Incipient vortex-ring condition

Empirical measurements suggest that for a conventional helicopter significant penetra­tion into the vortex ring condition can be made without hazard. It is possible therefore

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Fig. 2.22 Thrust fluctuations in powered descents (adapted from [2.27]).

to examine the incipient stages of the condition without compromising a safe recovery. Any theoretical vortex ring boundary should therefore reflect this reality by showing contours of increasing vortex strength. Model tests on a rotor, with — 8° of linear twist, operating at a range of collective pitch settings in a vertical descent highlight a correlation between vortex strength and thrust fluctuation [2.24], see Fig. 2.22.

The trend in thrust fluctuations implies that the worse case situation occurs with a RoD equal to approximately 0.8vih, slightly lower than the theoretical value of vih. Figure 2.22 also shows that for rates of descent less than 0.3vih and greater than 1.5vih only 1% thrust fluctuation occurs which might be equated to the incipient stage of the vortex ring condition. Based on the vertical flight data shown in Fig. 2.22 and using the boundary shape suggested by Peters and Chen [2.22], a series of contours can be drawn (Fig. 2.23). Note that if the rate of descent is low (around 0.5vih) glideslopes steeper than approximately 60° will cause the helicopter to enter the incipient stage of the vortex ring state. Also note that if an approach angle of less than 10° is used vortex ring can be almost completely avoided thus the simple rule-of-thumb of ‘no more than 500 ft/min below 30 kts’ is sound since it gives an approach angle of 9°. Figure 2.24 presents the same information in a slightly different manner and also illustrates the 30 kts/500 ft/min rule of thumb for vih between 15 m/s and 50 m/s. Here normalized glide velocity (Vg/vih) and glide angle (y) are used to show the vortex ring boundary. The figure clearly shows that for glide angles of less than 60° incipient vortex ring can be completely avoided. Also if a modest rate of descent (around °.3vih) is generated from the hover (glide angle equals 90°) then the vortex ring condition can be explored with a degree of safety provided the helicopter is sufficiently high to initiate recovery by generating forward speed. Note that for rotorcraft with high disk loading and consequently high values of vih the rule of thumb cannot be guaranteed to keep the helicopter clear of the vortex ring condition.

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Fig. 2.23 Vortex-ring boundaries – forward speed versus vertical speed.

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Fig. 2.24 Vortex-ring boundaries – glide slope angle versus glide speed.

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