1、.- NASA TECHNICAL NOTE NASA TN D-3583 =+“ -0e3 e,/ -I 00 -r lh 0-I+= = T w- * n 0- = e= 4 z -* x c LOAN COPY: RET1 3B“ -4 AFWL (WLIL. Ew z KIRTLAND AFB, r GROUND EFFECTS RELATED TO LANDING OF AIRPLANES WlTH LOW-ASPECT-RATIO WINGS by WiZZium B. Kemp, Jr., Vernurd E. Lockwood, und W. Pelhum PhiZZips L
2、ungley Reseurch Center LungZey Stution, Humpton, Va. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION OCTOBER 1966 f Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-TECH LIBRARY KAFB,NM I111111 11111 11111 11111 IIIIIllllllllll11llIll1 GROUND EFFECTS RE
3、LATED TO LANDING OF AIRPLANES WITH LOW-ASPECT-RATIO WINGS By William B. Kemp, Jr., Vernard E. Lockwood, and W. Pelham Phillips Langley Research Center Langley Station, Hampton, Va. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION For sale by the Clearinghouse far Federal Scientific and Technical Inform
4、ation Springfield, Virginia 22151 - Price $1.00 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-GROUND EFFECTS RELATED TO LANDING OF AIRPLANES WITH LOW-ASPECT-RATIOWINGS By William B. Kemp, Jr., Vernard E. Lockwood, and W. Pelham Phillips Langley Res
5、earch Center SUMMARY Some results of a study of the influence of ground-induced aerodynamic effects on the landing maneuver of airplanes with low-aspect-ratiowings are presented. The fundamental mechanism of ground induction is reviewed and a simplified landing-flare analysis is used to illustrate t
6、he signifi cance of the ground-induced pitching moment, the load factor just before touchdown, and the ground effects on the elevator characteristics. Some effects of wing planform and airplane size are shown by use of dynamic cal culations of airplane motions during the landing flare. A constant-pi
7、tch attitude landing flare is shown to be possible for some large airplanes with low-aspect-ratio wings. INTRODUCTION Many airplane designs proposed for supersonic missions have employed low-aspect-ratiodelta-related wing planforms. The achievement of appro priate lift coefficients for landing with
8、these wings requires angles of attack so high that provision of adequate ground clearance is a serious problem and may possibly necessitate lower wing loadings or longer landing gears than would be desirable otherwise. Some wind-tunnel measurements on low-aspect-ratiowings have indicated that the ef
9、fects of ground proximity may allow the angle of attack at a landing touchdown to be several degrees less than that required to obtain the same lift coefficient away from the ground, and thus may significantly alleviate the ground.-clearanceproblem. Furthermore, since this angle-of-attack change due
10、 to ground proximity may be of the same order of magnitude as the flight-path angle in a normal landing approach, the execution of a landing-flare maneuver without changing airplane attitude appears within the realm of possibility. The constant- attitude flare is viewed in some quarters as being con
11、siderably easier and therefore safer than a conventional landing-flare maneuver. With these considerations in view, a study has been initiated at the NASA Langley Research Center using wind-tunnel experiments and analytical procedures to determine the influence of several configuration parameters on
12、 the ground effects on low-aspect-ratio wings. It is the purpose of this paper to present some highlights of the findings to date as they relate to the landing maneuver. lpresented at the classified “Conference on Aircraft Aerodynamics,“ Langley Research Center, May 23-25, 1966, and published in NAS
13、A SP-124. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SYMBOLS A IC h hh3 n S t V W a 6 Mi,G hF BF Y 9 A 2 wing aspect ratio wing mean aerodynamic chord, ft elevator chord, ft lift coefficient drag coefficient pitching-moment coefficient rate of c
14、hange of lift coefficient with elevator angle at constant angle of attack, per degree rate of change of pitching-moment coefficient with elevator angle at constant angle of attack, per degree height above ground of a point on the wing chord plane at the longi tudinal location of the center of gravit
15、y, ft height of landing gear above ground, ft normal load factor wing area, sq ft time, sec airplane velocity, knots airplane weight, lb angle of attack, deg elevator angle, deg ground-induced increment in any coefficient Ci angle-of-attack change during the flare, deg elevator-angle change during t
16、he flare, deg flight-path angle, deg airplane pitch attitude, deg leading-edge sweepback angle, deg Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Subscripts: A conditions in free-air approach G conditions at ground contact DISCUSSION Comparison of
17、Conventional- and Delta-Wing Airplanes The first two figures compare the ground effects on two airplane configu rations having widely different aspect ratios. Figure 1 illustrates the ground effects measured on a wind-tunnel model typical of a conventional subsonic jet transport with an aspect-ratio
18、-6wing. Drag coefficient, angle of attack, and pitching-moment coefficient are shown as functions of lift coefficient. The solid curves represent the characteristics in free air and the dashed curves correspond to the wheels touching the ground. The data of figure lwere obtained with a tail incidenc
19、e of -6O relative to the wing chord plane and a trailing-edge flap deflection of 600. The flap was designed for use with blowing at the slot lip but the data of figure 1were measured without blowing, and thus the maximum lift coefficient is lower than would be expected from a flap system designed fo
20、r use without blowing. At the lower lift coefficients, the ground effect produces a small increase in lift at a given angle of attack. The maximum lift coefficient, however, is significantly reduced by proximity to the ground. These trends are typical Of the ground effects observed on configurations
21、 with wings of moderate to high aspect ratio. For comparison, figure 2 shows the corresponding characteristics of a tailless model having a 55 clipped delta wing with an aspect ratio of 2.26 with no twist, camber, or flap deflection. Again, the drag coefficient, angle of attack, and pitching-moment
22、coefficient are plotted against lift coef ficient for free air and for a height representative of a wheel touchdown condi tion. Since the low-aspect-ratiowing does not exhibit a true stall, the ground effects on maximum lift coefficient need not be considered. Lift coef ficients appropriate for a la
23、nding approach are indicated for each configura tion. Observe that at the approach lift coefficient, the ground effects on the low-aspect-ratiowing allow a reduction in angle of attack of more than 3O from free air to touchdown, whereas the corresponding angle-of-attack reduction for the subsonic je
24、t configuration is only about 1/2O. Both configurations show significant drag reductions in ground effect. Although these drag reductions would affect the speed bleed-off in a landing flare, further analysis of the ground effects on drag is beyond the scope of this paper. The pitching-moment charact
25、eristics show that both configurations experi ence a modest increase in static stability in proximity to the ground, with a resultant increase in nose-down moment at the approach lift coefficient. The 3 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,
26、-effect of trimming out this moment change is discussed in a subsequent section. The data of figures 1 and 2 show that the angle-of-attack increment produced by ground proximity is of greatest interest for the low-aspect-ratio configuration. The present study has, therefore, emphasized the low aspec
27、t ratios. Mechanism of Ground Induction Consider an airplane flying in close proximity to the ground (fig. 3). The effect of the ground is to prevent the existence of any vertical air velocity at the ground plane. If the ground is replaced by an inverted mirror-image air plane flying under the groun
28、d, all vertical velocities induced by the airplane and its image are canceled at the plane of symmetry. Thus, the effects of the image airplane are identical to the effects of a ground plane. Figure 4 shows the airplane in side view with a typical chordwise distribution of lift due to angle of attac
29、k. The aerodynamic center is at the centroid of this distribu tion. This same distribution of lift is represented on the image airplane as a system of lifting and trailing vortices. The image vortex system induces upwash velocities in the wing chord plane that may be distributed somewhat as shown in
30、 the middle sketch. The induced upwash over the region of the wing has an average value which is equivalent to an angle-of-attackchange and a gradient which is equivalent to a camber change. The equivalent camber would induce a lift whose center would be near the rear of the wing. The combined groun
31、d-induced lift may be distributed as shown by the lower sketch and its center of pressure would be expected to lie behind the aerodynamic center and produce a nose-down ground-induced pitching moment. Experimental Program The experimental program recognized the importance of the ground-induced pitch
32、ing moment by placing some emphasis on the configuration of the elevators used to trim out the moment. Figure 5 illustrates two of the wind-tunnel models used. The wings were of delta planform with clipped tips and had leading-edge sweep angles of 55 and 70. Some data obtained on this 35 wing were s
33、hown in figure 2. Elevators having chords of about 10 percent and 20 percent of the wing mean aerodynamic chord were examined. The models were tested in a wind tunnel at various heights above a ground plane. A moving-belt ground plane was used to remove uncertainties even though a correlation discus
34、sed in reference 1 indicates that the moving belt was unnecessary for these models. The ground effects measured on these two models are compared in figure 6. The ground-induced increments in lift and pitching moment at zero elevator deflection, and the ground-induced increments in the elevator lift
35、and moment parameters, are shown as functions of height above the ground. The ground- induced increments were measured at a constant angle of attack of 12O. The lift increment and the increments of the elevator parameters are each normal ized by their respective free-air values. 4 Provided by IHSNot
36、 for ResaleNo reproduction or networking permitted without license from IHS-,-,-For the 55 wing, shown by the solid curves, all of the parameters increase continuously with decreasing height. For the 700 wing, the lift at zero ele vator and the lift due to elevator deflection also increase continuou
37、sly but the two moment parameters show a reversal in the ground-effect trend at the lowest height. Although this trend reversal is not fully understood at present, it may be associated with the formation of an effective venturi throat between the ground and the wing trailing edge that may cause nega
38、tive pressures on the underside of the wing near the trailing edge. It is possible that the 55 wing may also have shown some tendency toward trend reversal if it had been tested closer to the ground. The lowest points shown for each wing, however, represent heights that are appropriate for wheel tou
39、chdown. The ground-induced lift increment at touchdown is seen to be nearly the same for both wings. Simplified Landing-Flare Analysis In order to assess the importance of these ground effects on the landing maneuver, the analysis procedure illustrated in figure 7was used. Two flight conditions are
40、assumed. The first is a steady-state landing approach glide out of ground effect on a straight descending flight path. The second repre sents the conditions at the instant of wheel contact with the ground. The flight path here may be curved end is usually at a flight-path angle less than that in the
41、 approach. The curvature of the flight path requires a normal load factor somewhat greater than 1. The normal load factor at ground contact may be expressed approximately as the ratio of the lift coefficient at ground contact to that in the steady-state approach. Now if the approach lift coefficient
42、 is known and a value for normal load factor is assumed, the lift coefficient at ground contact may be determined. The wind-tunnel data may then be used to find the trimmed angle of attack in the approach and at ground contact corresponding to the appropriate lift coefficients and ground heights. Th
43、e change in angle of attack during the flare is denoted by the symbol LhLF and is a sortof ground- effect figure of merit as determined in the wind tunnel. The significance of this parameter is indicated by the second equation of figure 7,which was derived by using the angle relationships shown in t
44、he sketches. For a smooth landing, the flight-path angle at ground contact should be nearly zero. Thus, for any given value of the approach flight-path angle, the change in airplane attitude required to achieve this change in flight-path-angle is determined by LaF. If, for example, the value of AUF
45、were -2lo, a very reasonable landing flare could be achieved with no change in airplane attitude. The change in elevator angle during the flare can also be determined from the wind-tunnel data and is a measure of the required pilot activity. Application of this analysis procedure to the data for the
46、 55O delta wingyields the results given in figure 8. The angle-of-attack increment ce=o.zc A, DEG -55 -U- 70 b k-a b a -2 .4 0 -.02 -.04 -.06 0 .2 .4-.2 0 .2 LO( .a .6 WE .4 .2 0 ACL,G Acm,G AcL8,G “m8, G CL,A %,A Cm8,A Figure 6 11 Provided by IHSNot for ResaleNo reproduction or networking permitted
47、 without license from IHS-,-,-BASIS OF DATA ANALYSIS “G =-CL, G CL. A APPROACH,FREE AIR GROUND CONTACT Figure 7 EFFECT OF TRIMMING AND FLARE LOAD FACTOR 55O DELTA WING ; c, z 0.2 F -.8 .6- -CL, A .4 -.2 “ -6 -4 -2 0 -8 L0-4 AaF,DEG ASF, DEG Figure 8 12 Provided by IHSNot for ResaleNo reproduction or
48、 networking permitted without license from IHS-,-,- - - COMPONENT BUILDUP OF GROUND EFFECT .8 .6 CL,A .4 .2 .8 .6 CL,A .4 .2 0 55“ DELTA WING ;TRIMMED ; nG = 1.1 GROUND EFFECT INCLUDED IN : c, (a) C,(a), c, (a) C,(a,8), C,(a,8) k 1 -8 -4 AF,DEG Figure 9 EFFECT OF ELEVATOR CHORD 55“ DELTA WING ; nG 1
49、.1 ce /E 0.2 0.I k -4 -2 0 -8 -4 AaF, DEG ASF, DEG Figure 10 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- CONSTANT-ATTITUDE LANDING FLARES ce r 0.1 E ;VA= 131 KNOTS ;YA= -2.75O 4 WING AREA, W/S, DEG SQ Fi LWSQ FT 70 8000 35 55 8000 40 55 889 40 -10 I
