Blade Section Variation on Small-Scale Rotors

A technique for small-scale rotor testing incorporating the use of a three-component balance, torque measurements, and stroboscopic speed measurement is described. Fifteen rotors using a related series of airfoil sections were tested. On the basis of maximum L/D, the optimum thickness ratio and the optimum camber were found to be 12 and 2 per cent, respectively. The small-scale rotors proved to be extremely sensitive to a change in both thickness and camber. Pointed or sharp-nosed sections are undesirable. I t was determined tha t the position of maximum camber greatly influenced rotor characteristics. The center of pressure travel before and after the stall point is reflected in the pitching moments of rotors with the horizontal hinge at the axis of rotation. SYMBOLS The terminology and symbols used in this paper conform to those used by the N.A.C.A. as set forth in their Technical Report No. 552. a — rotor angle of attack (angle between plane perpendicular to axis of rotation and relative wind)* $ — rotor blade pitch angle, measured from zero lift chord line of blade section L = rotor lift, lbs. D = rotor drag, lbs., from N.P.L. balance D' = drag derived from rotor torque M — pitching moment of rotor, ft.lbs. Q — rotor torque, ft.lbs, n = rotor speed, r.p.s. & — angular velocity, rad. per sec. V — wind speed, ft. per sec. ^ R — rotor radius, ft. CL = coefficient of lift, L/(P/2)TR V CDQ — drag coefficient from N.P.L. balance drag, D0 = D/(p/2)xRV CD' = drag coefficient from D' CD = total drag coefficient, CDQ plus CD' CM = moment coefficient, M/(P/2)irR V p, = tip speed ratio, V cos a/oiR INTRODUCTION I AN EFFORT to extend the findings of the N.A.C.A. from their rotor tests of 1936 in which four different airfoil sections were used, as well as to determine the sensitivity of small-scale rotors to blade section variation, 15 rotors using a related series of airfoil sections were tested in the University of Kansas wind tunnel. This related series included the following sections: Thickness variation—N.A.C.A. 0009, 0012, 0015, 0018. Received January 11, 1943. * Assistant Professor, Aeronautical Engineering Department. Now, Associate Professor of Aeronautical Engineering, University of Wichita. Camber variation—N.A.C.A. 0012, 2412, 4412, 6412. Thickness and camber variation—N.A.C.A. 4409, 4412,4415, 8318. Thickness shape variation—N.A.C.A. 23012, 2301233. Camber shape variation—N.A.C.A. 0012, 2412, 23012, 2R212, 4412, 43012, 6412, 6712. This group of sections was used by the N.A.C.A. in their investigation of the effect of Reynolds Number on the characteristics of airfoil sections. CONSTRUCTION OF BLADES For each section a template was made from stainless steel by scribing the outline of the section on the thin steel and cutting it out with fine files. These templates were used to form the basswood blades. Since the blades were very close to final dimensions after forming, little or no thickness could be added. Therefore the blades were finished with a thin coat of shellac, which was sanded and buffed to a high polish. ROTORS The rotors had a diameter of 36 in. with a blade chord of 2.5 in. This produced a solidity of 0.0885. The center of gravity of the solid blades at approximately 44 per cent of the chord necessitated placement of the vertical pin at that position. The pins were offset from the axis of rotation by l/i6 in. The blades were carried on a horizontal hinge that passed through the axis of rotation, the flapping axis of both blades being the same. The hub contained adjustments for pitch and also an adjustment for moving the blades forward and backward in the plane of rotation. This latter adjustment was necessary in order to secure proper balance of the rotor by placing the centers of gravity of the separate blades directly opposite to each other. Pitch adjustment was secured by rotating the blades about their span axes by means of pitch adjustment screws incorporated in the hub. The test rotor is shown in Fig. 1. TESTING APPARATUS The rotor was driven by a y3-hp. electric motor which was mounted on a support of streamline tubing, offset so as not to interfere with the flapping of the blades. The plane of rotation was vertical with the center of rotation on the centerline of the tunnel, which 83 84 J O U R N A L O F T H E A E R O N A U T I C A L S C I E N C E S — J A N U A R Y , 1 9 4 4 -HORIZONTAL HINGE AXIS