ARE Ko. LkE22 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED May 192(.l). as Advance Eestricted Report IAE22 WHJD-TDMEL TESTS OF DDAL-EOTATIHG EROEEIIiERS WITH SYSTEMATIC DIFFERENCES IN NDMBER OF BLADES BLADE SETTHKJ, AHD EOTATIOHAL SPEED OF FROKT AMD EEAR PROFKT.T.ERS By W. H. Gray Langley Memorial Aeronautical Laboratory Langley Field, Ta. 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 war 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. 80 DOCUMENTS DEPARTMENT Digitized by tine Internet Arcliive in 2011 with funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation , http://www.arch ive.org/details/windtunneltestsolang 1/ » ^ KACA A^R No. LkE22 RESTRICTED NATIONAL ADVISORY COMIvHTTEE FOR AERONAUTICS ADVANCE RESTRICTED REPORT ¥/IND-TU.Mi-TEL TESTS OF DUAL -ROTATING PROPELLERS WITH SYSTEMATIC DIFFERENCES IM NUMBER OF BLADES BLADE SETTINC^, AND ROTATIONAL SPEED OF FRONT AND REAR PROPELLERS By W. H. Gray SUMMARY The advent of dual -rotating propellers has created a need for information concerning the effect of the nuriiber of blades of the front and rear prooellersj rela- tive rotational soeeds, and small changes in the blade angles of the rear propellero Results of aerodynarrdc tests of seven-blade propellers, vifhich were considered as a possible arrangement to avoid vibration difficul- ties, are presented herein. Variations of relative blade angle and rotational speeds of the front and rear components of a six-blade dual-rotating propeller vjere also investigated. The test program was an extension of previous work on dual-rotating propellers at the NACA propeller-research tunnel; the propeller blades and test body were those used in the previous tests. The results indicated that envelope efficiencies of a seven-blade nroneller with three blades in the front hub and four in the rear Vifere from to 1~ percent lower than envelope efficiencies for the six-bl.ade dual-rotating prooeller; four blades in the front hub and tliree in the rear resulted in efficien- cies 1/2 to 5— nercent lower than those obtained with the six-blade propeller. This conclusion applies to blade-angle settings of the front and rear propellers to absorb equal power at peak efficiency when the rotational sneeds were held equal. FEST'^IGTED NACA ARR Ko, lI'S22 Chang3.s In rotational speed of the rear Go;r;oonent of" a six-blade dual-rotating propeller to maintain t±ae torque equal to thet of the front coTioonent over the entire range of advance-diameter patio rather than onl7/" at peak efficiency resulted in no appreciable effect on efficiency for either the low- or high-speed condition. The saine conclusion was made for operation at constant speed ratios differing from unity by as m.uch as tlR per- cent. The rear blade angles were adjusted to provide equal front and rear propeller torque for each ratio at peak efficiency. ■".Yhen the rear blade angles of the six-blade pro- peller were adjusted to provide equal torque at values of advance-diameter ratio other than that for peak efficiency and when rotatlonel speeds of front and rear blades were eqaal, the effect on efficiency v;as negli- gible e?"cept at pe&k efliciency, at which a higher rear blade angle resulted in a higher efficiency. IKTRODVCTION 'The use of dual -rotating propellers introduces complications into the problem of propeller-perf orm-ance estimation. Pro-oeller diam.eter and the njumber of blades are the most i'.i-oortant param.eters in tiie selection of a dual-rotating propeller to meet specific operating con- ditions. After the number of blades has been decided, there still remain the differential blade angle and the differential rotational speeds, which may ue varied over the operating range. Experiments have shovm that im- provement in the vibration characteristics of dual- rotating propellers would also be desirable (reference 1). At present, dual-rotating propellers consist of tro comj- ponsnts having equal diamreters and number of olades and operating at equal rotational speeds. The use of tvjo components having different num.bers of blades and uheq-aal rotational speeds might miaterially I'edace the vibration. Aerodynamic effects of such a deviation from, standard practice rem.ain to be established. The results of tests of systematic variations in blade num.ber, differential blade setting, and differential I'otational speed are presented herein = The effect of unequal numiber of blades has been investigated by tests MCA ARR No. lIlE22 '-'^ of seven-blade dual-rotating propellers with three blades in the front and four in the rear and with four blades in the front and three in the rea^^. The operating modes contenvplated for dual -rotating propellers are those in v/hioh, throughout the operating range, the sioeed of the rear propeller v-ias continuously adjusted to '-reep the torque of the two propellers equal, the Sioeed of the rear propeller was held at a fixed ratio to that of the front propeller, and the blade angle of the rear propeller differed from that of the front pro- peller by a fixed amount. These conditions have been covered in this investigation for a six-blade dual- rotating pror)eller. These .modes of operation cover the most irportant variables relating the front and rear parts of dual-rotating propellers: nuraber of blades, blade angle, and rotational speed. APPARATUS AND METHODS The test setuD v;as that used in previous tests of dual-rotating pro-oellers in the NACA propeller-research tunnel (reference 2). A photograoh of the setup with a single-rotating nropeller is given in figure 1 and out- line diTnensions are given in figure 2. The symmetrical- airfoil v.'lng, shown in the photograph and the drawing, was in place for all the tests. The propeller blades, for which blade-form curves and the plan form, are given in figure 3> were Plam.ilton Standard 3155-6 for the right-hand blades and Hamilton Standard 3156-6 for the left-hand blades. Both six- and seven-blade propellers v^rere maounted in two hubs spaced I5 inches. The shape of the front soinner was Identical with the one previously used (reference 2), but the rear s-oinner was elongated 5 inches because the front hub y^as m.oved forward to increase the spacing from 10 to 15 incnes. This change required a slightly altered spinner and forward body section to give a faired body. A plot of the difference in blade-angle settings for dual-rotating oropellers of four to eight blades for the condition of equal torque of front and rear oropellers at Deak efficiency is given in .figure li,. 1| NACA ARE No. LiiE22 Settings for the four-, six-, and eight-blade dual- rotating propellers wez^e those used in tests reported in references 2 and J-. The rotational speed of the front and rear pro- pellers was j-naintained. equal for both the seven-blade and six-blade propellers for the investigation of the effect of changes in rear blade angles only. The speed- ratio and equal-torque tests, ho\^'ever, required a varia- tion of the front and rear propeller rotational speeds that gave differences up to JO percent. The limiting conditions of tunnel speed (110 mph) and propeller rotational soeed (55'-' rprn) resulted in a tip speed below JOG feet per second and a Reynolds number of about 1,000,000 for the 0.75^^ section, where R is the propeller radius. T'.i'o effects of corripressi- bility, therefore, would be expected. RESULTS AND DISCISSION The results are presented in nondimensional form. Coefficients and symbols used are defined as follows; Crp thrust coefficient (T/pn^D ) Cp power coefficient (P/pn^D-^) ( -in-. \T Ti propulsive efficiency -rr- -^ C„ speed-power coefficient {-^ I — ~ or V/nD advance-diameter ratio v/here T effective thrust, pounds P power absorbed by propeller, foot-pounds per second IvACA ARR No. LiiE22 V TO D P Pp Pr airspeed, feat per second propeller- rotational speed, rps propeller diairieter, feet mass density of air, slugs per cubic foot front blade angle at O.J-jV. rear blade angle at 0.75'^ po'.'irer coefficient foi' front propeller power coefficient for rear propeller The effective thrust is tlie measured thrust of the propeller-body cornbination plus the drag of the body without a propeller. For tests in v/hich the rotational speed of the tvjo coniponents differed, the follov/ing coefficients based on tlie rotational speed of the front prooeller were used: 2 '' ^P = 2TTnpQ,p ■where Hp front-proneller rotational speed, ros ITACA ARE l\To. l1+E22 n_, rear-oropeller rotational s-psed, ros Qp front-prooeller torque, foot-pounds Q-o rear-nropeller torque, foot-pounds It will be seen that these coefficients reduce to the usual ones for the condition of equal rotational speed of front and rear components. The figures showing propeller characteristics and efficiency cortiparisons are given in table T. Seven-bl ade prop e lle r.- The tests of seven-blade propeirers v'ere -nade "'ith~the t-^'o possible combinations of three blades in the front hub and four in the rear (designated hereinafter the three-four combination) and four blades in the front hub and three in the rear (designated the four- three combination), 'Rotational speeds of front and rear componentG were m.aintained equal throughout the tests. The blade angles of the front and rear propellers were set to absorb equal power at peak efficiency. The characteristic curves fir the seven-blade propellers are given in figures 5 'to I-'j. The curves of Crp and Cp ^f3gs. % b, 10, and 11) indicate a m_ore gradual stall for the three-four combination than for the four-three combination. This stall is more gradual probably because th^ larger number of blades requires a lower rear blade angle, which results in a lighter loading of the inboard sections of the rear propeller for the three-four combination. The efficiency envelopes for the two combinations (fig. 15) are only from to J-i- percent lovjer than for the six-blade dual-rotating propeller jn the tractoi condition (from reference 2). The efficiency envelope of the three-foiir combination varied from no higher at a front blade angle of 20^ to 1— percent higher at 60 thg.n the envelope for the four-three combination (fig. 15) • A comparison of efficiencies oi\ a basis of constant Gp (fig. 1-) indicated little or no con- sistent relationship betv/een the efficiency curves e>'ce"ot for the condition of peak efficiency shown by the envelopes of figure 15- MCA ARR No. l1iE22 7 Equal- torque t ests .- In previous tests of six-blade dual-Fota"'triig propellers (reference 2), the rotational speeds vifere maintained equal throughout the test and the rear blade angles were adjusted to attain equal torque at peak efflolency. In the present investigation , tests were made- to determine the effect of varying the speed of the rear component to maintain equal torque through- out the range of "^'/n'O as well as at peak efficiency; the blade angles V7ere the sam.e as in the tests of ref- erence 2. These tests may therefore be compared with the tests of reference 2 for peak efficiency. The V/nD intercepts of the thrust ciorves must coincide in order that the curves obtained from the present tests may be com_pared ¥;ith results obtained from previous tests. The intercepts did coincide for olade angles of 20' and 30^0 The intercepts differed slightly, however, for the blade angles of hi)^ , ^0''^ , and 60'-' and new thrust- and r)Ower-coef f ici ent curves (figs. I7 and lo) were obtained from, interpolation by assum.ing that the slight shift was equivalent to an effective shift in blade angle. The individual povver-coef f iclent curves obtained from test data without interpolation are presented in fig- ure IQ = Few efficiency curves for the blade angles of lj.0' , 50'''', and oO'-' v./ere computed and olotted from, the inter- polated thrust- and oov/er-coeff icient curves, so that all efficiency plots on figure 20 are effectively those for the corresponding blade angles used in the previous tests. Only a slight increase in efficiency due to adjusting the propeller speed, to provide equal torque is indicated in the range of cli:rib. Because the present condition of testing is the sarnie as the previous con- dition at peak efficiency, the efficiency envelopes from, the two sets of tests should coincide. There is a difference of 1 to 1— percent between the envelopes (fig. 21); the envelor)e obtained from the equal-torque tests is lower. This difference is within the lim.lt of agreement that can be expected of tests intended to reproduce conditions of much eax-lier tests. Speed- ratio tes ts.- Adjustra.ent of the rotational s'oeed of "one iTomponeTrt of a dual-rotating propeller within small limits should provide adequate m.eans for control of front- and rear-propeller operating con- ditions; that is, equal povjer absorption could be 8 NACA ARR No. Ll-i-E22 obtained for any of the various conditions of flight . Tests at constant values of the speed ratio were inade with the si.x-blade dual-rotating propeller. The front blade angle was set at Li.O° and the rear blade angle was adjusted to provide equal torque at peak ef ficlenc3''. The character- istics for rotational-speed ratios froin O.S5 to I.I5 (figs. 22 and 23) indicated no appreciable effect on efficiency. Figure <^h. is a cross plot of the characteristics and indicates the manner in vv-hlch the torque ratio changes with speed ratio. At a v/nD of O.'^', decreasing the speed of the rear propeller JO percent decreased the rear-propeller torque and increased the torque ratio Qp/Qp by 9 oercent. 3 1 ade - angl e tests ,- The effect of a fairly Isrge difference in rear blade angle with equal front and rear rotational speeds was investigated for a blade-angle range of 20° to 60'-'. Figure?! 25 to 54 present the char- acteristic curves of these tests. As ^vould be expected, a decrease in rear blade angle, corresponding to an increasMig inability to rerrove t>ie i-otatlon imparted to the slipstream by the front propeller, resulted in decreased efficiency; the higher the front blade angle, the larger the difference. For each value of front blade angle a value of rear blade angle was selected such that equal torque would be absorbed by front and rear propellers approximately at a V/nD for peak efficiency. Other rear blade angles were then selected to give equal pov/er absorption at values of V/nD below the value for peak efficiency and dowm to values that would occur in the climb con- dition. Within the experimental accuracy there v;as apparently no effect on the power absorption of the front propeller due to a shift in blade angle of the rear propeller. For each value of front blade angle, therefore^; only one faired avei-age curve of Cp for the front prooeller was used (figs. 26, 2c, 50, 52, and 5I1 ) . The three envelopes of figure 55 b.ave been obtained bj- fairing envelopes through the peaks of efficiency curves obtained from blade -angle settings giving equal front and rear power absorption at O.Jii), 0.65, and 1.00 times the value of V/nD for peak efficiency. These FACA ARR Fo. l1;E22 envelopes emphasize the necessity for readjusting the blade angles to those pi'oviding equal power absorption at v/nD for peak efficiency, when the propeller is operating in the high-speed range. Except for the high- speed condition, there ?;as little apparent effect of blade-angle change; figure 56 ( Cm st constant C-q) shows little deviation e:?ccept at V/nD for peak effi- ciency ffor exam-ole, at V/nD of aporox. 3-6 for Cp = O.h) , Figures 57 "O 39 =-re cross-plotted data of thrust- Dov/er ratio a;?ain3t Co ^''•^i^ t— e conditions of Cr, = C-n at 0.553 0,6'3, and 1.00 times the value of V/nD for peak efficiency. The curve for a v/nD of 2.5 appears to be somewhat in error, especially in figure 57s ^■^•'^ should be lower. The error is experimental, however, andL could not be faired out. Superposition of these plots again showed that at constant C-^ and V/nD there was little or no effect of blade-angle difference on thrust-oower ratio for efficiency at constant power) until the high values of V/nD were reached. C NCLUDI NG REMARKS Tests were made of a six- and a seven-blade dual- rotating propeller of the same size and blade design. The seven-blade prooeller was tested with three blades in the front hub and four in the rear (three-four com-- bination) and with four blades in the front hub and three in the rear (four-three com,binatlon} . 1. The results of tests at equal rotational speeds with the blade angles of the front and rear components of the seven-blade propeller adjusted to give equal power absorption at peak efficiency may be surcm.arized as follov./S'> (a) Peak efficiencies were from to 5"5' p6^~ cent less than for the sim.ilar six-blade dual- rotating oropeller, depending on blade- angle setting and the type of seven-blade com.blnation. 10 , NACA ARR Fo, lI^EZZ (b) Peak efficiencies of the three-four com- bination vera from to 1— percent higher than 2 those of the four-three combination, depending on the blade-angle setting. ( c "i There was no appreciable difference in efficiencies of the two co-mbiriStions except in the range of advance-dia;r:etoi' ratio near peal' efficiency. 2. For the sl^'-blarie dual-rotating propeller, adjusting the speed of the rear conponent to maintain its torque equal to that of the front component had no significant effect on efficier^cy. 5. For the six-blacie dual-rotating propeller at a blade angle of hO^ , operating at fix5d values of rotational- speed ratio from 0.65 to I.IS had no oonsideraole effect on efficiency v;hen the rear blade angle was set to give a torque equal to that of the front propellex' at peak efficiency. L. For a six-blade dual-rotating propeller at equal rotational soeeds of front and rear comiponents, changes in rear blade angle did not affect the low-speed (take- off and climb "^ operating eff5.ciency but reducing the rear blade angle did result in a lower high-speed efficiency. The effect was greater at the higher front blade angles. o-^ Langley Memorial Aeronautical Laboratory, National Advisory Committee for AeronauLics, Langley Field, Va. , MCA ARR No. LiiE22 11 REFERENCES 1. Miller, Mason F.: Wind-Tunnel Vibration Tests of Dual-'^otatlng Propellers. NACA ARP No. 3111, 191,5 . 2. Biermann, T5avld, and Fartman, Ed-'Adn P.: Wind-Tunnel Tests of Four- and Six-Blade Single- and Dual- Rotating Tractor Prooellers. NACA Reo. No. 7kl > 19i!.2. 3. Biermann, David, and Gray, W, H. : ^Vind-Tunnel Tests of Eight-Blade Single- and Dual-Rotating Prorjellers in the Tractor Position. NACA ARR, Nov. I9I1.I. rJACA ARR No. lUE22 12 o CO H < o o o o M CO o M ^ CO M K E^ O < o ^:! a, o PC p:! a a o d o CO to to • P-, f-. 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