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NickLappos
18th Nov 2007, 16:30
Time to start the arguments and get this web site fun again! The below was generated by some informal consensus about 2 1/2 years ago:

Helicopter Urban Myths

These Urban Myths pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.

1) Vortex Ring State (VRS) can happen at as little as 300 foot per minute descent, it does not have to be a higher descent rate

2) VRS is more likely at high altitude and high gross weight

3) Hovering with the nose off wind consumes much more power

4) Blade stall is always preceded by vibration

5) Winds affect the power we require when we are in forward flight

6) Downwind takeoffs are absolutely forbidden

7) The Height Velocity curve is a precise guide to the engine failure danger zone

8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.

9) The legal definition of VFR is sufficient to assure flight control and safety using outside references

10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.

11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.

12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees

13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.

14) NVG are dangerous and should only be used by gifted military pilots.

15) You have to first learn to fly fixed wing before you take helicopter training

16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.

Bravo73
18th Nov 2007, 17:08
Nick,

Didn't we discover a new 'myth' recently? That hydraulic lock and jack stall are the same... ;)

manfromuncle
18th Nov 2007, 17:30
Errr, isn't number 5 correct?

As I see it, to maintain a given groundspeed requires more power if you have a headwind.

??

206Fan
18th Nov 2007, 17:53
In Num one should it not say VRS occurs at a descent rate of 300ft and at an airspeed of around 30Knots??

Bravo73
18th Nov 2007, 18:06
Welcome to the party, gents.



Errr, isn't number 5 correct?
As I see it, to maintain a given groundspeed requires more power if you have a headwind.
??

Where does no 5 say anything about maintaining a constant groundspeed? :confused:


In Num one should it not say VRS occurs at a descent rate of 300ft and at an airspeed of around 30Knots??

Those 300/30 numbers are one of the very reasons for the growth of these myths. But no, Davy, you can't get VRS at 300ft RoD and 30kts.

NickLappos
18th Nov 2007, 18:09
Davy07,

The books say so, but they are quite wrong. VRS can only be induced by descending at least as fast as 75% of the downwash velocity of the helicopter and at a forward speed of not more than about 8 knots. For a Robbie this is at least -750fpm rod. For an S76 it is about -1500 fpm.

Why is 300 taught? Because one can start a VRS event by entering a hover with too little power, slip into an overpitching event and in short order, enter VRS. In a helo with scads of OGE hover performance, the 300 fpm is truly a myth, in one without, it is misleading as a VRS cue, but good word as an overpitching warning.

Manfrom Uncle, what you are correctly saying is that going faster uses more power. The wind has nothing to do with it. 80Kts airspeed uses the same power both up and down wind.

kevin_mayes
18th Nov 2007, 18:18
Hi Guys.
What would be the downward velocity of my Bell-47.
I was also taught the 300/30 rules, it put the willy's up me every time I land? Which I know it shouldn't... so I end up making very long low landings, which are often not ideal.
Cheer
Kevin.

ARRAKIS
18th Nov 2007, 18:20
14) NVG are dangerous and should only be used by gifted military pilots.


Well, I would cut this one to

14) NVG should only be used by gifted military pilots.

With lack of proper training, correctly adapted cabin/cockpit it could be dangerous.

Arrakis

206Fan
18th Nov 2007, 18:21
Aye cheers for that nick, just checked the Principles of Flight book there..

Vortex State Ring can develop when the helicopter has:

1 A low or zero airspeed (below translational lift speed)

2 Some power in use (collective input) 'As you mentioned'

3 A rate of descent that is in the region of 400 to 800ft/min, depending on aircraft type!

Duno were i got 30 Knots out of!

Simul8
18th Nov 2007, 18:42
1) Vortex Ring State (VRS) can happen at as little as 300 foot per minute descent, it does not have to be a higher descent rate


If you are in a hover, I could buy that. Just can't imagine that happening with forward airspeed, above ETL.
2) VRS is more likely at high altitude and high gross weight

Possibly, however, demonstrations in the 407, with half a bag of gas, myself and the Instructor were the only two in the aircraft, it was very easy to get it into VRS once the airspeed was at ETL or below. 3) Hovering with the nose off wind consumes much more power

Much more? Where is the wind? Less power with right pedal application, because you are using the engine torque (in Bell aircraft that is).
4) Blade stall is always preceded by vibration

Haven't the slightest idea, I'm not a test pilot and hope I never find out...got to be honest and know my limitations!
5) Winds affect the power we require when we are in forward flight

Doubt it...fuselage and tail will weather vane into the wind during slight crab angles...if the wind is 90* off the fuselage, there wouldn't be any further power requirements, and if the wind was off the tail, we would have a higher ground speed.
6) Downwind takeoffs are absolutely forbidden

Well, it's not a smart thing to do with smaller A/C (206B,L, schwietzer 300's, MD 500"s, etc.) If you've got an A/C with good power, I see no big deal. I see Blackhawks, 53's, 47's do it all the time. Fact is I was taught to use the wind to your benefit, and it is a safety concern in the A/C I fly.
7) The Height Velocity curve is a precise guide to the engine failure danger zone

Not precise, just a limitation by the manufacturer to prevent lawsuits ;)
8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.

Nope, Pilot error is the most common. CAT A is a relatively safe way to take-off and depart, but Pilot error could still screw that up.
9) The legal definition of VFR is sufficient to assure flight control and safety using outside references

Go here http://www.aviationtoday.com/rw/services/other/1835.html It's a great discussion on the subject, in the U.S anyway...
10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.

"They" need a life and perhaps get laid.
11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.

I could by that...the reason for adding a tiny bit of collective on takeoff before and during ETL.
12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees

Sure, in a two bladed helicopter.
13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.

I am sure some will come on and say "only in Bell OH-58, Bell 206B,L etc. etc.) But I have seen videos of other manufacturers experiencing the same thing, especially the Aircrane video, that's at the top of my list! So yeah, it can happen with single rotor heli's with a tail rotor.
14) NVG are dangerous and should only be used by gifted military pilots.

What a crock...that's an improvement on safety...especially when the Pilot receive proper training.
15) You have to first learn to fly fixed wing before you take helicopter training

I didn't...and quite frankly, fixed wing Pilots have the hardest time, having seen those coming through the Academy.
16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.

Well, I'm in it not only for flight, but to live to fly again. If I am in flight and exceed the flight time, or my survival depends on it, that I can live with...but anything intentional or my survival option is within my boundaries, I trust the engineers, having seen what they do to test those limits.:ok:

Bravo73
18th Nov 2007, 18:50
Simul8,

Here's one of the many definitions of 'myth' that Google just threw up:

"Something not true, fiction, or falsehood. A truth disguised and distorted."

Here's what wiki has to say about 'urban myths':

"An urban legend or urban myth is similar to a modern folklore consisting of stories thought to be factual by those circulating them."



In light of that, do you understand what Nick's point is? :p

Simul8
18th Nov 2007, 19:07
Sure do...however, Nick first stated "Time to start the arguments and get this web site fun again! The below was generated by some informal consensus about 2 1/2 years ago:"...so Bravo73 have you got any arguments to state based upon my submission?:):ok:

Bravo73
18th Nov 2007, 19:46
Nick then went on to say:

These Urban Myths pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.

However, from a lot of your 'arguments' it would appear as if you believe that the majority are true. The 3 words, 'hook', 'line' and 'sinker', spring to mind! :p


PS LTA is different to LTE.

hihover
18th Nov 2007, 20:15
Now we're having fun!

tam

jayteeto
18th Nov 2007, 20:54
When the RAF taught using 30kts for vortex ring, it was for a very good reason. The ASI on Gazelle started reading accurately at 30, so anything below was classed as zero. We were told the figure was MUCH lower, but that was the lowest speed we could read.

TwinHueyMan
18th Nov 2007, 22:36
"When the RAF taught using 30kts for vortex ring, it was for a very good reason. The ASI on Gazelle started reading accurately at 30, so anything below was classed as zero. We were told the figure was MUCH lower, but that was the lowest speed we could read."

I got told the same thing.

However, this VRS rumor thing confuses me a bit. I read up everything out there on VRS and how to get into it, and really believed that you'd have to be falling at a great rate with low airspeed to get into VRS (think 900fpm vertical descent), until I went out with the instructor and got full blown bucking bronco VRS that started with pulling collective at 200fpm descent rate indicated (figure 300-400 with the VSI lag) and maybe 0-5kts airspeed in a robbie. After seeing that, I'm a firm believer of <300fpm at <30kts. Granted, it's still that elusive goblin that's gonna come and git'cha, but without complete practical experience and understanding, and only what I've seen during training evolutions, I'm convinced that the point at which you could get into it is a lot closer than some make it out to be.

Anybody's thoughts?

Mike

A.Agincourt
18th Nov 2007, 23:19
5) Winds affect the power we require when we are in forward flight

The question is badly phrased in that 'winds affect the power we require.......' - to do what?

Any movement of air in relation to the disc will alter the relationship of IF/RAF and therefore the maximum potential of lift will also be affected corresponding to that and pilot induced variations. So you will only need more power if you are trying to maintain datums precisely. This of course assumes W/V is constant in value and origin and it never will be. So the answer is NO.:}

MSP Aviation
19th Nov 2007, 00:41
But no, Davy, you can't get VRS at 300ft RoD and 30kts.

What about when in a strong updraft? Pressure altitude may be decreasing at 300fpm, but a 450fpm updraft will give a Robbie that 750fpm descent Nick cites above.

paco
19th Nov 2007, 05:37
Without going into individual myths, Nick is right in suggesting that a lot of helicopter activities are carried out without much analysis as to why they are done.

I would add "a shallow approach is the only one to use in mountains (or anywhere else)" to the list.

Phil

sunnywa
19th Nov 2007, 07:18
Hi all,

Shouldn't the myths read:

1) Vortex Ring State (VRS) can only happen when gifted military pilots are conducting NVG operations

2) VRS is more likely when the operaor has drunk 4 pints of heavy (preferably NOT warm) mixed with tequila chasers while going down the back stairs to the carpark.

3) Breaking wind with the nose off wind consumes much more power from the chilli night

4) Blade stiffness is always preceded by vibration of the 1R variety on the mattress

5) Winds affect the power we require when we stick our heads out the window

6) Downwind farts are absolutely forbidden

7) The Height Weight (BMI) curve is a precise guide to the heart failure danger zone

8) Alcohol is the most common accident cause, so a breath analysiser ignition lock thingy is the most cost effective safety enhancement we can incorporate into new helicopters.

9) The legal definition of a bundy (rum) and coke is sufficient to assure your flight control and safety using outside references

10) "They" always hide things from us. We should not trust them, and insist that the water always comes from a bottle.

11) The socially lubricated helicopter pilot is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when he/she moves forward to chat up the bird.

12) A midnight stagger is always caused by gyroscopic precession, and is always exactly 90 degrees out from where you are attempting to get to

13) LDE (Loss of Drinking Effectiveness) is when you run out of your allowances (either from work or from the home financial controller) and can be experienced by any single or married helicopter pilot.

14) NVG are dangerous when driving home to avoid detection by the local plod, and should only be used by gifted pilots with good insurance coverage.

15) You have to first learn to not take yourself too seriously before you take helicopter training

16) Head spins, vomit, and damage from jumping out of buildings (seemed like a good idead at the time) limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.

paco
19th Nov 2007, 07:32
LOL! I hear you about downwind farts.....

Phil

Bravo73
19th Nov 2007, 08:28
The question is badly phrased in that 'winds affect the power we require.......' - to do what?

Any movement of air in relation to the disc will alter the relationship of IF/RAF and therefore the maximum potential of lift will also be affected corresponding to that and pilot induced variations. So you will only need more power if you are trying to maintain datums precisely. This of course assumes W/V is constant in value and origin and it never will be. So the answer is NO.:}

:D Well done, 'Ace'. The answer to most of the statements (if there is actually an 'answer') is 'NO'. They are myths, ie NOT TRUE.


What about when in a strong updraft? Well, yes MSP, that is obviously the exception that proves the rule. But for Davy's sake (considering that he's got a couple of hours training under his belt and is currently getting his answers from Wagtendonk), being at 30kts and 300ft RoD will not get you into VRS. :ok:


sunnywa - :D:D:D :ok:

Galileo
19th Nov 2007, 08:37
Outstanding. LOL.

I thought 12 - 'midnight stagger' - was actually an inverse square law. The ability to get where you want to stagger is adversely affected (inverse) by the square of the number of beers consumed. This in turn is affected by the gyroscopic progression described but which can act at any angle both in a vertical and horizontal plane. :}

Flying Binghi
19th Nov 2007, 09:51
Damn, looks like I got most these questions wrong... now dont anybody tell the ol bovines that or they may think I dont know how to fly and decide to ignore me buzzing over head. :E:)

ATN
19th Nov 2007, 12:16
Nick,

You forgot to mention the 'low-skid-in-hover-due-to-the-main-rotor-and-T/R-not-being-on-the-same-plane'. You raised up that one a while ago. I will keep believing it until otherwise demonstrated.
point #3: in Utopia, considering the rotor only with nothing underneath, no, the wind direction does not influence the required power. In reality try to maintain hover with a 30 Kt crosswind and then look at the engine parameters.
Another one that woud be worth discussing is the pitch down attitude when entering auto. Many think it's the airstream pushing up the horizontal stabilizer.

Cheers

ATN

hihover
19th Nov 2007, 12:16
Fantastic!

re # 11 - I have found that I don't even have to move to fall off, I can fall off just by opening my mouth to speak.

manfromuncle
19th Nov 2007, 12:22
How about number 13

13 - I can get my PPL in 45 hours, my CPL in 150 hours, then get a job flying VIPs around in an S-76. All for £40,000.

Sliding Doors
19th Nov 2007, 14:23
manfromuncle,
You could simplify No. 13 to ' With a new CPL and low hours your chances of getting any job are good' :eek:

Or my all time 'favourite' from some flying schools ' With 2000+ hours TT in a Robbo you are at a stage where the industry feels any job would be open to you' :uhoh:

When what they really mean is ' We'd love to sell you an FI course'

Max_Chat
19th Nov 2007, 14:41
Well done Nick, hats off to you.

Re number 14, are there any other sorts of military/Ex military pilots? :p

heliski22
19th Nov 2007, 16:32
I had the pleasure of meeting Bell man OK Moore in the early nineties. What he said in conversation was "We've been flying around in single-engined helicopters for forty years and suddenly they're dangerous!"


Found the piece below in my archives regarding Singles and Twins, just to add to the mix!! It was written in 1994. The Tables didn't paste in as saved, I hope they make sense.


************************************************************ ********
There are two important studies in the matter, both carried out by Bell Helicopter over the last 24 years.

The first considered the number of fatal accidents involving Bell helicopters during the period January 1970 to March 1987. The survey covered 19,700 single turbine and 1,800 twin turbine helicopters supplied to both military and civil customers and divided the causes of fatal accidents into three categories of Accident Initiator. They were, Engine Material Failure, Non-Engine Material Failure and Non-Material Failure (Pilot Error, usually). 6% of single turbine helicopter fatal accidents were caused by Engine Material Failure, 3% in the case of twins. However, Non-Engine Material Failure was the cause of 12% and 22% of fatal accidents in singles and twins respectively. Thus the total percentage of fatalities for all mechanical failures was 18% for singles and 25% for twins.

The table illustrates these figures clearly:

Accident Initiator
Single/Twin




Engine Material Failure

6%/3%


Non-Engine Material Failure

12%/22%



Sub-Total

18%/25%




Non-Material Failure (Pilot Error, Usually)


82%/75%



It is apparent, therefore, that any advantage offered by the presence of the second engine is more than offset by the reduction in reliability of the aircraft as a whole brought about by the additional and more complex systems to be found aboard the twin-engined helicopter.

18% and 25% represent a ratio of 4:5 for accidents involving singles and twins respectively.

The second study counted the number of accidents per 100,000 flight hours for single turbine and twin turbine helicopters. These show that accidents, all caused by mechanical failure, were at the rate of 5.49 for singles and 4.37 for twins.

The ratio of 5:4 for accidents involving singles and twins respectively is almost precisely the reverse of that shown above.

Farmer 1
20th Nov 2007, 08:47
It is apparent, therefore, that any advantage offered by the presence of the second engine is more than offset by the reduction in reliability of the aircraft as a whole brought about by the additional and more complex systems to be found aboard the twin-engined helicopter.



Careful, Heliski. You know what they say about lies, damn lies and them other things.

Shawn Coyle
20th Nov 2007, 08:56
Nick:
Well done - you're going to both restore a sense of fun and smoke out more myths (and their believers).

heliski22
20th Nov 2007, 11:00
Farmer 1

Like Nick said at the start, the idea is a little debate!

Sitting fat'n'happy, fully coupled in a corporate twin, clear blue sky above, cotton wool to the horizon just below, it's easy to look back on the "good old days" of powerline patrols and aerial photography/filming in an old JetRanger which fit like a well-worn boot!

And remember -

"Old men like to give good advice when no longer able to give bad example!"

scooter boy
20th Nov 2007, 15:53
13) LTE is when you run out of power pedal

:confused:
Please forgive my ignorance but I thought this was the case.

And Bravo 73 what is the difference between LTE and LTA?

SB

Simul8
20th Nov 2007, 16:08
From recent discussions with our pilots, I was told that LTA is the precursor to LTE. Meaning you will first loose the authority of the pedal, however I understand this could lead to LTE, but could happen by itself without experiencing LTE... I have been told you will experience LTA first before LTE...

After Bravo73 pointed it out, I had to investigate...:ok:

Bravo73
20th Nov 2007, 17:59
scooter boy and simul8,

re LTA vs LTE

This is how I summed up the difference a couple of years ago (in another thread (http://www.pprune.org/forums/showthread.php?t=202277)):

There's a difference between LTE (Loss of Tail Rotor Effectiveness - in it's simplist terms, the TR is not 'big' or effective enough to provide control within the entirety of the flight envelope) and LTA (Loss of Tail Rotor Authority - TR doesn't have enough RPM to maintain control due to mechanical problem or the pilot 'overcooking' things).

This has been covered in depth in previous threads.

The B206A was particularly prone to LTE whilst most helis can be subjected to LTA if the pilot allows the RRPM to droop too far.

This thread, however, covers the topic in much more detail:

http://www.pprune.org/forums/showthread.php?t=143383


HTH

:ok:

thekite
20th Nov 2007, 21:45
Like Twin Huey Man I got my VRS education in a Robbie.
At the factory instructor course I happened to mention, in flight, that I could only occasionally achieve this elusive VRS at >300' + <30kts. to teach my own students.
"Follow me thru!" And the factory pilot used (from memory), 16" with AIS flickering10-20kt, and yep it is the bucking bronco ride. :E A great demonstration, and confirming what I had earlier learned with my own dangerous experiments. It is not only autorotation entry that gets you out; transitioning fwd with cyclic movement will do just as well with much less height loss. :ok:
thekite

southernweyr
20th Nov 2007, 21:48
I was wondering if anyone could elabortate on this myth. I am new here and I read this thread this morning then went out and hovered and noticed my altimiter reading lower in a hover than when on the ground. It seems that my altimeter is showing higher pressure when in a hover. Also, why do you get that dip in altitude after accelerating just prior to ETL? It seems like it takes more power at that point than any other during take off.

SASless
21st Nov 2007, 01:01
Statistics....oh Lordy!

If it takes 22 Cobra crashes to kill the same number of people as can be killed in 1 Chinook Crash....does that make the Cobra 20X safer than the Chinook? Does that mean Cobra pilots have 20X less chance of buying it than does a Chinook pilot?

How many crashes of two engined helicopters occurred due to a second engine failure as compared to the number of crashes that occurred in single engine helicopters which had a single engine failure?:uhoh:

Simul8
21st Nov 2007, 01:56
I've seen many videos which claim to be LTE, and it's quite a wide range...OH58, 206 series including the 206LT, Dauphin in the ice next to the ship (is that really considered LTE if it doesn't have a conventional tail rotor?), the Aircrane, AS350, Sea King just offshore, darn it's really hard to remember all of them...So, this leads me to think not many aircraft have a tail rotor effective enough, or big enough, to prevent LTE...(I haven't heard of or found any incident with the 407, does the asymmetrical airfoil of the TR blade have anything to do with that? Have not heard of any L models with the High Altitude tail rotor either, but I am not familiar with other manufacturers)

Bravo73 thanks for the links...I was interested in the CAA document...Looks like they regurgitated much of what Don Bloom found back when the Army was having problems. Which leads into my next example...I know he had found that LTE can be initiated by two distinct causes (amongst others)...The main rotor vortex produces a downwash that disrupts the airflow into the tail rotor (washing out the effectiveness of the thrust produced?), and the second to mention, is the prevailing winds themselves can cause the tail rotor to experience VRS. I have seen a video of those wind tunnel tests and it was quite convincing...however, after PPRune, I am:
1. Somewhat convinced the true cause is still a myth, or
2. Not all causes have been discovered, and last but not least
3. The pilot had a brain fart:ooh:

;):)

Back to investigating those other posts...repetition is the key you know

Farmer 1
21st Nov 2007, 09:44
Heliski22,

No offence meant, honest. Yes, I remember them good old days - in my dreams. Nightmares, actually.

heliski22
21st Nov 2007, 10:27
Farmer 1

No, no, I didn't assume you were doing anything other than adding to the mix - fair play!! I just remembered I had that piece of information stashed away and thought it opportune to throw it in!!

A bit off-thread but the memory that stands out from the powerline work is my first experience of blade tape coming off a 206 blade after flying in heavy rain. Not having experienced it before, I thought the aircraft was about to come apart and the 50kts at 50ft became a very slick run-on landing and wind-down to idle in what seemed like an instant. This was very much to the amusement of the Electricity Company observer who, after 30 years doing the work, had seen it before, knew what it was and was able to tell me before I shut down to inspect it!!!

I still don't view those days as "nightmares" but I agree they do look rather better when viewed from a distance! :-)

fly_boyz
21st Nov 2007, 20:03
**disclamer: this relates primarily to longline work and VRS** :O

Just to add to the discussion; working seismic and shakeblocks on 150' longlines teaches a lot about VRS. It seemed I'd get into it once a day on some jobs (particularly on the Astar). Because of the nature of the work it was a risk that we were aware of and managed. Here's a link I found that gives you a basic idea - it's a 500D dropping off some bags at staging.

http://youtube.com/watch?v=FDJkllGnN5k

Schools teach students to push forward on the cyclic (as do some books) to recover from VRS (along with lower the collective, etc), however, chances are you got into it because of the conditions already mentioned or a combination of those conditions and being even just slightly downwind (seems to me to be the biggest contributor). To me, pushing forward is the wrong response because you are looking for clean air to get the disc into. If you're getting into VRS then you can be sure that your clean air is not in front of you. When you only have 150' or, in a lot of cases, much much less (150' longline - 80' trees = 70' to recover), lowering collective and pushing forward will not provide recovery before hitting the trees. A little peddle and some lateral cyclic (without touching the collective) has always solved the problem for me. The idea is to get the disc into clean air.

deeper
21st Nov 2007, 22:02
fly boyz,

You must have all the vrs allocated for the entire industry if you get into it every day.

Must be scary descending at 1500 feet a minute with 150 feeet of long line under you, what rubbish.

Of course you fly into clean air if you are nose into wind, you are probably over pitching, try carrying a lighter load or handle your aircraft better.

I personally have never read so much ill informed crap in my long flying life, I hope no one takes any notice of your ideas or your techniques.

If the video is supposed to demonstrate flying that could induce this fabled problem it does exactly the opposite.

Dont you read Nicks writings, they are correct and I am sorry to inform you that there is absolutely no argument you could put that can change this fact.

Get over this VRS fetish and get on with flying. I would hate to be a young flyer being fed this tripe, i would have to go through the rest of my career living in fear for no good reason. I know a hundred long line pilots who have worked in the most hostile environments and to a man they have never experienced VRS. Try to keep it over your side of the pond, we don't need it here.

Another ill informed pilot was mustering the other day and said "I am getting a bit of LTE over here, are you gettig any over there?"

I am so glad for you that you think the books and years of experience are wrong, write one of your own. :confused:

delta3
21st Nov 2007, 23:39
I share the same feeling fly_boyz.

On high hovers with some amount of wind, I am almost sure to get one when making a 360 (as I stated before on a R44-II, did not experience that on the R44-I). So I tend to agree with some kind of TR/MR or other interference (slow translation unawareness) that catalyses VRS.

I cannot comment on your recovery, because I always had hight and got out of it according to the books (using more than 150 ft...).

My statistics are so bad that I would not do a high 360 without a positive vario of at least 200 ft/min

d3

before landing check list
22nd Nov 2007, 01:45
I have to call BS on this one:
5) Winds affect the power we require when we are in forward flight.

A helicopter does not know which way the wind is coming from. You would not either if you could not see the ground nor had any instruments to reference position over the ground.

MD900 Explorer
22nd Nov 2007, 02:31
No. 7 - Only applies if you are one of those pilots that uses the kebab house / Curry house more than you should...hang on... oooppps... Sorry guys :E

No. 16 has to be a Coriolis thing.... shouldnt it?.... dunno much about this spraying about a circle thing.... thought it was a part of training...ooops :(

MD :=

Blackers.
22nd Nov 2007, 06:37
5) Winds affect the power we require when we are in forward flight.

Of course this is correct. The only thing that affects the power required in forward flight is Gross Weight and Density Altitude. :ugh:


However, Hovering IGE is a different matter :E

Bravo73
22nd Nov 2007, 09:23
I have to call BS on this one:
5) Winds affect the power we require when we are in forward flight.


5) Winds affect the power we require when we are in forward flight.

Of course this is correct. The only thing that affects the power required in forward flight is Gross Weight and Density Altitude. :ugh:




Ladies/Gents* (*delete where applicable),

Just to remind you what the opening post actually says:

These Urban Myths pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.


:p

before landing check list
22nd Nov 2007, 11:56
When you are flying (Not at a stationary hover ref the ground) there is no relative wind. The helicopter is moving through a moving airmass. Imagine a free balloon in a 100 kt wind. If you could see the ground you would know you were moving however you could strike a match since there is no wind relative to the balloon.
If you took away all ground referencing devices and you could not see the ground and did circles all day long the helicopter would not care however if you were able to draw your ground track afterworlds you would see connected elongated circles over the ground if you had a wind. If at the same time you could have your track drawn in the air you would see one circle. (Assuming you kept a constant speed, rate, bank turn)

Jerry

Marc Tower
22nd Nov 2007, 14:04
BLCL, I disagree. There is always relative wind. In your balloon example above the RW value is zero.
In flight, you create a RW result by combining the aircraft flight vector and the wind vector. The the resultant relative wind is the airflow observed relative to the observer or object (aircraft, airfoil or arm out the car window).
Or I could be wrong. Happy Thanksgiving to all. Fly safe.

SASless
22nd Nov 2007, 14:45
Does not "relative" connote a comparison to some reference....thus would not there be an infinite range of values possible?

The key would be to identify the reference being used to form the comparison.

Shawn Coyle
22nd Nov 2007, 15:13
I would like to add one thing to the discussion about VRS.
There is the chance of getting into VRS whenever the downwash velocity is equal to the rate of descent.
Don't forget that in a descent at less than hover power, the downwash velocity will likewise be less, so that there are a large number of ways to get into VRS.

before landing check list
22nd Nov 2007, 16:15
Marc, you are correct and I was not clear with my explanation. Yes there is relative wind, that is relative to the helicopter or the car etc. However the wind that is relative to the ground given the circumstances that I mentioned earlier is not a factor because it does not exist to the helicopter.

Matthew Parsons
22nd Nov 2007, 16:20
"11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward."

If it walks like a duck and talks like a duck, call it a duck.

I would like to see hard evidence whether there is or is not high pressure air beneath, but whether there is or not, this is how it feels to hover, especially for a new pilot.

I'm happy keeping it an urban legend, but how about one with some value?

delta3
22nd Nov 2007, 16:42
Add for instance to this helicopter loading. A very lightly loaded heli will also produce a smaller induced velocity, its almost as simple as that : at 750 kg you have approx 16% less induction than at 1100 kg (R44), the weight difference being 30%. Induction goes approx with the square root of weight: at hover this is 6.3 m/s resp 7.5 m/s which is a difference of 16% .

SAS, of course. My question is how do you determine you have near zero speed, based on what ? IAS doesn't help, because near zero, GPS doesn't help because of wind, no visual clues, unless there is hot air balloon in the vicinity...

d3

delta3
22nd Nov 2007, 17:01
Matthew

The power curve works positive in all directions, so in that sense the top of the curve can indeed be visualised as a half spere.
But fysically there is no pressure ball...

I would at least call it a good analogy. And as said many times before these simplifications have pros and cons...

d3

NickLappos
22nd Nov 2007, 17:27
Matthew is right, if it works for your basic understanding, then the myth has uses. Just don't let someone sell you a "pressure enhancer" for your Robbie to increase payload.

Matthew Parsons
22nd Nov 2007, 17:32
delta3,

I'm not totally convinced. The rotor downwash is blown downwards, and then deflects horizontally. Something has to act on the air to change its velocity. The ground doesn't do it...that would lead to upwards air. In the end, you need air pushing air away from center. I'd say a pressure gradient would be the culprit.

Just because I think there is a gradient, I'm not saying there is a 'ball of air' as discussed above. Hard numbers would be required to determine the magnitude and extent of this gradient.

Matthew.

Matthew Parsons
22nd Nov 2007, 17:35
"Pressure Enhancer"

Careful, Nick. DaveJ will consider that idea as public domain and you won't be able to patent it. :E

Maybe if we hovered an R22 over an empty hot tub we'd get more pressure....

delta3
22nd Nov 2007, 17:40
Matthew,

got to leave for the bar for now, I'll try a scientific reasoning later...

cheers

Graviman
24th Nov 2007, 10:04
Matthew,

Don't forget that at these speeds the air is to all practical intents and purposes incompressible. I like to visualise ground effect as a mirror image virtual upwash which is going back up through the rotor system. The rotor system causes this virtual upwash to expand to cover the whole disk, in the same way the downwash suffers wake contraction. The net effect is that for the same lift thrust the blade pitch, hence rotor torque, hence power is reduced.

What causes the air to travel out horizontally at the ground? Well the real downwash and the virtual upwash meet at the ground to produce zero vertical velocity. The downwash is effectively suffering complete wake expansion at the ground, which results in the horizontal outwards flow. In truth streamlines require extensive computing power to work out, since you need at least the Biot-Savart law to consider the resulting flow from the blade tip vortices. This is why before CFD, wind tunnels were required.

Don't forget that Navier-Stokes also tells us that the air has viscosity, so the downwash will also reduce in stregth as you get further away from the rotor. As a physicist you will appreciate that this is just one of those problems where the complexity soon becomes such that the average person needs a much simpler model to visualise. The pressure bubble is just that reduced model, but no aerodynamicist will seriously consider it when doing calculations.

However, the mirror analogy works well to explain the long grass phenomena, where you need more collective (thus manifold pressure (~0.5") or torque) over long grass. Effectively the ground is no longer a perfect mirror, but has some mechanical impedance. This reduces the strength of the virtual upwash, so hover power goes up slightly in comparison to concrete.

On balance, the reflected virtual helicopter is more intuitive to me.

Flying Binghi
24th Nov 2007, 10:26
Graviman,

I read it before the edit, and after - and I still dont understand. Perhaps I need to understand what the Biot-Savart law is - or perhaps I need more beer.

I do like the idea of virtual upwash as an explanation - it is something I havnt come across before.

Graviman
24th Nov 2007, 10:38
FB,

Like Mathew i have studied physics, and so refer to techniques that will make sense to him. I have also studied engineering, including aerodynamics, so often refer to principles which are probably not widely understood. Interestingly i find that physicists often do not understand engineering, and engineers often do not understand physics! There is at least a common ground in Newton/Bernouilli physics.

I think this forum would be a good place to try to develope a new model which, while compatible with the aerodynamics, would also help all pilots gain intuitive understanding of what is happening. Visualisation is useful, because it stays with you in an emergency.

The other two visual models i would like to develop are VRS (which i see as a bound vortex or aerodynamic doughnut), and LTE (which i see as the tail rotor caught up in it's own bound vortex). Considering tail rotor downwash vorticity also, i believe, explains why a tail rotor with marginal power works better with front-up rotation (Magnus effect reduces TR VRS).

This post has probably confused half it's readers, but may lead to a clear diagram or two...

Graviman
24th Nov 2007, 14:43
1) Vortex Ring State (VRS) can happen at as little as 300 foot per minute descent, it does not have to be a higher descent rate

2) VRS is more likely at high altitude and high gross weight

3) Hovering with the nose off wind consumes much more power

4) Blade stall is always preceded by vibration

5) Winds affect the power we require when we are in forward flight

6) Downwind takeoffs are absolutely forbidden

7) The Height Velocity curve is a precise guide to the engine failure danger zone

8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.

9) The legal definition of VFR is sufficient to assure flight control and safety using outside references

10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.

11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.

12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees

13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.

14) NVG are dangerous and should only be used by gifted military pilots.

15) You have to first learn to fly fixed wing before you take helicopter training

16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.

I'm happy to believe our friendly neighborhood test pilots for most of this stuff.
Number 10) i'll put down to aliens - they seem to get into everythang.
Number 11) has frightened away the other players - again. :ouch:

Number 12) looks like fun. I would say that each blade on the rotor behaves as a system in resonance, so its maximum movement occurs about 90 degrees after the maximum input. The exact phase depends on the blade natural frequency compared to revolution frequency, and the damping of the resulting frequency. This damping is dependant on the aerodynamic forces to inertia (Lock number).

Number 16) Miner's law of fatigue damage says that if you're an occasional bean nibbler, the jar will soon be empty...

Actually i'd like to put in a couple of my own:

17) In hover the cyclic controls the direction the lift vector is pointing.

18) There are certain regimes of flight where smaller diameter rotors are advantageous.

Matthew Parsons
24th Nov 2007, 18:07
Graviman, not sure where you're going with #17.

For #18, the implication is that greater rotor diameter is always better. Not so. An increased diameter also means heavier blades. At some point you'd get diminishing returns.


As far as the pressure "bubble". I didn't want to go down this road. But since we are...incompressible doesn't mean you can't have a pressure gradient. It is an early assumption made to make calculations easy, but it doesn't change the physics. Also, viscosity is not derived from the N-S equations, it is an input.

Your explanation for ground effect sounds great, but it doesn't address whether there is a pressure gradient. You can't say that you'll ignore the pressure gradient, so therefore there is none. Remember, I'm not claiming this is necessarily something causes a flight phenomenon, just that it seems like it must exist.

Instead of talking about what happens to the helicopter, why don't you tell me how a chunk of air changes its direction by 90 degrees when it hits an open surface squarely (ie downwash hitting ground).

NickLappos
24th Nov 2007, 22:29
For the record, there is no pressure gradient below the rotor, in fact, that lame conventional explanation doesn't even hold water for a millisecond when you realize that ground effect only works on the induced power. How does a "pressure bubble" single out induced power as the only recipient of its wonder?

In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.

http://www.anav8r.com/pelican.jpg

southernweyr
24th Nov 2007, 23:21
What would be a better explenation than the conventional one?

I can see the point that ground effect slows induced flow and therefore increase the AoA, but it does seem as though the pressure is higher in a hover than on the ground when not producing lift.

So how does ground effect diminish the size of the vortex rings? I was taught that it was the downwash pulling the vortex rings out away from the helicopter. I was told this is the reason that tall grass diminishes ground effect, the grass slows the outward flow and lets the vortex rings get bigger. Of course after reading some of the explanations so far it seems this is not true.

Of course, all of this is in an effort to explain, at least in part, why the helicopter requires more power at the point just before ETL than any other time.

Matthew Parsons
24th Nov 2007, 23:35
Fine, there's no pressure gradient. Just tell me what changes the direction of the air.

By the way, this myth was about a sensation the pilot felt while hovering in zero wind conditions, not about power required or ground effect.

southernweyr
25th Nov 2007, 00:05
Interesting, my 15 year, and 7000 hour chief pilot uses the pressure bubble explanation to explain how to do a normal take off.

"Start accelerating slowly so as not to fall of the bubble and pull it along with you. As you continue to accelerate you will start to out run the pressure bubble and experience a dip just before ETL. At that point you may use a slight amount of up collective to avoid dipping too much, then lower collective to previous position."

I am very interested in learning a more accurate explanation for all that we have been discussing.

Phil77
25th Nov 2007, 01:28
southernweyr:

Maybe you should get a hold of Shawn Coyle's "Cyclic and Collective":

http://i159.photobucket.com/albums/t145/CNN77/Low-Altitude-Vortex.jpg?t=1195957331


Dear Shawn, Mr. Publisher, dear mods: if this should violate any copyrights and isn't appreciated to be posted, please remove it immediately or have me remove it - I just can't explain it better :ok:)

southernweyr
25th Nov 2007, 02:25
Thank you for the post. It makes sense now. I suppose that information is not in question due to testing in a wind tunnel or something?

Any ideas on why the vortex rings are diminished when in ground effect? Is it just due to slower induced flow, or does it have anything to do with the down wash moving horizontally outward and "pulling" the vortex rings away?

Matthew Parsons
25th Nov 2007, 04:50
Maybe Shawn could tell us how the vortices are formed (think pressure).

Graviman
25th Nov 2007, 09:21
I'm enjoying Shawn Coyle's C&C now! It's worth getting hold of...

Mathew, difference between static pressure and stagnation pressure:
Static pressure is pressure moving with air free stream (static port).
Stagnation pressure is pressure when stream is stopped (pitot tube).

This bothered me about meteorological systems too. What you have to remember is that at subsonic speeds there is no real resistance to flow. The viscosity affects how the air flows over a surface, but the bulk air just finds a different path. Air hitting the ground tries to set up a static pressure gradient, but this never develops before the air has been diverted. Otherwise there would be a change in density.

What you are thinking about is the stagnation pressure. I've included a link for everyones benefit (particularly mine :\ ). A "perfect" altimeter will read the same static pressure altitude under a hovering helicopter as in free space (otherwise airspeed would affect it). The static port is positioned where local flow is close to free stream velocity (a wing reduces static pressure because the velocity is much higher than free stream).

http://en.wikipedia.org/wiki/Stagnation_pressure

For more information about pitot-static systems:
http://en.wikipedia.org/wiki/Static_port

Actually, this post has helped my understanding! :ok:

Graviman
25th Nov 2007, 09:47
Just, easier to break this into two posts.


Graviman, not sure where you're going with #17.
For #18, the implication is that greater rotor diameter is always better. Not so. An increased diameter also means heavier blades. At some point you'd get diminishing returns.

Agreed about your point on #18, there is eventually a point where parasitic drag takes over. For a practical design (droop stops etc) i can see that mass would increase as length^2 for strength or length^3 for stiffness. What is not always appreciated is that rotor diameter is often limited to less than ideal by the mission requirement. This may include transport, or stiffness requirements. This is where the Mollers of this world get a look in...

What i was trying to get at with #17 is that the simple model is that the cyclic controls the tilt of the rotor, which explains why hover is so hard to master. The problem is that it just is not true. The cyclic actually does control the pitch and roll rates of the aircraft, but the compliance in the rotor system leads to a delay in response. It is this delay which makes hovering such an art. I'm often met with suprised reactions when i discuss this with other pilots - it took me a while to get it.

delta3
25th Nov 2007, 10:55
Matthew
Took some time, but I did some detailed pressure calculations concerning the pressure ball myth
Basis for calculations: My Scientific simulator for R44 (detailed dynamic model, precision better than 1%)
Assumptions R44-I, ISA , sea level, TOW 1000 kg
OGE IGE (3 ft) IGE (0ft)
Induction speed vi m/s 7.1 6.0 5.2
Pressure below rotor mB 1014.14 1014.23 1014.29
The ground effect blocks the air flow of rotor wake, this reduces induced speed and as a result increases pressure below rotor by 0.09 mB.
What are the dynamics now.
1. Vertical changes
The pressure ball is stable with respect to vertical variations :
(a) If hight increases pressure drops from 1014.23 to a minimum of 1014.14 and the heli sinks again.
(b) Conversily if heli sinks until skids touch ground, pressure increases to 1014.29 and the heli will climb
2. Changes in cyclic
Move cyclic 2 degrees forward produces two effects
(a) Decrease of lift by 0.1% because rotor trust is not fully aligned with vertical and starts producing a forward acceleration of 0.035g
(b) Slight decrease of pressure (increase of induction), also of the order of magnitude of 0.1%, because rotor wake is less blocked (wake deflects and starts escaping more to the rear, see publish drawning of Shawn by Phill77)
3. Changes in forward speed
Forward speed has two opposing effects:
(a) gradual disappearence of ground effect with loss of pressure and increase of induction speed (again because wake starts escaping to the rear)
(b) gradual increase of translational lift, with increase of pressure and decrease of induction speed
Calculation IGE (3 ft), maintaining IGE collective
Forward speed Knts 0 5 10 15 20 30 50
Induction speed vi m/s 6.0 6.0 5.5 4.9 4.3 3.1 2.0
Pressure below rotor mB 1014.23 1014.23 1014.29 1014.31 1014.34 1014.38 1014.43
Climb potential (ft/min) 0 10 91 200 306 416 576
4. Conclusion
The negative effects of pressure loss and effective lift loss because of cyclic movements are rapidely (as of 5 kts) counteracted by the positive effects of translational lift (the power curve). If cyclic were used more aggresively (say 5 -10°) the first effect will of course be more pronounced


Edited : don't seam to get the tables aligned

Graviman
25th Nov 2007, 11:34
Wow, this is good work D3! I'd like to know more about your Scientific simulator for R44 - it sounds interesting.

Lets use your figure of min to max pressure change in hover, so from 1014.14mB to 1014.29mB. Now an R44 has a rotor radius of 198" or 5.03m. Lets assume wake contraction and hub cutout half the disk area, or that pressure change is halved at rotor. This pressure change of 0.15mB or 15N/m^2
produces a lift increase of 1192N/2 or 61kg.

http://robinsonheli.com/r44iispecs.htm

This all looks pretty convincing for the pressure ball theory, but what you haven't stated is are you quoting static or total (stagnation) pressure.
Yes the total pressure under the heli does increase, since an ASI would register a reading. No the static pressure under the heli does not increase, since an altimeter at ground height in hover center (stagnation point) would not register a change. To get into the aerodynamic nitty gritty the terms have to be used precisely.

Matthew, this has got under my skin and i'd like to provide a good answer. I'm going to discuss this in detail with some aeroengineers. I'll get back to you when i'm convinced that i know what i'm talking about! :ugh:

Meanwhile...
http://www.1000pictures.com/aircraft/helos/sh-60b-seahawk-hover.jpg

delta3
25th Nov 2007, 12:25
Thx.

1. The pressure quoted is static pressure, as inferred from Bernoulli. If uniform induction applies (ideal hover rotor, no tip losses), then an underpression of apprx 1/4 of the weight (converted to pressure using rotor disk area T/A) is found above the rotor. The rotor gives an impuls to the pressure equal to the trust (or weight in hover), resulting in a 3/4 T/A over pressure (see also Leishman p 42)

2. In that sense I think there is a fundamental difference between the plank analogy Nick made an the heli. For the plank indeed the pressure change only comes from a very local change in speed (slowing airflow below the foil). In a heli the same blade pulses over and over again on the same air mass.

3. The pressure increase under the rotor cannot be used directly to compute gains in trust, because the pressure above the rotor may (and will) also increase, it is the differential that counts.

4. For me the induction speed is the important parameter.


5. Restictions of my model

5.1 It does not used aero-elasty. I am sure I could build this in, because right now I already use a full 4th order aerodynamic blade section model with full stall modelling that is input to full dynamic equations (to "all moving parts"). Computing power however is already max for real-time simulation (which was the prime motivation of my work, see the delta3 discussion with good old LU, the low-g roll simulations, different autorotation simulations, retreating blade stall simulations etc).

3.2. The ground effects are modelled as average effects, also consistent with empirical data by Leishman. As can be seen from the drawings by Shawn, two effects can be distinguished
(a) a more overall effect of more or less blocking of wake, observed quite a bit downstream of the wake (5.03m radius versus 4.60 m rotor to the ground in the given example)
(b) a temporary indigestion of the forward vortex by a small section (forward sector) of the rotor.

Both produce in the given scenario an increase of induction, so a decrease of trust with fixed collective.

Since my data seams to be consistent with my personal (restricted) measurement capabilities, I think the R44 may be less susceptable to forward vortex ingestion, because of low induction speeds and a relative high position of the rotor. This may be different for other geometries.

d3

NickLappos
25th Nov 2007, 15:15
So basically what delta 3 says is:

1) The idea that ground effect reduces the induced power is wrong, the rotor is just a pressure pump.

2) That the SAME effect on an airplane's wing at 250 knots simply doesn't fit, so we must simply ignore it!

3) That he can ignore the effect of the reduction in angle of attack and suppression of the induced drag losses (which are the REAL cause of ground effect) because he calculates a pressure rise on his desktop universe.

Nice. Delta 3's popcorn aerodynamics PROVES the hardiness of the MYTH!

May the pressure balls be with you.

Matthew Parsons
25th Nov 2007, 15:49
Nick, that's a bit of a leap. All that delta3 has shown is that there are pressure changes beneath the helicopter associated with height, forward speed, and cyclic activity. This didn't surprise me. What hasn't been shown is effect of the pressure gradient. Just because we can predict something with formulae or even measure something with instruments, doesn't mean that measurement has any effect on anything.

As far as urban myth status, I think we'll have to refer to Mythbusters terminology and call this one plausible (choices are busted, plausible, or confirmed). However, even though it feels like falling off a pressure ball to a pilot, and it appears there might be a ball of pressure, I still think that the pressure gradient is not big enough to cause this effect on its own.


As far as ground effect machines at 250 kts, I'd be interested to know how far in front of the machine they cause changes to the air. Anyone have a computer simulation that could help?

Hey, speaking of mythbusters, we could try bust this. In a few months I should have an instrumentation pallet available to me with a trailing static bomb. It might be possible to give the bomb a short leash and measure static pressure closely under a helicopter during transition to forward flight. I'll let you know if I get a chance to do this.

Shawn Coyle
25th Nov 2007, 18:51
From my observations - in a Bell 206, at a medium weight, the difference in pressure from rotors stopped to just lifting off the ground (which is where the pressure difference is most noticeable) is about 50' on the altimeter.
Isn't that easier to use as a measure - it's really insignificant as far as performance goes.
An interesting effect, but of little practical value.

And for whoever it was that copied the stuff from my book - I don't mind. The plugs are appreciated!

As far as the 'bubble' stuff goes - in a perfect no wind day, there may be a slight tendency to push the helicopter when the thrust vector has been tilted, but I would defy anyone to measure it.

I seem to remember there used to be a requirement to measure static stability in the hover - turned out to be impossible to do.

delta3
25th Nov 2007, 23:56
Shawn

That look OK to me. The figures I published for the R44 show 1.09mb or 33 feet.
Be interested in imperical proof. The static tube should be reasonally close to the MR, and averaging the 5-7 Hz range correctly or be fast enough to register all pulses to allow for averaging out.


Nick, ho-ho...

1. I did say the induced power REDUCES because of ground effect : the induction SPEED is reduced because of the blockage, so is the induced power, and because of Bernoulli's fault the pressure increases below the rotor.
I suggest you take more time to read my stuff before you jump on it.

2. You say the induced drag is reduced because the angle of attack is reduced. FULLY correct, don't seem to have stated anything against that, see the posted vector diagrams in the stated scenarios that just illustrate that:
OGE:
http://www.portmyfolio.com/prive/heli/OGE.JPG
IGE 3ft:
http://www.portmyfolio.com/prive/heli/IGE%203ft.JPG
IGE 0ft:
http://www.portmyfolio.com/prive/heli/IGE%200ft.JPG

You know all these equations are inter-related and there are many interdependent variables. This reduces to a chicken and egg situation. Why is the induction flow reduced, why is the angle of attack reduced, what is the impact on the static pressure...
And just to help understand the pictures : in the given attitude the effective aerodynamical angles of attack (solid lower mesh) are nicely symmetric around the rotor axis.
The mechanical angles (transparent upper mesh) are not symmetrical because the rotor is "flapping" to adjust its angle with respect to the hub-plane, needed for the given load/attitude of the heli. The mechanical angles are also greater because of induced flow.

3. I will not comment on the desktop universe. I had fun and I am proud that during many months I put 5500 lines of Math together, including detailed foil models of MIT applied to the R44 blades, fins and hull and including very complex 3D rigid body dynamics.
The results of the model gave me many sleepness nights, but all proved to be correct once I got over some apparently wrong predispositions.
I know math doesn't sell well and do not expect any changes soon. It's a relieve to me that history showed that some mathematical theorems took 100 years to be accepted. So I firmly and patiently stand by the figures I published.

4. I got the time despite some wintery weather out here to do some testing this afternoon in the following conditions
R44-II, 1 pob 2/3 fuel, 5-10 knts of surface wind.
(a) Hover Up wind : 19 map, steady hover at 11 feet over concrete (measured by radalt), keeping collective and moving over to long grass gives 14 feet.
(b) Starting from 11 feet up wind with gentle cyclic and fixed collective: take off without sinking: In line with prediction that once above 5 knts the translational lift, and in your words -Nick- reduced induced drag, takes over from the loss of ground effect
(c) Doing same down wind : needed to abort (increase collective) because of risk of touching down.
Cheers,
d3

NickLappos
26th Nov 2007, 03:52
delta 3,

OK, your work is truly great, and I can see the reduction on angle of attack and power. It is truly a desktop universe, and I mean that in a good way.

The problem is that without careful explanation, one could actually think you show how the "pressure bubble" under the rotor is what ground effect is, and that would be untrue.

Ground effect is when the ground plane flattens out the flow, causing the rotor to behave as if the blades were much longer. As a result, the tip losses for the blades are sharply reduced, causing a sharp reduction in the induced power requirements.

The reason why ground effect does not show very well at 100 knots is that induced power is very low there, but ground effect is still present at 100 knots.

For an airplane, the case is easier to picture. As the wing comes down into ground effect, the tip wash reduces sharply, and the wing behaves as if it were infinitely long. This requires less angle of attack (which had caused a large drag rise prior to entering the ground effect). The induced drag wastes power, so ground effect shows itself as a reduction in angle of attack and a reduction in the power required.

Ground effect is not caused by a pressure rise, nor is the advantage to the aircraft due to any pressure that pushes the aircraft up (or any other such pressure bubble nonsense.)

delta3
26th Nov 2007, 08:13
Nick,

Thx, the discussion made me think of the electrical resistor equations : is it the voltage that makes the current go, or does the current create a voltage differential...
People have personal preferences or (thought) habits to take one route or the other. For some scenario's one route may seem more appropriate : voltage creates current looks "more intuitive" when hooking a resistor to a battery, but current creates voltage looks more appropriate when hooking up the resistor to a coil. Some people may even talk about electrons jumping orbit and take great fun at using this route (could be freaky on this forum)


In the pressure bubble case, my preferred route is looking at the induced speeds, I only calculated pressure because of your myth (so blame Bernoulli and yourself...)


Concerning the plank / helo :

- difference between a line and a circle : locally on segments not a big difference, globally totally different topolygical (read mathematical) objects. So at some point this will show up somewhere.

- ground effect : to have the effect of a plank wing close to the ground (say cord width to height) I personally think we are more talking about a grass machine or an dangerously low flying inverted heli or a RC-model. If it so low as a landing plane, I even think some people would be surprised of the aerodynamical results (vortexes, vibrations etc???) But yes you are right the effects are there in both cases, but at some point it matters wether they are significant or not with respect to the other phenomena. Just as an example : in the three hover conditions above (OGE/IGE3/IGE0) rotor trust is slightly different because of the effects of down wash on the frame...

d3


added : some russian mathematicians take the coordinate references at the rotor blade and make the world go round. This is even difficult for me to grasp.

Dave_Jackson
26th Nov 2007, 19:00
Matthew says;"Pressure Enhancer"
Careful, Nick. DaveJ will consider that idea as public domain and you won't be able to patent it.


Dave says;"****! Someone already has patented it.US Patent 5,197,446 ~ Vapor Pressure Enhancer and Method"____________________


Nick says;In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.Me says; The reason for this is very well understood by the Mongolian physicists. This physiological phenomenon is also apparent in the reducing diameters of a streamtubes. Simply explained; - the vortices compress the air. Then the gravitational force draws these denser masses of air toward each other. This is also very simply explained by three other forces (http://home.fnal.gov/~cheung/rtes/RTESWeb/LQCD_site/pages/strongforce.htm).
_____________________


Urban Myth says;12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degreesLu mental telepathes;"The guys just don't get it. Everything is now very clear." _____________________


Is this an Urban Myth, a Rural Myth or a Non Myth?19) Gross Weight’s Effect on Autorotation (http://www.aviationtoday.com/rw/issue/departments/engineering/)

delta3
26th Nov 2007, 19:45
Dave


"This is also very simply explained by three other forces"

I always believed that quarks are responsible for VRS. I am quite relieved now that at last some physicists proved that... I leave it to you to convince the rest...
Personal experience: if somebody emphazises his point by using "very simply", look very sceptically at the claim...


19) http://www.aviationtoday.com/rw/issue/departments/engineering

No yoke but my simulator not only confirms the claim, it also predicts better autorotation with weight. Perhaps the next though test for the simulator. So I'll second this as a myth.


d3

southernweyr
26th Nov 2007, 22:10
Thank you all for your post, I am learning a lot.

Delta3 it is interesting that you mentioned that the R44 may not be affected by the Low Altitude Vortex because of its high rotor blades. The helicopter that I fly most definitely is affected by the Low Altitude Vortex because to perform a normal take off without getting the big dip is not possible unless certain wind conditions exists. The distance from the rotor blades to the ground is usually about 10 ft with a 2 ft skid height.

It is also interesting that you got more altitude at a given power setting over the grass than over the concrete. (If I read it correctly.) Was there a building closer to the concrete than the grass? Or is the idea that you have less performance over grass just another myth?

So, if I understand right, ground effect causes the air at the tip to flow more horizontally which increases AoA and also reduces induced flow. Both of these things help reduce the vortex rings generated at the tip.

delta3
26th Nov 2007, 23:31
southernweyr

I refer to studies done by Sheridan&Weisner 1977, also refered to by Leishman p189 explaining difference in accelerating forward flight between a OGE hover versus IGE hover. Their measurements shows that in OGE you just ride the power curve, whereas from IGE the trust will first decrease, requiring more power to compensate up to 10 knts, from which point it starts resembeling the OGE curves. To quantify: many studies show a 10% gain at Z/R of 0.75 (relative height of rotor above ground), and a max power needed at 8-12 ktns.

As stated before "one way to model" that is to look at average induced velocities. My simulator uses the Cheeseman&Bennetts formula for calculating induced velocities that is consistent with the above observations.

It is however my believe that higher loading (higher induced speeds) and closeness to the ground will make the effect more pronounced : not only will it create more ground effect -which I think nobody will contest- but also will delay the effect to greater speeds. As a result I would put it for a R44 at the 8 knt range. Caution: it requires quite precise measurements because we are getting below my 1% precision target, so I personally am not equipped to back this up with precise measurements other then it is in the 5-10 knt range.

With respect to the effect of grass you claim the opposite of what I observe. My explanation is that the grass creates more ground effect because it slows down the downwash more than concrete does. I also remember that some pilots quoted that the bubble the downwash creates in water also produces the same effect (don't remember the thread anymore)


d3

Graviman
28th Nov 2007, 11:46
D3,

That is a nice piece of work on the simulation. Is it using something which would run on a generic PC, or did you write it in (say) delphi? is it realtime, or do you have to set up the conditions?

I've been doing some homework to make sure i understand this properly. What really bothered me is is that since altimeter static port is in an airflow then, like a carburettor, shouldn't the static pressure go down (hence altitude up). My understanding now is that the freestream stagnation pressure remains the sum of atmospheric pressure P0 and reference frame dynamic presure 1/2RhoV^2. So for an altimeter static port pressure only varies from atmospheric if the flow past the port varies from TAS (near to a wing for example).

For your simulation this means that the stagnation (or total) pressure will be the same as atmospheric reference frame anywhere in the system. In the flow field the velocity, hence dynamic pressure, will vary. This means that you would expect static pressure to drop where the velocity increased. I think what your model shows is that in ground effect the downwash velocity slows down (as expected), and so the local static pressure goes up. However the stagnation pressure does not alter.

Basically the ground effect bubble has burst - or myth busted, Matthew...

delta3
28th Nov 2007, 15:07
Graviman


The logic (which is in fact used by the impuls theory) is as follows:

1. Start upstream at infinity : we have Pinf = Patm (inf cannot of course be that far that we have atmospheric pressure drop conditions, so a few hundred ft or so will do)

2. Just before the rotor we have vi and a static pressure drop according to the Bernoulli formula you quote.

3. The rotor system now interferes with the wake tunnel in that sense that it gives an average pressure impuls equal to T/A

Completing the impuls theory which calulates the vi results is (same numbering as before):

1. Pstat = Pinf = Patm

2. Pstat = Pinf -1/4 T/A

3. Pstat = Pinf -1/4 T/A + T/A = Pinf + 3/4 T/A

Simulator : written in Mat Lab, running at approx 600 frames/second, enough to have full blade dynamics. Calculates vi, pressure in derived via Bernoulli. Runs real-time on a PC (see the AVI-films I published on low-G roll over


d3

Graviman
28th Nov 2007, 17:01
Post a link, i'd like to see this work. Maybe i should buy MatLab...

I'm bothered about your number 3 condition - this just doesn't make sense. In a carburettor there is just no way to make the static pressure go above atmospheric (otherwise why design an expensive mechanical turbo charger).
The only way you can go above atmospheric, if this is the Pinf condition, is to push dynamic pressure way up (which leads to the interesting condition of negative static pressure - that's how low vacuum capture pumps work). For example where extremely high speed downwash impinges the ground could be where measured pressure went up, but not in the free stream.

I've done a few simulations in my time, and can vouch for the fact that sometimes you get the right result with an incorrect assumption. Clearly you have spent a great deal of effort getting this to work (and well deserved congratulations are in order), but the physics just doesn't make sense to me.


Mathew, for your static pressure bomb i would try to keep the bomb at the centre of the rotor downwash. For several different lengths get a measure of static pressure for hover at different heights above the ground. For completeness if the bomb also had a pitot tube you could determine stagnation pressure. If you feel that flight safety isn't compromised maybe get some records of pitot (stagnation) and static pressures at various alternate positions in the downwash (ie hand held). The same could be done in transition. Clearly i don't need to comment to record ambient pressure of the day.

My bet (one international standard engineer's pint) is that if you work out stagnation pressure (dynamic + static) for that flight condition, then you always get the same pitot pressure result, and static pressure is never above ambient.

delta3
28th Nov 2007, 17:16
Graviman


I'll pm you later tonight or tomorrow on the sim.

As a scientist, I fully agree with your point : (nice) theory, but needs to be backed up with hard data = measurement. I am not aware of publications in that field, and as a pilot, I realise that my altimeter does not really prove the theory, although Shawn did mension some readings in this direction, perhaps an idea to do some "static" flying with the robby.
I wonder though that people doing model tests should somehow have instrumented that.

So I guess the eyes are on Matthew, looking forward to some measurements.

d3

Matthew Parsons
28th Nov 2007, 17:52
This has left the urban myths world and ventured into the area where engineers and scientists think about the same things in different ways.

Try to forget about how to measure pressure, because that introduces so many other effects that it becomes easy to lose sight of the basic principles.

Think more about Newton's first law. Take one molecule of air moving swiftly toward the surface of the earth. It encounters ground. More molecules are charging up behind it. If you want to ease calculation and make the air incompressible, you have to increase pressure (and temperature will increase as a result). If you decide to be a realist and make the air compressible, you either increase pressure or temperature or both, however at some point you'll find you have to increase pressure. You're probably thinking that you don't need to increase pressure in either case because there is somewhere for the molecule to go, hence the pressure "spills" out. However, that molecule is going to keep going downward unless "compelled to change direction by a force impressed upon it". So we need a force that is pointed upwards (to reduce the vertical component) and outwards (to accelerate away) from the center of where the downwash strikes the surface. That force comes from the interaction with the molecules in the higher pressure area (its 3D so if you want to call it a bubble, go ahead). You can call it stagnation pressure if you like, but that is still a real pressure.

Now for the measurement. If I get a chance to do this (bjc, you in on this discussion?), then the biggest issue will be in measurement error. The differences in pressure we'll be looking for will be quite small, we will require much more precision in height than normal, and normal local variations in pressure could be greater than some of the numbers we try to measure. Or, we could discover a giant bubble of pressure (not likely).

Now for the urban myth. I haven't changed my stance. I do believe there is a pressure gradient formed underneath the hovering helicopter. It is an essential part of determining the flow of air in the vicinity of the helicopter. Also, when you move the helicopter forward, there is a sinking feeling. When you are trying to keep the helicopter still, it feels statically unstable, as though trying to balance a baseball on top of a basketball. In fact, it feels like you're hovering on a bubble. Two different facts, but I think this is where the urban myth holds to an extent. Just because the pressure "bubble" is there, doesn't mean that is why the helicopter feels like it does.

Of course, next time I describe how to hover, I still might refer to this sensation (and will caveat that it may not be scientifically accurate).

Matthew.

delta3
28th Nov 2007, 18:57
Two remarks

1. The fact that I claim that the static pressure under that rotor is higher in IGE than in OGE does NOT mean I am advocating an extrapolation of the metaphore to the point of stating that the heli sits on a (spherical) pressure bubble. May be a better metaphore could be : put a propeller on the top part of a tube that you put vertically on the ground. On the bottom side of the tube you have some air escape holes, that you can make smaller (IGE) or bigger (OGE)

2. Instability comes from a combination of all effects, among which the reaction of rotor efficiency with respect to height and cyclic angles as inputs to a comprehensive dynamic model.

Personally I never used pressure as any "driver" in the model nor in my "personal" thinking, I fully stand behind Nick's reasoning of induced flows, less induced drag etc as "a thinking" model".

But as I stated before impuls theory implies the above claim....


Two "ex aburdo" reasonings (without the need of going to the molecular level, which -as you state- is of course also possible, but only will make the model and convincing others more difficult).

1. If pressure does not increase how do axial pumps work, in particular all the turbines many of the PPruners use every day...

2. If impuls theory is wrong many schools will have to rewrite their books...

d3

MightyGem
28th Nov 2007, 21:00
why don't you tell me how a chunk of air changes its direction by 90 degrees when it hits an open surface squarely (ie downwash hitting ground).
Because it's got nowhere else to go...???

deeper
28th Nov 2007, 22:03
Oh Nick,
What have you done,

Urban myths have now become Global myths,

Some of these posts show an astonishing lack of general knowledge and add to the overall confusion of what is "simple aerodynamics", now we have low altitude vortex, what next.

Some of these posts make me feel that i have been in imminent danger for years of flying and was totally unaware of it.:confused:

Backward Blade
28th Nov 2007, 23:23
:)Buddy if you don't think that you are in "imminent danger" anytime flying a helicopter then maybe your in the wrong buisiness:}...All the above is a discussion which I'm sure all of us can use so that we mitigate the risk of that bustup that's always over the horizon for every flight we do

BWB

On with the thread

just the driver
29th Nov 2007, 05:10
Couple of points guys:

Isn't all of the above just a theory ? - It was called theory of flight when I was taught it anyway.

Correct me if I'm wrong, but wasn't this thread meant to be a light-hearted fun thing. Some people have either A) Too much time on their hands or B) No life! You should get out more.

Graviman
29th Nov 2007, 11:49
Despite the fact that we seem to be at odds, actually this counts as a light hearted discussion! My motive here is not to disagree with Matthew and D3, but to get to the true nature of what is happening. I have a great deal of respect for both. :ok:

The point about axial flow compressors is a good one, the point about what causes the air to change direction is also a good one. Basically the answer, for me, is in the fact that a helicopter is operating in a free stream while a turbine stage isn't. The rotor also tries to generate the increase in static pressure, like the turbine stage, but the pressure leaks out. This is what causes the tip vortices, and is also why turbines have many overlapping blades. In a fenestron that tip vortex becomes recirculation around the tail fin - so again i would expect no static pressure diffence for the same flow velocity. For change of flow directtion, the ground is subject to the total stagnation pressure. The ground will see a pressure greater than atmospheric (after all it is supporting the weight of a flying machine), so a pitot tube would also register this increase in total pressure. The static pressure flowing with the airmass would not increase, and for any flow velocity should decrease. This stuff gives me headaches too...

Regarding the hover ball, well whatever imagery works for you. To me the dynamic instability is just driven by a finite response time and the risk of pilot overcontrolling. Tail last fixed wings have the same problem with longitudinal response - that's what often causes the landing bounce.

Matthew Parsons
29th Nov 2007, 14:28
Graviman, would there be any pressure on the front of a kite? If not, why is it harder to hold the string when there is wind?

Graviman
29th Nov 2007, 17:17
Matthew, the reason a kite stays airborn is usually because it is not streamlined so the flow becomes turbulent in the wake (ie static pressure is not recovered on lee side). If air was inviscid then a kite would fall to the ground and also there would be no parasitic drag! Unfortunately the lack of coanda effect would mean our precious helis would not get aiborn either...

Assuming kite flies as an aerodynamic wing: The flow over the kite requires wind so the total stagnation pressure is higher than atmospheric (static + dynamic pressure). The kite induces vortices which by various methods (either magnus effect or vortex shedding) speeds up flow above the kite. This increased local velocity means an increase in dynamic pressure hence a reduction in static pressure to keep total stagnation pressure constant. This is why a wing produces more lift at higher airspeed.

Don't forget there is always a net change in momentum. I have a nice video somewhere of some Airbus wind tunnel work. Once the model flies through the smoke field the wingtip vortices are generated as expected. Afterwards the vortices slowly work their way to the ground, as each vortex causes a downwash on the other.

Dave_Jackson
29th Nov 2007, 17:39
Does this mean that 'weather' is a myth?
Does this mean that meteorologists can throw away their barometers and now think of 'bars' as a place to drink?

Pilots will definitely be happier, 'cause they no longer need to get a weather report.



Just joking.

Graviman
29th Nov 2007, 18:53
Dave, weather is the result of a fluid trying to equalise warm and cold regions. The sun shines equally with 1.4kW/m^2 across the entire earths surface. However, the corners of the flat earth are unable to dissipate heat and so cause an accumulation of thermons. These thermons quantum tunnel through the air particles, causing no end of mischief. The reason that meteorologists concern themselves with isobars is this indictes regions where this mischief is likely to accumulate. In regions of low pressure there is a distinct absence of mischief, while high pressure leads to definate high jinx.

Now if i could just get the lid off my jar of bernoulli particles...

Matthew Parsons
30th Nov 2007, 01:00
Matthew asked, "Graviman, would there be any pressure on the front of a kite? If not, why is it harder to hold the string when there is wind?"

Graviman answered, "...The flow over the kite requires wind so the total stagnation pressure is higher than atmospheric..."

Or more simply, yes.


Your answer is not why a kite flies. It is one way of describing it. There are many ways to describe these things.

Graviman
30th Nov 2007, 09:45
Mathew, stagnation pressure at infinity for a given condition is defined as:

Pstagnation = Patmos + 1/2RhoV^2, where v describes the wind velocity.

So it must be higher than atmospheric, but will be the same in all places for incompressible and inviscid flow. But, yes this is just one way to describe what is happening to the kite.

NickLappos
30th Nov 2007, 10:30
Ground effect explained
Misinformed Instructors who propel the pressure bubble myth should be asked to do a quick Yahoo search using "induced drag ground effect" so that they can learn how to tell their students the truth, and not convenient myths. We would be all the better for it. Pressure bubblers, please note:

Induced drag explained:
http://galileo.phys.virginia.edu/cla...ero/node5.html (http://galileo.phys.virginia.edu/classes/311/notes/aero/node5.html)

http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html

Ground effect explained:
http://www.faatest.com/books/FLT/Cha...oundEffect.htm (http://www.faatest.com/books/FLT/Chapter17/GroundEffect.htm)

http://www.pilotfriend.com/training/...g/aft_perf.htm (http://www.pilotfriend.com/training/flight_training/aft_perf.htm)
"Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."

The quiz for today, for the bright student:
Define induced drag
define aspect ratio
explain how aspect ratio affects induced drag
explain how proximity to the ground affects aspect ratio

puntosaurus
30th Nov 2007, 18:26
I thought induced drag had two components, the first as described in the article and the second being the vector consequence of tilting the wing relative to the airflow to produce lift. In other words I thought induced drag was defined as all the drag consequences of lift production, as opposed to one particular part of it.

NickLappos
30th Nov 2007, 19:52
puntosaurus,
You are right, but by definition, drag is in one direction, the free stream direction.

puntosaurus
30th Nov 2007, 20:27
Ah well, I confess I was thinking of rotor drag rather than drag. Does the argument change for rotor drag ? ie. is the vector consequence inline with the rotor plane of increased angle of attack actually defined as part of profile drag ?

In the big scheme of things it probably doesn't much matter, but even for something relatively arcane I'd rather not be the source of further myths !

Graviman
30th Nov 2007, 20:56
Using non-mathematical terminology, to start the discussion. :ok:


Define induced drag:

The drag which is produced from the tip vortices produced by a finite size of wing, and is directly proportional to useful lift.


define aspect ratio:

The wing span to average chord, with >10:1 being required for an efficient wing.


explain how aspect ratio affects induced drag:

By reducing the strength of the tip vortices, the induced drag goes down, so the wing becomes more efficient.


explain how proximity to the ground affects aspect ratio:

Although there is no physical alteration of aspect ratio, the ground restricts the downwash and hence the strength of the tip vortices. The effect on the flow field is as if the wing aspect ratio had been increased.


Hmmm, good links.

froggy_pilot
1st Dec 2007, 01:45
Nick I don't really understand : "Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."

all your links refer to fixed wings not helicopters

What about an helicopter at zero speed ? so why at zero speed the ground effect is different on grass or concrete?

Graviman
1st Dec 2007, 02:40
Froggy, the rotor system can be seen as a wing in a rotating reference frame and the fixed wing in a linear reference frame. The fixed wing may be easier to visualise, and is also discussed more on the web, but the basics of flight apply equally to rotorcraft... ;)

Puntosaurus, in answer to your question i would say that the change of lift vector is just as valid for rotorcraft as it is siezed wings. In both cases the cause is the downwash across the wing/rotor, which results from the unbounded tip vortex.

puntosaurus
1st Dec 2007, 04:13
I wasn't making a fixed/rotary distinction, just trying to confirm my understanding of the definition of induced drag (Since Nick asked the question).

So I went back to Prouty, and he defines it (Fig 1.2) as the 'horizontal' component of the lift vector. Since he was talking about the hover we can avoid arguments about what horizontal means. Wagtendonk seems to be working along the same lines (Fig 4-6).

My point is that as defined by Prouty, induced drag is simply a mechanical consequence of the definition of lift (ie. that force which is perpendicular to the local velocity) and the wing being tilted out of the horizontal plane.

Now I know the stuff about tip vortices and I'm sure that's an additional souce of lift induced drag, but coming back to my original point, the tip vortices (as I understand it) are not the only cause of induced drag.

To put it another way if you had no tip vortices (eg. by extending the wing through the walls of the wind tunnel) would an aerodynamicist still say that there is induced drag on a lift producing wing ?

waspy77
1st Dec 2007, 07:03
Induced drag is a consequence of producing lift. Lifting line theory defines this in terms of circulation. But in simple terms, if you are producing lift, then you are producing downwash, you can't escape from that.
When looking at an aerofoil section, the apparent angle of incidence is increased by the downwash, and so a tiliting of the lift vector backwards.

So what's all this about vorticies then? At the tip of an aerofoil, the air rolls round from higher to lower pressure (in simple terms). This produces an increase in the local downwash towards the tip adding to the induced drag effect. In a low aspect ratio wing, this is proportianally much higher than in a high aspect ratio wing to the total lift/drag. When not interested in optimising for compressibility effects etc. the aim is to produce a uniform downwash across the span. This represents the minimum induced drag any given aerofoil can achieve.

Graviman
1st Dec 2007, 10:29
Puntosaurus, i have wondered the same thing about the change of lift vector in an unbounded wing in a wind tunnel. I'll just consider this, rather than rewrite Waspy's excellent post.

Basically in all flight conditions the induced drag is always the result of the angle of attack revectoring lift rearwards. Don't forget AOA is defined from the datum of tip to tail, so even a cambered aerofoil will produce no lift at zero AOA (they will be optimised for say 8'). Put another way zero AOA can be defined as the attitude of the wing section to produce no lift.

When the wing section is in a wind tunnel, increasing the AOA both increases the lift vector magnitude (since Cd increases with AOA) and revectors it rearwards. The drag is thus the result of the induced drag, and a parasitic term associated with the increasing turbulence as you approach stall. If the same wing section is operating in free air the tips spill off the circulation as vortices. These vortices mean that the section is now effectively operating in a local downwash. Thus the AOA must be increased a little more, to produce the same lift, and thus the induced drag goes up some more.

So yes, if you had no tip vortices an aerodynamicist would still say that there is induced drag on a lift producing wing.

puntosaurus
1st Dec 2007, 10:51
Many thanks for that. I agree because at least it is internally consistent. Whereas from the NASA site
This additional force is called induced drag because it faces downstream and has been "induced" by the action of the tip vortices. It is also called "drag due to lift" because it only occurs on finite, lifting wings and varies with the square of the lift.
The emboldening is mine, and encouraged by your support I rather disagree with it. The Virginia article also focuses exclusively on the tip vortex issue, and ignores the mechanical issues.

To quote again from the NASA article
The derivation of the equation for the induced drag is fairly tedious and relies on some theoretical ideas which are beyond the scope of the Beginner's Guide.
Maybe someone who has endured the tedium can shed some light on this. Tip vortices, revectoring, or both ?

Graviman
1st Dec 2007, 16:13
Puntosaurus, don't forget the wing or rotor is only required to generate enough lift to lift the aircraft. For a bounded wing in free air it would need to be infinately long, so each segment would be required to produce zero lift. So for a practical wing it is fair to comment about the induced drag being a function of the vortex strength. As for the equation being unfathomable - that first link put up by Nick is well worth a peruse for some of the equations.

Overt Auk
1st Dec 2007, 16:36
Yesterday Nick asked:

"Explain how proximity to the ground affects aspect ratio."

The correct answer is that extreme proximity reduces it. (You knock your wingtips off):)

delta3
1st Dec 2007, 18:53
Nick

What is the most common FORMAL definition of induced drag ?

Why am I asking this apparent simpel question?


When calculating the detailed dynamics for my simulator, I needed a formal definition to aggregate the precise data so as to obtain a precise global figure (=integrated over one revolution) for induced drag.

In more detail:

First consider Plank, linear aerodynamics:

Given a certain flow we have ONE force on the airfoil, this force is "arbitrarily" decomposed to drag and lift with respect to some "conventional" reference frame. This is the frame that is conventionally taken with reference to some points of the airfoil profile. Rotating this reference frame gives different meanings to drag and lift (without changing the resulting total force). This of course will also change the definition of induced drag which is the drag caused by the lift because it is projected in a different (rotated) coordinate system.
Putting a wing at a rotated reference with respect to the plane may alter this reference and thus the precise NUMBER of induce drag.
Having the plane fly at a different pitch angle may again change the reference system and thus the notations of lift, drag and induced drag.

Puntosaurus used for instance "horizontal", can he give a precise definition ?

Now go to the heli

We can take the hub plane (that is perpendicular to the physical rotor shaft (this is what I took, because its the most natural one to calculate moments and torques around the rotor axis)
or the effective rotor plane or disk plane (once it starts flapping), which looks more natural when analysing all flow components around a particular blade element, or the direction the body goes (plane of motion)

So even the "induced drag" line of reasoning may not be as simple, question of references.


Back to the pressure bubble.

Again Nick I fully endourse your arguments (conditional to theabove caveat), but reiterate that this is not the only possible "angle". We can talk pressure, angles, molecules... all lead to the same: less power needed. I guess it is up to your personal preference.

May be the analogy to electricity does not appeal the the rotary world, but take for instance a car: why does it accelerate
- because the driver hit the pedel
- because the engine gives more power
- because carb pressure increase
- because sheer force between ground and tyres increases

I think all are correct...


d3

puntosaurus
1st Dec 2007, 19:35
Well that's why I stuck to the hover, I wanted to isolate the problem for discussion without getting bothered with other stuff, and in the hover there's a little less debate about what horizontal is.

But this all comes back to the definition issue. Clearly the boys who worked out the formula for induced drag had a view about what was included and what frame of reference they were using. I was rather hoping my last post would smoke someone out who could explain that !

Dave_Jackson
1st Dec 2007, 20:07
Overt Auk says that Nick says; ""Explain how proximity to the ground affects aspect ratio.""Then Ovet Auk says that he says;"The correct answer is that extreme proximity reduces it. (You knock your wingtips off)." Then The Shadow is about to says;"Everyone is grammatically incorrect in the use of aspect ratio, because Wikipedia says; ""In linguistics, the grammatical aspect of a verb defines the temporal flow (or lack thereof) in the described event or state. For example, in English the difference between I swim and I am swimming is a difference of aspect."" Then The Shadow throws in a couple of extra quotation marks to make sure that the above quotes have all been properly closed off, grammatically speaking."""

:)

waspy77
1st Dec 2007, 20:12
The consistent frame of reference is that of the direction of the aerofoil through the air. Nick referred to it earlier as the free stream direction. This is generally used as it is the one real constant, whether on an aircraft or in the wind tunnel.

Other frames of reference can be used and in a fixed wing are usually closely mechanically coupled to the above. The fun thing about designing helicopters is that these many frames of reference often change dynamically with respect to each other.

Apply the equations concerned to the aerofoil, and then translate them anywhich way you want. Afterall it's the aerofoil that flies, everything else just hitches a ride.

delta3
1st Dec 2007, 20:18
Induced drag according to the simulator


R44-I, see figure for detailed boundary conditions


IGE (arbritarily choosen as 3ft):


http://www.portmyfolio.com/prive/heli/Induced%20Drag%20IGE.JPG


OGE (30 ft):


http://www.portmyfolio.com/prive/heli/Induced%20Drag%20OGE.JPG


Remark that the profile drag is (almost) the same, but induced drag is higher in the case of OGE.


So I guess my definitions must be similar to Nick's ones.


d3

PS remark the drag also becomes "induced lift", at small angles this is mostly neglectable


Added defintions according to my doc (is so complicated that even I need to read back!)

2. MAIN ROTOR DRAG

Rotor DM equation (rotor drag small angle approximation used in force calculations for non stalled profile)
In the precise calculations stall is incorporated. The precise and approximate calculations are performed at the end of each simulation to give an idea of approximation errors.

Drag = Profile Drag + Induced Drag
= Drag along IF projected by cos(alfaIF) + Lift by sin(alfaIF)
where cos(alfaIF) is almost= 1 and sin(alfaIF) = alfaIF

and alfaIF is the inflow angle of the relative airflow with respect to the HP plane, that is the plane perpendicular to the rotor axis. Remark that this plane is tilted forward by approximately 3 degrees in the case of the R44.

Dave_Jackson
1st Dec 2007, 20:53
:=. . . . :uhoh: OK No more jokes.


delta3,

You might find this information and the links (http://www.unicopter.com/B263.html#Drag) of value.

Dave

delta3
1st Dec 2007, 21:11
Dave


Yes and .... no

I agree with your general definitions but I did not see a precise coordinate reference

The reference I choose is :

1. Project everything to the HP plane (= perpendicular to MR axis)

2. Reduce(=project) all flow-speed components a foil section sees to that reference plane (= rotational speed, inflow of air (aircraft motion+induction), flap and when fully dynamic rotor pitch and roll rates)

3. Calculate foil drag and lift with respect to that inflow angle

4. Induced drag is then defined by the angle of that vector makes with respect to the HP-plane

d3

Graviman
2nd Dec 2007, 10:15
D3, you will have to explain those diagrams more clearly. The text is hard to read, and we are not familiar with your simulation. The polar plots slightly differ from one plot to the next, but i have no way of understandiing what this is telling me. Why is the plot slanted?

Agreed that in hover it is the induced drag which goes down. To me, the reason for this is the induced downwash velocity goes down. The blades will see a similar static pressure difference top to bottom (since heli weight has not changed), but the pitch will be less. So if you use axis plane as datum then the lift vector is less rearwards, so torque hence power is reduced.

It would help if you described how you accounted for the reduction in downwash velocity from ground effect. To me this is a complex CFD problem (using vortices or finite difference), and i am not sure how you can do such a complex calculation real time. Prouty would describe the way that the downwash wake contrction becomes reduced as the ground is brought nearer, so velocity goes down.

If you wish email me directly, since i am worried that this thread may be losing the pilot community for which it is intended.

puntosaurus
2nd Dec 2007, 11:07
I think I've just realised the wisdom in Nick's and Waspy's comments about relating drag to the free stream.

I'm guessing the aerodynamicists definition of induced drag is quite narrow, ie. the cost of tip vortex production in the direction of the free stream. Therefore without tip vortices (ie with an unbounded wing) there can be no induced drag as far as an aerodynamicist is concerned.

I'm further guessing that Prouty, Wagtendonk et al and anyone else responsible for educating pilots may have appropriated the term induced drag to cover ANY gain in ROTOR drag as a result of lift production from the pilot's perspective.

If my guesses are correct the circle is squared, and everybody can be right according to their different definitions.

delta3
2nd Dec 2007, 11:49
Graviman,

For publication puposes resolution is reduced, I thought still readable, but I guess this may not be the case.

Your comments on boudary conditions are right : same weight, just a different hover height.

The graph shows :

Blue : profile drag
Green : induced drag
Color : total drag

all according to the previous definitions.

Why is the induced drag so slanted, up to a negative dip on the retreating blade, this is because the rotor is already flapping given the CoG of the load, see figure: cyclic is between 2.12 and 2.25 degrees backward. This can lead to a situation that induced drag gets propulsive when the blade is flapping down. The simulator allows me to look at all data, even point per point blade elements, but then again details get so volumenous, that this would be diffult to present clearly.


d3

Graviman
2nd Dec 2007, 12:31
D3, thanks for the clarification. I think the slant is an example of where this can get confusing, depending on whether you reference axis plane control plane or tip-path plane. For the purpose of this discussion it is fair to comment that 1D momentum theory describes a uniform downwash relative to tip-path plane (which would be slanted in axis plane). Once the machine has flapped to equality, we are both agreed that this average velocity goes down with ground effect.

What i am still confused over, and central to the discussion, is how have you accounted for the way the ground plane affects the downwash velocity? Prouty shows how induced downwash Vige/Voge is a function of height, but this data appears to be either from wind tunnels or CFD simulations. I am guessing that you have used an empitical curve, possible taken from very accurate flight test data, to simulate ground effect in your model.

My only concern here is that we are trying to establish the cause of ground effect reducing Vige/Voge in a hovering helicopter. There are two competing ideas:
1. The complex 3D flow means that ground effect reduced downwash velocity through the rotor by reducing wake contraction.
2. The viscosity in the flow means that the ground resists the outwards flow, causing a buildup of stagnation pressure under the rotor.

Although your model is clearly very good, and probably quite accurate, it's reasonable to comment that we can not use it to bust (or otherwise) this myth. I get the impression you had reached a similar conclusion.

delta3
2nd Dec 2007, 12:47
Right on Graviman (see PM)

whether this busts the myth or not, is a personal call.

I would go for plausible, because static pressure under the rotor disk does go up. I had by the way the possibilty to do my "static" flying test yesterday, all beith not very scientific
at 2/3 of fuel and 1 pob the alti lowered almost 20 ft just before lift off. (surprised me that as a pilot I didn't really notice this before, perhaps to buzy doing other things in lift off, thanks Shawn...).

The alti cannot be used as a reference any more when climbing and is not precise enough to use it as an instrument to validate lower static pressure OGE...

The increased static pressure also may not be used in an extrapolative way to extend to some spherical ball kind of picture, this is why I also would be willing to rest my case.


d3

Graviman
2nd Dec 2007, 13:10
D3, we missed each other there. For reference i hope you don't mind if i put highlights of your PM on this post. I think this will increase interest in your model:


I think my model is best classified as : Blade element augmented with impuls theory. That is iterations are performed to balance impuls(=induction) and local aerodynamics on each full rev, according to Glauert.

Near ground I put an extra rule on (OGE) impuls theory consistent with Leishmans formula's (p187), which is backed up by experimental data:
Tige/Toge = 1 / ( 1 - (R/4z)²)

no CFD calculation of the wake is done.

To improve precision, I can add aeroelasticity and tips losses (this can already be activated), but compairing during check mode of the program those reveils differences that are of second/third order and as such of less "full heli dynamic" interest.


Interesting that the altimeter goes down 20'. Don't forget that the altimeter will only read correct when the flow past the static port is the same velocity as the free field. Although i would have thought the downwash velocity would increase the altimeter reading, not decrease it. On the other hand if you had sideslip, from say cross wind, the static port would pick up some dynamic pressure (no instrument is perfect).

I think we have to consider other effects here too. There will be a lag which causes the altimeter to low pass the pressure signal. This is caused not least by the capacity in the VSI.

The other assumption in the aerodynamic model is that the freefield flow is static. It is quite possible for an acoustic wave to build up under the rotor, since the rotor would literally act as a very large diameter loudspeaker as you changed collective. In fact it would not supprise me if a microphone nearby would pick up a subaudio pressure wave, every time collective is changed. This is obviously nibling away at the incompressibility assumption.

I think the best test would be for a 2nd pilot to determine how an altimeter set for Qfe would vary in a low hover into wind, but then if the downwash is not over the static port, does the test count? The more i think about it the more i realise that Mathew's comment about instrumentation accuracy is the limitation. There may just be no practical way to determine if the air just under the rotor compresses slightly in a stable hover... :{

Shawn Coyle
2nd Dec 2007, 13:28
My observations on the pressure altimeter are that (in a no-wind situation) as the collective is raised from flat pitch, the altimeter starts to decrease in reading. This indicates an increase in pressure sensed by the static ports. The altimeter reading continues to decrease as the collective is raised, reaching it's lowest reading just as the helicopter lifts off.
The altimeter then starts to return towards the 'correct' reading and when the helicopter is out of ground effect, the altimeter is once more reading correctly (using known external references for height, as a radar altimeter is not accurate enough for comparison).
So, when OGE, the pressure altimeter is pretty accurate.
The difference in height/pressure at the 'just lifting off point' is minimal and makes no difference to any performance data. It's of academic interest, aside from modeling ground effect as has been so spectacularly done in earlier posts.

delta3
2nd Dec 2007, 13:41
Shawn,

I fully agree what every word you state, for the intended use the altimeter resumes correct operation OGE and in forward flight because ..... the bubble got away (not the intent to put these words in your mounth, just picking up)

I also tend to believe in IGE it is a correct read out of the pressure increase.

1. read out in line with theory (could call this of course result biased reasoning...)
2. looks to me that given its position and function, it is capable to measure correctly under the scenario you described and which I litterally replicated.


d3

southernweyr
2nd Dec 2007, 14:25
After all that has been said it seems to me that the pressure does increase under the rotor when IGE and this is what slows the induced flow. Would Bernoullis principle be at work here? Slower velocity would increase pressure, right? Of course, the pressure above the rotor probably increases as well. Please correct me if I am wrong.
I definately see the altimeter lower when IGE and my static ports are at the bottom of the helicopter on both sides just in front of the rotor mast.

Delta3, I read correctly and the idea of less performance over grass is a myth according to what you are saying. I do not claim otherwise. I have heard others claim that the tall grass slows the outward flow of air underneath the rotor system which allows the vortex rings to enlarge.

As to the rest of your post when you replied to me . . . I think you are saying that the R44 would dip at 8knts because of the distance of the rotor above the ground and the value of the induced flow velocity.

delta3
2nd Dec 2007, 15:08
Southernweyr

Q1. Bernoulli : right on, at same trust levels, the pressure above MR proportionally increases with the pressure below when induction speed (and induced drag) decreases

Q2. I claimed the opposite, IF the grass slows down the outflow it may reduce induced speed, so I claimed a few feet higher hover when compaired to sleek concrete, but this is SPECULATIVE, as some pointed out I don't model this with CFD, so need to rely on experimental data. I observed this experimentally but not using a scientific set up.

Q3. Indeed, I claim that from 8-12 knts onwards indeed no dip will occur, starting at lower headwinds, a dip should occur

d3

Graviman
2nd Dec 2007, 17:23
I'm stumped for words! So there is an increase in static pressure in ground effect. The next trick is to understand whether this effect is the same from ground level to rotor height. Good work, Delta3. :ok:

Matthew, i concede - but i'm at a loss to understand what causes this. I guess incompressibility is an assumption to make the maths simpler, but it shows that aerodynamics always finds ways to suprise you...

Shawn, what is your experience of torque or manifold pressure over long grass vs tarmac?

Shawn Coyle
2nd Dec 2007, 21:58
I have only had the opportunity to look at this a couple of times but in all the cases I've tried moving from hovering over hard concrete to hovering over even short grass without moving the collective resulted in a definite decrease in height over the grass, and a return to the same height when back over the concrete.

Probably too much flight time at my disposal without definite things to do.... so I can pay attention to trivia....

Dave_Jackson
2nd Dec 2007, 22:29
definite decrease in height over the grassSpeculation: Perhaps the reason for this is that grass has a lesser surface friction (smooth) than does concrete (rough).
_____________

A thought that just keeps bubbling up to the surface.

Meteorological charts show pressure and each line represent a difference of 4 hectopascals (hPa)
Tire pressure is measured in PSI.
A tire pressure of 36 PSI = 2,500 hPa
We tend to think of pressure in the context of the much higher tire pressure.
A meteorological pressure difference of a few hectopascal can create a wind.
This wind can remove the 'whats the thing' out from under a helicopter when it is hovering in ground effect over a fixed point.
I would suggest that meteorically induced air pressure is blowing away the rotor induced air pressure.Perhaps aerodynamists do not talk in terms of pressure because 99% of their computations relate to aerodynamic activities that have nothing to do with the ground.

However; pressure is pressure.

Dave

Graviman
3rd Dec 2007, 11:50
Weather is actually a good example. It is definately free stream yet the centre of a hurricane is not moving and is at low pressure.

I just had a discussion with another chap who has fluid dynamics experience (dredging ships, so slightly larger than helicopters). The conclusion we reached is that directly below the rotor at ground level you would expect an increase in static pressure (due to change of direction of dynamic pressure), but further out you would expect a decrease as venturi effect took over. Certainly i would expect this on the ground, but i'm still amazed that the air itself actually compresses under the rotor.

Probably too much flight time at my disposal without definite things to do.... so I can pay attention to trivia....

Lucky so&so! C&C is an enjoyable read BTW.

delta3
3rd Dec 2007, 16:47
Graviman

That is correct, according to Impuls theory / Bernoulli, the wake tunnel below the MR in OGE contracts and will progressively half surface down stream (so square root for diameter), whilst induction doubles from vi to 2vi with the corresponding static pressure drop starting from below the rotor

In IGE the geometry is different, very turbulent, so too complex, so the whole argument starts upstream from the rotor (once we agreed upon the effects on trust and vi-IGE) and continues down stream to just below the rotor up to that point where the wake stops OGE-like contracting because of the ground proximity. I did not make any prognosis on the speeds/pressures from there on.

This is why measurements should be reasonably close to the MR, but not too close to reduce the "loadspeaker" pulses (a few feet i would say) and increase setup safety.

d3

Graviman
3rd Dec 2007, 17:53
D3, i will read up on impulse theory and blade element theory - you are right i am a little rusty. Agreed that the downwash velocity will be lower with ground effect. The suprise for me is that the stagnation pressure ends up higher than in the free stream.

At some point i intend to study CFD, and this strikes me as an interesting project. It's clear to me that aerodynamics is more complicated than can be easilly captured in a simple calc. The fact that tall grass consumes hover power, although you feel it should help, is an indication that even this is more complex than it first appears...

Graviman
26th Apr 2010, 11:57
Mmmmm, this thread takes me back! :ok:

I have made good on my promise and read up on blade element momentum theory (Prouty & Leishman), and am now attempting to model helicopter rotor aerodynamics. This is the best way i know to get fully to grips with the issues discussed here. I did enjoy the discussions, D3.

My current model (BEMT & vortex panel) assumes a number of streamlines that pass through different positions on the rotor plane, to be later deflected by the ground. Each streamline has it's own total pressure which does not have to be the same as any other streamline. The helicopter is then kept in the air because of the sum change in total pressure across the rotor, so sum of static pressure and dynamic pressure increases below the rotor. So far the code appears to produce accurate results - that could be a fluke.

This next part is subject to some contention, but should be explained to complete the model: Near the ground the total pressure is the same as under the rotor but is stagnated due to a direction change. This means that altimeters will register an increase in static pressure. This direction changing happens because the airstream sees a mirror image helicopter under ground plane. The mirror image helicopter causes helicopter to see a slight updraught so collective goes down.

My intention is to keep this discussion offline for the time being until i can establish the validity of my approach with D3. :8

delta3
30th Apr 2010, 08:18
see PM,

d3

AnFI
30th Apr 2010, 16:12
30 ft on the altimeter ~= 1mb = 1 hPa ~= 10kg/m^2

Area of R44 disk ~= 5^2 x 3 = 75m^2

75x10kg = 750kg

Therefore an r44 'weighs' 750kg

(plus approximately 1/3 rd of that for pressure drop on top) ~= 1000kg
:rolleyes: