Votex Ring In Huey and Blackshark

[louisv]

Should the VRS be as bad in the KA-50 as it is with the Huey ?

[Maverick_]

No, IIRC because of the advantages of the multiple rotor blades and coaxial design in the shark the VRS in Huey would be more pronounced

[Sundowner.pl]

There is no effect I know about that would make the co-axial rotor less susceptible to VRS. What does matter is the rotor disc loading, and those also are almost identical in both cases (30 kg/m^2 +/- 0.2), therefore my guess is that - yes, the VRS in both helicopters should be rather similar.

[Griffin]

I don't fully understand what the manual says but here it is:

 

The vortex ring's top boundary for both the upper and lower rotors is the same; the right and lower boundaries of the vortex ring (where the characteristics of this mode are minimal) are somewhat more extensive in coaxial-rotor helicopters

 

When a coaxial-rotor helicopter enters a vortex ring state, it is best to use available altitude to gain forward airspeed and exit the state. Adding power may only exacerbate the problem. The same is true for the single-rotor helicopters.

[Fuzzy_bear]

I find that the stability control/auto-pilot damping function in the shark makes getting out of VR state a lot less troublesome than the Huey but I get into VR with the shark just as easily with as I do in the Huey... but most of the time I don't survive it in the Huey. but that's partly due to me flying the Huey lower than snake s*** on the sea floor at the best of times.

[TimeKilla]

I read somewhere that the way the Huey is designed makes it hard to get it into the VRS and is not really a issue for it in real life.

 

I'll need to search all over and find that might not be correct but was from a very detailed document.

 

Also a'lot of people around here seem to think that if you're moving forward you can't enter into the VRS this is untrue any speed under 40 knots its possible.

 

A computational study by the U.S. Army Aviation Laboratory, in May 1971, developed vortex ring-envelope diagrams for the Vietnam-era Huey as a function of the aircraft's descent angle, horizontal speed, and induced velocity. This information subsequently was carried over to the primary training helicopters for naval aviation and may have been the genesis of the 800-fpm/40-knot guideline. At their respective normal operating gross weights and "Pensacola, Fort Rucker" standard sea-level conditions, the TH-57 and UH-1E/L yield VRS diagrams with approximately an 800-fpm upper threshold (Figure 1). The 40-knot guideline most probably was based on the inherent limiting characteristic of antiquated pitot-static, airspeed-sensing systems that are unreliable or erratic below 40 knots. For some reason, the 800-fpm/ 40-knot guideline was adopted by virtually every rotary-wing NATOPS manual without consideration for the vast range of gross weights and rotor-disk areas of modern helicopters. The guideline also ignored technological advances in low-airspeed sensing systems.

 

VRS Heli moving forward.

The helicopter was reported to have been maneuvering at a low airspeed at an altitude of approximately 150 feet above the ground while on a patrol flight near an international border. Local authorities reported that the helicopter impacted a parked pickup truck in a nose-low attitude while in a right turn, coming to rest on its right side.

 

Witnesses said that the helicopter "appeared to dip the nose-down and enter a spin to the right nearly straight down. One witness added that the helicopter appeared to have recovered from the spin and initiated a climb before it began to spin to the right again, impacting the ground in a near-vertical attitude. Other witnesses added that the engine appeared to be "screaming." A witness, who reported having experience as a helicopter mechanic, added that the engine was screaming, but that the rotor system sounded as though it was slowing down. One witness stated it sounded like it was sucking or chopping air.

 

The recorded weather near the accident site was reported as winds from 260 degrees at 12 knots, visibility 10 statute miles, clear skies, temperature 23 degrees Celsius, dew point 1 degree Celsius, and a barometric pressure of 29.94 inches of Mercury. Density Altitude was computed to be 5,433 feet MSL, No discrepancies or pre-existing anomalies were found with the helicopter or the engine that could have precluded normal flight.

 

A performance study was conducted to determine the controllability and maneuvering capabilities while operating in the flight environment during the assigned observation mission. The study concluded that at the approximate altitude of 150 feet AGL and an airspeed of 20-30 knots, may not have allowed sufficient time or altitude for the pilot to recover after the helicopter entered a vortex ring state.

 

A fully developed vortex ring state is characterized by an unstable condition where the helicopter experiences uncommanded pitch and roll oscillations, has little or no cyclic authority, and achieves a descent rate which, if allowed to develop, may approach 6000 feet per minute. A vortex ring state may be entered during any maneuver that places the main rotor in a condition of high upflow and low forward airspeed

 

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

 

The pilot's encounter with a vortex ring state and his inability to maintain control of the helicopter.

 

DoD News Briefing - Lt. Gen. Fred McCorkle, USMC, DCMC (Aviation)

http://www.defense.gov/transcripts/transcript.aspx?transcriptid=1545

 

Q: Just to give us a sense, what -- a helicopter like the 53 or 46 or whatever, what is the maximum rate of descent in the NATOPS for that?

 

McCorkle: Eight hundred feet per minute in a 46 or a Cobra or a Huey when you're below 40 knots. The 53 Echo and the 53D don't even give a rate of descent or an airspeed; it just says high rates of descent and low airspeeds can cause you to go into a vortex ring state.

 

Q: Is it correct to say that the power settling did not actually start until the nacelles were tilted to some degree?

 

McCorkle: He was already, I think -- and 95 percent is a full helicopter mode, even though you would think it would be a hundred percent, 95 percent is a full helicopter mode, not just the attitude of the airplane and where it is. And it's my opinion, and you might read something different in the JAG, but I think from the CSMU data that he was at full or 95 percent nacelle before he ever slowed below 40 knots, so that at the time -- and he never got into a vortex ring state or even approached it until he was below 40 knots. So when he was below 40 knots he was in the full helicopter mode.

 

Q: Was the tilt essentially at 95 percent the whole time, or did he adjust the tilt during the flight?

 

McCorkle: No, I think that once he went in, once he started going into raising the nacelles, at 800 feet he was still -- I'm not sure where the nacelles were. You can read it in the findings of fact. But wherever they were, he was still well above 40 knots, so he still had a fixed wing airspeed. But once he raised the nacelles all the way up and he was at 95 percent, I'm pretty sure that he was at 95 percent -- I know he was at 380 feet, which is where he first started getting into trouble. But I think that he was at 95 percent when he was passing through 500 feet.

 

Q: So then it's correct to say there was no sign of trouble until after he had converted into that mode and --

 

McCorkle: That's correct. Until after he was below 40 knots indicated airspeed, he was not in trouble.

 

Q: At the moment he got in trouble at 380 feet or whatever it was, what could he have done to get out of it? Flip the rotors into forward and -- would that have done it, or -

 

McCorkle: At that time I think that he would have still hit the ground with that high rate of airspeed at 380 feet. If at that time he had lowered the collective all the way down and had gone into 65 percent or something, he may have been able to come out of it with a very hard landing.

 

Q: Any changes to the flight envelope as a result of this or did the review find the flight envelope for the aircraft was fine?

 

McCorkle: No. All the experts that I've talked to at NavAirSysCom, they feel like that the 200-foot separation between aircraft is sufficient. They also feel like that the 800 feet per minute and 40 knots or below 40 knots is more than sufficient. Fred McCorkle's opinion that if we went and spent a great deal of money on testing that it would probably go in the other direction, you know, that we could say that you could do a thousand feet per minute at 35 knots or whatever, and I'd just pull that out of the air, and I'll say please don't quote me it's on the record or on the tape or whatever. But at the 800 feet per minute and 40 knots is -- has a large Jesus factor in it, in my opinion. And anybody that don't know that, in aviation the large Jesus factor means that Jesus takes care of you a little bit after that, you know?

Edited May 27, 2013 by TimeKilla