1、NATIONALADVISORYCOMMITTEEFOR AERONAUTICSTECHNICAL NOTE 2676SUMMARY OF STALL-WARNING DEVICESBy John A. ZalovcikLangley Aeronautical LaboratoryLangley Field, V%WashingtonMay 1952Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TECHLIBRARYKAFB,NM :v11111
2、1111111;illlb5458NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS “-”-TECHNICAL NOTE 2676SUMMARY OF STALL-WARNING DEVICES By John A. ZalovcikSUMMARYThe,principles involved in the operation of several types of stall-warning devices sre described and conditions under which difficultymaybe experienced are p
3、ointed out. In the discussion, stall-warning devicessr”egrouped as special stall-sensing devices anddevices. Methods of transmitting the warning tocussed. Some specific examples of stall-warningand described.INTRODUCTION.angle-of-attack-sensingthe pilot are also dis-devices are illustratedUnder cert
4、ain flight conditions, such as landing or acceleratedmaneuvers, the pilot may be forced to fly as close to,maximum lift asspossible in order to effect a desired change in airplane path. Sincestall and its attendant changes in attitude are to be avoided, the pilotm must have some indication of the pr
5、oximity of stall. A few airplanes dohave adequate aerodynamic.stall warning in the form of wing or tail buf-feting which is apparent to the pilot through shaking of the entire air-plane structure, the stick, or the rudder pedals. For airplanes thathave little or no aerodynamic warning, the use of an
6、 artificial stal.l-warning device appears to be one solution (alfhoughperhaps not the mostdesirable).Numerous devices have been proposed over the past 25 years in anattempt to provide a satisfactory warning of the impending stall of anairplane. In order to he generally acceptable a stall-warning dev
7、icemust meet rather stringent requirements. The device must provide a con-sistent margin of warning not only under various flight.conditions suchas airspeed, wing loading, power setting, and airplane configuration butalso under icing conditions. With schemes involving only a single-warningstage, a c
8、onstant margin of warning; between 5 and 20 percent of thestalling speed (depending on the particular airplane), has been considereddesirable. The device, in addition, must be exceptionally reliable. For.,example, the reliability of an airspeed installation is envisioned bysome operators as a practi
9、cal goal. ,.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2Because ofa compromise isthe difficultysometimesmadeNACA TN 2676of satisfying all these requirements,to accept satisfactory oyeration of astall-warning device under limiting conditions, gen
10、erally the landingcondition in absence of icing.While a variety of stall-warning devices are available, most ofthe devices operate on a few basic principles. The purpose of thispaper is to describe the principles involved in several types of specialstall-sensing devices and angle-of-attack-sensingde
11、vices and to pointout some conditions under which difficultymay be experienced. Somemethods of transmitting the warning to the pilot are also discussed anda few examples of special stall-sensing devices and angle-of-attack-sensing devices are given.PRINCIPLES INVOLVED IN STALL-WARNING DEVICESSpecial
12、 stall-sensing devices.- Speci.alstall-sensing devicesusually operate on flow characteristics associated either with the move-ment of the stagnationpoint or with flow separation on the wing. Thosedevices based on the movement of the stagnationpoint make use of eitherthe large change in pressure or t
13、he 1.800change in flow direction as thestagnationpoint moves past a given position on the wing leading edge.The change in pressure can be detected by means of one or”more static-pressure orifices. The change in flow direction can be determinedbymeans of a small pitot-static tube, a free-floatingvane
14、, or by reversalof aerodynamic force on a small body placed close to the leading edge.The location of the sensing device relative to the position of the stag-nation point at stall determines the margin of warning that is provided.Devices that operate on the basis of flow separation near the leadingo
15、r trailing edge make use of the decrease in local velocity, total pressure,or aerodynamic force on a small body near the surface of the wing asstall is approached: Since the spanwise position at which separationoccurs initially varies with flight condition and airplane configuration,several such sen
16、sing devices distributed over the span are required togive adequzitewarning. The use of a single unit to sense stall is madepossible, however, by artificially inducing separation to occur at agiven spanwise station at the desired margin-of warning (angle of attackor speed).Leading-edge devices, in g
17、eneral, operate on larger changes in pressureor aerodynamic forces for small changes in airplane speed near stall thando devices located.near the trailing edge (fig. 1). The mgin of winning -provided by leading-edge devices, therefore,may be expected to have theadvantage of being less sensitive to s
18、mall changes in the details of thedevice, in wing contour, or in reference pressures. .Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACA TN 2676 3. Since flow separation and the position of the stagnationpoint arerelated to angle of attack, the ma
19、rgin of warning provided by sensingdevices based on these principles may be affected to some extent by“trailing-edge-flapdeflection and engine power. In the case of partial-span trailing-edge flaps the effect of the flaps may be minimized andperhaps eliminatedby locating the devices, when possible,
20、outboard ofthe flaps. With full-span leading-edge slats the margin of warningprovided by leading-edge devices, if such installations were practical,would probably be affected by slat extension. On some propeller-equippedairplanes the maximum lift coefficient is appreciably affected by appli-cation o
21、f power. In such cases the margin of warning in terms of angleof attack as provided by a single sensing device may not change withpower but the margin in speed above stalling speed,may be appreciablyincreased. One scheme of eliminating the effect of power on the speedmargin makes use of a second sen
22、sing device located within the propellerslipstream on the wing behind the down-going propeller blade whichdecreases the local angle of attack. The resulting later actuation ofthis second unit when power is applied is used to delay the warningsignal of the first unit.Ice formation on the leading edge
23、 is expected not only to changethe sequence of stall of the vsrious spanwise sections but also to changethe angle of attack and lift coefficients at which stall occurs. Therelation between the position of the stagnation point, leading-edge. separation, and angle of attack would obviously be altered.
24、 Devicessensing separation near the trailing edge, however, may still be usefulwith ice formation on the leading edge provided the devices (severala71 distributed over the span) are heated to prevent ice formation on them.Angle-of-attack-sensing devices.- An angle-of-attack-sensingdevicemeasures the
25、 angle of local flow relative to an arbitrary reference line.A device of this type may consist of a vane pivoted in such a way as toaline itself with the local flow or of a pressure head equipped with apair of orifices or slots symmetrically disposed and a positioning mech-anism to maintain zero pre
26、ssure differential. A fixed pressure headcould also be used but would require a pressure-ratio meter similar tothe Mach meters now in use in order to provide an indication of theratio of pressure differential to dynamic pressure and hence of thelocal flow angle.The problems associated with the effec
27、ts of power, airplane config-uration, and icing on stall warning mentioned previously in connectionwith the special stall-sensing devices also apply to angle-of-attack-sensing devices. In addition, considerationmust be given to the effectof pitching velocity on the measure of angle of attack when lo
28、cations.forward of the fuselage nose or wing are used and to the effect of thebending of the support in accelerated maneuvers. In the null-pressuretype of angle-of-attack-sensingdevices there may be a tendency for the.Provided by IHSNot for ResaleNo reproduction or networking permitted without licen
29、se from IHS-,-,-NACATN 26764head toable ifhunt about the null point. Such a characteristic is objection-the amplitude corresponds to an appreciablemargin of warningin speed. Some difficultym- also srise in acceleratedmaneuvers ifthe lag of positioning mechanism is large.Forms of stall warning.- Stal
30、l warnings may be transmitted to thepilot by any one of several methods based on sight, hesring, or senseof feel. The visual methods include lights and dial indicators;variouscombinations and colors of light have been suggested and used in the past.Unless the lights are near the direct line of sight
31、, experience has indi-cated that the waning may be disregarded. When located in or near thedirect line of sight, however,othelights should not be so bright as tointerfere with the pilot:s view through the windshield and of the flightpanel instruments. Dial indicators,which are used primsrily in conn
32、ec-tion with angle-of-attack-sensingdevices, provide a progressive indi-cation of the proximity of the stall. However, in order to avail him-self of this progressive wsrning, the pilot must direct appreciableattention to the indicator,perhaps more than most pilots are willingto give to it, particula
33、rly in the landing condition.The auditory forms of wsrning include the horn and the radio. Onairplanes where a horn is already used either in connection with thelowering of the flaps or extension of the landing gear, an additionalhorn for stall warning may be confusing. The radio-audio warning mayno
34、t be desirablebecause of possible interferencewith radio receptionduring landing.The rudder-pedal shaker and the stick shaker sre examples of methodsof transmitting the warning through feel. The shakers may consist ofrotating unbalanced weights or solenoid-operatedvibrators. The frequencyand amplitu
35、de of the shake should be readily distinguishable from thoseassociated with airplane vibration. The amplitude, however, should notbe so great as to shake the control surfaces.EXAMPLES OF STALL-WARNING DEVICESA few examples of special stall-sensing and angle-of-attack-sensingdevices, some of which ar
36、e commercially available, are illustratedbysketches in figures 2 to 4. The sketches sre intended to illustrate afew arrangements and operational characteristics. The drawings sre notmade to scale. Additional information on stall-warning devices may beobtained in references 1 to 11.Provided by IHSNot
37、 for ResaleNo reproduction or networking permitted without license from IHS-,-,-NACA TN 2676 “5 b Special Stall-Sensing DevicesLeading-edge orifices.- The device shown in figure 2(a), tested.by the Royal Aircraft Establishment, England (reference 1), cons:stssimply of a static-pressureorifice locate
38、d near the wing leading edgeand operates on the basis of the movement of the stagnation point. Theorifice is connected to a pressure diaphragm referenced to cockpit staticpressure. In operation, as stall is approached, the static pressure atthe orifice rapidly increases towards cockpit static pressu
39、res thespeed gradually decreases (fig. l(a). When the static pressure at theorifice equals cockpit static pressure, a warning of impending stallis provided through the closing of electrical contacts in the pressurediaphragm. The margin of warning in speed above stalling speed isdeterminedby the loca
40、tion of the orifice near the leading edge. Incases where power affects the maximum lift coefficient a second ori-fice within the propeller slipstream is used.Leading-edge tab.- The stall-sensing device, illustrated in fig-ure 2(b), consists of a small tab about 1/2 inch squsre projecting nor-. .mal
41、to the lower surface very close to the leading edge. The chordwiseposition of the tab is selected so that at low angles of attack thestagnationpoint is ahead of the tab and the tab is forced back againsta stop. Near stall the stagnationpoint moves behind the tab with theresult that the tab moves for
42、ward against a light spring and closes anelectrical circuit actuating a stall warning. The margin of warningis adjusted principally.throughthe chordwise location of the tab and”also to some extent by adjusting the light restraining spring force onthe tab. Another version of the leading-edge tab is a
43、 dual-stage devicewhich consists of two tabs located side by side The chordwise locationis selected as with the single unit to provide the initial warning withone of the tabs. The restraining spiing force on the second tab is thenadjusted to provide a second and more intensive warning at a desiredma
44、rgin.Spoiler and pitot-static tube.- The device, tested by the NationalAdvisory Committee for Aeronautics (reference 2) and illustrated infie 2(c)consists of a pitot-static tube mounted on the upper SU-face of a wing immediatelybehind a leading-edge spoiler of 4- to 6-inchspan. This arrangementmay b
45、e located near midsemispan as shown infigure 2(c). The total- and static-pressureleads from the pitot-statictube are connected across a pressure diaphragm equipped with electricalcontacts. In operation near stall a very limited region of local sep-aration develops immediatelybehind the spoiler and c
46、auses the localimpact pressure measured by the pitot-static tube to decrease very rap-idly (fig. l(b) for small decreases in airplane velocity. The electri-cal contacts in the pressure diaphragmmay be adjusted to close anelectrical circuit and give a warning when the local impact pressureProvided by
47、 IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 6 NAC!ATN 2676has dropped to a predetermined level. The msrgin of warning in speedabove stalling speed may be changed by altering the span of the spoiler,its leading-edgeradius, the height of the tube and its
48、distance fromthe spoiler, and the spanwiselocation of the spoiler.Trailing-edge pitot tube.- The device shown in figure 2(d), testedby the NACA (reference3), consists of a pitot tube mounted about aninch above the upper surface of a wing near the trailing edge. Thepressure lead from the tube is conn
49、ected to a pressure diaphra equippedwith electrical contacts and referenced to a source of free-stream staticpressure. As stall is approached the boundary layer near the trailingedge thickens and finally separateswith the result that the total pres-sure at the tube vsries with speed above stall somewhat as shown infigure l(c). The wsrning Is given when the pressure drops to a prede-termined level.Trailin
