1、I=NASA TN D-970iTECHNICAL NOTED-970EFFECT OF GROUND PROXIMITY ON THE AERODYNAMICCHARAC TE RISTICS OF ASPE CT-RATIO- 1 AIRFOILSWITH AND WITHOUT END PLATESBy Arthur W. CarterProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-l IFNATIONAL AERONAUTICS AND S
2、PACE ADMINISTRATIONTECHNICAL NOTE D-970Li695EFFECT OF GROUND PROXIMITY ON THE AERODYNAMICCHARACTERISTICS OF ASPECT-RATIO-I AIRFOILSWITH AND WITHOUT END PLATESBy Arthur W. CarterSUMMARYAn investigation has been made to determine the effect of groundproximity on the aerodynamic characteristics of _spe
3、ct-ratio-1 airfoils.The investigation was made with the model moving over the water in atowing tank in order to eliminate the effects of wind-tunnel walls andof boundary layer on ground boards at small ground clearances.The results indicated that, as the ground was approached, the air-foils experien
4、ced an increase in lift-curve slope and a reduction ininduced drag; thus, lift-drag ratio was increased. As the ground wasapproached, the profile drag remained essentially constant for each air-foil. Near the ground, the addition of end plates to the airfoilresulted in a large increase in lift-drag
5、ratio. The lift character-istics of the airfoils indicated stability of height at_positive anglesof attack and instability of height at negative angles; therefore, theoperating range of angles of attack would be limited to positive values.At positive angles of attack, the static longitudinal stabili
6、ty wasincreased as the height above the ground was reduced.Comparison of the experimental data with Wieselsbergers ground-effect theory (NACA Technical Memorandum 77) indicated generally goodagreement between experiment and theory for the airfoils without endplates.INTRODUCTIONThe large thrust augme
7、ntation obtainable with annular-jet configura-tions in ground proximity has promoted considerable interest in ground-effect machines (GEMs) as possible transport vehicles. Although thisthrust augmentation can be obtained in ground proximity during hovering,the inlet momentum drag of the air required
8、 to produce the jet resultsin relatively high drag at forward speeds and relatively low lift-dragProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-/. “ 2kratios (see refs. i and 2). The inlet momentum drag will probably haveto be reduced if reasonably
9、high speeds and long ranges are to beachieved. This drag reduction may be accomplished by transferring someor all of the lift from the jet thrust and base lift to somethingapproaching an airplane-type wing.In order to obtain some data for use in predicting the performanceof ground-effect machines at
10、 forward speeds with the annular jet andthe inlet momentum drag completely eliminated, an investigation of theaerodynamic characteristics of airfoils in close proximity to the groundhas been made in Langley tank no. i. The investigation vas made withthe model moving over the water in the tank in ord
11、er to eliminate theeffects of wind-tunnel walls and boundary layer on ground boards at %hesmall ground clearances desired. Inasmuch as most of the ground-effectmachines built or contemplated at present have aspect ratios of i orless, the present investigation has been made on a_pect-ratio-i airfoils
12、only. Lift, drag, and pitching-moment data were obtained on 22-percent-thick and ll-percent-thick airfoils. In addition, data were obtainedon the ll-percent-thick airfoil with vertical end plates attached belowth_ lower surface. A related investigation on wings in close proximityto the ground is pre
13、sented in reference 3.Li693SYMBOLSThe positive directions of the forces and moments are shown infigure i.Abb2aspect ratio, -_airfoil span, ftc airfoil chord, ftCD drag coefficient, D1 2_ov sCL lift coefficient, LCm pitching-moment coefficient, MyProvided by IHSNot for ResaleNo reproduction or networ
14、king permitted without license from IHS-,-,-_i_!_U_ L 693_CD iDLhhMySVD(LID)=Subscript:max max S_numchange in induced drag coefficientairfoil drag, ibairfoil lift, ibheight of c/4 above ground plane, ftheight of trailing edge of airfoil above ground plane, ftairfoil pitching moment, ft-lbairfoil are
15、a, sq ftfree-stream velocity, ft/secangle of attack, deg ground-influence coefficientmass density of air, slugs/cu ftlift-drag ratio of airfoil out of ground effectMODEL ANDAPPARATUSThe airfoil sections tested and ordinates are shown in figure i.The 22-percent-thick airfoil is the Glenn Martin 21 se
16、ction (ref. 4)with the lower surface modified to have a flat bottom between the30-percent-chord station and the trailing edge. The ordinates of thell-percent-thick airfoil were obtained by dividing the 22-percentordinates by 2. Both airfoils had a 48-inch chord and an aspect ratioof i.Vertical end p
17、lates were attached to the ll-percent-thick airfoilfor some of the tests. These end plates were made of 1/16-inch-thicksheet metal. As shown in figure l, the end plates were flush with thetrailing edge and the bottom edges were parallel to the water surface.The end plates were changed for each angle
18、 of attack so that the bottomedges of the plates remained parallel to the water surface.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-I i_i _iThe investigation was made in Langley tank no. i. A descriptionof the tank and the apparatus used in the t
19、est is presented in refer-ence 5. For these tests the airfoils were attached to the towing gearby a single streamline strut as shown in figure 2. Lift, drag, andpitching moment were measured by three external strain gages. Thepitching moment was measured about a pivot point on the gear above theairf
20、oil and then transferred to the moment center at the quarter chordon the lower surface (fig. i). All tests were made at a forward speedof 72 feet per second, which corresponded to a Reynolds numberof 1,840,000. Data were obtained through an angle-of-attack range from-6 to 18 at heights of the traili
21、ng edge of the airfoil above thewater surface ranging from 0.015 chord to 2 chords. The height varia-tion was obtained by changing the water level in the towing tank aswell as by raising and lowering the airfoil through a limited range.RESULTS AND DISCUSSIONThe results showing the effect of the grou
22、nd on the aerodynamiccharacteristics of the aspect-ratio-i airfoils are presented in fig-ures 3 and 4. The variations of CD, _, and Cm with CL for the22-percent-thick airfoil and for the ll-percent-thick airfoil with andwithout end plates are presented in figure 3 for a range of height-to-span ratio
23、s. The variation of CL, CD, and Cm with height of thetrailing edge of the airfoil above the ground is presented in figure 4for several angles of attack. Lines of constant height of the quarter-chord point are also shown in this figure.L1693LiftThe data of figure 3 show that, at small angles of attac
24、k, thelift-curve slope increased as the ground was approached. This increasein lift-curve slope was accompanied by a change in the angle of attackfor zero llft. As the ground was approached, the angle of attack forzero lift became progressively less negative.The lift for both the ll-percent-thick an
25、d 22-percent-thick air-foils near an angle of attack of 0 was essentially invariant withheight of the airfoil above the ground. At positive angles of attack,the lift was increased as the ground was approached, whereas at negativeangles of attack, the llft was decreased. These results suggest thatthe
26、 increase in lift at a given positive angle of attack, as the groundwas approached, may be due to the ram air on the lower surface whichincreased the positive pressure on that surface. Pressure-distributiondata presented in reference 6 for a wing with an aspect ratio of 5Provided by IHSNot for Resal
27、eNo reproduction or networking permitted without license from IHS-,-,- L16935indicate that the increase in lift at positive angles of attack was dueto an increase in lower surface pressures_ the upper surface pressureswere essentially unaffected as the distance above the ground was changed.The loss
28、in lift as the ground was approached at a given negativeangle of attack apparently was due to venturi action which increased thenegative pressures on the lower surface as the ground was approached.The pressure-distribution data of reference 6 show the rapid increasein negative pressures on the lower
29、 surface near the airfoil leading edgeas the ground was approached; whereas, again the upper surface pressureswere essentially unaffected by changes in height above the ground atthese negative angles.The additional lift obtained by the airfoil with end plates (com-pare figs. 3(b) and 3(c) apparently
30、 was due to the reduction of flowout at the tips of the airfoil, which greatly increased the ram-pressureeffect on the airfoil lower surface, especially at heights very near theground.Near an angle of attack of 0, the lift coefficient for the22-percent-thick airfoil (fig. 3(a) was approximately twic
31、e that forthe ll-percent-thick airfoil (fig. 3(b). For the ll-percent-thickairfoil, the lift coefficient was only about 0.15. These low liftcoefficients near an angle of attack of 0 and the fact that lift wasessentially invariant with height at this attitude suggest the desir-ability of operating a
32、ground-effect machine which has an airfoil-“shaped body at positive angles of attack so that a reasonably highOperating lift coefficient may be obtained. A further reason foroperating a ground-effect machine only at positive angles of attack canclearly be seen in figure 4. These data graphically sho
33、w that a reduc-tion in height caused a loss in lift at negative angles of attack andan increase in lift at positive angles of attack. This lift character-istic would provide stability of height at positive angles of attackand because of the venturi action at negative angles of attack wouldlimit the
34、operating range of angles of attack to positive values.Pitching MomentThe data of figure 3 show that the pitching moments became lessnegative at an angle of attack of 0 as the ground was approached. Thepitching-moment data also show that, for positive angles of attack, thestatic longitudinal stabili
35、ty was increased as the height above theground was reduced. This increase in stability apparently resultedfrom the ram pressure on the lower surface of the airfoil. As theground was approached, the increase in lift due to the ram pressure wasdistributed more or less uniformly over the lower surface.
36、 (This effectis shown in ref. 6.) The center of this lift increment was, therefore,Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- “ i _I ! “ , _ _i_ /_i _:_k6L -near the half-chord point and thus tended to move the center of totallift (aerodynamic
37、center) aft and thereby increase the longitudinalstability. This effect was particularly noticeable for the airfoilwith end plates (fig. 3(c).DragThe data of figure 3 show the effects of the ground on drag. Asthe ground was approached, the induced drag was reduced although theprofile drag remained e
38、ssentially constant for each airfoil. Near theground, the addition of end plates to the ll-percent-thick airfoilresulted in a large decrease in the induced drag. (Compare figs. 3(b)and 3(c).)Lift-Drag RatioThe“ results showing the effect of the ground on lift-drag ratiosof the airfoils are presented
39、 in figures 5 and 6. Lift-drag ratios areplotted against lift coefficient in figure 5 for various heights of thetrailing edge above the ground and in figure 6 for various heights ofthe quarter chord above the ground. The angle of attack for maximumL/D was about 2.5 for both the 22-percent-thick and
40、ll-percent-thickairfoils. The addition of end plates increased the angle of attack formaximum L/D to about 3. Maximum lift-drag ratios have been obtainedfrom figures _ and 6 and are plotted against-height-to-span ratio infigure 7. A reduction in thickness from 22-percent to ll-percent chordincreased
41、 the value of L/D approximately 45 percent at _- = 2.00b(no ground effect) and approximately 55 percent when the airfoil was inclose proximity to the ground = O.O1 . This increase was largelydue to the lower profile drag of the thinner airfoil. The addition ofend plates to the ll-percent-thick airfo
42、il resulted in a large increasein L/D when the airfoil was in close proximity to the ground becauseof the increase in lift caused by the increase in ram pressure. Theeffect of end plates became negligible when the trailing edge was 15 per-cent of the span above the ground or when the quarter chord w
43、as 2_ per-cent of the span above the ground.The theoretical treatment of ground effect presented by Wieselsbergerin reference 7 indicates a method for predicting the reduction in induceddrag for a wing at various heights of the quarter chord of the wing abovethe ground. According to reference 7, the
44、 reduction in induced drag ofa monoplane in ground effect is given by the equationL1693Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-/ 7LH i693twhere q is defined as the ground influence coefficient. At values ofh/b between 0.033 and 0.25, g may be
45、 obtained from the followingformula:i- 1.32hb1.05 + 7.4 hbwhere h in the present investigation is equal to one-half the qu/ntityh defined in reference 7. The variation of _ with h/b (for h asdefined in the present investigation) is shown in figure 8.The results of the present investigation are compa
46、red with theoryin figure 9, where the ratio of maximum L/D in ground effect to maximumL/D out of ground effect is plotted against height-to-span ratio at theairfoil quarter chord. At maximum values of the lift-drag ratio, thetheory of reference 7 reduces to the following formula (for finite aspectra
47、tio):(L/D)max i(LID)o%max _-The theory is plotted as a solid line.represents the range of values of h/bThe dashed portion of the curvefor which the author of refer-ence 7 considered the theory inapplicable 0.033 K 0.2 . The agree-ment Between experiment and theory appears to be generally good for th
48、eairfoils without end plates.Data for the aspect-ratio-i wing of reference 3 are shown in fig-ure 9 for comparison with data from the present investigation. Datafrom the two investigations of aspect-ratio-1 airfoils without endplates appear to be in generally good agreement. Although only limiteddat
49、a were available for the model with end plates from reference 3, theeffect of end plates on lift-drag ratio was considerably less than thatof the model with end plates from the present investigation. The reasonfor this lack of agreement between the two sets of data is not known.The wind-tunnel data of reference 3, however, were o
