ARE No. IAH28 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGNALLy ISSUED October 19W- as Advance Restricted Report IA-H28 APPLICATION OF SPRING TABS TO ELEVATOR COITEROLS By WlUlam H. PMlllps Langley Memorial Aeronautical Laboratory Langley Field, Va. NACA WASHINGTON 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. L - 122 ^ DOCUMENTS DEPARTMENT Digitized by tlie Internet Arcliive in 2011 witli funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/applicationofsprOOIang nn oh^ ^^ '::rCA AKR No. r.i|H^8 3^' NATIONAL ADVISORY COMMITTEE ?0R AERONAUTICS ADVANCE RESTRICTED REPORT APPLICATION OF SPRING TABS TO ELEVATOR CONTROLS By Vv'illiam H. Phillips SmiMARY Equations are presented for calculating the stick- force charactsri etics obtained with a soring- tab tyne of elevator control. The inain orohleras encountered in the design oT a satisfactory elevator soring tab are to provide stick forces in the desired range, to maintain the force per g sufficiently constant throughout the speed range, to avoid undesirable ''feel" of the control in ground handling, and to prevent flutter. Exajriples are presented to show the design features of spi'ing tabs required to solve these problems for airolanas of various sizes. It apnears 'oossible to provide satisfactory elevator control-force chai'acteristics over a large center-of -gravity range on airplsnes weighing from about 16,000 to 500,000 pounds. On airplanes v;eighing less than 16,000 pounds, some difficulty may be encountered in obtaining sufficiently heavy stick forces for rapid n-ovements of the control stick. Some special tab designs, including geared and preloaded spring tabs, are discussed. The geared spring tab is shovm to offer a means of obtaining satisfactory ground control v^ithout introducing excessive variation of force per g with speed. By the use of spring tabs on elevators, the control forces may be made more closely predictable and the variation of stick-force characteristics among different airplanes of the same tyoe may be gi-eatly reduced. One of the princioal objections to the use of spring tabs is the aiP.ount of weight required for mass balance to prevent flutter. "MTip' ^.ODUCTION Difficulties have been encountered in obtaining desirable control-force characteristics on large or NACA ARR No . li^23 high-speed airolanes, because the hinge moinents on the control surfaces must be very closely balanced and because slight changes in the hinge-moment parameters result in large changes in control forces. The advantages of spring tabs in overcoming these difficulties have been -pointed out in reference 1 and other reports. It has been recognized, hovvever, that the use of a spring tab on an elevator results in a decreasing value of the stick force Dsr g normal acceleration with increasing speed that might be considered undesirable. An analysis is presented herein of the effects of spring tabs on elevator forces for airplanes of various sizes. The results indicate that an elevator equipoed with a suitably designed spring tab may avoid any serious disadvantage from this effect and may still obtain the advantage of having the control forces predictable and relatively insensitive to variations in the elevator hinge -moment characteri sties . SY^^BOLS W weight b span S wing area c chord I tail length S^ tall area slope of lift curve of wing w e downwash angle q dynam.ic pressure qrp dynam.ic pressure at tail T elevator effectiveness factor ' "^ dCLrp/6a^ HAGA AIU( llo. LliBSG Ct lift coefficient V,^ stalling Gpeed I elevator monent of inertia K ratio of stick raovemont to elevator deflection, -'• tab fixed J nornially positive Kp ratio of stick r.iovsment to tab deflection, elevator fixed j normally negative K, ratio of stick force to tab anrle at zero 3 airspeed, elevator fixed; normally positive H hinge moment H C-, hin£,e-moment coefficient \qbc2/ 5 elevator defloction (positive down) 5^ tab defloction (positive; down) x„ stick movement (positive for-.;ard) F stick force (pull force positive) a an£;le of attack of v/in£; a.. ancle o£ attacl: of tail p mass density of air n normal acceleration in g units g acceleration of r;,ravity (32.2 ft/sec^) X distance betv/een center of gravity and stick- fixed neutral point in straight flight (positive when center of gravity is rearward) variation of elevator hinge-moment coefficient with angle of attack of tail, measured v/ith tab free k KACA ARR Ko. Li+Ji23 variation of e?-evator hinge-moment coefficient with elevator angle, measured with tab free distance between tab mass -balance weight and tab hinge line distance between elevator hinge line and tab hinge line wfi - ^^ dc ^^ / v; ^- ^ — —x^ -.1, c^Cl T It -' St J 7^^ Subscripts T tail t tab e elevator SQUATIOKS ?0R ELEVATOR FORCES The method of deriving the equations for the elevator control force in maneuvers with a spring tab will be briefly outlined. These equations are similar to equations given in reference 2 but have been arranged to give a clearer pnvsical significance to the various terms . The change in elevator hinge mom.u-nt caused by any change in angle of attach:, elevator angle, or tab angle is given by the following formula: NACA ARR No. LI4II28 The corresnonding change in tat hinge raoment is given by the expression Allf, O Ur-i - Cy ( ht h-. ^h, + A£t J^^jlT^t Ot (2) The change in elevator angle and the corr in angle of attack at the tail - both o the calculation of the change in elevator be derived for any type of maneuver. angle required to insert in equation particular linkage arrangement under present discussion will consider the arrangement shown in figure 1. 7or this relation betv\feen ohe st: ck force, the ele moment, and the tab hinge moment, when th equilibrium, is given by the formula may tab the The e spending change vvhich enter into hinge moment - The change in (1) deoonds on consideration. spring -tab arrangement , the vator hinge e system is in A J" AH, AH^ + ■'iO- K2"5 (5) in which the constants Ki and K2. are the gearing ratios between the stick and elevator and betv/een the stick and tab, resriectively, defined by the formula xg = Ki5e + ::^5t iU) and the constant Kz This spring constant the stiffness of the spring. ■ an unpreloaded spring tab is defined in terms of the stick force required at zero airspeed to deflect the tap with the elevator fixed; thus. K iSt (5) By simultaneous solutions of equations (1), (2), and (5), the stick force required in any maneuver for an elevator equipped with an lan'nreloaded spring tab may be derived. The elevator force required to produce a given change in acceleration in gradual pull-ups is used as a criterion of the elevator control characteristics. In a pull-up. NACA ARR No, lUEZQ the change in angle of attack at the tail is given by the formula Aa.ji = / + '/; 1 - ^.] ,.£7 V da , ' ^ &2'' I — ) qS \aa /„ w (li - 1) (6) and the change in elevator angle required is given by the formula A5^ = Wx dCr T 05, q.-pSrr, l 1 ^2" (n - 1) (7) In order to show the relation between the forces required with a soring tab and the force v;ith a conventional elevator, the equations for per g In a pull-up are derived first for an ele without a tab, then for an elevator with a serv finally for an elevator vifith a spring tab. In of a conventional elevator, the change in eleva moment may be derived from equation (1). By us values for Aarp and A5e obtained from equati and (7) and by setting A6t ~ ^> acceleration is found to be the force per g elevator 2 obtained the force vator otab, and the case tor hinge fc of the ons (6) normal 6P -iCei^ ^ r?'^ dn K]_ [^ car^, n,-. 65e _J qrp bgCg' (8) where .. 4 - 1^) E = Vda vvx 3 w 6Cr.rp qm — ^-StI q 66, ^^ (9) NACA ARR No. lIlH28 7 The second case considered is th£.t of a servotab, v;hich is defixied as the system shown in figure 1 with the spring oraitted. In this case, the stick force in a pull-up may be obtained from equations (1), (^), and (3) by setting the spring constant K^ equal to zero. The I'elation obtained for the force per g is r^n .l-C,':fiv-\ I /tf q^. "^e^e^ (10) -bpc, 1 _ A^ h Ki c56t -btct This equation differs from that for the force v;ithout a tab in two ways. The first difference is that the terras '^Che /6o.rn and oGhe/063 are replaced "oj the corresponding values v;hich would be measured on the elevator with the tab free. These vnlues for the tab- free condition are given bv the exoressions cC 'ht ^Ch. he 7.aT ^ot dan -^t d6t ^^Cht dChe cC ht > (11: 65 1 Tf the tab does not have any floating tendencies, the values obtained with equations (11) are the same as those obtained for the elevator with the tab fixed. The second difference is that in the denominator a term is added which depenas upon the ratio of the elevator ■dimensions to the tab dimensions, the ratio of the effectiveness of the tab to its aerodynamic hinge moment, and the ratio between the tab and elevator gearing constants. This added term, which in cractical designs may range in value from five to several hundred, effectively \ . NACA ARR No. li^H23 divides the elevator stick force that v;ould be obtained without a tab by a large factor. The force per g for a servotab, like that for the elevator without a tab, is essentially independent of speed. The force ner g for an elevator equipped with an unpre loaded spring tab is found to be 1_ 2l 6n ,^ChA l3 tf K2K3 qi'btc *l — beCe 1 - Ko 6Chp ^5t bec e + K2'-'j Ki C St btc t" CCht - qibt^ .t^ (12) Thx'ee terms are added when the tab- taken into account. all tliree term the same form and contain the dynam the denominator. At very low soeed three terms will be very large comp term.s in equation (12) and, in this reduces to the form of equation (6) the elevator without a spring tab. the three added terms in equation ( the equation for force oer g reduce servotab (equation (10)). The actu oer g with sneed for various values constant K3 is shown for a tyoica latlon in figure 2. constai:Lt is s are seen to be of ic pressure q-p in s, therefore, these ared with any other case, the equation , the force oer g of .nt very high speeds, 12) appi'oach zero and s to that derived for al variation of force of the spring 1 spring-tab instal- DESIGN PR0 3LEJ;'S The main problems that arise in connection with the design of a spring tab for an elevator are as follows: (a) To -provide stick forces in the desired range (b) To !:aintain force per g sufficiently constant through the soeed range NACA ARR No. ri|H28 9 (c) To avoid undesirable "feel" of control for ground handling (d) To prevent flutter These four conditions will be shown.-, to restrict the design characteristics of a satisfactory elevator spring tab to a rather narrow range for any particular type of airplane . Some additional discus; si on may be necessary to clarify ooints (b) and (c). The force per g obtained v;ith a servotab has been shown not to vary with speed. A servotab has been found to be undesirable, hov/ever, because the elevator does not follow movements of the stick smoothly when the airplane i s on the ground or taxying at low soeed. The use of a spring tab provides a mechanical connection between the stick and the elevator and relieves this difficulty. One of the m:ain oroblems in connection with the design of a spring tab is to avoid an undesirably large variation of force oer g with speed in flight and still to provide a sufficiently rigid connection between the stick and the elevator to give control while the airplane is taxying. The variation of force per g with speed in flight may be reduced to a small value by using a spring sufficiently weak that, in the normal-flight speed range, the control behaves essentially as a servotab. It is necessary to decide upon some criterion for the manimum value of spring stiffness required for control while the airplane is taxving. •'o ' The response of the elevator to a sudden stick m.ovem.ent depends upon the elevator hinge raom.ent that results from a unit stick deflection. If the elevator is held fixed, the variation of elevator hinge m.oment v/ith stick deflection for an elevator equipped v/ith a spring tab is given by the following equation: '""Che p " '^^"ht V, 2 (13) oxs ~ '-'2 ■'^2 Y.,2^^ At zero speed the elevator hinge m.oment com:es entirely from the spring but, as the soeed increases, the 10 FACA ARR No. l1lH23 aerodynamic hinge mo'nent due to tab deflection Is added. The initial angular acceleration of the elevator, which occurs after a sudden stick movement, depends on the ratio of elevator hini=^e moment to stick deflection divided by the moment of inertia of the elevator about its hinge line. In flight tests of a small fighter airclane, the minimujn value of spring stiffness required for satisfactory feel of the controls on the ground corresponded to the value (at zero airspeed) 1 d^i^ '^I^'''^ -J ^-^ = „ ' = 200 foot-oounds per foot per slug-foof^ I 0X3 >^Zi This value is, of course, many times smaller than the degree of rigidity present in a conventional control system but has nevertheD.ess been shown to be satisfactory for the case of the sma].l fighter airplane. For a large alrolane, oarticularly one equioped with a tricycle landing gear, elevator control should not be required until speeds aoproaching the take-off speed are reached. In such a case, then, a lower value of the ratio might be acceTDtable at 2ero alrsneed. The value of -^ 0x3 should, however, bo reasonably large at spetds approaching the taice-off speed. EXAI'.IPLES Design considerations .- In order to illustrate the application of spring tabs to elevator controls of airplanes of various sizes, the stick-force charac- teristics in maneuvers have been calculated for four airplanes ranging in size from a scout bomber to an airplane weighing 5'^0>'^'^0 pounds, which represents about the largest type of airolane now being contemplated by aircraft designers. ■ In each case, a practical spring- tab design has been arrived at that orovides stick-force characteristics which satisfy the requirements of ref trance J* These examples show what design features of a spring tab are required to obtain stick forces for maneuvering within the range required for each class of aii'Dlane and indicate also special problem.s that may arise in the design of spring tabs for aircraft of NACA ARR No. Li^H23 11 particular sizes. The characteristics of the airplanes chosen as examples are given in table I. Certain factors that were considered in designing the spring tabs are as follows? (a) The spring stiffness has been selected on the basis of providing satisfactory ground control 1 e by mal-:ing the value of — at zero airspeed I 6;: 3 e iual to or greater than 2 00 foot-pounds per foot p per slug-foot except where otherwise noted. (b) A reasonable degree of aerodynamic balance of the elevator, corresponding to a value of = -0.002 or -G.OO5, has boen assuir'.ed so that c5q large elevator deflections raay bo obtained without having the tab size or deflection exceed practical lirilts. The \'alue of = 0, which has been dOrp used in all calculations, may be attained In •!->ractice bv suitable choice of tlie elevator contours. Variation in the value of will not, however, alter the effects of the spring tab but will simply shift the stick-free neutral points in straight and turning flight by the ss^ie amounc for a spring tab as for a conventional type of balance. fc) The tab binge -rr.CTient characteristics were assigned the representative values = -0.00^ dCh. dc^ ast or -0.005, -:: - 0, £ind -r: — - 0. By suitable o 5 Oa„, e !• modification of the tab design, considerable variation in these values may be obtained. The effects of such changes on the stick forces may be determined from formulas (11) and (12). Scout bomber (w eight , l6,000 l b).- The variation of force per g with speed and with center~of-gr-avity position for a scout bomber weighing l6,000 pounds is shovjn in figure ^. The desirable range of stick forces (shown by cross hatching in figures) is Indicated in accordance 12 NACA AKR No . l1|-H23 with the requirements of reference 3" A center-of- gravity range of 10 percent of the mean aerodynamic chord has been assumed. The h7/pothetical curve of force per g at zero speed, which also represents the force oer g throughout the sneed range when a spring tab is not used, shov/s that a conventional elevator with the degree of balance used would give heavy stick forces and an excessive variation of force per g with center-of -gravity position. The assum.ed soring tab reduces the variation of force per g with center-of-gravity position to an acceotable amount. The variation of force rjer g with soeed, for the spring stiffness chosen to give satisfactory ground control, also apoears to be desirably small. Somewhat larger values of force oer g are obtained near the Liinimum sueed, but this fact is thought to be uniinportant because tiie airplane stalls at lovv values of nornial acceleration in this speed range. Tiie stick forces were generally too low with a soring tab alone but have been raised to an acceutable value by tl^e use of a sinall bobweight that requires a pull force o'^" about thj:^ee oounds on the stick. Although the combination of spring tab and bobweight gives stick forces that satisfy the requirements, recent flight tests have shown that such an arrangement might be considered unsatisfactory to tlie pilots because of the lightness of the stick force required to make large raoid movements of the stick. This lightness, of course, r-v,3ults from the small effective value of dCh^/b^e , which is necessary in order to obtain a small variation of force per g v;ith center-of-gravity position. The requirement for light stick forces over such g large center-of-gravity range on an air-^lane of this type seems, in fact, to be incompatible v.ith the pilot's desire for forces large enough to prevent inadvertent movements of the control stick. The problem of providing sufficient heaviness of the control stick for quick movements (wi th the resultant undesirable variation of force per g with center-of- gravity nosition) v\^hen a soring tab is used may present some difficulties on an airolane as small as a scout bomber. The following oossibilltit-.s art; available for making the forces heavier: NACA ARR No. LLjI28 15 (a) To decrease K2j the mechanical advantage of the stick over tne tab (b) To increase the tab chord (c) To increase dCht-/^^t ^7 ^^® °^ strips on the tab trailing edge (d) To reduce the amount of aerodynamic balance on the elevator Of these Dossibilities, (a) and (b) may excessively increase the amount of mass balance required to prevent flutter, a subject that will be discussed in a later section of the paper. Only a limited advantage is gained by method (c). Method (d^ will require the use of a larger tab to obtain large elevator deflections. 3y a combination of those methods, hov/ever, it appears practicable to obtain a sufficiently large centering tendency of the stick on an airplane of the scout-bomber class. For a given value of — r— ^ at zero airsiDeed, changes (a), (b), and (c) give a lavorable reduction in the variation of force per g with speed. Satisfactory control feel might possibly be provided, even on an airolane that has no variation of force per g with center-of -i^ravity position, by suitable inertia weights or damning devices in the control system. Several systems for accomplishing this result have been proposed, but none has yet been tested in flight. Medium bomber (weight, ^0,000 lb) .- The stick- force characteristics of a medium bomber weighing 50,000 pounds with the assuir.ed spring-tab design are shown in figure ii. The spring stiffness, chosen on the basis of ground control, provides a sufficiently small variation of force per g with speed. The stick forces lie v/ithin the desired limits. It is believed that the centering tendency of the control stick associated vvith these forces would be considered sufficiently large, although no tests have been m.ade of an airplane of this size to verify this belief. Heai^y bomber (weight, 123,000 lb) .- The calculated stick-force characteristics of a heavy bomber (weight, 125,000 lb) are shown in figure 5- I^* order to obtain ill NaCA ARR No. ri;H28 stick forces within the desired range, a tab of rather narrow chord and an Increased value of K2 (the mechanical advantage of the stick over the tab) have to be used. When these measures are adopted, it is no longer possible to meet the criterion for ground control I— — ~ at zero soeed = 200 foot-oounds oer foot oer slug-f oof^ j and still maintain a sufficiently sm.all variation of force per g with speed. Although the spring stiffness required to obtain the characteristics shown in figure 5 is greater than the stiffness used on the smaller airnlanes, the value of ^ — ^ at zero sneed is I ^Xg considerably reduced but reaches a value of 200 at a speed of 80 miles per hour. This condition would probably be acceptable, however, on a large airplane with a tricycle landing gear. Airplane of 300?Q'^0 ^^^ounds weight .- The calculated stick-force characteristics of an airplane weighing approximately 500,000 pounds are shown in figure 6. On an airplane of this size, considerable care must be taken to balance aerodynamlcally both the elevator and the tab in order to obtain sufficiently light stick forces. A very small value of — — — ^ at zero soeec. I ,^Xs must also be acceoted in order to avoid excessive variation of force oer g with speed. The value of ■i -^^^^ for this tab arrangement exceeds 200 at soeeds I dxp above 102 miles oer hour. The stick forces on an airplane of this size depend rather critically on the elevator and tab hinge -moment characteristics. In view of the rather limited data available at oresent on the hinge-moment characteristics of tabs, special tests 'would undoubtedly be required to develop a design that provides the desired hlnge-mom.ent parameters. The degree of balance required is not so high that small variations in contours among different airplanes would cause excessive variations in the stick forces. It therefore apoears that a spring tab may be used to provide satisfactory elevator control on an airplane of at least 300*000 oounds gross weight. The limiting size NACA ARR No. LkHZS 15 of airplane that could be adequately controlled by this means is difficult to estiraate, inasmuch as factors such as the resoonse of the elevator to stick movements, rather than the magnitude of the stick forces, would rirobably set the ucper limit on the size of airolane that could be cot^trolled. The increasing importance of the elevator inertia en large airplanes is caused by the fact that the moment of inertia of the elevator tends to increase as approxirr.ately the fourth power of the linear dimension, vvhereas the aerodynartiic hinge raom^ents due to the tab vary as the cube of the linear dimension. DISCUSSION QF EXAMPLES The abllit-5'- of the soring tab to provide desirable stick-force characteristics over a large center-of- gravity range on airplanes weighing between about 16,000 and 500,000 oounds has been shown by the preceding examples. The lower limit on the size of airplane that can be controlled is determined by the requirement for a definite centering tendency of the control stick. The upoer limit is not clearly defined but probably is set by the ability of the elevator co follow rapid stick miovem.ents. One advantage of the spring-tab control is that any variation in the stick-force characteristics betvv-een airplanes of the same type, cauaed by slight differences in the contours of the elevators, would be much less I'or a spring-tab elevator than for an elevator equipped with a conventional type of balance such as a balancing tab or an inset hinge. This difference m.ay be explained as follows! In order to obtain desirable stick forces with a conventional tyoe of balance, the elevator hinge- m.om.ent parameters '^Che/'^5e and dCh^/do.rp must be reduced to very small values. Variations of these parameters caused by slight differences in the elevator contours are likely to be of the same order of magnitude as the desired values. Such variations would cause changes in the stick-force characteristics of 100 percent or more. In the case of the spring tab, hoiwever, a high degree of balance of the elevator is not rfcq.uired. The stick forces are reduced to desirable values by the action of the tab. A pronerly designed soring tab has been shovm to act essentially as a servotab at normal flight speeds. The formula for the force per g with a l6 NACA ARR No. Li^H23 e.ervotab ( equati on (10) ) shows that the force per g is reduced by a large factor in the denominator that depends on the tab and 15nkage characteristics. The effects of any variations in the values of 6Che/'^<^T ^.nd c^Che/'^5e will be reduced by the sa?ne ratio. Inasmuch as this ratio varies from about 1:10 in the case of the scout bomber to 1:100 in the case of the 30C)>000-'nound airolane, the snring tab should effectively eliminate any difficulties caused by variations in elevator hinge-moment param.oters. Errors in the predicted stick-force characteristics for a pronosed sr)ring-tab design, caused by failure to obtain the desired elevator hinge-moment characteristics, are likewise reduced by this ratio. As a result, the control characteristics of a soring-tab elevator should be more closely predictable than those of a coiaventional elevator, especially on a large airplane. This advantage is somewhat offset bjr the fact that the stick forces obtained with a spring, tab deoend on the hlnge-ruoment parameters of the tap, as well as of the elevator. B.t present, inform.ation on the hinge-moment characteristics of tabs is not very complete. The spring tab should provide an effective means of control in high-speed flight, especially as regards recovery from high Mach number dives, where the control forces on a. conventional •;,levator may become excessive. It is knoiwn that trim tabs may be used to recover from dives, at least at the Mach numbers reached by present- day airolanes, but this procedure is known to be extrem.ely dangerous because, when the airplane reaches lower altitudes and T-.'ach numbers, excessive accelerations m.ay be experienced before the trim tabs can be returned to neutral. The spring tab directly controlled by the stick should eliminate this difficulty. Furthermore, the stick forces with a snring tab would not be lilcely to become excessive in the pull-out. The effects of com'oressl bill ty m,ay in many cases be considered as a large rearvard shift of the neutral point (of the order of 20 to $0 Dercent of the mican aerodynamic chord) . Figures 5 to 6 show that such a shift would lead to excessive stick forces for recovery with a conventional elevator but to reasonable forces for a spi'ing-tab control. In order to effect recovery, the elevator and tail would have to be built sufficiently strong to withstand the large loads imoosed. NACA ARR No. l1;H28 1? PREVENTION 0? FLUTTER H theoretical investigation of the flutter of spring tabs is presented in reference J+ and the practical results are given in reference 5- These reports shov/ that both the elevator and tab must oe mass -balanced about their hinge lines and that the tab mass-balance weight must be placed closer to the tab hinge line than a certain distance defined by the relation ah) In order to be raost effective, the cab mass-balance v/eight should be olaced about half this distance ahead of the tab hinge line. Equation (lU) shows that, if the mechanical advantage of the stick over the tab K2 is reduced to a small value, the tab mass-balance v;eight must be nlaced so close to the tab hinge line that a prohibitively large y;eight may be required. Equation (i|) indicates that Ki and K2 cannot be reduced sim.ul- taneously without unduly decreasing the stick travel. A small val\ae of the mechanical advantage of the stick over the tab has been shown to be advantageous on small airplanes in order to orovide sufficiently large stick-force gradients and sm.all variation of force per g v;ith s-ceed. An exoerimental Investigation to determine the validity of equation (ll(.) is, therefore, urgently required. Because of effects of flexibility in the control linkages, the apolicability of equation (lii) is open to some question in cases in which K2 is small. In some instances spring tabs without mass balance have been used v.-ithout the occurrence of flutter. Special devices v/ith a sm.aller penalty due to v;eight have also been proposed to prevent flutter. STICK-PORCE CHARACTERISTICS IN STRAIGHT FLIGHT In figures 3 to 6, the rear limit of the assumed center-of -gravity range 'was taken as the stick-fixed (actually, elevator- and tab-fixed) neutral point in l8 NACA ARR No. Li+1128 straight flight. Because - — - was taken equal to zero, Oarp this point also represents the stick-free neutral point. For all center-of -gravity positions ahead of this point, the stick-force variation with speed will be stable and the gradient will be reduced by the spring tab in the saiTie proportion as the maneuvering forces. The effects of changes in the hinge-moment parameters §• and -; ^ and the effects of altitude on the neutral point and maneuver point may be shown to follow the same rules v;ith a spring tab as with a conventional elevator. SPECIAL SPRING-TAB ARRANGH/iENTS The formulas set up for the stick forces obtained in maneuvers with a spring tab may be used to determine the characteristics of several special arrangements. Tab controlled inde p endently of elevator . - The mechanism for a tab controlled indeoendently of elevator is shown diagramatically in figure 7(a). This arrangement is a sToecial case of the previously used system in v;hich the elevator gearing constant K] equals zero . The stick-force characteristics may be found from equations (12) and (13) by setting Ki equal to zero. If K]_ equals zero, the value of K2 must be large enough to require full stick travel for full, tab deflection. For airplanes v;/eighing about 50^000 Dounds or less, a small value of K2 was required to provide sufficiently heavy stick forces. The tab controlled independently of elevator would therefore be considered satisfactory only on large airplanes. Formula (15) furthermore indicates that, when K]_ = 0, the elevator will not be constrained to follo'-v stick movements at zero airspeed no matter how stiff a spring is used. The system of figure 7 (a-) will thus have no advantages over a servotab from the standDoint of ground control. The spring should therefore be omitted in order to avoid a force per g that varies with speed. This system is more likely than an ordinary spring tab to result in Instability of the short-period oscillation of the airplane with stick fixed, because the stability of the elevator Itself NACA ARR No. Li,H28 v;ith stick fixed is essentially the same as with stick free. As a result, the dynanuc stability of the airplane with stick fixed is no greater than \vith stick free. With a conventional suring tab such as that shown in figure 1, on the other hand, the effective restoring moment on the elevator with stick fixed is greatly increased by the leading action of the tab, so that the stick-fixed dynamic stability of the airplane is close to the elevator-fixed value. The only benefit that appears to result from the use of the system of figure "J ( a) is a oossibile reduction of stick forces on a very large airplane because of the increased allowable mechanical advantage of the stick over the tab. Use of this alternative does not appear to be necessary, hov;ever, for the largest airplane considered (JOG, 000 pounds weight). Geared spring tab .- The mechanism, for a geared spring tab is shown diagramatically in figure Jib). This device differs from^ an ordinary spring tab in that, when the elevator is m.oved (at zero airspeed) with the stick free, the tab deflects with respect to the elevator in the same m.anner as a conventional geared tab or balancing tab. The stick-force characteristics for a geared spring tab m.ay be calculated by means of the same equations as those derived for an ordinary spring tab, if certain substitutions are made for the hinge-moment pai'ameters of the elevator. These substituted values may be interoretsd physically as the hinge-moment parameters of an elevator equipped v/ith an equivalent balancing tab, which is defined as a balancing tab that has the same gearing ratio as would be obtained on the geared spring tab v/ith stick free at zero airspeed. By means of a geared spring tab, the force per g at low airsoeed may be reduced without decreasing the force per g at high speed and without reducing the response of the elevator to raoid stick movemients on the ground. This device, in fact, presents the theoretical possibility of obtaining a force per g that does not vary with speed regardless of the spring stiffness used. This result may be attained by making dChe/'^ciT ^^'^ -Cht/cciT equal to zero and by using a tab-gearing ratio such that the force ner g at low speed is reduced to the value which would be obtained at very high speed, where the characteristics of a servotab are approached. In practice, it is unlikely that the exact values of hinge-iuomient characteristics required could be obtained. Some variation of force uer g with speed would result if these characteristics 20 NACA ARR No. lLh28 differed slightly from the desired ones. The variation of force ner g ".vith speed would be smaller, however, than that obtained with an ungeared spring tab with the same spring stiffness. Tt therefore appears that the stick- force characteristics shown in figures 5 to 6 could be improved by the use of geared, spring tabs. Stiff er springs, nroviding improved, ground control, could, be used, alternatively for the same variation of force per g with speed. Eri'ors In obusining the desired value of 6Che/'^6e for the geared spring tab may be compensated by adjustment of the tab linkage by trial on the actual airplane. Preloaded spring tsb .- If the tab spring is preloaded- to prevent deflection of the tab until the stick force exceeds a certain amount, the stick force per g will equal that of the elevator without a spring tab up to the point where the stick force reaches the preload. Beyond this point, the force per g will equal the force calculated for an unore loaded spring tab. The force variation with acceleration will therefore be nonlinear, a characteristic usually considered to be undesirable. If friction is present in the tab syster:, an unpreloaded spring tab may not return to a definite equilibrium position and, as a result, the pilot raay experience difficulty in maintaining a specified trim speed. A small amount of preload may be used to center definitely the tab in trimmed flight and thereby to overcome this difficulty. In view of the mechanical complications involved in the use of a preloaded spring, as well as the nonlinear force characteristics mentioned previously, it appears desirable to avoid the necessity for preload by reducing friction in the tab system to a minimum. CONCLUSIONS An analysis of the effects of spring tabs on elevator forces frrr -'^irolanes of various sizes has indicated the f ollowiii,: j'^nclusions : 1. 5v tha use of spring tabs, satisfactory elevator control-f j"oi^ chu.-'^actc.ri sties may be obtained over a large cencor--of -gravity range on airplanes verylng in v/eight fro:';i about l6,ubo to at least. 300,0UC pounds. NACA ARR No. Ll4Ji23 21 2. The soring tab offers the possibility of greatly reducing the changes in stick forces that result from small variations in contours of the elevators on different aii'planes of the same type. 3. The elevator control-force characteristics resulting from the use of a soring tab should be more closely predictable than those with other types of aerodynamic balance such as a balancing tab or inset- hinge balance; in order to take advantage of this effect, however, more complete information on the hinge -moment characteristics of tabs is required. [i-. One of the chief objections to the use of spring tabs is the amount of v/eight required for mass balance to prevent flutter. Exoerimental vi/ork is recomjriended in order to find means of reducing the amount of balance weight required. Langley Memorial Aeronautical Laboratory National Advisory Committee for Aeronautics Langley Field, Va . 22 MCA ARR No. Li].H28 REFERENCES 1. Gates, S. B. • Note? on the Spring Tab. Rep. l\io . B.A. 1665, British R. A. E. , April 19i|l. 2. Greenbei'g, Harry: Calculation of Stick Forces for an Elevator with a Spring Tab. NACA RB No. jJiFO^ , 19144-' 5. Anon.: Stability and Control Requirements for IT. S. Army Airplanes. AaP Specification, June 10, 1943 . h,. Prazer, R. A., and Jones, '.'V. P.: VVing-Aileron-Tab Flutter - (Parts I and II). 5668, O.251, British N.P.L., March 1?, 19i|2. 5. Collar, A. R.: The Prevention of Flutter of Spring Tabs. Ren. No. S.M.E. 3214.9, British R. a. E. , May 19i|3. NACA ARR No. L4H28 23 TABLE I.- CHARACTERISTICS OF VARIOUS AIRPIANES NATION*!. ADVlSOm COMMinEE FOR AEBOtWITlCS Scout bomber Medium bomber Heayy bomber 300,000-pound airplane ^r^ Scale, ft 9 50 9 , ipo 9 , ipo 9 . 1 U 0) ■d CO a o o ■p fl (D 5 0) a OiS •s C! a I • H 4) (< NACA ARR No. L4H28 Fig^ r 1— ADVISOR t AERON/ CD LU 2 S s / / ^'' 'N / . 4^ n / \ \ / / — ^ / / 2 \ A //, <5 Si i M // / ,*v / l // / L ¥ / V I I Cb 3 Si ^ ^ ^ ^ ^ ^J ^ ^ to •d (3 O O U O. O. O CO at > ^^■^ n o o g 0) O a to to u o a o o u o 3--I O O 0] o ^ •»» o ^4 m r-l O aJ j:l r^ « ad o d - O "^ Vt .O Vi • Cri o « Vi -P p o I a d to (D OS d d :3 o -H iH ^ •p a>-H cd cd • tt > O CO •H COr-l 1 a O (tf • >> d o oj -p Vl•r^ Vl CO u oPif d Vl o o. to NACA ARR No. L4H28 Fig. 3 ^ I <90 40 O Confer- of- cjray/fy posif/on, pe rcer?/ /^.A.C. X -^O ■~ •^ ^s ^^^S^S^S s Desired /im/h o /oo ;200 300 400 /nc//c<^/'ec::/ <:?/rspeecf, /Dp/i NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 60 40 ZO O Sp&^c/, m/p/i I S/-/c/r~ f/xeaf r?et/fra/ po/r?t No sjbr/n^ tab^ ^^^^-.003 1 Desired ' l/m/ts -8-4 4 8 Forward Bc?ck Cenfe'r-of'^ra^/fy posif/cn , percenf M.A.C. Figure 3,- Variation of force per g with speed and with center-of -gravity position for scout bomber (weight, 16,OoO lb). NACA ARR No. L4H28 Fig. 4 SO 4-0 O Cent<^y- of- Qrai^/ty po^/ ^/<^^'_ percent M.AC. i Desired ^ ///77//v5 :fi O /OO ^00 300 ^00 /20 _ /ne/zcafecf a/rspeec/, ^^^^. /OO 80 C>0 SffcA' f/xed heufra/ ^ po/nf /n sfra/^/)t ^O ■.-003 -a 'A o 4 & C<^nfer' cf- ^ray/f/ pos/f/on^ percent A7.A C. Figure 4.- Variation of force per g with speed and with center-of -gravity position for medium bomber (weight, 50,000 lb). NACA ARR No. L4H28 Fig. 5 400 \ ZOO o C^^ter-of- /O ys cyr^y'/fy po^z/yor), /pe rcent MAC. i I I O /OO zoo JOO 400 NATIONAL ADVISORY COMMinEE FOR AERONAUTICS Desired linntts 4O0 300 ZOO /OO rjec/f^a/ point = -. 003 Desired ^ //rr?/ts -3 -^ O 4- & For^varc/ Bac/^ CG'nf&r-of-gra>^/fy pos/t/on, p^rcerit AfAC. Figure 5,- Variation of force per g with speed and with center-of -gravity position for heavy bomber (weight, 125,000 lb). 4 I NACA ARR No. L4H28 6O0, Fig. 6 400 200 I Cenfer-of-^rak'/fy pos/f/on, percent M.A.C. ^77T -U o\ \. ^ \ ^SS Des/nsd /frn/Z-s \ I /OO 200 300 400 /n^/catec/ o/rspeed, mph NATIONAL ADVISORY SOO, COMMinEE FOR AERONAUTICS 400 St/c/(-f/xed _ meufrc:// po/nt in sfy^a/^/it 300 ZOO K /OO o Sp a o j5 cd • t ^ cU 5 ^ « ^ ■P ■H w ;h fl a •H CD h -d CO (U U^ Va^ o 03 b* i UNIVERSITY OF FLORIDA 3 1262 08105 000 6 UNIVERSITY OF FLOR/nA DOCUMENTS DEfwTT^pr n- 120 MARSTONS(S^?L GAINESVILLE FL ??rii ^^ •^'- ^2611-7011 USA