Autorotation, Ground Effect and Settling w/ Power

[Dave_Jackson]

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

[Ascend Charlie]

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?:ooh:

[Senior Pilot]

Ground effect in an auto?

This was comprehensively covered in this thread (http://www.pprune.org/rotorheads/70866-autorotation-ground-effect.html) some time ago :ok:

[Ascend Charlie]

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?:(

[Shawn Coyle]

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.

[eagle 86]

"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.
GAGS
E86

[Dave_Jackson]

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. :ok:

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. 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

[Rich Lee]

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?


Interesting propulsive idea. Reverse vortex ring state. Reverse vortex ring state that caused the aircraft to ascend might almost ;) be called lift.

[before landing check list]

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

[Torquetalk]

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

[VfrpilotPB/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?

Vfr
Peter R-B

[Hughes500]

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 !!

[tony 1969]

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.

[DTibbals53]

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?

Yes, the rotor does not care from whence the clean air cometh.

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.

[Dave_Jackson]

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

[jab]

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.

[Dave_Jackson]

Jab

Thanks for the offer.

Rotor & Wing has a search engine but the article did not come up.

Dave

[jab]

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."

[rotorfossil]

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.

[Hughes500]

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 !

[Dave_Jackson]

Hi Nick,
It appears that membership is required to enter the site that has your information.


There is no intent to be rude by not responding to the interesting posts. It just seem prudent to keep quite and listen to those with the knowledge.

Dave

Edit,
Nick's web page is accessible here.[http://webpages.charter.net/nlappos/VRS.pdf]

[eagle 86]

Nick,
Can you explain to me what happened the other day when I was demonstrating the INCIPIENT stage of VRS following this setup:
ROD 300 ft/min
IAS <20kts
near flat pitch then,
without changing any other parameter, near full MCP application within about 2 secs, which resulted over a very short period of time a ROD going beyond 1,500 ft/min at which time I did what have been doing for the last 35 years that is applied recovery techniques, which thankfully as usual, worked.
GAGS
E86

[Hughes500]

E86
Out of interest what type of heli were you in.
I reguarly like you have to do vortex ring recovery. Normally with a 300 ac back to 20 kts rod up to 500ft a min with 18 " mp the ac will only just start to show incipient vortex ring and sometimes ( 50/50) with application of power she will haul herself back to a hover ! Which makes the demo sometimes difficult as you havent been in incipient vortex ring!!!!!!!!!

[Ascend Charlie]

The old Bald Eagle was probably in an EC135, with nice brown seats! Or at least brown on the left side.

The old recovery techniques are still the best, aren't they...

[topendtorque]

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. http://static.pprune.org/images/smilies/thumbs.gif

That's a pity, he sounded like a good dude to be talking with on a double muster, about any damn thing when one runs into a storm front, just twenty minutes from the yard, scrub bulls and cleanskills trying to climb all over you, forty minutes from dark and wind gusting like you wouldn't believe, lightning flashing all bloody everywhere. And, also a good dude to have a beer with over a heap of dribble after the excitement has settled.

Where would he be now 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.

No, you can, if you get caught in the dreaded downwind area, especially if you are rash enough to practice it.

It goes like this, Holy s**t, cyclic quickly forward, wait until ground is seriously close, Heave, then, "what in tarnation was that?"

Well even if it wasn't intended the subtle downwind or calm winds that you may have projected in front or yourself with your own flare, and then sunk into, may have caught you.

The procedure is then taught, if one is stuck with downwind auto always run on so's you don't get caught with the short time powered flight, virtue of pitch pull, at the same time as the other symptoms briefly appear.

BTW I waited until Nick succintly stated the obvious before I replied.

[topendtorque]

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?



Yep.

In true VRS the A/C will briefly be sinking at an alarming and accerarating rate in its own cocoon of air, which is recirculating around the M/R blades.

That is of course until it sinks fast enough to have the onrushing relative airflow simply blow the cocoon away as it accelerates downward.

If you would like to load your A/C laterally heavy one way, establish VRS, hold the cyclic central, and watch the aircraft FALL toward the heavy lateral load, it will simply come out of VRS in that direction.

Or, if the A/C is loaded level in a lateral sense then you can fly it out in any direction that you choose as per the standard recovery technique.

But don't try that at the bottom of a downwind auto, as by the time you mess around with that then the real ground effect, which is the crump at the sudden arrival, will have caught you.

In auto, if in doubt as anywhere, always shoot with cyclic toward the A/C inertia. (that's the direction the A/C is travelling in)

[rotorfossil]

Bent skids was a euphemism. The rest was pretty bent as well. R of D was about 1800 fpm. The initial pull resulted in a bit of shuddering as per VR, then it impacted. No it didn't hurt (much), which is a tribute to the vertical impact absorption of the R22. I seem to remember a video somewhere of an R22 drop test where they hoisted it up on crane and turned it loose. It seemed to be pretty well OK afterwards although the skids spread to a lot and then sprung back. It is diagonal loads as in touchdowns with drift that R22's don't seem to like too much.

[Dave_Jackson]

topendtorque
And, also a good dude to have a beer with over a heap of dribble after the excitement has settled.

Where would he be now Dave?


Rumor has it that he never left the pub. http://www.unicopter.com/5shots.gif

He met a reformed tea totaller (http://www.englishforums.com/English/TeaTotaller/mxrl/post.htm) called SASless and they reminisce about politics and other unPPRuNely subjects.


Dave_

[generalspecific]

presumably if you terminate to a hover as opposed to heading to the ground, then the chance or vortes ring is increased. If you are going for a low or zero speed this could become quite dangerous ??

[eagle 86]

GS,
No, because the flare will wash off the ROD - an essential ingredient of POSSIBLE entry into VRS. In another life I instructed an exercise which involved a no flare, 35 kt constant attitude night autorotation to the ground, ROD around 1500 ft/min - all washed off with a one shot pitch pull. No hint of VRS.
Hughesy, try it downwind!
GAGS
E86

[Ascend Charlie]

Was that the one which Ted Bach wanted to improve with the "broomstick method?" You cut a hole in the floor, under the collective, and insert a broomstick pointing down.

In the constant attitude auto, no need to do anything, as the machine got close enough to the ground, the broomstick pushed the collective up, beautiful pitch pull, perfect touchdown.

The Bissell company and even Hoover and Dyson were interested but the newer plastic handles weren't up to the job. And a vacuum cleaner certainly didn't cut the mustard.:8:ooh:

[baobab72]

hello
a couple of questions about settling with power:
how does wind direction and in particular tailwind will effect settling with power?
what about poor attitude control while hovering?

many thanks

baobab72

[[email protected]]

baobab - do you mean settling with power (not having enough power to maintain an OGE hover) or Vortex Ring Syndrome (descending into your own downwash causing large tip vortices and stalling the root of the blade)?

If you don't have enough power to achieve a stable OGE hover, the aircraft is likely to descend which will prompt you to pull more collective which will decay the Nr and the aircraft will sink further and so on until the ground gets in the way or you take corrective action to lower the lever and flyaway using forward cyclic (if you have enough height to do so).

This can lead to VRS but VRS can be encountered with an adequate power margin just by descending with low airspeed (under 20 kts) and getting to a rate of descent in the region of 50 to 70% of your downwash speed.

The two conditions are not the same but VRS is often called settling with power, especially West of the Atlantic.

Do a search for Nick Lappos' explanation.

If you want to avoid settling with power, ensure you have an adequate power margin (minimum of OGE hover power) and make a gradual level transition to the hover - any rate of descent has to be arrested by power and any tendency to quickstop to the hover will mask the real power requirement with flare effect until you drop off translational lift and start to fall out of the sky.

In answer to your actual question, if you only just have enough power to hover OGE, then overcontrolling will increase the likelihood of settling with power. If you are already established in a downwind hover then it should make no difference to the power required as the disc doesn't care which direction the wind is coming from - the only rider to that is that a crosswind will require additional pedal to hold the heading which may sap some power if you end up using more 'power' pedal.

Hope that helps.

[Aucky]

Agreed 100% with Crab - except to add a subtle difference. If, as crab said, you are already established in a downwind hover then the hard bit is done and the helicopter doesn't care where the wind is coming from. If, however, you are making a downwind approach, and have not yet lost translational lift (you have 30+ kts AIRseped) this is where you can get yourself in trouble if you don't have HOGE performance. That's to say, during the earlier stages of the approach, with sufficient translational lift you have plenty of power margin and all feels normal, as ground speed decays at a normal rate, airspeed will be much less than an equivalent into wind approach and relatively early in the approach, perhaps 150-300 feet AGL you will be losing a big proportion of your translational lift. If you don't have HOGE performance, or dont realise you are downwind this is where it can get nasty. You start to sink with the loss of TX lift, instinctively pulling power to contain sink, engine reaches max deliverable power, your still sinking, raising collective, rpm decays, sink accelerates and you have settled with power with decaying RPM, which may well lead to VRS. If you have HOGE power this should not happen as you will have sufficient power to contain the sink... BUT VRS is still a distinct possibility if you don't spot your low AIRspeed and high ROD.

[baobab72]

Hi thanks for the prompt reply.
I was referring to the tendency of the rotorcraft to descend in its own downwash.
Why must the collective be lowered as a recovery procedure? I understand the reason behind moving the cyclic forward to the fly away from the downwash and to increase the speed, but i dont understand the reason behind the necessicity of lowering the collective as a recovery procedure.

Thanks

Baobab72

[puntosaurus]

I don't know anyone that teaches lowering the collective as part of the VRS recovery, and was never taught myself to do so. You've got enough problems usually diving away from the vortex without lowering the collective as well.

[army_av8r]

VRS is a condition where your rate of decent is approximately 75% of your induced flow (Downwash) while also at a low forward speed. if your downwash is1000FPM it would require 750fpm ROD to have a fully developed VRS. vibrations associated with VRS are generally felt around 25% or 250fpm for this scenario. quick application of power early on will then change all of this math. if you double your power early... say increase to 2000fpm worth of downwash. you now need 1500 fpm decent in order to be fully developed in VRS. once VRS is fully developed, application of power only aggravates the situation. if you reduce the collective you could enter into autorotation and fly out of the situation that way. HOWEVER, this is generally not the preferred/ Taught method because a reduction in collective on an approach would put you even deeper in the corner. always get your power in early on the approach to reduce the chances of VRS and to determine that you do indeed have HOGE power.

[the coyote]

As we know, the recovery from VRS requires airspeed. But in VRS, the rotor disc may not respond to cyclic inputs as you would expect, and in fully developed VRS, you may have little or no control over the rotor at all.

Lowering the collective and entering autorotation will give you cyclic control back, and therefore the ability to lower the nose and recover airspeed.

If you don't need to lower collective to regain airspeed, then don't, but if you go to lower the nose and nothing happens, then you must lower the collective to regain some cyclic control.

[JohnDixson]

There is a Pprune Vortex Ring thread that started in 1999 which contained some posts by Nick Lappos that are worth reading. Army av8r if you felt something in a Hawk at -250 ft/min, it wasn't VRS. PM me for further.

Thanks,
John

[army_av8r]

Good point J.D. I used the 1000 fpm to easily divide the ROD into percentages... 250 fpm for the initial indications is a bit too low, I agree. the 25%, and 75% ROD vs. downwash however have been proven by NASA wind tunnel research. Good luck calculating your downwash velocity on the fly! Haha

[[email protected]]

It is theoretically possible to muscle your way out of VRS if you have enough spare horsepower to overcome the rotor drag caused by the vortices at the tips and the stall at the root - it's just that not many helos have that much spare power.