1、k c. “ RESEARCH MEMORANDUM THE TORSIONAL DEFLEC TIONS OF SEVERAL PROPELLERS UNDER OPERATING CONDITIONS By W. H. Gray and A. E. Allis Langley Aeronautical Laboratory Langley Field, Va. - NATIONAL ADVISORY COMMITTEE FOR AERON-AUTICS CON Ft DENTIAL Provided by IHSNot for ResaleNo reproduction or networ
2、king permitted without license from IHS-,-,-NACA RM L51A19 NATIONAL ADVISORY CO- FOR AERONAUTICS RESEARCH MEMORANDUM TEE TORSIONAL DEFLECTIONS OF SEVERAL PROPELLERS UNDER OPERATING CONDITIONS By W. H. Gray and A. E. Allis SUMMARY Propeller-blade torsional-deflection data obtained during an inves- ti
3、gation of the pressure distribution on constant-chord solid aluminum- alloy blades differing only in camber and thickness are presented herein. Blade-section aerodynamic moments obtained from the measurements of pressure distribution have been used to compute the blade torsional deflections, and the
4、 measured and computed results are compared. The magnitude of blade torsional deflection was not negligible and varied with blade design; the thinner the blade, the greater the deflection. The deflections could be computed with good accuracy from a knowledge of the section physical characteristics,
5、the aerodynamic forces,acting on the blade, and propeller operating conditions. This work indicates that blade torsional deflection should be con- sidered especially in the design of thin propeller blades. A large effect on power coefficient was encountered for a condition at which the effect on eff
6、iciency was small. INTRODUCTION The constant effort to maintain good propeller efficiency even at high subsonic flight speeds has resulted in the usegf very thin propeller blade sections. Thin sections, however, reduce the ability of a blade to resist torsional deflection and, consequently, greater
7、importance must now be attached to torsional deflection. Previous optical measurements of blade torsional deflection under operating conditions, such as presented in reference 1, indicated negligible torsional deflections. The blades used Fn these earlier investigations, however, were considerably t
8、hicker than are now considered desirable. Prior to the present research a theory also existed for the Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2 - calculation of blade deflectlow of propellers operating lift and centrifugal forces, but no exte
9、nsive comparison calculated data had been made. - NACA RM L5lAl9 .- - with combined of measured and “ “ t: The present trend in the -design of efficient propellers is in the dhrectioa to increase the Fmport,artce of blade-twist. Unless attention is given to designing for a condition of no twist-by t
10、he arduous process of selecting sections having the properaerodynamic as well as geometric characteristics, thin propellemroperating at high speeds will experience large values of blade twist. Consequently, the effect of twist may have an important bearing on-the expe:cted perfoiiiece of a propeller
11、. “ - “ - “ “ - - -. The purpose of the present investigation was to determine by experi- ment and by theory the magnitude- of blade de-flection. This determination of blade torsional deflection wa,s a necepary contribution t-o the enl- uation of the results of a pressure-distribution investigation
12、in the Langley 16-foot high-speed tunnel. Experimental values of blade tor- tional deflections were obtained concurrently with the pressure- distribution data. The investigation was therefore comprehensive. A desirable cmiider8“tn in the present invest-igation is to show the possible application of
13、the propeller-deflection theory in the pro- peller design stage as it is conceivable that torsional deflections may influence blade design. . . b cl “d rn cn D G h SYMBOLS - blade chord, feet“ - blade-section lift coefficient blade-section design lift coefficient “ “ section pitching-moment coeffici
14、ent about- quarter-chord . . . point section normal-force coefficient , propeller diameter, feet shear modulus of elasticity, pounds per square foot . . 1 I blade-section maximum thickness, feet- Ib Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-NAC
15、A RM L5W9 rr, 3 P 5 z IX moment of inertia about axis through center of gravity parallel to chord line,. feet4 J moment of inertia about axis through center of gravity perpendicular to chord line, feet4 advance ratio (V/nD) J torsional stiffness constant, feet 4 M Mach number of advance Ma aerodynam
16、ic torsional moment, foot-pounds % tensile torsional moment, foot-pounds MP M, planipetal torsional moment, foot-pounds helical section Mach nuniber M 1 + ( J) N propeller rotational speed, revolutions per minute n qX R r propeller rotational speed, revolutions per second resultant dynamic pressure
17、at a radial station x, pounds per square foot (1/2pWO2) propeller-tip radius, feet radius to a blade element, feet I S distance froin leading edge to any point on chord, feet .C - s distance from leading edge to flexural center of section, P. feet a centrifugal stress, pounds per square foot t - sec
18、tiqn thickness perpendicular to the camber line, feet velocity of advance (corrected for wind-tunnel wall- interference effects), feet per second .- Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-4 WO - REA RM L5M9 -4 resultant section velocity vect
19、or, feet per second (dq$) f . -. resultant velocity at blade section, feet per second induced velocity at blade section, Feet per second wi fraction of propeller-tip radius (r/R); also used as a subscript to denote any section X a angle of attack of blade element, corrected for induced flow and blad
20、e deflection, at radial station x, degrees (j3 - + 43) I .“ a geometric angle of attack of blade element at radial statfon x, degrees (f3 - fro) induced angle of-attack, degrees static blade angle, degrees static blade angle at 0.75 tip radius, degrees blade torsional deflection or blade twist, degr
21、ees inclination of sighting station to vertical, degrees mass density of air in Pee stream, Slug8 per cubic foot maas density of-blade material, slug8 per cubic foot aerodynamic helix angle, degrees geometric helix angle, degrees (tan-I (Jinx) angular velocity, radians per second DESCRIPTION OF APPA
22、RATUS General.- An investigation was made of the torslonal deflections on the blade sections of four two-blade constant-chord solid aluminum- alloy propellers which differed only in thiGkness or section camber. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from
23、 IHS-,-,-NACA RM L5lAl9 - 5 The investigation was conducted in the Langley 16-foot high-speed tunnel on the 2000-horsepower dynamometer which is described Wly In refer- ence 2. Propellers.- The propellers investigated are identified by Roman numerals and are designated by their blade design nurnbers
24、 as follows: I WA 10-(3) (049)-03 I1 NACA lo-( 0) (066) -03 I11 NACA lo-( 5) (066) -03 IV NACA lo-( 3) (09.0) -03 Using the NACA lo-( 3) (049),-03 propeller as 89 example, the digits . in the first group of numbers indicate a nominal 10-foot-diameter pro- peller with the following design parameters
25、at the 0.7 radius: section design lift coefficient of 0.3, thickness ratio of -0.049, and solidity ofO.03 per blade. The NACA 16-series blade sections are used in all four propellers, each propeller having a constant value of design lift . coefficient along the radius to x = 0.95. Blade-fora curves
26、are shown in Tlgure 1 and values of section blade angle for all propellers are given in table I. In the subsequent discussion of the propeller blades %he Roman numeral designation will be used. One blade of each propeller contained tubes which were installed for the pressure-distribution investigati
27、on as reported in references 3 to 6. . Optical deflectometer. - The optical deflectometer was employed during the torsional-deflection investigation of each one of these four blade designs and is described in the appendix. Mirrors.- The small, rhodium-coated, first surface reflecting mirrors were fi
28、xed to the thrust face of the untubed blade for each propeller at rhodium surface proved most satisfactory because of its ability to main- tain a good reflecting surface after continuous progeller operation. The reflecting surface was about 5/16 of an inch square, and the mirror thickness was 1/32 o
29、f an inch. A conibination of mirror sizes and bonding materials was investigated before a positive method of securing the mirror firmly to the blade was achieved. The most satisfactory conibi- nation was found to be the mirror size noted above bonded to the blade with a thermopolymeric cement with t
30、he mirror edges faired to the blade three radial stations, x = 0.45, 0.70, and 0.90 (fig. 2) . The hard 1 . by plastic metal. TESTS Blade torsional deflection, which, for brevity, will hereafter be referred-to as blade twist, was measured at three radial stations, - *I Provided by IHSNot for ResaleN
31、o reproduction or networking permitted without license from IHS-,-,-6 NACA RM L5lAl9 c .- I x = 0.45, 0.70, and 0.90 for a nominal blade angle setting of 45O at- the three-quarter radius (table I). Advance ratio J = - was varied by maintaining the rotational speed constant and varying the tunnel air
32、- speed for some of the =: The-remainder of the runs were made with the tunnel-air-stream Mach nuniber held constant, and a range of advance ratio was investigated by changing the propeller rotational speed. Tests were run from zero torque to full laaCto facilitate operation ofthe deflec- dynamomete
33、r would not overheat if the deflec-tometer operator required any length of time to locate the initial test point and (2) the deflec- tions were normally small at zero torque and therefore little movement - of the deflectometer was required to locate the initial point. ( (2) the tunnel vibration was
34、high, especially on the constant Mach-number runs, and it was necessary for the deflectometer operator - to average .the reflected images. . The most desirable method of measuring blade twiet would have been to record simultaneously the twist values for the three stations inves- tigated. It would al
35、so have been desirable to obtain the pressure- distribution measurements simultaneously at dl nine stations. Neither of these two conditions was feasible. When correlation of these non- simultaneous groups of runs in order to conrpare measured pute-d data was attempted, some differences in .velocity
36、 or Mach number were present. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-7 A general discussion of the lmge amount of data presented for the four propellers requires that a brief resume of the various factors which contribute to twist, as well a
37、s the method of computation of the twist, be presented. The theory used herein in calculating blade twist is essentially the same as presented in reference 7. Three moments are considered to act on a rotating propeller blade to produce twisting: aerodynamic torsional moment s, planipetal tor- Sioa m
38、oment %, and tensile torsional moment %. Blade twist at a given radius r is then equal to w The aerodynamic moment can be positive or negative and therefore can act in a direction to increase or decrease blade twist. The posi- 3 force coefficient are the principal factors entering into the magnitude
39、 and direction 6f twist of the aeroaynamic moment. The basic equation used in calculating this moment at station x is - tion of the center of pressure and the magnitudeand sign of the normal- r As may be seen from this equation, the magnitude and direction of twist of the aerodynamic moment are dire
40、ctly affected by the location of the flexural center. By definition, the flexural center of a section of a beam is that point in the section plane through which a transverse load must act if bending deflection only iB to be produced with no twist of the center having line, I m section. An appFoximat
41、e fornula- for the location of the flexural of an airfoil section is given in reference 8. For thin sections small camber where the chord line may be considered as the median “ .- . t3s ds - Jo s= t3 ds JO - Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IH
42、S-,-,-The flexural axis. .of the b.lade Wuld then be considered to pe the loci of all such points. along the blade. Norr+ly it is most convenient and- sufficidly accurate to assume that the center“of gravity coincides with the flexural ceder of an airfoil section. For the case-of tkre- 16-seriea. se
43、ctions, the flexural “center and the center of gravity are located at s = O.#b, afid-.$quatjon (1) reduces to the form .* “ JX . which was used for the computed data presented herein. The planipetal mment- at each section is produced by a component of centrifugal force acting in a,plane,normal to th
44、e-flexural axis. This moment tends to decrease the section blade angle under all o cn and cm were measured af%er the blade had twisted. If two-dimensional data are used to compute blade twist, the aero- dynamic moment must be determined by a series of approximations. This procedure is necessary beca
45、use blade twist is assumed equal to zero in the initial computation so that the value of a = p - 8, - ai. When blade twist is considered, a = f3 - 8, - ai + AB. The.number of succes- sive approximations required however, in the J range 1.875 to 1i825 the change in C, was only 0.013. This phenomenon
46、occurred when the range of section Mach number from station x = 0.78 to the tip was from 0.70 to 0.82. Simultaneously with the change in increment of cn with J, the values of Cm decreased slightly. The same phenomenon occurred at a rota- tional speed of 1350, rpm but was less pronounced than at 1500
47、 rpm. The net effect on twisting moment of the foregoing changes in Cn and is apparent from an inspection of equation (la) for aerodynamic twisting moment. - Propeller 111.- Poeitive and negative values of twist were measured for propeller I11 (fig. 7) for the constant rotational speed runs. For . t
48、he constant Mach number runs the measured twist was mostly negative. Maxim positive twist of 1.02O occurred at-1500 rp-for a J of 2.00. A maximum negative twist of l.50 occurred at a Mach number of 0.65 for a J of 2.40. Propeller 1V.- The values of measured twist for propeller N, the thrckest propeller, are given in figure 8. At 1.600 -rpm and a J of 1.80 a maximum positive twist of 0.710 was recorded. The small values - “ Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-7- of twist for this propeller may be associated with the high values of
