EB Ko. L5I05 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WAllTIMIi! REPORT ORIGINALLy ISSUED Octoljer 191+5 as EeBtricted BiOletln L5I05 USE OF 7AELAELE -RATIO GEARED TABS TO IMPROVE STICK-FORCE CHARACTERISTICS IN TURJJiNG FLIGHT By Harold F. Kleckner Langley Memorial Aeronautical Laljoratory 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 wax 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. ®^ DOCUMENTS DCPAPTM^NT Digitized by tine Internet Archive in 2011 with funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/useofvariableratOOIang ^^j ^-}^H ^'r NACA RB "Jo. L5IO5 I'ATIOITAL ADVISORY COMMITTEE FOR AKROKAUTICS RESTRICTED BULLETIN USE OP VARIABLS-RaTIO GEARED TABS TO IMPROVE STICK-FORCE CHARACTERISTICS IN TURNING FLIGPiT By Harold F. Kleckner SUMMARY In xlight tests of an experimental elevator with geared tabs, a cockiDlt control over the tab gear ratio was found to be satisfactory for adjusting the stick force cer g in turning flight according to the pilot's preference. This type of control appears to have appli- cation for increasing the center-of -gravity range for satisfactory stick forces in turning flight. Sample cal- culations made for a fighter airplane indicate that sat- isfactory stick forces in turning flight can be obtained for any center-of -gravity position at Vifhich the elevator control meets other requirements. INTRODUCTION Because of the increase in weight and in altitude range, most recent airplanes meet Army and Navy specifi- cations for satisfactory stick forces in turning flight for only a lim.ited center-of -gravity range. Attempts have been made to provide satisfactory stick forces for an increased center-of-gravity range by the use of spring- tab elevators and by a reduction in the variation of ele- vator hinge moment with elevator deflection Chs - If any appreciable increase in center-of-gravity range is obtained by either of these methods, it is then necessary to add a bobweight or to make the variation of elevator hinge moment with tail angle of attack Cho, positive in order to increase light stick forces in turns at normal center-of-gravity positions. Arrangements of this kind have introduced undesirable control feel in rapid maneuvers and poor control characteristics in rough air. With spring tabs, in addition, difficulty in preventing KACA R3 No. L5IO5 flutter and in maintaining the stick force per g suffi- ciently constant throughout the speed range may be encountered . A method is suggested for increasing the center-of- gravity range for satisfactory stick forces in turning flight by the use of geared tabs and a cockpit control over the gear ratio. Flight tests have been made ^;vith an arrangement of tills kind, and pilots' opinions of such a control are nov/ available. SlY:BOLS bg elevator span, feet "c iving mean aerodynamic chords feet Cg elevator root-mean-square chord, feet Cb^ variation of elevator hinge m.oment -with elevator deflection — =i Cbn variation of elevator hinge moment with tail angle of attacK I — — j Cj^ elevator hinge-mom.ent coefficient h .1^e~e^y H hinge moment, foot-pcunds q dynamic pressure, pounds per square foot C Liji t al 1 li ft c ef f 1 ci ent '^I,Ti tall lift-curve slone, Toer raaian ( —\ \ • Clrp / K ratio of sticl-: m.ovement to elevator deflection, feet Der radian NACA RB No. L51O5 3 lip tall length, distance from center-of -gravity position to elevator hinge line, Test Sip horizontal--Dail area (exclusive of area through fuselage), square feet W airplane weight, pounds aq\ tail angle of attack, radians 5q elevator deflection (positive down), degrees 5^ tab deflection (positive down), degrees 5t/Se tab gear ratio elevator effectiveness factor PLIGHT TESTS A cockpit control over the tab gear ratio was used during flight tests of an experimental all-movable hori- zontal tail on a fighter airplane. 'With the all-movable tail the tabs were used as geared unbalancing tabs, and ratios of tab deflection to elevator deflection 5t/5e of 0.6 to 1.0 were available. At the center-of -gravity location tested, the corresponding stick force per g In turning flight varied approximately from 6 to 10 pounds at an altitude of 5OOO feet. The control was found to be satisfactory for adjusting the stick force per g to the pilot's preference. The pilots preferred a force of about 8 pounds per g in the present flights, which did not Involve high accelerations. The pilots reacted favorably to the control and did not think It confusing in any way. It is believed that, after elementary education in the function of the control, pilots './oTild use It as instinctively as they use the trimming controls. !(. NACA RB Eo. L5I05 TYPICAL APPLICATION AND DISCUSSION In the flight tests of the all-inovahle tall with variable-ratio geared tabs, the control characteristics could not "be detemined ever a large center-of -gravity range. In order to illustrate the increased range for vtThich satisfactory stick forces in turns can be obtained, therefore, calculations have been made for a typical fighter airplane with conventional elevators. The char- acteristics of this airplane are given in table I. The variation of stick force per g vjith center-of -gravity was calculated frori the ecuation Stick force per g WcbgC = O.OlChg - e^e Percent change in c. g. position " Kt /"CLq^j S,j.Jrr, In this equation the hinge moments of the tab are neg- lected. Calculations were made for t^vo sets of values of the elevator hinge -moment coefficients Ch^ and Ch- , For the first example it was assumed that Cjiq - and Chg - -0,002. The value of Chg^ = may be obtained in practice by a suitable choice of the elevator contours. For the second example it v\/as assuijied that Cha ~ -0.001 and Cyi- ~ -O.OOJSl these values are typical of those obtained on elevators of several current fighter air- planes. For both exam.ples the elevs^tor hinge-moment coef- ficient due to tab deflection was assumed to be -O.OC55> values close to -0.0055 have been obtained in practice with tabs the same size as those of the airplane con- sidered for the calculations. The cvirves showing the variation of stick force per g with center-of -gravity position for the two examples are presented in figure 1. The range of satisfactory stick forces is indicated in figure 1 in accordance with the requirements of reference 1, The locations of the stick-fixed neutral point and the stick-fixed maneux'er points were estimated from flight-test data on an air- plane of this type. The stick-fixed neutral point is the center-of -gravity position at which the variation of ele- vator an'^le with airplane lift coefficient is zero in NACA R3 No. L5IO5 straight flight. The stick-fixed maneuver point is the center-of -gravity position at v/hich the variation of ele- vator angle with airplane lift coefl'icient is zero in turning flight. Figure 1 shows that a greatly increased center-of - gravity range for satisfactory sticV. forces in turning flight is possible with a variable-i-atlo geared tab con- trol. In addition the control pemits the pilot to adjust the force to his own preference at any intermediate center- of -gravity position. The center-of -gravity range can be extended forward any arbitrary airiount if sufl'icient tab pov'/er and elevator balance are provided. The most forward center-of -gravity position then will bs determined not by limits on stick forces in turning flight but by other con- siderationsj such as the elevator reflection available to meet landing requirements. Figure 1 also shows tnat the most rearward center-of -gravity position for which the variable-ratio tab control can be used to obtain satis- factory stick forces in turning flight is somev\?hat forward of the stick-fixed maneuver point. This limit will be at least as far rearwai'd as the limit set by the stick-free or stick-fixed neutral points. iVith a control over the gear ratio of geared elevator tabs, thsrefore, satisfactory stick forces in turning flight could be provided for any center-of -gravity position at which the elevator control v/ould meet other requirements . When Cho, is negative, a condition of stick-free instability in turns would be obtained if the control over the tab gear ratio were used in the position to give light stick forces at a rearward center-of -gravity position: for example, if a tab gear ratio of -0,5 (fig. 1(b)) were used at 52 percent mean aerodynaniic chord. This condition v/ould be dangerous in take-off and, although an item, could be included in the check-off list for take-off to have the control in position to give heav^'- stick forces, the possi- bility of this condition should be avoided by the use of an elevator with Cho. ~ ^ •■ A second advantage is gained if an elevator with Cha = is used. When Ch.Q^ is negative, larger negative values of Ch^ are required to obtain sufficiently heavy stick forces at rearward center-of -gravity positions and larger tab gear ratios or larger tabs are required to provide a given increase in centor-of-gravity range. (Compare figs. 1(a) and (b).) 6 NACA RB No. L5IO5 It has recently been found that zero is the approxl- Kiate limit to which Cho^ can be changed in the positive direction when Chs is small imless soine aujciliar:/ device is used to eliminate the undesirable control feel in rapid maneuvers. Elevators v.ith Ch.Q ~ have been found sat- isfactory when Che was sufficient to give 5 to 8 pounds per g. An elevator with Ch^^ = and with Chg large enough to give only 2 or 5 pounds per g would probably give uncomfortably light stick forces. vVhen the tab ratio is changed, the stick-free sta- bility in straight flight (variation of stick force with airspeed) will change as well as the stick force per g in turning flight. If the variable-ratio tab control is used to maintain an essentially constant value of stick force per g through the center-of -gravity range, more nearly constant stick-fi'ee stability in straight flight through the center-of -gravity range will result than is obtained v.'ith conventional elevators. In addition to improving stick-force characteristics in turning flight, a variable-ratio geared tab control may be useful in compensating for variations in Ch^ and Chs due to manufacturing tolerances and other factors that prevent accurate prediction of hinge-mioment characteristics . DHSICtN C0FSID3RATI0MS Tab power greater than is normally provided for trimmilng may be necessary if the elevator tabs are employed both as geared tabs and as trim tabs. If a reasonable amount of elevator balance is provided, however, the necessary tab gear ratios will be small and the added dem.and for tab power will not be difficult to meet. The required tab ratios may be reduced by pro- viding sufficient elevator balance to allow the tabs to be used as unbalancing tabs for rearward center-of -gravity ■nositions and as balancing tabs for forward center-of- gravity positions. Tab power additional to that normally provided may also be necessary to trim the stick force in the landing NACA RB -^lo. L5I05 approach if the center-of -gravity range is extended for- ward. The amount of additional tah pov;er for tiiis purpose will of course depend on the amount of elevator unbalance and the size of the tabs, but estinates from flight-test data indicate that a tab deflection of approximately 1^ per percent center-of -gravity movement wtsuld be needed for the elevators assumed in the first example. In order to avoid large deviations from linearity in the curves of elevator hinge moment with elevator deflec- tion at large elevator deflections , the maximum tab deflec- tions should be limited to reasonable values, probably not greater than 20°. A small-chord tab of large span is, in general, preferable to a large- chord tab of small span. The job of providing a cockpit control ever the tab gear ratio is structurally and mechanically similar to that of providing trim-tab control. A diagrammatic sketch of an installation that would provide variable tab gear ratios and tab movement for trimming is shown as figure 2. Definite stops should be provided to limit the adjustment of the tab gear ratio to the amount needed to give saisis- factory stick forces over the cent er-of -gravity range that is to be used. If stops are not provided, a condition of zero or positive Chg might be obtained. This condi- tion, of course, must be avoided. CONCnJDING RSI.'ARKS In flight tests a cockpit control over the gear ratio of a geared elevator tab was found to be satisfactory for adjusting the stick force per g in turning flight according to the pilot's preference. This type of control appears to have application for increasing the center-of -gravity range for satisfactory stick forces in turning flight. Samole calculations made for a fighter airnlane indicate that satisfactory stick forces in turning flight can be obtame,-^ for any center-of -gravity position at v/hich the elevator control '.ncets other requirements. Langley J'emorial Aeronautical Laboratory National Advisory Committee for Aeronautics Langley Field, Va . NACA RB Ko. L5IO5 RIIFEREliCE 1. Anon.; Stability and Control Characteristics of Air- olanes, AAF Srecification No. R-l8l'^-A, April 7, 19^5- NACA R3 No. L5IO5 9 TABLE I AIRPLANE CKARACTERISTIC3 USED IN CALCULATIONS Airplane weight, pounds ...,,..„ 11,000 Vvlng area, square feet « 35[|. Wing span, feet , o ..,...., = . 1|2 ,8 vVing mean aerodynamic chord, feet 8.12 Tail length, feet 21.Ii. Horizontal-tail area, square feet .69.5 Elevator area, square feet '25.8 Elevator span, feet . I8 . 5 Elevator root-mean-square chord, feet . 1,L;.6 Tab span, feet i;,85 Tab chord, foot . O.I4.2 Tail lift-curve slope, (^Ln) > '9^'^ radian .... 5.7 Elevator effectiveness factor, T ... O.5 Elevator deflection, degrees ........... ±25 Ratio of stick movement to elevator deflection, feet per radian 1 .72 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS NACA RB No. L5I05 Fig. la,b /6 IZ I I 1 k n \ 1 \^ Stick - //x.?^ \ 1 Alfihde neutral point 1 1 \ \ /•^•y y / x^y^ r// e t ■^ ff^/ . • •• . • •yy, yyy^, yy ••. ■/• • y y yyyy ■^ •^ 000 '^N'"' \ \ •v 1 1 1 ^e^/'o/? of 20. C ;o5"- •- \ \ sof'/s factory \ ^//ijXr forces ^■~. \ \ 1 ""■" ^ i::;i"^ stick - , f/xed mar?euyer po/nfsy^ 's-- "^■. /^ /6 20 24- 28 32 36 Center-of-yrcf^^/f/ position j percent t/t.A.C. Ao (a) Ch^ = 0; Chg = -0.002. 16 NATIONAL ADVISORY COHMITTEE FOt AERONAUTICS ^ 12 3 St/Se = -O.S y///y/ /yy/ N ^ // / /y,\ h /Se -■ 20, OOO // /yy ^ / / / yV //f y ^S,ooo Altitude (ft) £;ooo f , t I \ r //■ ^f / // f / / df ff/i ■ f f y ^ '/ y / f ^ -Stick 'f/xed neutral point N 20, OOO ^eyfan of satisfactory st/'ck forces T" I im^ \ '> > "" -nrrr.'ii// I.I Dili TT-T-nTTTTT ' 11?/^/// f/^r I /i/\i 1 1 1)^1 1 — :^i I /y N \ ^. \^ \. \ \ \ -~ \ N \ I St/ck -f/xed 6 /naneu/er points^ "n 12 IG 20 24 28 32 3G Center- of -yrov'ity position , percent tvl.A (b) Chj^ = -0,001; Chj = -0.0035. Figure 1 . 40 .C. Variation of stick force per g wit position for a .typical fighter ai; ;nter-of -gravity ne. Fig. 2 NACA RB No. L5I05 .Ci V N •^^ H. ^ j::^ ^ V. Vj .1 ^ v^ «o '^ ^ ^^ ^ t ^ o C^ ^t J< ^ ■K V^ tS 8 u > K a. 3 O z '£l o -i 9 < **- Z uj o ^ < z o rH C ca g > +^ 0. BOX 11 70 11 GAINESVILLE. PL 32611-7011 USA ' I