This is the HTML version of a paper presented at:
The American Helicopter Society International Specialists' Meeting, Stratford, Connecticut, October 11-13, 1995.
Flowfield Measurements
The distribution of total pressure in the rotor wake was calculated from the flowfield survey data for the isolated rotor. The presence of the body was found to have a negligible influence on the flowfield survey data. The total pressure coefficient was defined as (Ref. 21):
Figure 3: Distribution of C_p0 at z_h/R = -0.14 for µ = 0.10
A small low pressure region can be observed in Fig. 3 just behind the center of the rotor disk. This is due to the presence of the hub wake, which appears on the advancing side of the rotor due to the (small) swirl flow in the direction of rotor rotation. This hub wake was found to be convected further downstream as the advance ratio was increased, but at no time did it impinge on the empennage region. Previous research (Ref. 4, 23) has shown that the rotor hub wake can have a considerable influence on the flow environment of the empennage at high advance ratios. However, for the measurements made here, which were for advance ratios of 0.3 and below, the hub wake was convected downward well before it approached the empennage. This was also evident from the flow visualization, which showed that the tip vortices passing near the stabilizer were not affected by turbulence generated by the hub wake.
The region behind the rotor wake, where a horizontal stabilizer may typically be located on a helicopter, was characterized by fairly low total pressure. Examination of the individual velocity components revealed that there was a significant downflow at this position. This is illustrated by Fig. 4, which shows the contours of the vertical component of measured velocity V_z, non-dimensionalized with respect to the free-stream velocity.
Figure 4: Distribution of vertical velocity V_z at z_h/R = -0.14 for µ = 0.10
The two low pressure regions trailing from the edges of the rotor disk in Fig. 3 provided evidence of the wake roll-up into two larger vortex bundles. Examination of the V_z component in Fig. 4 shows the presence of these vortex bundles, with high velocity gradients occurring when the vortex bundles were close to the measurement plane. This is particularly visible near y_h/R = -0.80 and y_h/R = -0.80 just behind the rotor disk. Note that the flowfield was not fully symmetric, and in general the velocity peaks in V_z on the advancing side were larger and more concentrated, which suggested that the trailing vortex bundle on the advancing side was stronger. This was also observed in a recent test by Ghee and Elliott (Ref. 24), where flow visualization using the laser light sheet technique was used to determine the roll-up and position of the trailing vortex bundles.
Figure 5 shows the distribution of the non-dimensional lateral velocity V_y, again illustrating the fairly significant asymmetry of the flowfield near the empennage location. As observed before with the V_z component, the effect induced by the tip vortex bundles was much more pronounced on the advancing side of the disk, and is visible at x_h/R = 1.28 and y_h/R = 0.80. In general, the flow direction between the vortex bundles was oriented towards the centerline of the body when the measurement plane was above the vortex plane, and away from the centerline when the measurement plane was below the vortex plane.
Figure 5: Distribution of lateral velocity V_y at z_h/R = -0.14 for µ = 0.10
The measurements of lateral and vertical velocity have been combined in Fig. 6 to give another impression of the influence of the trailed vortex bundles. In this plot, the flowfield is shown for a cross-plane at x_h/R = 1.58, which is approximately equivalent to 11% chord in the stabilizer coordinate system. The position of both vortex bundles can be determined without much difficulty. It is noteworthy that the vortex bundle on the advancing blade side was convected further down below the rotor at this cross-plane, a phenomenon which was also observed by Ghee and Elliott (Ref. 24).
Figure 6: Velocity distribution in yz-plane at x_h/R = 0.20 for µ = 0.10
Figure 7 shows the development of the wake velocities in a longitudinal plane at y_h/R = 0.20, just to starboard of the body centerline. This figure confirms that the wake in forward flight was skewed backward significantly, but still induced large downwash angles in the flow. In the vicinity of the horizontal stabilizer, just downstream of the wake boundary at this advance ratio, the velocities decreased considerably (as illustrated previously by the total pressure measurements) but the downwash angles remained quite large. It is likely that the flow over the lower surface of a stabilizer located in this position would be largely separated. However, it should be remembered that the wake trailed from the horizontal stabilizer itself will reduce the angle of attack, and Leishman and Bi (Ref. 5) showed that a wing in a rotor wake operating at quite large negative downwash angles can still have attached flow.
Figure 7: Velocity distribution in xz-plane at y_h/R = 0.20 for µ = 0.10
The presence of the hub wake is illustrated in Fig. 7 by a small region of low velocities just behind the rotor hub. It is clear from the figure that the hub wake is separated from the rotor wake boundary by some distance, making it unlikely that the hub wake was convected close to the horizontal stabilizer in the current experiment.
The wake skew angle and the asymmetry of the velocities in the wake increased as the advance ratio was increased. This is illustrated in Fig. 8, which shows the distribution of total pressure at an advance ratio of µ = 0.20. The values of C_p0 at this higher advance ratio were much smaller, as expected (recall that by definition C_p0 decreases with advance ratio for a given C_p). The low pressure region due to the hub increased significantly in size, and was convected further downstream, but still not close to the horizontal stabilizer. The high total pressure region found at the rear of disk at low advance ratios moved towards the retreating side of the rotor, indicating that the loading on the retreating side had shifted more towards the blade tips. Even though the rotor was trimmed, this still gave rise to a larger differential between the two trailing vortex bundles, and thus to a larger asymmetry in the flowfield.
Figure 8: Distribution of C_p0 at z_h/R = -0.14 for µ = 0.20
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