PPRuNe Forums - View Single Post - Vortex Ring / Settling with power (Merged)
Old 7th May 2006, 05:47
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BlenderPilot
 
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Just recently I read an article in a well known helicopter magazine by a Mr. Mott Stanchfield in which he regarded that one of the conditions that would most likely promote VRS would be "high altitudes near or above the helicopters HOGE ceiling"

And then I thought if learned anything on this forum (from Nick Lappos) is that VRS is actually more difficult to enter at higher altitudes because to produce the same amount of lift you need to move air downwards at a higher velocity the higher you are, thus your downwash it going faster and it's more difficult to equal in a descent to enter VRS at higher altitudes.

I wonder if the article is keeping alive some of those old myths that say that the higher you are the easier it is to enter VRS, confusing Settling With Power, with Settling For Lack of Power.

I have lately seen a couple of articles on this subject, at Rotor and Wing in which also the author mislead the readers into saying that it's actually easier to enter VRS when hot and heavy that when your flight condition is the opposite, and this is just not true, its the other way around acording to this . . . .

Same thread earlier . . .

Thoughts on the matter, I think we should clear this up once and for all.

Nick Lappos

VRS is only encountered when the downward speed of the helicopter matches the downwash velocity from the rotor. In the cases mentioned by vfrpilotpb it is likely that the VRS was passed through, and then a zero knot autorotation was flown, thus the 3500 ft/min descent rate.

The flow around the rotor is established by the balance between the rotor's push on the air, and the upwind matching that push. Tales of VRS sort of locking the rotor up aerodynamically, and causing tremendous fall rates are simply not true. Those descents are experienced, we can be shown them, but they are not VRS. The reason for VRS is that the rotor downwash is pushed upwards by the free stream and then recirculated back down through the rotor. If the ROD is very much higher than the downwash velocity, the free air just passes through the rotor, and you have to raise the collective to keep from overspeeding the rotor.

For Gaseous, the autorotation first could be a difficult way to experience VRS. If you are into auto, the rotor must be transitioned to powered state, and you must try to trim the descent rate at somewhere between the range of 50% to 150% of the downwash speed. The rate of descent varies a lot depending on how heavy the disk loading is. For a Robbie, the VRS range is 700 fpm to 2200 fpm, for a Black Hawk, it is 1400 fpm to 4000 fpm.

One of the difficulties comparing what your instructor shows you with the actual VRS is that there is no telling what your instructor knows about actual VRS, and there is not standardization of techniques for the demo. The texts are poor, and much pilot lore surrounds the maneuver. I am sure many well intentioned instructors show a descent, some vibration and then an awesome vertical autorotation, and call the whole thing VRS. Why not, mine did in flight school back in 1968.

For VRS to be established in a rotor, the rotor must be lowered to about half the downwash speed under powered conditions. By 75% of the downwash speed (R-22 = 1100 fpm, H-60 = 2000 fpm) the VRS will show its head, the thrust will oscillate (you will feel low frequency vibration like turbulence, with maybe 3/10 of a G of magnitude, really big) and the aircraft will pitch and roll somewhat, the cyclic will be sloppy, and raising the collective will not necessarily produce a reduction in ROD. If the descent is increased to about 150% of the downwash speed, VRS is gone, clean air passes through the rotor, and you are in a vertical descent.

Any nose down (or even lateral tilt to slide out sideways) will help break up the VRS, and a climb will probably start (or at least the rate of descent will reduce somewhat. If you are falling at 3000 or 6000 feet per minute, you are not in VRS, you have slid through it and are now into a vertical descent or autorotation.
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