Eeect of Solidity and Inclination on Propeller-nacelle Force Coeecients 6. Authors

A series of wind tunnel experiments was conducted to study the e ect of propeller solidity and thrust axis inclination on the propeller normalforce coe cient. Experiments were conducted in the Langley 14by 22-Foot Subsonic Tunnel with a sting-mounted, counterrotation, scalemodel propeller and nacelle. Con gurations had two rows of blades with combinations of 4 and 8 blades per hub. The solidity was varied by changing the number of blades on both rows. Tests were conducted for blade pitch settings of 31:34 , 36:34 , and 41:34 over a range of angle of attack from 10 to 90 and a range of advance ratio from 0.8 to 1.4. The increase in propeller normal force with angle of attack is greater for propellers with higher solidity. Introduction Although decades of experience exist for propeller driven aircraft, this experience has been for con gurations having signi cantly lower power loadings than those presently being considered. Investigations (refs. 1 through 4) indicated that wingand aft-fuselage-mounted advanced turboprop con gurations appear feasible and that con guration selection depends on further information regarding acoustic treatment requirements, structural weight, and engine-airframe installation aerodynamics. This research indicates that one impact of the high disk loading associated with advanced turboprop installations is increased aircraft stability during operations which expose the propeller to high in ow angles in either pitch or yaw. Such operations include the takeo , climb, and approach phases of ight and ground operations in crosswinds. These increases in stability are not always bene cial since they may require higher levels of control to maneuver the aircraft. The problem of an inclined propeller is one of many installation problems that are related to the nonuniformity of the ow past the blades. A nonuniform in ow can alter vibrational and aeroacoustic behavior of the operating propeller. Other examples of these problems are counterrotating propellers where the aft blade row is exposed to a highly nonuniform wake produced by the upstream blade row and pusher con gurations where the blades are exposed to the wake of the upstream wing-pylon. For the pusher con gurations, because of the asymmetrical variation of the blade section angle of attack, the loads experienced by the blades are cyclic (ref. 5), and thus the propeller blades experience time-dependent forces and moments. These cyclic loads (ref. 6) may cause additional noise (ref. 7) or vibrational problems (ref. 8). In the present report, the focus is on the nonuniformity of the in ow caused by the propeller inclination. The investigation discussed herein is part of a broad NASA research program to obtain fundamental aerodynamic information regarding advanced turboprop installation e ects. Data from early research (ref. 9) on lightly loaded propellers showed a strong dependence of propeller normal force on blade solidity. Also, limited data on more highly loaded propellers (ref. 10) showed that a counterrotation propeller at thrust-axis (nacelle) angles of attack produced substantially higher values of normal force than did a single rotation propeller with the same solidity. The present investigation was conducted to extend the research to provide baseline information regarding the e ect of changing the solidity by changing the number of blades on the force and moment characteristics of an isolated counterrotation turboprop-nacelle combination operating over a range of angle of attack from 10 to 90 , a range of advance ratio from 0.8 to 1.4, and at blade pitch angles of 31:34 , 36:34 , and 41:34 . Tests were conducted in the Langley 14by 22-Foot Subsonic Tunnel (ref. 11). Symbols a1 induced velocity fraction in axial direction a2 induced velocity fraction in circumferential direction B blade area, ft CN normal-force coe cient, Normal force qS CT thrust coe cient, Thrust nD CY side-force coe cient, Side force qS c blade section chord, ft cn sectional load in normal-force direction, Section normal force wc=2 ct sectional load in thrust direction, Section thrust wc=2 cy sectional load in side-force direction, Section side force wc=2 D propeller diameter, ft J propeller advance ratio, V 1 nD N number of blades n propeller rotational speed, rps q free-stream dynamic pressure, lb/ft R propeller radius, D 2 , ft r distance along propeller radius, normalized by R S propeller disk area, ft t time, sec V 1 free-stream velocity, ft/sec va section axial in ow velocity, ft/sec vq section rotational in ow velocity, ft/sec w section velocity, ft/sec x distance along X-axis, in. section angle of attack, deg p propeller inclination (nacelle angle of attack), deg blade pitch angle, deg 0:75 nominal blade angle at 0.75R, deg free-stream density, slugs/ft solidity, NB=S in ow angle, deg azimuthal position rotational frequency, rad/sec Test Apparatus Propellers Photographs of the propeller-nacelle model used in this investigation are shown in gures 1 and 2. The single rotation propeller blade design, designated SR-2, used for the tests reported in reference 1 was used in counterrotation arrangement for this study and the one in reference 12. The detailed geometry of the SR-2 blade design is documented in reference 13. In order to simulate a representative ratio of propeller diameter to hub diameter with the single rotation blades of reference 13 in a counterrotation arrangement, the SR-2 blade coordinates were scaled to a diameter of 15 in. and then shifted radially to accommodate the hub requirements. This resulted in a hub diameter of 2.25 in. and a propeller diameter of 16.1 in. The reference chord in the original single rotation model (ref. 13) was located at the 0.75 radial station. For this model, this reference point was moved to the 0.79 radial station. To obtain the blade pitch angle at the 0.75 radial station of the current con guration, an increment of 1:34 was added to the 0.79 angle setting. The hubs allowed 0, 2, 4, or 8 blades on either or both blade rows. Blade angles were adjusted with a collective pitch-change gear which permitted a continuous range of blade angle setting with an accuracy of 0:25 . The blade angle used in this investigation is the average angle of the blades at the 0.75 radial station. For these tests, both blade rows of the counterrotation system were set at the same pitch setting. The spacing between the pitch-change axis of the blade rows was 2.31 in. (0.287R). The front row of blades was driven counterclockwise looking upstream. The counterrotation gearbox consisted of two gears and two pinions which drove the rear blade row at the same speed but in the direction opposite to that of the front blade row. Nacelle and Support System The dimensional characteristics of the propellernacelle are given in tables 1 and 2 and are shown in gure 3. The nacelle used in this investigation was a body of revolution with maximum outside diameter of 6 in. and housed a water-cooled electric motor which was rated at 29 hp at 10 000 rpm. A fairing which covered the counterrotation gearbox smoothly transitioned from the hub diameter to the nacelle diameter. The nacelle was mounted as a straight extension of a straight sting. Facility Tests were conducted in the Langley 14by 22Foot Subsonic Tunnel, which has a test section 14.50 ft high by 21.75 ft wide. This is a closedcircuit atmospheric wind tunnel and is described in reference 11. The nacelle was mounted on a model