Document
ACR Oct. 191.9 " -t , NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS ORIGINALLY ISSUED October 1939 as Advance Confidential Report . ~'IS OF PROPELLER3 AND OF VIBRATION ON THE EXTENT OF LAMINAR FLOW OU THE N.A.C.A. 27 -212 . ..yRFOIL · By Manley J. Hood and M. Ed uard Gaydos Langley Memorial Aeronautical La~oratory Langley Field, Va.
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- I viously held under a security status but are now unclassified. Some of these reports were not tech- .: {li cally edited. All have been reproduced without change in order to expedite general distribution.
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EFFECTS OF PROPELLERS AND OF VIBRATION ON THE EXTENT OF LAMINAR FLOW ON THE N.A.C.A. 27-212 AIRFOIL By ~ey J. Hood and M. Edward Gaydos SUMMARY The effects of propellers and of vibration on the extent of laminar flow on the N.A.C.A. 27-212 airfoil were investigated in the N.A.C.A. 8-foot high-speed tunnel by testing the airfoil in conjunction with a tractor and a pusher propeller and with a me c hanical vibrator. The Reynolds numbers of the investigation ranged from 3,500,000 to 7,600,000 for the propeller tests and to 10,300,000 for the vibration tests.
The results show that neither the pusher pro~eller nor vibration with amplitudes up to 0.094 inch and with a frequency of 1,650 cycles per minute had any consequential effect on the extent of laminar flow but that the tractor propeller had a very pronounced effect. The tractor propeller caused transition to move from approximately midchord to a position near the leading edge; the accompanying increase in drag probably exceeded 100 percent for the N.A.C.A. 27-212 airfoil. The corresponding drag increase for the N.A. C ·.A. 0012 airfoil would be approximately 25 percent beeaus~ this airfoil no~mally has a less extensive laminar boundary layer.
INTRODUCTION For some time it has bean suspected, but never definitely ascertained that tractor propellers increase wing drag by reducing the extent of laminar flow over the wing back of the propeller, nor has it been ascertained whether pusher propellers behind the wing or vibration of the wing produce similar effects. The investigation described in this paper was therefore made to evaluate the effects of propellers, both tractor and pusher, and of vibration on the extent of laminar flow as an indication of the effect on wing drag. The N.A.C.A. 27-212 airfoil, one of the laminar-flow airfoils recently developed by the N.A.C.A.
(reference 1), was used for the tests because airfoils of this type are especially sensitive to flaw disturbances.
APPARATUS AND METHODS The investigation was conducted in the N.A.C.A. 8-foot high-speed wind tunnel, a closed-throat tunnel of c i rcular cross section. Sphere- drag tests in this tunnel (reference 2) have shown an average critical Reynolds number of 380,000, indicating a relativel y low degree of turbulence.
The airfoil used was made to the N.A.C.A. 27-212 section and has a 5-foot chord. The maximum thickness is 12 percent of the chord and the camber line shape and the thickness distribution entail falling pressures in the downstream direction over the forward 70 percent of the chord, on both surfaces, when the airfoil is operating at the design lift coefficient of 0.2. Drag ~~d transition measurements for this airfoil without propellers or vibration are described in reference 1.
The model was accurately constructed of wood; both surfaces were lacquered and sanded to a smooth finish. In position for tests, it completely spanned and was rigidly supported by the test section of the tunnel, as shown in figure 1.
The propeller used for the tests was a left-hand, two-blade propeller of 4-foot diameter, the complete description and characteristics of which are given in references 3 and 4. Ths propeller was driven by a windmill mounted 7 feet downstream from the propeller on the opposite end of the propeller shaft. In the tractor position (fig . 1), the propeller was 20 percent of the chord (0.2c) ahead of the leading edge, the windmill then being 0.2c behind the trailing edge; in the pusher position, the propeller and the windmill were 0.2c and 1.6c, respectively, behind the trailing edge. In all cases, the axis of the propeller was parallel to and 7.5 inches (0.125c) below the chord ' of the airfoil at the center of the span. The investigation was conducted at values of the thrust coefficient C of 0 and 0.068; the propeller blade angle T was set at 40 throughout the tests. In order to obtain the desired thrust coeffiCient, the corresponding advance-diameter rat i o V/nD was estimated from the propeller characteristics (fig. 8 of reference 4) and the windmill was adjusted to drive the propeller at that value of V/nD.
The model was vibrated by two eccentric weights driven by a variable-epeed electric motor and spur-geared to rotate oppositely to ~roduce vibrations only in a vertical direction. Weights, gearing, and motor were mounted on the under surface of the airfoil. The amplitude of the vibration was measured by means of a shielded vertical rod firmly anchored at one end to the under surface of the a i rfoil; the total amplitude was read directly on a scale at the lower end of the rod with the aid of a magnifying glass.
The transition point was located by measuring the velocities in the boundary layer close to the airfoil surface. (See reference 1.)
Velocities 0.0035 inch from the surface were measured with small total- and static-pressure tubes mounted on the upper surface of the airfoil at the center line and 16 inches on either side (fig. 1). Because of the large damping of the tubes, the indicated velocities were the temporal mean values.
The propeller-removed data were obtained with the propeller shaft and the supports in place but with the propeller and the windmill remov ed.
The propeller shaft and the supports mounted beneath the model affected the general flow over the airfoil; in order to obtain a pressure gradient favorable to extensive laminar flow, the model was tested at 0 angle of attack. The resulting gradient and boundary-layer flow (fi g s. 2 and 3) were about the same as were obtained at an angle of attack of 0.5 , the angle of minimum drag, with no obstructions in the air stream. The static-pressure coefficient S, used in figures 2 and 3, is equal to ~ q where H free-stream total pressure p local static pressure q dynamic pressure of the air stream RESULTS AND DISCUSSION The results of this in v estigation, uncorrected for tunnel effects, are presented as curves of a transition parameter plotted against chord position at the following Reynolds numbers and corresponding air speeds: Reynolds number Air speed (m.p.h.)
5,000,000 11 6 7,5 ,000 177 10,000,000 243
u/uo
The transition parameter is where u is the velo~ty
vf:R Ylc'
indicated by the surface tubes; U , the free-etream velocit y ; R, the o Reynolds number based on the chord; y, the effective height of the total-pressure tubes from the surface; and c, the chord of the a i rfoil.
The transition from the low-drag laminar boundary layer to the h i gher- drag turbulent boundary layer produces a definitely higher velOCity near the surface, resulting in a marked increase in the value of the parameter.
A marked increase in the value of the parameter at any point, t herefore, indicates tha t the boundary layer at that point has changed from t he laminar to the turbulent type with a consequent - increase in drag.
Figure 4 shows the effect of a tractor propeller on the boundary layer as indicated by the transition parameter. Although it is difficult to judge the location of the transition from a single curve of the type shown in figure 4, a comparison of the curves for the different test conditions at common chord positions indicates that, with the propeller operating, transition has in every case moved forward to between the leading edge and the O.lOc position. Unpubli~hed plots of the tran s ition parameter as a funotion of Reynolds number showed that transition without the propeller occurred at about 0.40c and 0.50c at Reynolds numbers of 7,500,000 and 5,000,000, respectively. On the basis of unpublished test results, the corresponding increase in drag is estimated to be of the brder of 100 percent or more. With a conventional airfoil, the drag increase would be less. If it is assumed, for example, that a tractor propeller would mo ve the transition point on a smooth N.A.C.A. 0012 airfoil from its normal position (about 0.30c for a Reynolds number of 6,000,000) to the 0.05c position, the drag would be increased about 25 percent .
The change in the boundar y -layer flow with increase in the thrust coefficient CT from 0 to 0.068 was small. This result indicates that the turbulence created by the propeller even at zero thrust was sufficient to prevent any extensive laminar flow on the wing in the propeller wake and that the thrust condition at wh ich ths ~ropeller operates is, therefore, unimportant. The greatest increase in the val ue of the transition paramster occurred at the center line directly behind the propeller hub, which may be attributed to the poor aerodynamic shape of the hub and the adjacent blade sections and also to the fact that ths solidity of th e propeller is greatest at the hub and hence the flow is disturbed a greater percentage of the time than behind portions of lower solidity.
Additional tests showed that, at 0.60c, the effects of the propeller extended approximately 28 inches from the center of the span. The corresponding angle of spread of the disturbed reg i on was 7.5 on either side measured from the poi~ts on the leading edge directly behind the propeller tips.
Because the N.A.C.A. 27-212 airfoil is designed to have the pe ak pressure located at 0.70c, the transition position is more sensitive to disturbances than is the transition position on more conservative types of airfoils. The effects shown ma y, therefore, be larger than wo u ld occur on oth er more conservative types.
Figure 5 shows that the pusher propeller, even at a value of CT of 0.068, had very little effect on the flow in the b oundar y la y er and that the change in the boundary la y er, as indicated by the v alue of ths transition parameter, was small. With regard to its effect on transition, the pusher propeller had no consequential effect on the drag.
Figure 6 shows that the vib ra ti on of the airfoil had no appre ci a ble effect on the boundary-layer flow; c onsequen t l y , the vib ra ti on h ad no ap preci able effect on the drag. The intensity of vibr a ti on c an be e xpr e ssed nondimensionally as the roo t ~ean-square of the vibrption
ve lo cit y divid ed by the free-stream ve locity, /Y/u ThJ o 8 expression
o .
i s analo gous in form to th a t general ly u sed to express the inten si ty ...:t of turbulence . The scale of v i bration c an be con s ide red as a wa ve len gth co t'- based on th e free-stream velocity and exp ressed in terms of the airfo il ch ord. The following t able shows the c orrespondin g Reynold s number s, frequencies, amplitud e s, vib ra ti on intens itie s, and vibr a tion wa ve lengths: Frequency Total R eynolds W a ve l ength
Pluo
(cycles ampli tude number (percent) (chords) per min.) ( i n. ) 1,600 0.032 0.090 5,000 ,0 00 1.2 7 . 262 1.24 5,0 00,000 1, 650 .094 10,000,000 1, 600 2.68 .0 32 . 043 10,000,000 2 . 60 1, 650 .0 94 .130 Ai r-stream turbulence of an intensity equ al to the maximum vib ration intensity investigated (0.262 percent) wo u ld be expecte O d to have an appreciable effect on the extent of laminar flo w on t he N.A. C.A. 27 -212 ai rfoil. The vibrati ons investigated, how ever, were of mu ch larger scale (i.e., lower frequency) th an the type of a i r-s t ream tu rb ul ence t o w hich laminar f low is s en si tive . Laminar fl ow might possibly be disturbed by v ib ra tion s of frequen cie s much highe r th an the frequencie s used in the present in ves tig a t ion. It is also possible th a t l ocal vib ra ti on of part of the w ing surfa ce, as oppoged t o vibr a tion of the win g as a whol e , wo u ld increase the dra g because l ocal vibr a tion w ould con s titute t rans itory deformation of the pr ofile . It ha s been sh own in referen ce 5 th a t a small deforma ti on of the pr o file w ill ca u se premature t ransi t ion and a con seq uent increase in drag.
CO N CLUSIONS 1 . The tractor p ropeller ca us ed tr ans ition on the N.A . C.A. 27- 212 airfoil t o move from a pproximately mi dchord to a position ne ar th e leading edge; the a ccompanying incre as e in drag probabl y exceeded 100 ~ercent for this airfoil. The corresponding drag increase for the N.A. C.A. 0012 airfoil would be , 8:~roximately 25 percent because this airfoil normally has a less extensive laminar boundary layer.
2. The effect on the location of the transition point of a pusher propeller 20 percent of the chord behind the airfoil was inconsequential.
3. The largest vibration amplitude of the airfoil as a whole, 0.094 inch at a frequency of 1,650 cycles per minute, had no measurable effect on the laminar flow over the airfoil.
Langley Memorial Aeronautical Laboratory, National Advisory Committee for Aeronautics, Langley Field, Va., September 9, 1939.
REFERENCES 1. Jacobs, Eastman N.: Preliminary Report on Laminar-Flow Airfoils and New Methods Adopted for Airfoil and Boundary-Layer Investigations.
NACA ACR, June 1939.
2. Robinson, Russell G.: Sphere Tests in the N.A.C.A. 8-Foot High-Speed Tunnel. Jour. Aero. Sci., vol. 4, no. 5, March 1937, pp. 199-201.
• 3. Wood, Donald H.: Tests of Nacelle-Propeller Combinations in Various Positions with Reference to Wings. Part 1. Thick Winge - N.A.C.A. Cowled Nacelle - Tractor Propeller. NACA Rep. No. 415, 1932.
4. Windler, Ray: Tests of a Wing-Nacelle-Propeller Combination at Several Pitch Settings up ' to 420. NACA Rep. No. 564, 1936.
5. Hood, Manley J.: The Effects of Surface Waviness and of Rib Stitching on Wing Drag. NACA TN No. 7 24 , 1939.
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