Vortex Ring / Settling with power (Merged)
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.
The hecklers are still here!
The air slows somewhat and its pressure increases. Forces between this lower airstream and the wing's undersurface provide some of the lift that supports the wing.
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!!!!
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!!!!
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
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 is the mechanism by which the 'ram effect' increases pressure under the wing when there is no choke point, just a slightly convergent duct?
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
I challenge you to slightly converge the end of a hose pipe and NOT experience an increase in pressure
It is compressed between the boat hull and the sea surface.
My money is with the physicist and the Boeing dude -
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
Week one day one at pilot school
[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].
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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
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.
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
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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?
surface pressure is due to the weight of air above it squashing it down against the surface of the earth
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?
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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
Really interesting article.Thanks
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.
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.
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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
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
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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?
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
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
Last edited by Aucky; 4th Feb 2011 at 17:48.
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Let's keep this thread alive until 2100!
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...
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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.
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.
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.
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?
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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
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
Last edited by helisphere; 8th Feb 2011 at 02:35.
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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.
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.
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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.
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.