kJA^/V /_'/^r ^' 'Jl f NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS WARTIME REPORT ORIGINALLY ISSUED January I9U5 as Advance Confidential Report L5AII THE EFFECT OF TRAILIWG-EIXa; EXTENSIOH FLAPS ON PROPELLER CHARACTERISTICS By John L. Crlgler Langley Memorial Aeronautical Laboratory Langley Field, 7a. i J t •;J>/.IHlW|jl ^tSmm'-'SK^.-.r^'fM .*3SSS«' w WASHINGTON NACA W/VRTIME 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 axe now unclassified. Some of these reports were not tech- nically edited. All have been reproduced without change in order to expedite genersd distribution. L - 16! DOCUMENTS DEPARTMENT Digitized by tine Internet Arclnive in 2011 witli funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/effectoftrailingOOIang niooo^ -^ , 1 r*!^ •7t^ MCA ACR No. L5A11 CONFIDENTIAL NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS ADVANCE COiv'FIDENTIAL REPORT THE EFFECT OF TRAILIHCr-EDGE EXTENSION FLAPS ON PROPELLER CH •IRACTSRISTICS By Jolm L. Crigler SUMMARY An analysis v;as made to determi.ne the effect on propeller performance of extension flaps added to the trailing edge of a prooeller blade. A metliod of calcu- lating the changes in the J.deal angle of attack, the angle of zero lift, and the design lift coefficient of a propeller blade section having a trailing-edge e:-:ten- sion flap was utilized to calculate the performance of a six-blade dual-rotating propeller with extension flaps varying up to LO percent chord. The method was used to determine the angle that the flap extension must make Vifith the chord in order to obtain a particular load distribu.tion. Although the analysis in this report was made for a wind-tunnel propeller designed to operate at lov/ alvance-diameter ratio, the method is directly appli- cable to any propeller section under any operating condition. :ntroduction Inasmuch as tlie production of a propeller of a given design is an expensive manufacturing procedure, it is expedient to make each existing design useful for as many applications as possible. For this reason, several choices of diameter have been available with a given blade design. This flexibility of design has recently been increased by providing a procedure for adding an extension along the blade. The selection of the v.'idth of extension pcrm.its a choice of propeller solidity for a given blade design and diameter. The addition of the trailing-edge extension changes the section airfoil characteristics by an amount dependent CONFIDENTIAL CONFIDENTIAL NACA AGR No. L^All on the length and angle of extension. Some choice in the airfoil characteristics is therefore oermitted when the extension flao is added to the trailing edge. Reference 1 presents a method of anal^fzing the change in airfoil section characteristics accompanying changes in extension length and angle, and the ore sent report applies the method of reference 1 to the calcula- tion of propeller characteristics. The calculations given herein were made for a six- blade dual-rotating propeller for values of advance- diameter ratio (V/nD) that are enco-jntered in the operation of a propeller used to drive a wind tunnel. The sane methods are applicable, however, to propellers for any operating condition. Preliminary calculations were first made on the -cropeller with the original blades in order to study the distribution of loading along the blade for a power coefficient Cp = 0.51 at an ad Vance -diameter ratio v/nD = 0.33. For this value of V/nD, the inboard sections of the propeller were found to stall before the oiatboard sections and, furthermore, the whole prooeller was found to stall before a power coefficient of 0.31 was absorbed. In order to make the design suitable for these ooerating conditions, it was necessary to increase the solidity of the original propeller. Trailing-edge extension flaps were used for this purpose and were attached in such a manner as to change the angle of zero lift along the blade to increase the load on the outer sections. Inasmuch as the effect of such extension flaps is applicable to both tunnel and aircraft propellers, the method given herein for use in tunn.el -propeller design may also be used in determining the effect of trailing- edge extension flaps on propeller sections for aircraft applications. The method used to calculate the changes in the ideal angle of attack, the design lift coefficient, and the angle of zero lift resulting from a flat sheet attached to the trailing edge of an airfoil section is outlined in reference 1. Tliis method was used to calculate the lift as a function of angle of attack for the sections of a six-blade dual-rotating nropeller having Curtiss blades 856- and 857-1C2-13 with trailing- edge extension flaps. The calculated thrust- and torque-distribution curves for the propeller with a i|0-percent-chord trailing-edge flap are presented for tv/o ooerating conditions. CONFIDENTIAL I-TAGA aGR Lio. L5A11 CONFIDENTIAL 3^^':30L3 b chord of -orooeller blade element Cj;^ section lift coefficij "dF dC.j "dx" 'icient [ t "' ■ ■ \ Cl section design lift ccefficienti lift coefficient at ideal angle of attack Cp pov/er coefficient ^^p/pn-^D^) C^ torqiie coeffic.'ent (q/pn^D^) Crp thm.st coefficient (T/pn2D^+) D propeller diameter elenent ':orque coefficient element thrus /dQ/dx\ i^pn^r^y /d "■ /d xN t coefficient ( ' ' ] Vpn^DV h tbiclmess of -^roreller blade element L lift of blade section M Mach number n propeller rotational speed p geometric -oitch of propeller P input power to propeller Q torque of propeller r radius to any blade element R tip radius T thrust of propeller CONFIDENT LAL COWFIDST^TTTAL ¥..kCl\ AC:? uo. ■ =^ .11 V airspeed X radial location of bl£-de elerent Cr/R) a angle of attack a-j angle of zero lift ''o a-j- ideal angle of attack ^ propeller blade angle at 0,75 radius e propeller blade angle at radius r p mass density of air "■•• 1,, Subscripts- F front propeller R rear propeller 0.7 at 0.7 radius ANALYSIS The propeller analyzed is a six-blade dual-rotating propeller with Gurtiss blades 836-102-13 (front, right hand) and S37-1G2-13 Trear, left hand). Blade-form curves for the propeller are given in figure 1. The propeller conditions analyzed vary fror: a value of Cp = O.5I at v/nD =0.53 to a value of Cp = O.O95 at V/nD = 0.26. Preliminary calculations showed that the propeller with the original blades vjould stall at an advance-diameter ratio of 0.33 before absorbing a power coefficient of 0,31. In order to use the available propeller for this condition, ■ "" it was necessary to increase the oropeller solidity by the use of extension flaps attached to the trailing edge. Extension flaps cause a change in the angle of zero lift, which results in an effective change in the propeller pitch distribution. It was necessary, therefore, to calculate the lift of the sections as a function of angle of attack for use on the propeller. The method of CONFIDSJWIAL NACA ACR No. L5A11 C0NFID3NTIAL 5 reference 1 was used to calculate the change in lift characteristics caused by extension flaps, and the results show the angle that will be required between the extension flap and the chord line of the original airfoil to produce zero change in pitch distribution for several sections along the blade. Certain assumotlons regarding the airfoil character- istics of the propeller were necessary in order to mahe the calculations. Experimental data are usually used in analyzing protjeller performance. Inasmuch as the Curtiss blades 836- and 837-1C2-13 are of NACA lo-series airfoil section, the section lift characteristics (fig. 2) for the original nropeller were obtained by extrapolating the experimental data of reference 2. The design lift coefficients and the operating Mach numbers for several sections along the blade for the limiting condition of operation are shown in figure 2. The calculations for the sections with extension flaps were made on the assumption that the addition of flans did not change the slope of the lift curve for a given section. Inasmuch as no experimental data were available for the airfoil sections with extension flaps, it was necessary to use theoretical calculation in analyzing the performance of propellers with these sections. The calculated and experimental values of a? for ''o the original sections are not In perfect agreement. Since experimental data were used for the original sections, the differences between the calculated values for the original and the extended airfoil section characteristics were acplied to these experimental data. The corrected valiies were then used in calculating the perforniance of the propeller with extension flaps. RESULTS AND DISCUSSION Compi^tations were made to determine the effect of the trailing-edge extension on the lift characteristics of an airfoil as a function of angle of attack. Curves showing the results of some of these computations are presented in figures 3 to 6 . Tb.e calculated angle of zero lift a^ , the ideal angle of attack aj, and a J - cc, (on which the design lift coefficient depends) are given for the propeller section at the CONFIDS'^ITIAL ■ •CONFIDENTIAL NACA AC R No. L^Ml .ij.5 radius in figure J. The calculated angles are measured from a straight line iolning the extremities of the mean camber line of the extended airfoil section but are plotted against the angle betv/een the extension flap and the straight line joining the extremities of the mean camber line of the original airfoil section, as was done In reference 1. The effects of a 10-percent extension, a 20-percent extexision, and a l(.0-percent extension are comoared. In the use of this information for propeller calculations, it is mere convenient to refer the calculated angles to the line joining the extremities of the mean camber line of the original airfoil section. The angular difference a]_ between the two reference lines is given by the following formula: tan~-^a. /Extension length\ . , , „ . . % 1 --; Z — - — ) sm (Angle of extension) \ Chord / ° 1 /Extension length 1+ [-^^ — ^ '—^ — ) cos (Angle of extension) \ Chord / The results for the O.li^ radius plotted in figure 3 are replotted in figure l^ but in figure 1+ the calculated angles are measured from the chord line of the original airfoil section. The calculated and extrapolated ex-oerimental values of a? and ar - ai for the ''O -'- ''o , original section (without flap) are shown in figure 4.. The points on the curves also show the calculated angles at which the extension flap ynust be set to the chord line of the original section to give the same value of a? or qt - a?, for the extended section as for ''O -^ ''o the original section. If the values of a? for the sections with the extension flap are the same as for the original sections, the pitch distribution is unchanged. Since a-p for the original section (16 series) is zero, "the crossing of the aj curves with the zero ordinate gives the angle of extension for an lonchanged aj. Figure 5 shows similar curves for a 20-percent-chord extension flap and for a i].0-nercent-chord extension flap at the 0.95 radius. In figure 6 the curves at several radii are coirroared for the 20-percent-chord extension. From curves of this type, any desired change in the pitch distribution of a propeller may be made by properly setting the extension-flap angles. CON^^IDENTIAL NACA ACR ivo. L5A11 CONFIHEFTIAL 7 Since the inboard sections of the original Dropeller stalled much earlier than the o-.ithoard sections for low v/nD operation, it was decided to change the angles of zero lift ox the Made sections in order to shift more of the load toward the tip. This change in the angle of zero lift is obtained by setting the flap extension at the proper angle to the chord line. The angles of zero lift of the blade sections were changed by the amount shown by the solid line in figure 7« This curve nay be shifted up or down, as is shown by the dashed lines in figure 7> with no change in the load distribution; the only changes resulting are in the design lift coefficients and a constant shift in the angles of zero lift of the sections. In making the nropeller performance calcu- lations, however, a shift in the angles of zero lift results in a change in the '^ropeller ^itch setting for constant Cp and V/nD . The only change in C-p and r\ will result from the small effect of the change in the drag of the airfoil sections. Examination of the results (see figs. [[. to 6 ) shows that a 20--Dercent-chord extension to the Ciirtiss 836' or 337-102-13 blade should be set about 7.2° to the chord line at all blade sections to give the various angles of zero lift tViat would be obtained by adding Aa^ (solid line in fig. 7) to a^, of the original section. A J4.0-Dei'cent-chord extension should be set about 6.1).° at all blade sections. The angles of zero lift for the sections at all radii given in figure 2 were increased by the ainount shown by the solid line in figure 7 fcr making the calculations of the propeller performance 'with the extension flap. Analyses of propeller performance for several pro- pellers indicate that single-rotating propellers stall at section lift coefficients of about 1.0 for most of the blade and that the thin sections near the tip stall at section lift coefficients of 0,8 to O.9. The calcu- lations presented herein show that these lift coeffi- cients were realized for a pitch setting of 2lj.° at the 0.7 radius for operation at V/nD = 0.35 ^J^-d that a llO-percent-chord extension (lj_0-percent increase in solidity) is required to absorb the -oov/er. Experimental data on dual-rotating propellers, however, show that the diial-rotating propeller can be operated without stall at higher blade angles and at higher section lift CONFIDEMTIAL 8 CONFIDSFTIAL wACA ACK No. L5A11 coefficients than single -rotatin/:-; propellsrs. It is quite ;'-50S3i'ble, therefore, that the L|.0-percdnt- chord flap extension to the tuniel propeller that would be required for a 3ii-:gle-rotating propeller will not be necessary to prevent stall for the limiting condition of operation with dual-rotating propellers and that a lower solidity may be used. Nevertheless, In order to obtain conservative results, the calculations for the tunnel propeller hav3 been made on the basis of a ij-O-percent- chord flap extension and the results for two operating conc.itioris are given. Figure 6 shows the differential thrust and torque curves plotted against x for oneration at a \'/r\I) of 0,53 with the front-propeller blade angle set 2if° and the rear-propeller blade angle set 23° at the 0.7 radius, The element lift coefficients at several section radii are shown in this fi^nire. ?i.!,ure 5- shov;s siniilar curves for operation at v/nD - 0.26 with the front-propeller blade argle set 12*-* and the rear-rjropeller blade angle set 11° at the 0.? radius. CONCLUDING REMARKS The solidity of a six-blade du.al-rctating propeller having Curtiss 836- and S37"1C2-15 blades has been increased by adding extension flaps to the trailing edge. The method of analyzing the new blade-section character- istics in this case was applied to a particular propeller for oneration at a very lo.v advance -diameter ratio, but the method majr be a'cplied to any propeller section under any operating condition. The pitch distribution of the propeller with flaps mav be held constant or, if desired, may be varied for different design operating conditions by properly setting the flap angle, Langley Memorial Aeronautical Laboratory National Advisory Cotrcnittee for Aeronautics Langley Field, Va. CONFIDENTIAL NACA kCn No. L5AII CONFIDENTIAL REJ^ERENCES 1, Theodorsen, Tlieodoi-e, and Stickle, Geox''gc W.; Effect of a Tr-aillng-Sdge Extension on the Characteristics of a Propeller Section. NACA ACR No. Li+IZi, l^bX-* <— . Stack, John: Tests of Airfoils Designed to Delay the Compressibility Burble. NACA TN No. 976, I^ec . ! (Reprint of ACR, June 1959') CONPIDENITAL NACA ACR No. L5A11 CONFIDENTIAL Fig. 1 /o .40 /.O \ P/^ 08 ,3Z / -^ ^U .3 / \ t X'^ ■ ^ e 06 .Z4 \/ /' X \ A A \ 0^ ./6 / \ 4- / \ V .OZ _0Q ^ P " NATi tOMMin DNAL Al :e for / VISORY ERONAU- ICS u u ^ s .c / I ., .t 3 ■ i. ^ / X Figiire 1.- Blade-forir. curves for dual-rotating propeller having Curtlss blades 856- and 837-1C2-15. Diameter, 15 feet 5 Inches CONFIDENTIAL I NACA ACR No. L5A11 CONFIDENTIAL Fig. 2 CONFIDENTIAL Figure 2.- Lift characterlstlce for 16-eerlea sections for original Curtlss propeller blades 836- and S37-1C2-13. Diameter, 13 feet 5 Inches. Data extrapolated from reference 2, NACA ACR No. L5A11 Fig. 3 < O o NP «- ^ -o: ^ \ NATIONAL ADVISORY )Om\Tili. 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