specifications are
based on the behavior of the airplane at some specified
air spoed, Tho air spood in such cases shall be takcn
a s the indicated air speed, Vhere minimum air speed i s '
referred to, unless otherwise stated, it shall be taken
as tho minimum air s p ~ e do btainable w i t h flaps down,
power off ,
Vith the exception of part I11 of the requirements,
which deals exclusively w i t h characteristics at or c3oso
t o the stall, the roquircments pertain t o behavior of tho
airplane i n the rsnge of normal flight speeds at anglos
of attack below that at which the s t a l l would occur,
In the specifications which follom, tho lower limits
of the control-forca gradients are specified i n terms of
the a b i l i t y of the cont~ol-st o return t o t r i m positions
upon release from doflscted positions. This i s a very de-
sirable characteristic bocause f t assures a control fric-
tio~ns ufficiently low i n comparison w i t h the aoroclynamic
forces t o allow tho pilot t o f e e l the aerodynamic forces
on the controls. Eonevor, some addltional interpretation
of the specifications is nccossasy, because no control
system can be mado entirely free of friction and, there-
fore, there n i l 1 alnays bo some small deviation from so-
turn t o absoluto t r i m , At the present time, it is not
possible t o , f i x the allowable l i m i t s for thctsa dovfatioos,
It is known, howevzr, that controls rossonsbby free from
friction, as mcasurod on the ground, have satisfactory
salf-centoring characteristics i n the air as long as there
is a drofinitc forcc grnd.iont, For elevators, force gradi-
ents as lorn as 0,05 pound per milo per hour have been sat-
isfactory when the friction was small, For relatively
small aisplancs such a s fighters, trainers, and light air-
planes, it appears that about 2 pounds of friction i n the
elevatos control system and 1 pound i n the aileron repre-
sent an upper l i m i t , In several cases, mherc push-pull
rods w i t h ball bearings wero used throughout the control
system, friction i n both olevstor and aileron systoms has
been found t o be under 112 pound,
For large airplanes not intended t o msneuver where
visual o r instrument references are always available, self-
centering charac'tc?ristics aro not believed t o be essential,
although they are very desirable. In these airplancs,
control friction s3ould be kept zs i n n ?,s possible, ?,1-
t%ou<h there i s ind.icatioa that co~sidsrtt51:?n ore f T ~ C ~ ~ O Z
csn be tolcrzted. A representative ncouat of c o ~ t r o lf ric-
tion for a trnnsgort or ce+Liunj enbsr aould be about 10
G pounds ir t2e elevator systen 6 poxids io the 2.ilerors.
C-
Cd
I
14 Irreversible c ~ n t r c l sh e ~ eso ne-,~>ats imilar c3arac-
t e r i s t i c s ,t o controls v i t h hiqh friction; thet i s , t h e y
a r e aot self-centerirs 2nd therefore tend t c destroy con-
t r o l feel, They are ntjt co;-zsidered aasirable, sithou"; 02
very large cLrpla@es :vilere the rates of devls.tioa from
steady f1Lq;ht are sloa they kzve been used successfull2: on
~ l l e r o z s ,
I, Bequirecents l o r soa?ltu&inal Stabilf ty and Contrcl
-R-e-a-v irozezt (1-A! : Character%s tic s of ~ n c a nyto lled
--..,.. ---.-----
-3 onqiLudi.iaL riot lor,,
Then elevator confi*cl i s deflected and released quick-
1I ?, tLe subsequ-est variat.ion of ~ ~ o r n azclc eieratioc and
elevator r,nqle shov-12. h<?,:;ve cc.r~Letel:c~?i sap;2eared a f t e r
oze cycle.
. e n s o e TLe requirement s;7cc-
x1a.1 es tLe degree of dam-u ing required o? the shoi-t-geriod
I,on?itu&inaL oscillation nith controls free. A 2l~;h.d e<ree
of dzrngi;n? i s r e q - ~ L ~ eb&ec ause of t h e short 9erTod cf tbe
notion, T i t i s,j,rn:anes hayin? less &zx?iaq then that spec-
ITied, the oscillation i s. ~::;rcite?. bar ~ u s t sac centilatin~;
their ezf ect a,--& 3-c dusicg uns2,ti sfactorp TOUT>-air char-
acteristics, The ratio of coztrol fric5ion t o air forczs
i s the3 da~pir,g5 s generally reCiuce0 at 3.i<1., szeeds,
B3c.,n tLe ,osci?la";oon ha.s ag;?eared &t hi:7;h s?eeC?s .%s i n
dives a ~ ddiv e pu-ll-cats it nas, of course, 7err 335ectF021-
able be.cause of the accelerations in~olved.
The short-period ~ s c i l l aifc ns involve ~ a r i a t i o n so ?
the zi:~Le of a t t z c ] ~8 % essentiallsr constant speed and
siocld not be ccnfused a i t h the nell-knoan lonq-~erfod
(phuqoid) oscil.iation, ,,v>.ich involves variation of speed
at an essentially constsnt anyie of attack. ks shown 33~
tha t e s t s of reference 2, the characteristics of t l ~ ela t-
t e r node of lon$itudina,l notion ha& nc correlation ~ 5 t h
the ability of pilots to f l y an air;pla:rle effioiectL3~,t he
i ~ a g;2e rioi3. of the oscillation acking the d e ~ r e eo f dampin?
unim~ortaat, Subseqiler_t tesf,s in~ren ot 2,ltered t h i s conclu-
sion, Tie case of pure longitudinal Civergence of t3e air-
plane (static instabilit;,~)m i l l be covered later undar
req~rirexents0 2 the eleva$cr control i n steady flisht. ?TO
req-s-iren2nt f s r dr,nntl:~ of tie long-period n!~uo;oid cotion
,?ppe~~rjsus t:: ?.p-blc $: 33e pres~ntt ixe.
3, olli;i;:c o r s:~.:;incs del.i-Jerin;i; po;-rar f o r
levcz ,fliyk.$n i t i ?laps darn (zn ?;scc? ir; :ar;5-inz; ap-
-.
grcacs7.%, , ian$-fi;q gear donl, 3% 212, sp~c&Fs, -CO-TC the
stall,
?>+c uar<.a,tio;~o f elevator contz7ol ? ~ r c o* L7i%l%
5L ,
speed. ::10ulii be such p u l l Zcrcc?s 2.r~ ~ r : q u i r e da % ail
s;-: lecd-. s >clor? tkt; ti-:;: speed. 226 ^i.) v,~>f orces roqv-ired at a l l
.
sl2se&s abova t%c $rin s.-: ; ~ f,o~r ~z l~e c ~ ~ ~ ~ ~rct qfufcriln:q ~
S?t CG,, I & stsbilf,<gf zl iten 1,
C r,nL -C ::a,<zit~dn 05 L,:i;.-a e:e.rator cou-krol force s3.0uld
,,- -v? n,q ,-cCh' os re 32 sy-fficdl .* cnt to return the cos5ro2 t o i t s t r i n
p o c i t i ~ z ,
4.
positive static s t ~ b i l i t yf or flight conditions i n which
the airplaoe is flown for protracted lengths of time, or
where opportunity exists t o establish a t r i m speed so that
'3 stable characteristics can be realized. Positive static
,-- stability i s not consiclered particularly helpful t o a
21
'8 pilot a t very lcw speeds ~ i t fhu l l power on or with flaps
4 extended with f u l l power on, because of the large t r i m
changes due t o pomer usually experienced, The conditions
arc classed as emergency conditions because in actual
opcrn.tion they are entored suddonly from approach eondi-
Cions, where relativaly l i t t l e power is used, In thoso
cases the elevator forco and position changes, due t o
applied power and change of flap setting, aro usually
far groster than any inhorent stable or unstable forco or
position gradients nhich exist due t o the degree of s t a t i c
stability present. For these reasons, s t a t i c stability 5.n
these conditions i s not considcrod assential, at least not
until trim changes due t o power aro reduced t o much lowor
values than are experienced at tho prosant time. The mag-
nitude of allowable tria change due t o poner and flaps is
covered later in Requirement (5-F),
In other conditions of flight, however, s t s t i c stabil-
i t y is regardsd as an essential flight characteristic.
Item 1 pertai2s t o the ol~vator-fixed condition. This re-
quiremcnt insures that the airplane w i l l remain at a given
angle of attack or a i r speed as long as the clevstor is
not moved, and provided that qisturbed motion of tho air-
plane is not left uncontrolled for long periods of tfrne.
Positive stabf l i t y eliminates the need for constant con-
t x o l rnanipulatf on in maintaining given conditions and,
furthernore, simplifies the control manipulation mhon a
spnod chnagc is desired, because tho disection of control
mosanent required t o start tho rotation in pitch corro-
sponds to that required t o t r i m at the new sngle of attack,
A negative slope t o the elevator-angle curve is a necessasy
requirement for clcvator control feel, and thc dogree of
control Pool increases as the variation of elevator angle
mith angle of attack is increased (reference 3), In gan-
eral, it may be said that tho variation of elevator angle
~ i t ahn gle of qt tack should be negative and a s l-trgo na-
rnerically a s is consistant gith other sequirsments of el-
evator ccntrol.
Item 2 requircs tbst the olevator-free static longi-
tudinal stability shall always be positive. This spocifi-
cation insures thst the airplane ail1 not depart froa a
t r i m si-2eed except as a ~ o s u l tc f definite 3,ction on tho
part of the pilot,
Iterl Z rcq-nires tbnt t5e zLcvn%or cc:?trol ba solf-
,ecntcrin~);-a, e.3.-:,rr~c'tc:istic r~!iich i s ncceasarg foz: the ct-
t ?..ln;lcnt os" soct rol Zool,
-
m i.c,zus~af ox- Ltez fL i s 0%-j.fous.
-
2e-o2 AT-'r zS-gg~;~~.~-&grA, ~d~e-tn~.~llgc.d r3c~1;,;siso f $ 3 ~
st?,tTc lon~;itu.3.il=?,ls tn,biif .t,g c3nrlctcri sties of y;:l?ious
ni~-ol?,nccF brd in$luercc of 7;n-r i~us b e z i ~ nf ez?.turec 01
the obserrea charic";rf i n ~jei? iic ref erecce 4,
3). :
e : SLzracteristics c? the ele~rtzo r
con5rol fn acaelerated fligkt.
. .
I, . 3 ; ~-2 se of e'e-:ator c03'brOl ~ ~ I o n eLC, . ~ 2 0 ~ 1 6
..
5e p o s s z ~ l e develoi? either t4-,e a l i o ~ z b l el ord factor
$0
o r f3.e ::z:;frLur LiTt coefficfenf ~~te ver;? speed,
. .
2 Fho T;zriztio2 of elesator sn%le yztn c o r ~ 9 1
8C-
celt2?atfo:l il o",,%d- tur~lic<f; fgll.F, 2% 2~:;-, ;laex s+z ~ed,,,
sko'uld be a acooth c:Jrye ;.:jl7_i~:2.e ~er?~ry:herhea :: L?. stable
slope, .
5, Tke <sa$.iect of elevator ccntz-92. lorce. iz ~3ou3ds
pzr uait corsal ncco;eratlon: ZLS ~easurcd5.: : stead7 turn-
i?" <li$h+, shoi~>db e zi%hin fo>loyia?; lj.ai$s:
,,.$
3, F9r :- -jursuit ";qe~, ths :;raZient s2loul3. 3e
less $kzn 6 poands 2er ?.
c, 12o r an:? nirplnze, 5% should reoni:e 2" sf eadp
pull force of not loss tban XO z?olJ.n?Ls to c b t ~ ~ lthne
s.llo~.~nblLro n& factor.
% e ~ + s o ~X~eq~-aoirer x e~t( I-C) : Item 1 o? :his sycci-
f i cz<i_on rec;:3.ire tll,zt suf T ic iert elsyct o r colltrol s ~ ~ o5- T - ~
be .yLynilsjle t execqt e gsneiJyers of the ninin-uiz rn5ius
a
P- inhsrsnt 5 3 fjhs aeroclg.canic 226 ~ t r - ~ ~ t . cLrecsLl. 7;~of t.he
n., Il nirpl3nea Since the curvature of tLe fiivht ?nth is di-
i
1-2 rectlp seL?.te& to the nor:z~~al cceler~~tioni,t i s obvious
thst zttainment 0: either the 02uximum lift coefficiellt
or t h e r,lla\.rable load fzctor i s tke licitiaf co,rditioxc
Iter: 3 specifies *l..,e .-wr:ox:nt of stabilLt;i: requ-ired 03
.
nil :,xy;J-,~ cne. !.:zzich ::=st - 2 ~T .32cv-ljcreC :st or close to TLZ:~:L-
152% viS>cat yescrt t o -jisuzdl or i2skrunent refer-
c-~7-n
ci?ces, It ;?-s Sean &enons?,r-tefi -3:~ t ~ s t sof serernl pxr-
i;ui-i; ,:.il-:g.ly :es thG?-tl o ru3c .i- +- -a2bifi~,sl tz,.,bfl itr:: 3x5. contrcl
c h r , r ~ c t e i.:.s ~ l ci7~.s r... s,ccffiod. ,?*.,re~ ecosrr..r:: for nirplr;,::?~
t".r;,t .,ri5,qu.ire r, 52%: : 5.3gyc.e :jf costrcl f ccil, Tke ;>roci-
sior of suc2 :zhzrzctoris.k j. cs c l s o rcducso $3.2 :,ins re-
~-uL._-c$<0 cL;?,r$e ,q,c$lc of n",tack i n entc-iilg rF,yi?- tzlr:ls
0.. zco;.?s ?-LIC to the c,in;~l5fie$-c oatil.ol z~ni?~ul.-tior;.? ,ssoci-
atcd ~ i 5 2 .n $3: izif c1-y st,s'blc 2,f r?l,7une.