This is the HTML version of a paper presented at:
The American Helicopter Society International Specialists' Meeting, Stratford, Connecticut, October 11-13, 1995.
Wake Geometry Measurements
The survey measurements have shown the complexity of the flow field in the vicinity of the empennage location. Furthermore, the flowfield measurements showed that the position of the rotor wake and the trailing vortex bundles are likely to have significant effects on the flow environment encountered by the horizontal stabilizer.
Detailed positions of the rotor wake were determined from video images using the shadowgraph method, as discussed previously. The shadowgraph images were analyzed off-line to determine the position of the tip vortices, which identify the boundaries of the rotor wake.
The wake boundaries for different test conditions are shown in Figs. 9-12. Each figure shows the location of the rear wake boundary along the longitudinal centerline for different advance ratios. Outlines of the tail boom, vertical fin, and horizontal stabilizer are also shown. The reference point for these figures is the center of the rotor hub rather than the origin of the TPP, since the hub-plane is fixed with respect to the horizontal stabilizer. The TPP was located approximately 0.050 R above the hub-plane for a typical coning angle of 3° .
Figure 9: Wake boundaries for alpha_s = -2° , BL = 0.075, and high tail position
Figure 9 shows the baseline wake boundaries, which are represented by a shaft angle of alpha_s = -2° , a blade loading of BL = 0.075, with the high tail position. Figures 10-12 illustrate the effect of changing the test parameters.
From the measured boundaries, it was clear that the advance ratio had a significant effect on the wake geometry. At the lowest advance ratio of µ = 0.05, the tip vortices were initially convected down almost perpendicular to the TPP. As the vortices approached the body, however, they were convected almost parallel to the body surface. This process was observed during a previous test, and is discussed in detail in Refs. 25-27. An increase in advance ratio produced a higher wake skew angle and a higher streamwise convection velocity of the tip vortices. Changes in the wake skew angle were largest at low advance ratios, and as the advance ratio was increased, the change in wake skew angle became smaller, the angle remaining almost constant above µ = 0.25.
Due to the difficulty in obtaining high contrast video shadowgraphs, it was not always possible to follow the vortex filaments until they impacted on the vertical tail. However, the trajectories of the observed vortices were consistent enough to determine the position of the wake with respect to the horizontal stabilizer. In those shadowgraph images where direct impingement of the tip vortices on the vertical fin could be observed, they were found to disintegrate quickly.
The effects on the wake geometry of increasing the blade loading to BL = 0.085 were almost negligible. Essentially, the tip vortices followed the same path as for the lower blade loading, and they appeared to be convected away from the rotor at nearly the same speed, despite the slight increase in mean inflow velocity corresponding to the thrust increase. However, the shadowgraph contrast was slightly improved due to the increase in tip vortex strength (Ref. 28), allowing more of the wake to be observed at higher advance ratios.
Figure 10: Wake boundaries for alpha_s = -2° , BL = 0.085, and high tail position
Descent conditions in forward flight were simulated by tilting the shaft axis back to alpha_s = +2° . This resulted in a more substantial change to the wake boundaries, especially at high advance ratios. The wake boundary now passed very close to the horizontal stabilizer for µ = 0.25 and 0.30. The wake skew angle was almost constant at these advance ratios, and the observed wake boundaries were virtually identical.
Figure 11: Wake boundaries for alpha_s = +2° , BL = 0.075, and high tail position
Lowering the vertical position of the horizontal stabilizer had little effect on the wake geometry for a given set of conditions, and the observed wake boundaries were essentially identical to those observed with the high tail. However, with the lower position the wake now passed over the stabilizer at µ = 0.25, and impinged on the top surface. Such close interactions were not observed with negative shaft angles at the high tail position, where the wake passed below the horizontal stabilizer for all advance ratios.
Figure 12: Wake boundaries for alpha_s = -2° , BL = 0.085, and low tail position
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