View Full Version : Blade-vortex interaction and the Gyrocopter?
03-15-2006, 10:06 AM
Helicopters are susceptible to blade-vortex interactions in low speed descent. It occurs when strong tip vortices dominating the rotor wake strike or pass in close proximity of the rotor blades. This result in changes in blade loading that produce high noise and vibration.
Does a gyrocopter ever experience blade-vortex interaction?
03-15-2006, 11:23 AM
If I understand your question. Short answer, no. Because a gyro's rotors are not engine driven.
03-15-2006, 03:01 PM
You won't experience blade buffeting, wobbling or that sudden sinking feeling in a gyro, in any descent from vertical (airspeed=0) on up to speeds well above cruise.
This is probably because gyro rotors operate at such low loadings, RPMs and blade pitches. The vortices are likely to be gentler and slower.
The rotor is still losing some efficiency in a true vertical descent, however, as a result of descending into its own "used air." Gyro vertical descents are pretty quick -- 1200-1500 ft./min. are standard.
(It's a common over-simplification to say that "the air flows upward through a gyro rotor." On a net basis,, the rotor must accelerate air downward or there'd be no lift. It's this downward-accelerated air that the rotor re-encounters and recycles if it's descending straight down.)
03-15-2006, 03:36 PM
"Blade slap" is not usually a problem in gyros, although it can sometimes be briefly induced as a sort of poor man's "chopper" sound.
In a helicopter, large negative inflow causes the vortex system to be pushed under the rotor's tip-path plane. This normally occurs in high speed level flight and during climb, but during a shallow approach, Near zero inflow implies that the vortex system shed from the rotor lies directly in the rotor's tip-path plane leading to BVI.
"Large positive inflow causes the vortex system to pass above the rotor's tip-path-plane. This latter situation is often encountered in autogyro flight or during autorotation on conventional helicopters. "
excerpted from a patent description:
03-16-2006, 07:58 AM
Dave, what you are referring to only happens in powered descent. The aircraft is descending at the same speep the blades are forcing the air down. The V-22 is highly susceptable to it, so are the other large helicopters. It never happens in autorotative flight, the air off the blades is going the wrong way.
03-16-2006, 08:05 AM
ya, I was gonna say, the circulation is quite opposite
cept if your trying to land inverted
03-16-2006, 09:30 AM
Actually I think it is called "rotor-ring state". The US Marines re-discovered it the hard way on the V-22.
03-16-2006, 10:27 AM
I think "Vortex ring state" or "Settling with power" are the common terms. Effectively the air being forced downwards rotates around the blade tips and back onto the top of the blades. In a worst case situation the helicopter can be at full power but descending.:eek:
03-16-2006, 12:56 PM
Thanks guys for the enlightenment.
Al, the patent was quite interesting.
His means of reducing the BVI during descent was certainly different.
03-16-2006, 02:11 PM
Is vortex ring state resolved by acquiring enough airspeed to restore translational lift?
03-16-2006, 03:21 PM
I don't know if low levels of transitional lift would remove the rotor from the vortex ring state, but the following quotation appears to answer your question.
"Effective translational lift is translational lift advanced to the point where all air flowing through the rotor system of a helicopter is fresh or undisturbed air. That is air that has not already passed through the rotor system while the helicopter speed is slow or when it is in a hover."
03-17-2006, 09:19 PM
Vortex Ring State ensues when the helicopter descends into its own vortexes, its own downwash. Risks are tailwinds, fully-vertical descents, and any descent in reverse flight.
When the helicopter's rotors enter their own vortex, the appearance from the cabin is that the copter is accelerated abruptly downwards, with a whole lot of shakin' goin' on. The natural reaction (up collective, stat!) is the wrong one as it feeds more power into the vortex and you come down faster! You're at full power/collective in a machine that usually hovers no problem, yet you're falling like a stone. That's because the rotor energy is all going into spinning a vortex system around the copter... imagine a toroid or two of rotating air, bound to the helicopter.
The FAA has an explanation and an illustration in the Rotorcraft Flying Handbook beginning on Page 11-6:
"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. This condition is sometimes seen during quick-stop type maneuvers or during recoveries from autorotations."
It can't happen during unpowered flight, ergo, autogyros are immune from this phenomenon, but it's a major hazard for helicopters, particularly those involved in work that had them climbing and descending vertically (military, logging, EMS, police and fire, aerial cranes) or doing lots of hovering OGE.
Corrective action is to fly forward out of the vortex ring, and THEN arrest the descent. which is slightly unnatural. Apply forward cyclic and reduce collective (which you probably have instinctively cranked up to 11). When you're at ETL and climbing, you have successfully exited.
I don't know if civilian helo CFIs have to do vortex ring state demonstrations. Military IPs in all types had to until the late 1970s, when a consensus started to develope that it was too risky. At least in the heavier helicopters like H-53, H-47, it was stopped then. Note that the actual behaviour is somewhat type specific and the 300+ fpm descent the FAA notes is for light trainer helicopters.
Dual rotor helicopters have another risk, that one rotor might enter vortex ring state and one not. As you can imagine, such an event would normally not be recoverable. I am unaware of this ever happening to a Piasecki/vertol/Boeing design but it might have done. It was this phenomenon that caused the USMC MV-22 crash that killed 19 Marines in, I believe, 2000. The operating limitations of the V-22 with respect to abrupt descents were reviewed and restricted in light of this knowledge.
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