1、REPORT No. 695DETERMINATION OF GROUND EFFECT FROM TESTS OF A GLIDER IN TOWEDFLIGHTBy J. W. WETMORE and L. I. TURNER)Jr.SUMMARYAn investigation was made tojind the e$ect of the groundon the aerodynamic churacteristtis of a Franklin PSI?glider. The lift, the drag, and the angle of attack of thegltier
2、in towed flight were determined at several heightsfrom 0.14 to 1.19 span lengths and at various speeds foreach height. Two wing arrangements were tested: theplain wing, and the wing with a nearly full-span 30-percent-chord split flap de$ected 45.For both wing arrangements, the results showed adecrea
3、se in the drag coefiient and the angle of attack fora given liji coefiient when the wing was aected by theground; for the $apped wing, which was the only onetested at two di$erent heights near the ground (0.14 and0.33 span length), the reduction in drag was greater atthe smaller height but the chang
4、e in angle of attack wasapproximately the same at both heights.The experimental results for the plain wing were in goodagreement with theoretical values calculated by the methodof Wieselsberger for both the angle of attack and the dragcoefiient at a height of 0.21 span length; Tanis refine-ments of
5、the theory had a practically negligible eect onthe computed values in this case. .For the jlapped wing,the ground eJect on the drag coefiient as calculated bythe extended treatment of Tani was in better agreementwith experiment, in general, than the predictions byWieselsbergers method. With regard t
6、o ground efecton the angle of attack oj the wing with split$ap, the resultsdid not indicate either treatment as definitely preferablealthough it appeared that, in this case, Wieselsbergersmethod probably agreed better m“th experiment.INTRODUCTIONThe fact that the close approach of an airplane tothe
7、ground is accompanied by substantial changes inits aerodynamic characteristics has been known forsome time; and a considerable amount of research,both theoretical and experimental, has been the appear-ance of the smoke in the phototheodolite photographsthus afforded a means of synchronizing the reco
8、rds.During the tests, the wind speed near the ground wasmeasured by an indicating vane-type anemometer.TESTSThe towing tests were made on a concrete runwayabout one-half mile long. Approximately one third of theavailable distance was used in accelerating to the desiredspeed, attainiig the prescribed
9、 height with the glider,and then establishing as nearly steady conditions aspossible before taking records. During the secondthird of the run, the phototheodolite and the gliderinstruments were switched on for a period of 6 to 8seconds. The rest of the course provided space inwhich to land the glide
10、r md bring it to a stop. Testswere made only when the wind was less than 5 milesper hour and parallel to the coume in order “to avoid,as far as possible, discrepancies due to vertical currentsand yawing of the glider. This precaution also per-mitted making test runs in both directions.With the plain
11、 wing, two groups of tests at diflerentheights were made, each covering a range of speedsfrom 36 to 54 miles per hour. For one of these groups,the average height of the wing above the ground was0.21fI and for the other, 1.17b. Three series of tests atdifferent heights were made with the split flap.
12、Thespeeds ranged from 30 to 38 miles per hour and theaverage heights were 0.14b,0.33b,and 1.19b.The towing tests were originally expected to show theeffect of the ground on the maximum lift as well as onthe aerodynamic characteristics in the unstalled-flightrange. It was found impossible, however, t
13、o obtainsteady conditions in towed flight near maximum liftbecause the longitudinal control was insticient toovercome the nose-down pitching moment of the towingforce, which became relatively large at the higherangles of attack. Special tests made to investigatenaximum lift consisted in dete rmining
14、 the lift coefE-fient in actual landings and in simulated landings at aconsiderable altitude to which the glider was towed withm airplane. Before each of these maneuvers, the$.ider was released from the towline so that the difficultyiue to the moment of the towing force was avoided.Lhe simulated lan
15、dings at altitude were made onlyvith the plain wing because it was considered inadvis-ble to attempt an airplane tow with the split flapnstalled.REDUCTION OF DATAInasmuch as the duration of the instrument records)btained in difTerent runs varied appreciably, the.ecords of the glider instruments were
16、 divided into;ections, each covering 2 seconds of time in order that;he final values computed from the data might all bef equal weight. Mean values of the quantities meas-ed by the various instruments were then determinedior each 2-second period.r!FIGURE 4.Forms on glider in towed fright.The forces
17、acting on the glider in towed flight arehown in figure 4. The symbols used in reducing thedata are as follows:wLDTRRzA.e*AY;v,hi$0.CDgross weight.lift.drag.towing force measured by dynamometer.resultant of L, D, and T.component of R along normal, or Z, axis of glider.ratio RZ/W measured by accelerom
18、eter.angle of R relative to Z-axis measured by pendu-lum inclinometer.angle of T relative to X-axis measured by dyna-mometer.attitude angle of X-axis relative to horizontalmeasured by photoinclinometer.flight-path angle.angle of attack.air speed along flight path.vertical velocity.height of quarter-
19、chord point of wingaboveground.density of air.wing area.lift coefficient.drag coefficient.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-298 REPORT NO. 695NATIONAL ADVISORY COMMITTEE FOR AERONAUTICSThe X axis and the Z axis of the glider were define
20、das parallel and normal, respectively, to the angle-of-attack reference shown in figure 3, which was a linetangent to the lower suxface of the wing at two points.Values of Iift, drag, and angle of attack were derivedfrom the instrument data for each 2-second interval inaccordance with the following
21、procedure:The value of the resultat_obtained from the relationsR,= WA=andof L, D, and T wasB=- i. e., in the lift and the dragdirections, orL=R cos (e)+T * (1#-cx)andD=T cm (+cY)-.R sin (L9-a)The lift and the drag coefficients were found from theusual reationsc,.= andDc.= $V2.RESULTSThe experimental
22、 values of lift and drag coefficientsand angles of attack for all the test conditions areplotted in figures 5 to 9. Figures 5 and 6 present theresults obtained with the plain wing at heights of 1.17band 0.21b, respectively. Figures 7, 8, and 9 show theresults with the split flap at heights of 1.19b,
23、 0.33b, and0.14b, respectively.The faired curves for various conditions, defined bythe experimental points of the foregoing figures, areplotted together for comparison in figures 10 and 11.Figure 10 shows the effect of variation in height on theaerodynamic characteristics of thefigure 11 gives corre
24、sponding resultsplain wing, andfor the split flap.K additi and, consequently, the fairingof the data for the split flap was less certain. Thisdifference is probably the result, in part, of considerableunsteadiness in flight, apparently due to a reduction inlongitudinal stability of the glider caused
25、 by the splitflap.The probable deviation of the results, as defined bythe faired curves, is estimated to be as follows:With the plain wing: With the split flap:c., +0.01 c., thedifferences increased with increasing lift coefficient.With the split flap, the range of lift coe5cientscovered in the test
26、s was considerably higher thanwith the plain wing, as shown in figures 10 and 11.As previously explained, the reliability of the results atlift coe5cients above 1.5 is very uncertain; hence,such results will not be considered in this discussion.Below this value of lift coefficient, the angle of atta
27、clcand the drag coefficient for a given lift coefficient weredecreased when the wing was near the ground, as inthe case of the plain wing, but the reduction was con-siderably greater,Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-GROUND EFFEOT FROM
28、TESTS OF A GLIDER IN TOWED FLIGHT 299(a) Variation with angle of attack.FIGUBE 5.Lift and drag e.hamcteristfc h/b=l.19. Franklin PS-2 glider.and as a means of calculating its effect with reasonable results. Ground-effect theory is a particular case ofaccuracy. More recently the theory has been exten
29、ded multilane theory; the actual system composed of thoby the method of references 4 and 5 to include factors airfoil and the ground is assumed to be replaced by aProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-GROUND EFFECThypothetical biplane cellu
30、le consisting ofFROM TESTS OF A GLIDER IN TOWED FLIGHT 301the real wing I The change in the aerodynamic characteristics of theand its image reflected in the gr0 Q%0/.2 .28; L2%ukb“ 2 Go .24: -/.D.Q.$? t ub Qal $0 QJ8 o*. .20 U.8w $4.6 .16 .6.4 ./2 .4,2 .08 .2.(a) (b)4 o 04 0Angle of %fork, n!,8deg /
31、2 ./8 .22 .26 .30Dmg coefficient, CD .34(a) Variationwith angleofattsck. (b) Polar diagram.FIGURE 8.Lift and drng chamctcristi split flap deflected 45”; f=o.33. Franklin PS-2 glider.equal spans, equal chords, zero stagger, and a gap twice induced vertical elocity at the real wing due to thethe dista
32、nce of the real wing from the ground. The lifts. trailing vortices of the image wing; (2) reduction of theof the wings are of equal magnitude and opposite sign. longitudinal velocity at the real wing due to theProvided by IHSNot for ResaleNo reproduction or networking permitted without license from
33、IHS-,-,- . . . _302 REPORT NO. .695-NATIONAIJADVISORY COMMITTEE FOR AERONAUTICScirculation about the image wing; (3) change of circula-tion about the real wing due to the bound vortices ofthe image wing; and (4) change in the flow pattern due1.8 .40011.4H- Vlllll-xillt/i “ iI eH-+-t=Y”&v .8 .204.6 .
34、f6.4 ./2.2 .08(a)Q4 o .044 8 /2Angle of uffuck, e/, deg(a) Variation with angle of attack.(3),and (4) in the case of a, and (2) in the case of CD.The results of the investigation, as subsequently dis-cussed, indicate that the refinements had a practically1.800-.- 0f.6 YD 00 0 001.4 0/o(t1.2 & 0e 0*.
35、C1.o-w().-O.3c.1.2.3.4.5.6.7.8.9.10.11.12.13.REFERENCESLe Sueur, Maurice: Ground Effect on the Take-Off andLanding of Airplanes. T. M. No. 771, N. A. C. A., 1935.Pistolesi, E.: Ground Effect-Theory and Practice. T. M.No. 828, N. A. C. A., 1937.Wieselsberger, C.: Wing Resistance near the Ground.T. M.
36、 No. 77, N. A, C. A., 1922.Tani, Itiro, Taima, Masuo, and Sirnidu, Sodi: The Effect ofGround on the Aerodynamic Characteristics of a Mono-plane Wing. Rep. No. 156 (vol. XIII, 2), Aero. Res.Inst., Tokyo Imperial Univ., Sept. 1937.Tani, Itiio, Itokawa, Hideo, and Taima, Masuo: FurtherStudies of the Gr
37、ound Effect on the Aerodynamic Char-acteristics of an Aeroplane with Special Reference to TailMoment. Rep. No. 158 (vol. XIII, 4), Aero. Res. Inst.,Tokyo Imperial Univ., Aov. 1937.Sears, William R.: Ground Effect with Special Referenceto Pitching Moments. Jour. Aero. Sci., vol. 5, no. 7,May 1938, pp
38、. 281-285.Ti5nnies, E.: Effect of the Ground on an Airplane FlyingClose to It. T. M. No. 674, N. A. C. A., 1932.Recant, Isidore G.: Wind-Tunnel Investigation of GroundEffect on Wings with Flaps. T. N. No. 705, N. A. C. A.,1939.Reid, Elliott G . A Full-Scale Investigation of GroundEffect. Rep. No. 26
39、5, N. A. C. A., 1927.Nutt, A. E. Woodward, and Richards, G. J.: Cine-Photographic Measurements of Speed and Attitude ofSouthampton Aircraft When Taking Off and Alighting.R. & M. No. 1621, British A. R. C., 1935.Hutchinson, J. L.: Further Measurements of Ground Inter-ference on the Lift of a Southamp
40、ton Flying Boat. R. & M.No. 1747, Britiih A. R. C., 1936.Wallace, Rudolf N.: The Effect of Split Trailing-Edge WingFlaps on the Aerodynamic Characteristics of a ParasolMonoplane. T. N. No. 475, N. A. C. A., 1933.Reid, Elliott G.: Applied Wing Theory. McGraw-Hill BookCo., Inc., 1932, pp. 175 and 176.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
