This is the HTML version of a paper presented at:
The American Helicopter Society International Specialists' Meeting, Stratford, Connecticut, October 11-13, 1995.
Time-Averaged Pressures
Time-averaged pressure measurements were made at thirty-two locations, distributed spanwise along the leading- and trailing-edges of the horizontal tail. The flow about the horizontal tail was found to be inherently three-dimensional, and chordwise pressure distributions along a few spanwise positions could not provide an adequate representation of the flow state over the entire surface. This was previously shown by Leishman and Bi, (Ref. 5) who encountered a wide variation in pressure distributions along the span of a lifting surface located in the rotor wake. The three-dimensionality was expected to be as severe in the current test, especially since the horizontal stabilizer was situated on top of a vertical fin. The vertical fin was expected to generate some lift due to the effective side-slip angles measured in the wake survey, thus contributing to the asymmetry of the flow over the horizontal tail.
Spanwise pressure distributions measured over the horizontal tail are illustrated in Figs. 13 through 16. These pressure distributions correspond to the same test conditions as the wake geometries given in Figs. 9-12. Each figure shows both the pressure distribution along the leading- and trailing-edges. The solid line represents pressures along the upper surface, and the dashed line represents pressures along the bottom surface. Since the horizontal stabilizer was operating in a flow environment that was more dependent on the rotor than on the freestream, the measured pressures have been non-dimensionalized with respect to the rotor tip speed. The steady pressure measurements presented here have been non-dimensionalized with respect to rotor speed, such that:
The pressure distributions over the tail showed generally positive (stagnation) values of C_p' on the upper surface, and negative (suction) values on the lower surface. There was a clear difference between the advancing (starboard) and retreating (port) side of the stabilizer, especially on the lower surface. In general, the pressures on the advancing side of the stabilizer were slightly higher along the leading-edge of the airfoil. This suggests that the (negative) lift on the advancing side was higher as a result of the higher strength of the vortex bundle trailed from this side of the rotor. The asymmetry is also partly due to the physical separation by the vertical fin, which divided the flow over the stabilizer at midspan.
Figure 13: Spanwise steady pressure distributions for alpha_s = -2° , BL = 0.075, and high tail position
Pressure distributions for a shaft angle of alpha_s = -2° and the high tail position are shown in Fig. 13 (BL = 0.075) and Fig. 14 (BL = 0.085). These pressure distributions appeared qualitatively similar, although the observed pressures were slightly larger at the higher blade loading.
Figure 14: Spanwise steady pressure distributions for alpha_s = -2° , BL = 0.085, and high tail position
At advance ratios below µ = 0.15, the pressures along the leading-edge were quite low, and significant suction pressure existed along the trailing-edge. The flowfield measurements and flow visualization discussed previously indicated that the downwash angles at these advance ratios were quite high, and it is likely that the tail was fully stalled at these operating conditions.
As the advance ratio was increased to µ = 0.15, a significant increase in the leading-edge suction pressures was observed. At the same time, the pressures along the trailing-edge decreased considerably, except at the measurement point just to starboard of the vertical fin. The flowfield survey and flow visualization had shown that the downwash angles decrease with advance ratio, while the wake skew angle increased, and this resulted in a smaller angle of attack at the horizontal stabilizer. It was likely that the flow was now more re-attached, but the tail was operating at a high negative angle of attack.
At high advance ratios, a large suction peak appeared in the trailing-edge pressure distribution on the starboard (advancing) side of the tail. It is possible that there was a lateral flow towards the advancing side of the stabilizer, such that the vertical fin was operating at a negative angle of attack. This was implied by the flowfield measurements of V_y (see Fig. 5), which showed that the lateral velocity was generally positive in the vicinity of the horizontal tail. This, combined with magnitude of the trailing edge peak, suggested that a scarf vortex was present on the advancing side of the tail. This vortex originated at the junction of the vertical fin and the horizontal stabilizer, and extended to the trailing edge.
When the advance ratio was increased from µ = 0.15 to 0.30, the pressure increases at the leading-edge of the lower surface were found to be much milder. Recall that the pressures presented here have been non-dimensionalized with respect to the (constant) rotor tip speed, such that any increase in free-stream velocity affecting the horizontal tail results in an increase in C_p'. However, as the advance ratio increased, the downwash angles and corresponding angle of attack decreased, such that the net increase in C_p' was fairly mild.
It is noteworthy that at µ = 0.15, the leading-edge suction pressures on the bottom surface were slightly lower on the advancing side of the tail. This indicated that the flow had just started to re-attach here, while the flow on the retreating side was already fully attached. This can be explained by the asymmetry of the rotor wake. The stronger vortex bundle on the advancing side led to higher downwash angles, delaying re-attachment. As the advance ratio increased, the flow became fully attached, and the unsteady pressure coefficient increased with the higher dynamic pressure. Therefore, the generally higher velocities on the advancing side led to higher suction pressures at the leading-edge.
Little change to the pressure distributions was observed at low advance ratios when the shaft angle was changed from alpha_s = -2° to +2° , and the tail was still stalled. Between µ = 0.15 and 0.20, the flow became more completely re-attached, and the pressure distributions looked similar to those for negative shaft angles. However, the leading-edge pressures were slightly lower. This was expected, since the observed wake skew angles were higher, resulting in lower angles of attack. The trailing-edge pressures showed a slight peak on the advancing side, indicating that the scarf vortex at the horizontal and vertical surfaces was still present.
Figure 15: Spanwise steady pressure distributions for alpha_s = +2° , BL = 0.075, and high tail position
As the advance ratio was increased to µ = 0.25, the leading-edge pressures on the lower surface on the retreating side decreased, and suction pressures were observed on the upper surface. This suggested that the airfoil sections on the retreating side were operating at a very shallow angle of attack. Flow visualization of the same test conditions (see Fig. 11) confirms this, as the wake boundary was convected almost parallel to the horizontal stabilizer.
On the advancing side of the tail, the pressure peak at the trailing-edge disappeared, suggesting that the scarf vortex was no longer present. While the observed pressures here were low, no suction pressure was observed on the upper surface, confirming that the tail was operating at a small negative angle of attack
Figure 16 illustrates the effects of lowering the tail position. Again, the flow about the stabilizer was completely stalled at low advance ratios, and the flow on the retreating side was found to re-attach at about µ = 0.15. However, re-attachment on the advancing side of the tail did not occur until µ = 0.20, suggesting that the asymmetry in the rotor wake was significantly higher for this tail position, with higher downwash angles occurring on the advancing side.
Figure 16: Spanwise steady pressure distributions for alpha_s = -2° , BL = 0.085, and high tail position
As the advance ratio was increased to µ = 0.25, high leading-edge suction pressures were measured. The wake geometry measurements (see Fig. 12) showed that at this high advance ratio the wake was very close to the stabilizer, and it is likely that the stabilizer was encountering quite high local velocities, especially on the advancing side where the vortex bundle from the disk was stronger. A small peak in the trailing-edge on the advancing side suggested that the scarf vortex was present at high advancing ratios, but it was not as intense as for the high tail position.
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