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TECHNICAL NOTES NATIONA L ADVI S ORY COMMITTEE FOR AE~O~AUTICS l~o. 670 N . A . C.A. S~ALL-WAR_ I~G I~DICATOR By F . L . r~Oilipson Langley Memorial Aeronautical Laboratory i'fashington October 1938 E:JlJ~11 E:.S~, ~vl I & TECHNOLOGY DEf'T.
fATIO ;AL ADVI30RY CO~VITT~E FOR AERO TAUTICS
TECBYICAL NO TB r o o 670
N . A . C. A . STALL- VAR£ Il\G INDICATO -R By F . 1 . Thompson The stall - warning indicator emp!oys a total - head tube located close to t~e wing surface in a r eg i on whe rein local stallilg occurs before the main ortion of the wing stalls . The artificial production of a ., localize d stalled re g ion i s a ccomplished by means of a sharp leadin g edge extenc~ng a few i nches along th e span . An abrupt drop in the to tal pressure relative to a st at ic refere nc e taken at some onven ient po int occurs at the sta ll i~ this re e i on . Thi s drop j.n to- tal pressure causes the con raction of a pressure cell to which the total - head tube n_d t~e st . ~ic orifice are con: Get e dan del 0 S e s a ~ e lee t ric a le ire itt ~la t a tuates a warni~g signal .
2 1 . A . C . A . T echn i ca l l o te Ji o . 6 7 0 n~ · ' rR O D U C T rO f Wi t h s o me a i r p l ane s the appr o ach t o the stal l i s ac - co mpan i ed by cha n ges i n t: e be ha vio r , such as t ai l bu f fet - i ng o r c ha n ges i n t h e c o ntrol c ha r acte ri st ic s o f t h8 a i r - p l ane so tha t t he p ilo t ob ta i n s a wa r n i ng o f t he i mpend i :1.g stall . Wi th o t her a i rp l anes it is p o ss i ble to approach the sta ll w it h o ut any p er cept ibl e warn i ng othe r than the r ea d ing o f the a i r - spee d me t ' .:l r , i n which c ase the d ange r of inad v er t ent stal li ng is co n side r ab l y greater . A lthOU Gh i t is not with in t he scope of th i s paper t o discuss s t all - i ng cha r acte ri st i c s, i t i s des i red t o point out that in goneral tho dang e r of in advertent stall i ng is greatest ~ i th th o se ai r p l anes tha t b eha v e worse when the stalling occurs ; that i s , wi th a ir planes in wh ich the sta ll start s a t the wi ng ti ps . A wa r ning of t he impend i ng stall is de - sirab l e in any c ase, bu t is pa r t i cul a r l y desirab l e with airplane s o f t he latter type .
When the r e is no th i n g in the behavi o r o r c on tr o l cha r ac t er i st i c s of the a i rpla n e t hat i ndicates the impend - i ng stal l, g r eat depen d e n ce mu st be placed on the air - speed meter . Unfo r tu n ately , h o we v e r, the stall i ng speed varies with wing l o ad in g and the air - speed mete r the r efore is not a relia bl e s t a l l i nd ic a to r. W hen the l o ad changes or the a ir p l ane unde r g o e s acce l era tio n , as , fo r example , i n turns , the sta llin g speed i nc re ases i n pr o po r t io n to the s quare root of the load fac t ot
j loa d f a crt 0 r
= V ~
V sta ll ml n I n hor iz onta l tu r ns the lo ad factor is d i rect l y related to the angle of tank I
Load facto r = -----------------
COs angle of bank N . A.C . A . Technic.::!l IJote , Uo . , 6, 7 0 so that for horizonta l turns / _ _ I
Vstall =
~ cos angle of bank The load factor and the ratio of Vstall to V mi n in turns are plotted in figure 1 to show how these quantities vary with an gle of bank . The curve of stalling speed is an ideal one that do es not take into account the influence of errors in the air -s peed meter reading . In general, such errors will further complicate the situation because calibrations made in steady fli ght will not'apply in ac- celerated flight unless the correction for posit ion error of the air -s pood head is negligible . If an instru ment is to be used to gi ve a warning of the stall, therefore , it a ppea rs desirable that its functioning s"lould be de p endent on angle of attack rather than air speed . Furthermore, - it seems desirable in order to avoid the ne cessity for watch- in g the instru ment continuously that it be arranged to give a warnin g signal when a safe li mit has been reached.
S TALL- WAR ,T r NG DEVI CE A stall-warning device developed by the National Advisory Committee for Aeronautics is shown diagrammati - cally in figure 2 . It makes usc of a m odifiod typo of pitot - static head set close to the wing surface in a re- gion wherein stalling occurs (or is mad e to occur) at an angle of attack considerably below that at which the main portion of the win stalls . T ~ is head is connected to a b pressure cell fitted with electrical con tacts and for ob- servations during tests it may also be connected to a conventional air - speed moter. When the air flow in the re gi on in which the head is located breaks away from the wing surface, that is to say, when local stalling occurs, the pressure indicatroby the hoad drops sharply to zero .
Whon thi s occurs the electrical contacts close a circuit that operates a warning signal . The si gnu l as used in flight tests of this device was a horn such as that used with r etractable landing gears, but of c ourse any suitable type o f warning may be used .
4 lIT . A . C . A . Tec _ln ic a l ~~ ot e No . 67 0 In ce rtai n c as e s it may be know n as th e result of a ir - f lo w su rv eys w ith s il k tufts or othe r m c an~ that th o r e is a re gi on wherein local stallin g occurs suffi- ciently before the sta ll of the main portion o f the w in g so that the artificial pr o d u ct io n of suc~ a re g ion is unnecessary . Tho more gene ral c ase , howeve r, is one in whic h it is necessary to p ro duce suc h a rogion artifi - cially . The require men ts of th e arran g ement used i n suc h cases are that it s hou l d p ro duce loc a l sta lli n g early en o ugh so as to p rovide a wp le w ar nin g of the co mp le t e stall a nd t h at it should not ad v ersely affect the ae rod y - nam ic c ha r a cteristics of th e a ir p l ane . It a p p e ared that the mo st s uit ab le means f or acc o mp lishin g this res u lt was the use of a sharp l e a d in g ed g e over a small . portion o f the w in g span . Fli ght e ~p eriments, th e r e for e , were made with such an arran g e me nt .
The h ead u se d in the e xp er i ments with different nose p i ece s is shown in f i g u re 3 . Table I sh o ws the v ar iou s arrange m ent s of . s h a r p edge a n d head t h at were tested and fi g u re 4 sh o ws ph oto g r a phs o f co mb ination H. For the tests the contacting dev ic e wa s s e t to operate when the local air s p eed as in dica t ed by t h e s ta l l- warnin g dev ic e drop p ed below 70 m ile s p e r ho u r . As will be shown later, this setting was a p p reciabl y el ow rr hat would normally b e t h e loc a l vel ocit y o ve r t h e curv ed p ortion of the leading edge o f the wing at the m ini mum s p e ed o f th e air p lane on which the tests were made . A co nsid erabl e variation of thi s setting, however, would not h a v e altered t he results a pp reciably .
FL I GHT TESTS AND RESULT S T he airplan e used in tho t ests was a Fai rc hi ld 2 2 f i tted wi th a uin g o f r e ct a ng u l a r p lan form , having a ch ord of 6 6 inc ho s and an N- 22 section . Th is win g wa s eq ui ppe d w it h fu~ l - a pa n Za p f la ps . Th e h e ad of the st a ll - warnin g devi c e rr ~s pl a ced o utboard from t ho cent o r a dis ~ tance e q ual to a b cu t t u o - thirds t ho s Gm is pa~ wh e r e ear l y stal l in g di d not n or ma l ly occur . Thro ~ g hQut the test s an air - spe e d i11 0 ter va G connected in t he l i n e of th e sta ll- wa r nin g dev ic e so a s to o b ~ain m ore co ~ p l ete infor m atio n concer n in s t h e f u n ct ionin g of th e devi ce t h an c ould be ob- tained by use of t h e wa rn in g signal al on e .
The f li ght tests of differ ent arran g e m ents we r e m ade N .A.C.A. Technical Note No . 6 70 5 in order to determine one that was unobtr u si~e ' and at the sa me time w oul d give sufficie nt warning ' in straight fli g ht and in turns. For the s ma ll li g ht airplane on which the te sts were made, it . w as decided arbitrarily that a satis - f actory wa rnin g consisted of one gi ven . at rou gh lY 10 to 15 miles per hour above the stallin g sp e ed for any particular c0ndition; that is, it was considered that this warning should be ~ iv en with power on , power off, flaps uP. or fl ap s down: Eiact measurem o nts of a irplan e s peed were not made, the only readings bein g those observed by the pilot on the meter with which t he airpla~e was re gu larly equipped a nd which was known to rea d considerably 1 6wer than th e ac- tual speed of the airplane at t he lo we r end of the speed r ange . The extra effort involved in obt~ining exact meas - ur em ents seemed unwarranted since the results would ha v e no more genera l a pp lication to other airpla ne s of differ- ent form than would approximate readings that are quali - t a tively accurate. The results for the comp lete series of · ·t ests, therefore, as pres nn ted in table I are s h own in quali tat ive form; that' s , whe ther t h e arran g e ment is con- sidered t o be satisfac tory or ' uns a tisf a ct ory , with some additional re ma r ks pert~ining to cert airi arran g ements .
:.aefo r e a c a m pa ' ison is made of t h e re sults. obtained with different arr a ng e m ents, i t seems advisable t o de - scribe the gen eral chara ct e ri stics of the be h avior of the device by r ef e re nce to figure 5. This figure s how s the r eadings o f t e meter connected to th e stall - wa rnin g head p lotte d a g a i ll st t l 1.e readin g s of the a ir-speed mete r of t he air p l ane wit h arran g c n e n t F and al so with the sharp e dge re mo v ed . Theros ult s s~'lon-n in · this f i gu r e for arra ng e - mont Fare typic .a lof arran ements considered to be satis - f a ctor y . It w ~ll be noted that whereas t h e readin g of the air - s pee d mete r ' of t he airp la ne at mi n i mum speed is 42 m iles p er hour , the warnin g ' is o btained at about 59 mi l~s per hour, the s pe ed at which the local veloci~y fa lls b~ lo w 70 miles per hou r . Wit l lOut t Ole sharp edge t he loc a l ' v e loci ty over th e leadi ng ed g e of th e wing d o e s not fall b e lo w 90 miles po r hou r . It ma y also b e noted that . al- t h oug h the device was Bet 't o warn when th e local volocity reach ed 70 mi l es pe r hour, a ch ange of setting of several mil e s p er hour in eit h er dir ec tion would not have g reatly altered the airplan~ s p eed at which the warning occurred .
I n other words , the adjustment of the contactin g device was not critical . One o t h er point of prac t ic al i nte rest brou g hl out b y figure 5 is t h at in the normal fli g ht range the loc a l velocit y is considerably g reat e r than the air - -- --- --- -- -- -- ---- N.A.C.A. Technical dot e N o. , 670 , sp e ed of the ai r plane, so that the pressure unit used in the contacting device must ' be ' rugg~d enough to withstand pressures much larger than the dynamic pressur~ corre- sponding to the maximum speod of tho airplane relative to un~isturbed air .
It appears from a study of the results given in ta- ble I that the functioning of the stall-warning device is adjustable through a considerable range by means of alterations of the shape and size of the sharp edge and of the nose of the head . It appears to be very important that the dynamic open i ng of the head be close to the wing ' surface in order to avoid the necess i ty for a large, sha.rp edge . With the dynamic opening above the wing surface about one - half inch as it was with t he ori g inal nose piece, satisfactory results were obtained only with the large , sharp edgos of arrar.gements A and B. With the dy - namic o pening brought down close to the surface, satis~ factory results were obtain e d with the sharp edge redu~ed to a length o f 6 inches in a spn.nwise direction and an extension of 11/ 1 6 inch in a chordwise direction (arrange- ments F and H) . A further reduction in size might hav~ been possible but appeared to be unnecessary in view of the apparent insignificance of the alteration o~ the wing contou r.
APPLICATION OF STALL - WAR NING DEVICE In g eneral, the stall - warning device will require two heads instal le d on the airplane, one near each wing tip . With such an arrangement the asymmetry of the an.
gle-of - attack distribution in bankin g , which amounts tg something of the order of 50 or 6 , will be taken care of . Anothe r advantage of placing th e device at the tip is that it can be placed beyond the edge that is covered by the rubber overshoes that are commonly used for de - icers . The sharp edge probably will be required in gen- eral, but at least one example is known wherein the ex- treme tip portion of the win g was shown by silk tufts to stall well before t~ere was any loss of lateral control or appreciable disturbance of the total lift . , In such a case apparently it would be possible to mount the head within the re g ion that was observed to be stalled and dispense with any artificial means of stall produc~ion.
.J N . A . C . A . Technic a l Kote r~ . 6 70 7 The size of the s ha r p ed ge req u~ re d o n a larger wing than that used in the tests reported h ere in is open to question. A ge o met rically similar but s ca led-u p arrange- ment probably would be satisfactory, but it seems possible t h at a reiatively smaller s h arp edge mi ght be satisfactory .
T ~l e i mp 0 r tan t poi n t t 0 k e e p in min d a p pea r s t 0 bet h8. t, a s indicated by the present t e sts, the d yna m ic op e nin g should be placed close to the surface and c los e to the sh a rp ed g e.
I t is i nt eresting to note that on hi g hly tapered win g s, even on very large airplanes, t he tip sections would not be materially lar ge r tha n the section of the rectangular wing on which the tests were made .
Th e beh a ior of the stall - warning de vi ce under icin g co n ditions is unknown . I n des igning t h e head , provision was ma d e for h eati ng it elec t rically as is com mon practic e with air-spe e d heads, as proper functioning of the devic e would demand that it be k e pt free from ice formation. If i t d id become clo gg ed w it h ice, it would presu m abl y ive a v!arning signal as tho'lgh the wing were st al led a t all times . The infl uen ce of ice on the s ha rp ed g e in f ro nt of the he ad mi g ht or mi gh t not i mpa ir the fu n ctioning o f t he device. It is p os s ible that the ic e would cause the he a d to indicate a stall at a so mewh at lo wer an g le of at- tack than custo ma r y . In this case the effec t mi g ht be d e- sirable since ic e adh~ring to the main p or ti o n of the wing p robably would caus e t ho com p lete wing t o s tal l earlier t han usual.
It is of i nte re st to not e a p articular ap p lica tio n that t h e de vic e might have t o hig l ly man euv e ra b l e pu rsuit or fighter air p lanes. Several such airplanes are so easi- l y stalled without warning in viol en t maneuv ers that such stallin g is ofte n in ad v e rt ent . I t may o ccu r in dive p ull- outs, sharp turns , loo ps , or oth e r maneuve rs at s poe ds rangi ng fro m the m inimu m s peo d in lo ve l flight to about two- and ono-half times th is mi nimum s peed , d ependi ng upon t h e accel era tion imposed . When the st a ll occ~rs the air - plane rolls m or e or less vio lently , de pending o n how the stall develo ps and the sta bilit y characteristics of t h o air p lane. In fact, the manouverability of tho airplane may be seriousl y li mi te d by this chara cte r ist ic. It w ould appear that the stall-warnin g device would be very useful in such cas es and it appears desirabl e t ha t an ins t a lla - tion be made on an airpla ne of t h is cl a ss for flight trials .
It would be very important in such an install a tion th a t the movin g parts of tpe contact ing , device or o ther moving part 's ' be unaffected by the h i g h acceleration, 8 N . A . C . A . Technical Note No . 6 70 In conc l us i on , it i s desired to po i nt out that t h e a p plicat i on o f the principles involved i n · the sta l l - warn - ing device i s subject to a wide variation as re g ards the constructiona l d e tails . The head used in the fli g ht test s was bu i lt around a cyl i ndr i ca l heatin ele m ent of the type that has been used i n · ce r ta i n electrical l y heated pitot tubes . A neater i nsta ll at i on of ~he head is undoubte d l y possib l e i n v i ew of the fact that the only part that need be exposed to th e air stream is the dynamic opening . The static reference pressu r e cou l d be obtained fro m an ori - fice set f l ush with the wing surface or po s sibly t h e in - ter n al win g pressure or cabin pressure could be used as a sat is factory reference p ressure . It seems undesirable to use the air - speed static as a refere n ce because suc h a conn e ction would make the warnin g device de p endent on the p ro p er functionin g of the air-s p ee d li n e . It i s neces- sary , however, 'lith ",hateve r Gype of h e ad use , to provide sui t able drains and a heatin g ele m ent to p revent rain and ice from interf e rin g vith the functioning of the device .
These r equirements wil l, i n a l arge m easure , determine the design of the head .
Langley Me r oria l Aeronautical Laboratory , Na tiona l Advisory Comm ittee for Ae ronautics, Langley Field , Va . , F e bruary 1 0, 1 9 38 .
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Figure 4. - Ste . ll-warning device ins ta . lled on wing of Fairchild 22 airplane.
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