Autorotation, Ground Effect and Settling w/ Power
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Autorotation, Ground Effect and Settling w/ Power
A question regarding a full autorotation to the ground.
The flare during a landing reduces the forward airspeed and, with the help of ground effect, reduces the rate of descent.
The initial rate of descent was the craft's Autorotative rate of descent.
Would it therefore be correct to believe that at landing this craft has the advantage of ground effect, but it also has the simultaneous disadvantage of attempting to pass through the vortex ring state, in the reverse direction?
Thanks,
Dave
The flare during a landing reduces the forward airspeed and, with the help of ground effect, reduces the rate of descent.
The initial rate of descent was the craft's Autorotative rate of descent.
Would it therefore be correct to believe that at landing this craft has the advantage of ground effect, but it also has the simultaneous disadvantage of attempting to pass through the vortex ring state, in the reverse direction?
Thanks,
Dave
Going back to my university aerodynamics of 1971, I recall that the only time vortex ring was a factor in an auto was in a purely vertical auto, when the "autorotative vortex ring brake state" was a phase the system passed through on the way to an established auto. But only vertical. Forward speed left the vortices behind.
Ground effect in an auto? Has anybody been game enough to continue an auto to the ground without a flare or a pitch pull, to see if the ROD decreased BEFORE it hit the ground and burned?
Ground effect in an auto? Has anybody been game enough to continue an auto to the ground without a flare or a pitch pull, to see if the ROD decreased BEFORE it hit the ground and burned?
Chief Bottle Washer
Originally Posted by Ascend Charlie
Ground effect in an auto?
Interesting comments back in '02, and it seems Jacko still isn't convinced. Does Proon lack the challenges of a good argument these days, now that Lu isn't around?
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To be completely correct- the purpose of the flare is to reduce the rate of descent, and a nice side effect is to reduce the speed relative to the ground.
The autorotation title is due to the rotor being turned by air coming up from below the rotor, and so ground effect is just about non-existant.
Only at the very end of the touchdown phase, when the rotor RPM is being sacrificed to produce downwash is there any effect from the closeness of the ground.
Having had the pleasure (?) of doing these on absolutely flat calm days, there is just the briefest puff of downwash the hits the runway and produces a small and quickly gone blowing around of dust.
If there is any vortex ring state, it must be extremely brief and has not been seen or felt by anyone I've ever talked to.
The autorotation title is due to the rotor being turned by air coming up from below the rotor, and so ground effect is just about non-existant.
Only at the very end of the touchdown phase, when the rotor RPM is being sacrificed to produce downwash is there any effect from the closeness of the ground.
Having had the pleasure (?) of doing these on absolutely flat calm days, there is just the briefest puff of downwash the hits the runway and produces a small and quickly gone blowing around of dust.
If there is any vortex ring state, it must be extremely brief and has not been seen or felt by anyone I've ever talked to.
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"Ground Effect" is the reduction of the "Big Green Arrow" ie Induced Flow which means for the same rotor blade pitch angle you have a greater angle of attack thus more total rotor thrust therefore the closer to the ground you hover the smaller rotor blade pitch angle you need and therefore less engine power needed to hover. Hover IGE less power than Hover OGE.
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E86
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E86
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Senior Pilot;
Thanks for the link.
The thread is new to me. It was started by some Dave Jackson, who joined PPRuNe on 29th November 2000.
I, Dave_Jackson, joined PPRuNe on 31st March 2003, which was just after that mischief-maker got booted off for making political/religious/sexist comments.
Agreed, it was a good coverage of autorotation and ground effect.
This inquiry is a little more detailed. It is questioning the perceivable role that is played by the vortex ring state (settling with power) as the collective is applied and the rotor attempts to power is way out of the VRS when using up its angular momentum.
Ascend Charlie
This relates to the question, in that does a helicopter have any significant forward velocity just before an autorotative touch down?
Thanks Shawn. You have addressed the specific question.
The deeper question has to do with side-by-side rotors and their asymmetrical entry of into the vortex ring state. And, the possible asymmetrical re-entry into this vortex ring state upon an autorotational landing.
eagle 86. This question is very VRS specific.
Dave_J
Thanks for the link.
The thread is new to me. It was started by some Dave Jackson, who joined PPRuNe on 29th November 2000.
I, Dave_Jackson, joined PPRuNe on 31st March 2003, which was just after that mischief-maker got booted off for making political/religious/sexist comments.
Agreed, it was a good coverage of autorotation and ground effect.
This inquiry is a little more detailed. It is questioning the perceivable role that is played by the vortex ring state (settling with power) as the collective is applied and the rotor attempts to power is way out of the VRS when using up its angular momentum.
Ascend Charlie
I recall that the only time vortex ring was a factor in an auto was in a purely vertical auto, when the "autorotative vortex ring brake state" was a phase the system passed through on the way to an established auto. But only vertical. Forward speed left the vorticies behind.
Thanks Shawn. You have addressed the specific question.
The deeper question has to do with side-by-side rotors and their asymmetrical entry of into the vortex ring state. And, the possible asymmetrical re-entry into this vortex ring state upon an autorotational landing.
eagle 86. This question is very VRS specific.
Dave_J
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Would it therefore be correct to believe that at landing this craft has the advantage of ground effect, but it also has the simultaneous disadvantage of attempting to pass through the vortex ring state, in the reverse direction?
be called lift.
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The US Army aerodynamics class (Not definitive I know) says some power (20-100%) is needed, zero airspeed a vertical rate of decent is required to experience settling with power. Is a auto capable of all three?
Jerry
Jerry
Dave,
The flare should reduce the ROD to a value where VRS (as opposed to being over-pitched) is not possible.
The effect of pulling collective is to switch from an inflow from below to an induced flow (as in powered flight); further reducing the ROD at the expense of RRPM (power off)
This is why a zero airspeed auto to the ground is dangerous: no flare means high ROD; pulling pitch results in opposing forces. Whether it's a big bump because you run out of energy trying to stop the high ROD (power off) or [perhaps] because of VRS (power on), the result is going to be the same: BANG!
TT
The flare should reduce the ROD to a value where VRS (as opposed to being over-pitched) is not possible.
The effect of pulling collective is to switch from an inflow from below to an induced flow (as in powered flight); further reducing the ROD at the expense of RRPM (power off)
This is why a zero airspeed auto to the ground is dangerous: no flare means high ROD; pulling pitch results in opposing forces. Whether it's a big bump because you run out of energy trying to stop the high ROD (power off) or [perhaps] because of VRS (power on), the result is going to be the same: BANG!
TT
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I was always taught the Vortex Ring State can be eliminated if you push forward on the stick, would this also work if you went Left or Right?
Vfr
Peter R-B
Vfr
Peter R-B
Dave
Dont forget not only does the flare at the bottom reduce your rate of descent, it also reduces your forward speed to next to 0, but it also increases rrpm considerably ( loading the disc) This increase in rrpm means you have to check the disc to stop it overspeeding by pulling pitch, this futher decreases momentum of the ac.
Cant see how you would get vortex ring in a normal auto unless you were doing min fwd speed one and pulled the lever to max pitch at about 10 ft, even then there would be no time for the state to build up as you would be on the ground !!
Dont forget not only does the flare at the bottom reduce your rate of descent, it also reduces your forward speed to next to 0, but it also increases rrpm considerably ( loading the disc) This increase in rrpm means you have to check the disc to stop it overspeeding by pulling pitch, this futher decreases momentum of the ac.
Cant see how you would get vortex ring in a normal auto unless you were doing min fwd speed one and pulled the lever to max pitch at about 10 ft, even then there would be no time for the state to build up as you would be on the ground !!
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The way I understand it
all three things required for VRS
1) High ROD
2) Low Airspeed
3) Power applied( no power applied in auto so how can you get VRS) ???
P.S. Not convinced ground effect does much to reduce ROD at the bottom of an auto, surely it wouldnt have time to take effect, the extra lift comes from increase of rrpm (due to flare effect) which allows you to raise the lever to cushion touchdown at the expense of Rrpm.
all three things required for VRS
1) High ROD
2) Low Airspeed
3) Power applied( no power applied in auto so how can you get VRS) ???
P.S. Not convinced ground effect does much to reduce ROD at the bottom of an auto, surely it wouldnt have time to take effect, the extra lift comes from increase of rrpm (due to flare effect) which allows you to raise the lever to cushion touchdown at the expense of Rrpm.
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VRS
Originally Posted by VFRPB/2
I was always taught the Vortex Ring State can be eliminated if you push forward on the stick, would this also work if you went Left or Right?
Also, VRS takes some time to develop and the bottom of an auto has 2 factors which decrease the possibility of settling with power or VRS.
1. Ground effect disturbs the airflow to the outer portions of the rotor disc
2. Time is insufficient to generate the ring state. By the time one could realize the VRS, one is on the ground.
Another factor is power, in this case stored energy in the form of inertia in the rotor system. It is continuously being depleted in the pull of the collective as it reduces the ROD, ultimately at zero as the helicopter settles to the ground.
Just my thoughts. Shoot them down if you care, as I am here to learn as I grow old.
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Thanks for the thoughts and information.
It appears that the consensus is that there is insufficient time for a vortex ring state to develop, and if it should develop, there would be even less time (and perhaps insufficient stored power) to lift the craft up and out of the VRS.
Re side-by-side rotors;
This subject may also be of little or no relevance to the V-22 Osprey because of its apparent inability to transition the VRS and its very fast autorotative descent rate. However, Sikorsky is proposing their 'Variable Diameter Rotor' with it's lower disk loading. This may result in; the VRS, recirculation, and perhaps landings, becoming a subject of increasing interest.
Thanks again.
Dave
It appears that the consensus is that there is insufficient time for a vortex ring state to develop, and if it should develop, there would be even less time (and perhaps insufficient stored power) to lift the craft up and out of the VRS.
Re side-by-side rotors;
This subject may also be of little or no relevance to the V-22 Osprey because of its apparent inability to transition the VRS and its very fast autorotative descent rate. However, Sikorsky is proposing their 'Variable Diameter Rotor' with it's lower disk loading. This may result in; the VRS, recirculation, and perhaps landings, becoming a subject of increasing interest.
Thanks again.
Dave
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Mott Stanchfield, a very experienced former test pilot wrote an article, I believe it appeared in Rotor and Wing, wherein he describes experiencing VRS during a power off demonstration in a UH-1. He went into some detail but I dont remember enough to quote any. His reasoning and explanation appeared quite sound. I will try to get the article from him and post it here with his permission. If any other Prooners are aware of, or have this article, please cut and paste.
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I found the following passage written by Mott Stanchfield and it is not as comprehensive as I remember, he may have added detail in emails that I no longer have. This passage does not provide any answers to the original question but it does add food for thought regarding VRS in an autorotation. Mr. Stanchfield was a test pilot for Hiller as well as other manufacturers and some of his stories of test flying are hair raising. His knowledge of helicopter aerodynamics is phenomenal and he occasionally writes articles for well known magazines, well worth reading.
"The name belies the adversity that the VRS may cause a pilot if allowed to reach maturity. In my opinion, a matured VRS is the most hazardous condition that exists in the realm of helicopter aeronautics. I once gave a flight demo in Fort Rucker, Alabama to the Commanding General and staff. The demo was to end with a precision autorotation to the ground. During the last few feet, the ship fell through the flare, even though the flare and collective inputs were correct and well coordinated. It was suspected that a slight tail wind was responsible. Couldnt have been the pilot! In that situation, VRS instantly formed when a stable autorotative approach into the flare passed a massive inflow of air up through the rotor, coupled with the added pitch increase. This is a formula for VRS, the consequence of which was an instantaneous partially stalled rotor system. Such a phenomenon had never happened to me before, nor has it happened since, during many hundreds of autorotations."
"The name belies the adversity that the VRS may cause a pilot if allowed to reach maturity. In my opinion, a matured VRS is the most hazardous condition that exists in the realm of helicopter aeronautics. I once gave a flight demo in Fort Rucker, Alabama to the Commanding General and staff. The demo was to end with a precision autorotation to the ground. During the last few feet, the ship fell through the flare, even though the flare and collective inputs were correct and well coordinated. It was suspected that a slight tail wind was responsible. Couldnt have been the pilot! In that situation, VRS instantly formed when a stable autorotative approach into the flare passed a massive inflow of air up through the rotor, coupled with the added pitch increase. This is a formula for VRS, the consequence of which was an instantaneous partially stalled rotor system. Such a phenomenon had never happened to me before, nor has it happened since, during many hundreds of autorotations."
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To throw in my tuppence worth. Situation - student instructor allowed ias to fall below 35 kt on constant attitude EOL in R22 in flat calm. Pulling pitch had no effect on descent. Result - bent skids. A little later, circumspect investigation suggested that in R22's, ASI over reads considerably in low airspeed auto rotations, putting you close to the descent angle relative to the vertical that gets you into the VR zone. Good idea to maintain at least 40 kt in constant attitude EOL's in R22's.
rotorfossil
Dont understand your post ;
If pulling pitch had no effect then i suggest a little more than the skids would be bent, more likely a total write off and as for the pax ? Most constant atitude eol's will have a rod of 1800 to 2500 ft a min depending upon type ( I wont get in an R22 so dont know for it). Hitting the ground at that rod will probably hurt a lot !
Dont understand your post ;
If pulling pitch had no effect then i suggest a little more than the skids would be bent, more likely a total write off and as for the pax ? Most constant atitude eol's will have a rod of 1800 to 2500 ft a min depending upon type ( I wont get in an R22 so dont know for it). Hitting the ground at that rod will probably hurt a lot !