Hmmm,

The hecklers are still here!

Oh really? I thought the air was already stationary. Air does not flow past a wing, a wing moves through the air.

What is containing the miraculous increase in the pressure of the "static" air?

If there was an increase in pressure it would need to be contained by something and then react against the lower surface of the wing to lift it.

Part of that container would have to be the surface. The surface in the above photograph of the Ekranoplane is water which you would think would react in some way to the increase in "pressure". How heavy do you think this aircraft is and your theory expounds that it is somehow supported by the surface. Clearly it doesn't.

TC - as RVDT asks, what is the mechanism by which the 'ram effect' increases pressure under the wing when there is no choke point, just a slightly convergent duct?

I don't think you can ram air into a tube with 2 open ends and hope to create dynamic pressure..

If your reading on WIGs has been on bernoulli based articles then I am sure it does talk about increase of pressure but said theory would appear to be incorrect.

Decrease of downwash - Yes

Increase of pressure and ram effect - No

Apparently the earth is no longer flat!!!!:ok:

RVDT: I think you misunderstand me. I'd like to summarise, if I may as I don't want to lose my money and my keys!

Bernoulli and Newton (pressure and AoA) are BOTH right , but for different reasons.
Bernoulli is the populous explanation, but may have been 'modified' en route:

The air hits the aerofoil and is deflected upwards and downwards.
The down deflection pushes UP on the +ve AoA aerofoil and produces lift.
The up deflection initially causes a +ve pressure increase at the front of the section and has the effect of trying to slow the aerofoil, but this is minor compared to the remainder of the surface of the aerofoil which experiences a pressure drop due to the fact that the AoA offers a blind spot behind the leading edge. This zone of reduced pressure causes the air to rush in to fill the imbalance and as a concsequence of this, it accelerates.

So as I have said all along - on the upper surface, two things happen:
Pressure (overall) reduces.
Air speed increases.

[What Newtonians and Bernoulians dont say is that the path that the air has to travel is longer on the upper surface and hence the air has to speed up to join its counterparts flowing under the aerofoil].

Newton bases his argument on AoA:
The air hits the aerofoil and is deflected upwards and downwards.
The downwards air will hit a +ve AoA aerofoil and push the wing upwards. BUT it will also deflect down on a neutral AoA plank of wood flying thru the air.
The upward air will experience the coanda effect which causes the air to go down in +ve AoA chords, but even with neutral chords it still goes down due to the coanda effect AND vortices coming off the trailing edge. Ironically Newton also accepts that there is a pressure reduction on the upper surface.

So: BOTH are right! What is wrong is the longer route theory on the upper surface.
------------------------
WIG:

Rostislav Evgenievich Alexeyev would be turning in his grave:=

What????

The air entering the underside of his plane WILL be compressed (by the slightly convergent duct) and WILL push on the hull of his baby. It is compressed between the boat hull and the sea surface.

I challenge you to slightly converge the end of a hose pipe and NOT experience an increase in pressure:ugh:

And if the hose pipe was square, open at both ends, and two sides where missing and only the top surface is moving and the fluid in it is completely static - what is going to happen again? That's a new one!

Another doozy - compressibility at less than .3 Mach!

My money is with the physicist and the Boeing dude -

https://secure.steenaero.com/Store/i...ht_640x801.jpg

TC - convergent duct gives increase in velocity and decrease in pressure and temperature - remember intake icing?

Week one day one at pilot school:)

I don't think you have read that article by Anderson and Eberhart:)

I don't want to mess with the heavyweights here, but if we're talking ground effect with respect to helicopters below translational lift speed the air is not stationary, there is an obvious downwards flow of air, that undoubtedly builds an area of higer pressure beneath the disc in proximity to the ground, because it's flow is restricted, and it's direction changes, meaning a force needs to act upon it (newton), which must be due to the fact the pressure in that area is greater than the surrounding ambient pressure (pressure gradient).

I don't get this mention of the requirement to be sealed on all sides etc... because just looking at wx pressure systems, high's low's etc... they are not sealed and there are obvious changes in surface pressure due to expansion/compression upwards/downwards movement of air (like above/under our disc). If everything needed to be sealed wouldn't the surface pressure be equal around the globe (perhaps with some fluctuations due to temperature)

I think that the principle of a reduced induced flow in close proximity to the ground below tx lift speed must hold some truth as a result of higher pressure under the disc... having said that I haven't noticed the altimeter drop on take-off before

Aucky - surface pressure is due to the weight of air above it squashing it down against the surface of the earth. Variations in surface pressure are due to variations in surface temperature which cause pressure differentials at altitude leading to air moving from high to low (pressure and temp) at altitude which redistributes the air giving variations in the quantity of air above differewnt points on the surface. This is why the tropopause is higher at the equator and lower at the poles and is where all thermal winds originate.


this is exactly the urban myth that Nick and others have tried to dispel - it is a popular notion because it seems intuitive and is easy to understand - doesn't make it right though.

crab - Isn't that what our helicopter is doing? it sounds like the same thing to me only on a micro scale as opposed to a macro scale. There is more weight (atmosphere + helicopter) over that small area under the disc than there is over the area next to you (atmosphere) in the hover....

I'd also like to put this question out there. F=M.A if the air is changing direction under the disc (diverging), by definition it's accelerating (change of direction/velocity). So a force is being applied to that air. If this is not due to a pressure gradient, what causes it's change in direction?

Having read the "Newton vs Bernouilli"´s article, it´s assumed that the change of direction is due to air´s viscosity forcing the inner molecules of air flowing through the airfoil to slow down, actually until it reaches 0, thus changing direction of the free flow.

Really interesting article.Thanks :D

The weight of the aircraft does not impinge on the surface so only the weight of air above that point is acting on the surface.

The rotor downwash is not constrained by anything and so flows outwards - you can see that occur when you hover over grass.

The rotor imparts an acceleration to the air in order to create thrust to balance the aircraft weight - the air accelerates downwards because of the rotor and outwards because of the ground - still no requirement for a pressure increase below the rotor.

Bernoullii's principle states that when you slow an air mass, it increases in pressure, when the speed of an airmass increases, the pressure drops.

It is only a principle or a statement of physical activity, wrongly applied to why lift is generated. The principle DOES happen on an airfoil, but it is the laws of physics (Newton) that actually make it fly.

I would gesture that the only reason you have a wing in the typical airfoil shape instead of flat, is to more closley follow the natural flow of the airmass as to allow a clean laminar flow

Crab - you may be right, but I'm not convinced

Are you suggesting that the downwash holding us up (newton) has no effect of 'pressing down' on the surface before it's forced to diverge?

Except the weight of the rest of the surrounding atmosphere (ambient pressure), and the mass of air it must displace...(quite significant)

Agreed

So how does the stationary ground cause this air to change direction if it's not a build up of pressure?

Nick might just get his wish the way this is going though it seems to be wandering a little bit off topic.

I have to substantiate what Lama Bear and RVDT have said about longline and mountain flying. Nick's explainations are very good and very thorough but he does not account for any vertical air movement. And in mountainous terrain with any significant wind this is a given.

I can personally say that I have experienced symptoms of a loss of lift and power increase did not reduce the descent and the control of attitude became more difficult, and I was NOT overpitching, felt just like the VRS that you do in training at higher altitudes and descent rates, and it almost always happens with an empty hook or water bucket (meaning at lighter weights) but not always. I know my rate of descent (pressure altitude rate of change) was not high enough for VRS but that doesn't mean I didn't have enough vertical airflow to get into it. I've had it happen in Astars, 500s, and hueys.

Like Lama Bear said, talk to any experienced longline pilot, you will hear the same story from every one of them...

Also, it looks like Nick's example of the R22 for vertical and horizontal velocities assumes a level rotor disc which of course is not always the case.

So here is a scenario: I'll use a disc load of 4 which is close for light to medium turbine helicopters at reduced weights of pilot fuel and longline. This would give a downwash velocity, according to Nick's rule of thumb, sqrt(4*210)=29ft/sec = 1740 ft/min

At the 75% horizontal velocity this would be about 13 kts fwd and translates to a descent angle of about 37 degrees from the vertical or 53 deg below level and a total velocity on this path of 1740/cos37=2175 or about 22 kts.

So if we put the helicopter in a 15 deg nose up attitude descending with longline to pick up a load, the new flight path that would provide the same disc angle of attack and velocity would be 52 deg from the vertical or 38 deg below horizontal with vertical velocity at 2175*cos52 = 1340fpm and horizontal velocity of 2175*sin52=1714fpm or about 17kts. Then lets say that he is coming in cross slope and crosswind to the sling site of 14 kts blowing up a 45 deg slope, this is about 1000fpm of vertical airflow. Since this 1000 fpm flow is not moving atmospheric pressure gradient lines past the helicopter then it will NOT show up on the VSI. So here we have a situation where the velocity component normal to the rotor disc is 1740fpm and a VSI that will only read about 340 fpm and this is a 100% downwash velocity descent rate and Nick says you only need 50% to start seeing VRS.

Yes I know I neglected to factor in the horizontal wind component but I think the point is still valid.

With a 100% of your theoretical downwash speed coming up at the rotor from underneath, you are in autorotation or as near as dammit, not VRS.

I have sat in autorotation in an updraught, as have many others, but suddenly getting a descent is due to variations in the strength of that updraught not VRS - although allowing the aircraft to descend in that configuration then pulling a handful of power to arrest the RoD might get you a lot closer.

Crab,
Actually, at 100% Vi you are in fully developed VRS. The Vi ratio tells all. Autorotation (windmill brake state) is from about Vi ratio of 1.2 and upward.

I take your point Nick but does that mean that helisphere's figures are correct and you could in theory be in fully developed VRS with less than 500 ft/min RoD on the VSI?

I know I kind of threw those numbers together but here is a spreadsheet I made that lets you change all the parameters and see a plot on a VRS chart.

The white blocks are for user input and everything else is calculated. It lets you enter wind velocity and flight velocity in 3 dimensional vector form. Also, air density, nose up attitude, gross weight and rotor diameter. All velocities are in feet/min but its easy to see knots by just moving the decimal a couple spots in your head. The plot is on the old chart out of the Army Field Manual.

It's kind of interesting to play with the numbers.

Let me know if anyone catches any errors in it.

http://dl.dropbox.com/u/19282077/VRS.xls

Crab and helisphere,
I can't subscribe to the updraft theory of VRS, it is just not that clean. The precise disk angle needed to enter and sustain VRS is not something that an armchair calculation is likely to prove. True VRS is a flaky thing, often needs the pilot to work at staying in the condition. I would be interested to chat with a long line pilot to see what he/she experienced in the event.
I would bet dollars to donuts that power issues are much more the issue, and I must say that few "experts" know much about the distinction between VRS and SWP to differentiate, including experienced and well intentioned government safety experts and investigators.
To my knowledge, no tests have shown anything close to a VRS case where updrafts and such have triggered any events. I believe the conditions that would create the updraft would also create enough turbulent flow to disrupt all those calculations (but that is a belief, not based on data.)
The situation is perpetuated with a thousand "VRS" demonstrations by a thousand instructors who show SWP to their sutdents. I searched youtube for VRS demos on video, and found NONE that were VRS, all were SWP.
To recap, to get VRS, you must descend at about 70% of the induced velocity, about 700 fpm, and also have about 8 knots forward velocity (straight down will not do). If you enter from lower descent rates, it is not VRS, it is SWP, and if you do nothing to correct the SWP, it can degrade to SWP.
None of the above means there is no danger, just explains what the physics of the situation is. The cure for either is about the same, once in the situation, but the cure to prevent SWP is to retain some power margin, enough to allow some vertical maneuvering while OGE. THAT is not mentioned in the typical VRS discussion, making it somewhat misleading to spin on and on about VRS, descent rates and updrafts, when most cases can be solved with adequate attention to power and weight.

Nick, I'll agree with you that the situations I am suggesting are not likely to be able to sustain VRS but I think they can create the condition for a short time which probably isn't going to hurt you but it can sure scare the wee out of you.

It would also exlplain why they always use such a low number as 300 fpm as a rule of thumb for avoiding SWP or VRS because you can definitly have a velocity normal the rotor plane of higher value than the VSI would tell. It may also be leftover from the days of low disc loadings.

I have a question: Is there any kind of testing requirement for VRS in regard to FAA certification?

I wonder how the Kmax with its angled rotors handles VRS. Can it get into it? If so what would it's descent angle chart look like? Different I would think.

Helisphere,
Good thoughts, I really agree that the updraft could make things worse. Maybe not VRS, because SWP is strongly influenced by descents. I think you might be on to something.

FAA does VRS testing - no, not required, maybe not necessary.

KMax? Great question! Different flow patterns might yield different/no VRS.

BTW nice excel spread sheet, good work!

Well the spreadsheet certainly makes you think... I wouldn't have thought to make it if I wasn't reading through this thread.

A little background, I've got about 4000 hours and about 700 longline mostly in BHT205/210, AS350 and 369/500. In any type of normal flying I have never had any issues with VRS. But longlining in mountainous terrain has definitely givin me a few thoughful moments. I think with that experience I have a pretty good feel/sense for when it's an issue and I have to agree that I don't think it's near as big an issue as many instructors teach, and yes SWP without VRS is a more common problem. But I have to say that mountain longlining really makes me fly more respectfully regarding VRS or SWP than when I'm doing any other type of mission.

Nick, I have a question for you. When you are taking data with an instrumented aircraft specifically on VRS, does the aircraft have angle of attack or vertical airspeed (besides VSI/ASI) instruments? And what parameters do you then look at and analyze from the data?

I think it would be cool to instrument a logging helicopter at work. Might scare you manufacturing types though, I mean for the stresses your strain guages would record, I know it scares me. I don't know, maybe it's already been done but for airflow data and to see if they are getting closer to VRS than one might think.

Finally... I got it....
 

.... GOTCHA.... aint' I ???

Thankyou MightyGEM

That's ok. Errrr....thanks for what?

Vortex Ring
 

Going back to the original question, I heard about a chap in Aden back in the 70's who thought his rotary experience should include VR and recovery. He apparently put a Scout into VR at 10000' and finally came out of it, a very chastened aviator, somewhere below 2000'

So many half-truths in those Jim - at least Transport Canada highlights there is a difference between VRS and SWP, even if that guy doesn't understand it doesn't have to be a vertical descent for VRS, just a steep one.

The Vuichard video shows recovery action taken immediately which is really an incipient VRS condition as he never lets it develop. If he did, his technique wouldn't work any better than the standard recovery. Going sideways out of the dirty air or going forwards - the rotor doesn't know which way it is going - clean air is clean air.

I enjoyed the first video and found it refreshing for someone to differentiate SWP and VRS but he lost me when he started talking about the tips stalling during VRS.
My understanding is it's the centre of the disk that stalls and propergates outward whereas the tips are at very low angles of attacks due to blade washout and increased tip vorticies increasing induced airflow as well as the amount of the blade tips exposed to it.
Perhaps I'm hopelessly wrong but its logical to me and explained well in Wagtendonk.

... except for use of pedal which is a required ingredient of the technique, which makes a difference to whether you are poling forwards or sideways with the cyclic. Probably that's why the technique is careful to specify WHICH pedal is to be used, with opposing cyclic.

That just induces a sideslip which doesn't get you into clear air any sooner than just applying forward cyclic.

Note also that he grabs a handful of power which, in developed VRS would aggravate the condition but, since he only ever does it in the incipient stage, just powers out of the IVRS condition.

Exactly the same result would be achieved if you just pulled power at the incipient stage without any of the sideslip

Though I'd have to defer to your doubtless greater knowledge and experience, I believe that the pedal opposing the cyclic allows the additional translating force of the tail rotor to assist with moving out of the VRS sideways, whereas forward cyclic alone has no such benefit, and would rely solely on the result of cyclic, ie. main rotor vectored thrust only.

As for sideslip. I'm not sure I understand your point, as that would only apply in flight with significant forward (air)speed, whereas in this case, by definition, there's negligible horizontal speed. So it seems the technique is designed to use every means possible in order to translate out of the VRS ASAP, which undoubtedly is swifter with the assistance of the tail rotor thrust.

But I'll be happy to be corrected if I've missed something obvious in your reasoning.

Which control do you use to fly the helicopter in pretty much any direction? The cyclic - it has far more authority than the TR.

If the IAS is so low then full pedal will create yaw but not much else - the main movement will always come from the cyclic input.

Far more experienced people than me dismiss the Vuichard technique as window-dressing for IVRS recovery which can be achieved just by raising the lever.

Far more authority, yes. However the tail rotor can still add some more. This isn't an either / or call, it is all about adding more. Even if the TR can only add 10% more, that's a whole lot better than 0% more.

Did you ever turn down a 10% pay rise on the basis that you already earned far more than the pay rise being offered?
The TR is the sole cause of translating tendency. Yes, it is INTENDED for yaw, but without question, it causes translation too. Are we saying the cyclic alone is so very powerful by itself that there is no need for any further assistance from wherever that help might come? If that was true, we wouldn't even be discussing escaping VRS ASAP with minimum height loss as by definition, the only time nil further assistance is required is when we can already do it with zero height loss using cyclic alone. That clearly isn't the case.
I didn't even consider that to be a possibility!

How much angle of bank do you use in the hover to compensate for TR Drift/translating tendency? 2, 3, 4 degrees? Maybe 5 max in some aircraft - so a very small amount of cyclic easily overcomes the TR power - I think you are greatly over-egging the effect of the yaw - if you want more lateral movement just use more bank.

Back to the standard recovery of pushing the cyclic forward - you are far more aerodynamic moving forward since that is the way the aircraft is designed to fly, unlike trying to drag it sideways with opposing cyclic and pedal.

My point about the use of collective to power out of IVRS is that with enough power available you can do exactly that so why mess about with the roll and yaw. The problem is that not rasing it to max immediatley runs the severe rsik of aggravating the condition and putting you into VRS proper. You could also just take yourself into overpitching and Nr decay

If Vuichard can produce a video where he can go past the stage he normally recovers by just raising the lever enough to worsen the stall and enter fully developed VRS and then recover from full VRS using his technique in 50', I for one will be gobsmacked but I doubt it will happen.

He also needs to demonstrate his technique on bigger and heavier aircraft for it to be valid.

That was the bit I dismissed as being beyond the bounds of possibility....!:ok:
Returning to the technical discussion, you haven't taken on board my points, and you have argued against things which I have NOT said - I was not saying yaw had any particular purpose in this, nor was I advocating yaw for yaw's sake, if you read what I actually said. My point was in favour of adding the translating effect of TR in ADDITION to cyclic, which can only help the cause. However, your mind appears made up and closed to other ideas, therefore further discussion appears pointless, unfortunately.

Mike you misunderstand my point - why use a combination of yaw and lateral cyclic when simple forward cyclic will do the job just as well if not better?

People have got hung up on the 'magic' of this technique and, to be fair, it has been sold well with the glossy videos. If he just used the same amount of power and pushed the cyclic forward instead of laterally, he would fly out just as easily because it is IVRS and not VRS.

People can believe what they want but my concern over the publicity this 'technique' has gained, is that people might really think they can mishandle the aircraft sufficiently to get into VRS in the misguided belief that the Vuichard technique will guarantee recovery in 50'.

Just avoid the conditions in the first place.

You're bang on Crab....I've pushed the envelope on VRS many times (training) in the 350, 125, 130, 206, 407 & G2; & I can tell ya once Your fully established in full blown VRS the Vuichard technique does diddly squat, absolutely nuttin, the machine continues accelerating like a greased anvil towards Mother Earth. Sure it works ok (just) in IVRS, but only for very mild IVRS as the demo vid shows! But, nothing works as well as poling forward which is the way the machine is designed to go (not side-ways).....it's no surprise then how the Robinson company is now pushing this flawed technique as the new saviour for VRS?

As You stated Mate; VRS is best avoided with good airmanship, unless deliberate for the purpose of training.

Happy landings

We can move two controls simultaneously can we not?

Key is to do something....and that which best resolves the problem.

It might just be you need to turn to gain an advantage afforded by terrain....so let's not get locked into exactly one response can we?

VF - :ok:

Sasless - getting locked into exactly one response is exactly what Vuichard is selling. I agree that there needs to be flexibility but based around good techniques.