Vortex Ring / Settling with power (Merged)
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Easy way to learn SWP, is attempt to hover with less power than it takes, result, aircraft settles.
VRS, the difference between the two maneuvres is as startling as is the feelings of VRS. We used to call it shooting the tube. quite easy to figure that out. tears to the eyes and the wax outa yer ears.
crab how are you doin?
your quote.
Had another look at the numbers to refresh my memory, he went from 100 feet to 70 feet in 30 secs, not much descent rate there.
Very little descent next 11.4 secs as flare at 20 degrees nose up turning to a bank 0f 43 degrees, washing off airspeed.
At that point descent increased dramatically as did his realisation as within 1.1 secs he had got the nose to pitch to slightly nose down from 16.5 degrees up and impact was at between 1320 to 1800 fpm, another 1.5 seconds later. That is much too high a ROD to have built up just from his entry maneuvre or a simple overshot, as it happened from at or below 70 feet.
I feel that I can say again that he got himelf into VRS at about the time he caught up (slowed his ground speed down to) the downwind airspeed and reacted brilliantly to attempt recovery, except that as you say there was just not enough room to recover in the manner which he initiated, which to me was correct. a bit more height and the disc would have cut clean air.
Entering the maneuvre was the problem in the first place, and the reasons why,as we have discussed before, but perhaps a systemic breakdown of the command chain's understanding of the known, practised and aircraft capable maneuvres.
Check a previous post up the page a bit for a cue on this, you will see reference to the billy goat idea that if you have enough power you can pull it through VRS. Hah, the bigger they are the harder they fall
There is another one i'll dig out one day that demonstrates quite neatly the rotor system entering VRS.
VRS, the difference between the two maneuvres is as startling as is the feelings of VRS. We used to call it shooting the tube. quite easy to figure that out. tears to the eyes and the wax outa yer ears.
crab how are you doin?
your quote.
Tet - your link has far less to do with VRS than it does about mishandling when approaching downwind. It would appear that he simply ran out of power and didn't have enough height, speed or power to go around.
Very little descent next 11.4 secs as flare at 20 degrees nose up turning to a bank 0f 43 degrees, washing off airspeed.
At that point descent increased dramatically as did his realisation as within 1.1 secs he had got the nose to pitch to slightly nose down from 16.5 degrees up and impact was at between 1320 to 1800 fpm, another 1.5 seconds later. That is much too high a ROD to have built up just from his entry maneuvre or a simple overshot, as it happened from at or below 70 feet.
I feel that I can say again that he got himelf into VRS at about the time he caught up (slowed his ground speed down to) the downwind airspeed and reacted brilliantly to attempt recovery, except that as you say there was just not enough room to recover in the manner which he initiated, which to me was correct. a bit more height and the disc would have cut clean air.
Entering the maneuvre was the problem in the first place, and the reasons why,as we have discussed before, but perhaps a systemic breakdown of the command chain's understanding of the known, practised and aircraft capable maneuvres.
Check a previous post up the page a bit for a cue on this, you will see reference to the billy goat idea that if you have enough power you can pull it through VRS. Hah, the bigger they are the harder they fall
There is another one i'll dig out one day that demonstrates quite neatly the rotor system entering VRS.
Last edited by topendtorque; 1st Feb 2011 at 13:02. Reason: more detail
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A very effective tecnique to deal with VRS I have used and taught in light helicopters in mustering operations for a long time now is to fly the disc rather than the airframe. When you become used to recognizing the early stages of VRS onset it is quite simple to take the disc into clean air and avoid the problem, in most siuations it usually means you just have to move a couple of feet into wind regardless of which way the airframe is looking.
Also it is important to to keep your manuvering in a relatively flat plane so as to reduce your opportunitys to encounter VRS. Lots of height changes high and low are no good.
Also it is important to to keep your manuvering in a relatively flat plane so as to reduce your opportunitys to encounter VRS. Lots of height changes high and low are no good.
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waragee,
You are absolutely right,That is great advice. I find that to perform low speed maneuvers successfully, one must sense the attitude, speed and power state of the rotor, and not concentrate so much on aircraft attitude or stick positions.
An exercise that I've used to sharpen the skills with understanding how the rotor flies goes like this:
Set the aircraft in a 5 foot IGE hover on a long runway on a very still morning. Use enough collective friction to keep the collective from moving and then relax entirely on it. This is your fixed power point, from now on you will use only speed to govern altitude and approach angle. From this steady hover lower the nose slightly, accelerate forward a few knots, 2 or 3, and note that you begin to climb very slowly. Carefully hold the nose position and climbed slowly to 100 to 200 feet. Now carefully pull back on the stick and raise the nose just enough to begin a slight descent. You will note that this slow gentle descent looks very similar to the very bottom of that approach. Remember you will not move the collective at all. Using only fine adjustments of speed set an approach angle that allows you to descend back to the runway. If your angle is too steep lower the nose very slightly to gain a few knots (two or three) and flatten out the angle. If your angle is too shallow pull back very slightly on the stick raise the nose (loose one or two knots) and steepen the descent angle up. Having started from 200 feet you should find plenty of room along the runway to carefully exercise that area of rotor angle, speed and power right at the edge of translational lift. This is the key place for power management and safety on approaches.
On a long runway on a still morning I have flown with students four approaches (take off ,climbed 200 feet, descent, approach to steady hover) without ever moving the collective pitch. If you could do that you really understand how to fly the rotor's angle and stay out of trouble.
You are absolutely right,That is great advice. I find that to perform low speed maneuvers successfully, one must sense the attitude, speed and power state of the rotor, and not concentrate so much on aircraft attitude or stick positions.
An exercise that I've used to sharpen the skills with understanding how the rotor flies goes like this:
Set the aircraft in a 5 foot IGE hover on a long runway on a very still morning. Use enough collective friction to keep the collective from moving and then relax entirely on it. This is your fixed power point, from now on you will use only speed to govern altitude and approach angle. From this steady hover lower the nose slightly, accelerate forward a few knots, 2 or 3, and note that you begin to climb very slowly. Carefully hold the nose position and climbed slowly to 100 to 200 feet. Now carefully pull back on the stick and raise the nose just enough to begin a slight descent. You will note that this slow gentle descent looks very similar to the very bottom of that approach. Remember you will not move the collective at all. Using only fine adjustments of speed set an approach angle that allows you to descend back to the runway. If your angle is too steep lower the nose very slightly to gain a few knots (two or three) and flatten out the angle. If your angle is too shallow pull back very slightly on the stick raise the nose (loose one or two knots) and steepen the descent angle up. Having started from 200 feet you should find plenty of room along the runway to carefully exercise that area of rotor angle, speed and power right at the edge of translational lift. This is the key place for power management and safety on approaches.
On a long runway on a still morning I have flown with students four approaches (take off ,climbed 200 feet, descent, approach to steady hover) without ever moving the collective pitch. If you could do that you really understand how to fly the rotor's angle and stay out of trouble.
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Ah yes the disc always the disc, use it to save yourself, if you have climbed up not too high to have a look around leave it pointing at the ground, for airpseeed potential, it doesn't matter which way the A/C is pointing, same as a VOR just follow the disc if things become silent.
A corollary to your excercise Nick in disc awareness was one i enjoyed in the '47J. Simply just lock the cyclic and fly around for quite a while with just the smallest movements of throttle and pedals and sometimes collective, land it it like that also. I think the further forward seating position gave one the better feel for the torque, power on, power off. Surely that's stuff which you must have done in those slippery things we saw on the video. geat stuff thanks for showing it.
Flying like that gets to be a drug, on a beautiful crystal clear cool morning that you could cut with a knife, sky of azure blue, golden grass waving in the light breeze, fat cattle waddling along in front, big cattleman sitting just behind me on the side under his ten gallon hat, with a slight smile of pride on his face as he watches his men and horses trotting along around the cattle, bit like being addicted to this here turnout sometimes.
I'd have thought you'd be on hols on some sunny beach, it's about twelve months isn't it since you disappeared? Good to see you anyrate.
tet
A corollary to your excercise Nick in disc awareness was one i enjoyed in the '47J. Simply just lock the cyclic and fly around for quite a while with just the smallest movements of throttle and pedals and sometimes collective, land it it like that also. I think the further forward seating position gave one the better feel for the torque, power on, power off. Surely that's stuff which you must have done in those slippery things we saw on the video. geat stuff thanks for showing it.
Flying like that gets to be a drug, on a beautiful crystal clear cool morning that you could cut with a knife, sky of azure blue, golden grass waving in the light breeze, fat cattle waddling along in front, big cattleman sitting just behind me on the side under his ten gallon hat, with a slight smile of pride on his face as he watches his men and horses trotting along around the cattle, bit like being addicted to this here turnout sometimes.
I'd have thought you'd be on hols on some sunny beach, it's about twelve months isn't it since you disappeared? Good to see you anyrate.
tet
TeT - in the example accident - he is already running short of power, then he sets 20 degrees nose up and rolls to 43 degrees!!! where is the vertical component of rotor thrust coming from to balance the aircraft weight? No wonder he descends quickly - then he compounds it by stuffing the nose down to 16 degrees!!
Not VRS in any way shape or form just mishandling and poor awareness.
Nick, I have always found that a fixed power transition from the hover in still air gives a descent just before ETL is achieved as the rotor tries to battle its way through the tip vortices to clear air - are you saying this doesn't happen?
Not VRS in any way shape or form just mishandling and poor awareness.
Nick, I have always found that a fixed power transition from the hover in still air gives a descent just before ETL is achieved as the rotor tries to battle its way through the tip vortices to clear air - are you saying this doesn't happen?
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Crab,
Absolutely, there is a very definite settling that occurs during acceleration in ground effect as you get around 8 kn where you catch up with the vortex from the outflowing air. This causes a slight increase in power required. It is far more pronounced if you are a low hover perhaps 2 feet or less, and non existent when OGE. From low IGE conditions, this settling can cause you to momentarily touch the ground.
During exercise that I described you will note some settling but by being a slightly higher IGE altitude usually is not enough to touch the ground.
Absolutely, there is a very definite settling that occurs during acceleration in ground effect as you get around 8 kn where you catch up with the vortex from the outflowing air. This causes a slight increase in power required. It is far more pronounced if you are a low hover perhaps 2 feet or less, and non existent when OGE. From low IGE conditions, this settling can cause you to momentarily touch the ground.
During exercise that I described you will note some settling but by being a slightly higher IGE altitude usually is not enough to touch the ground.
Last edited by NickLappos; 1st Feb 2011 at 21:37.
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I was always under the impression that the sink you described was result of 'displacing the ground cushion behind you'. Is that just a simplified urban myth or is that a factor also?
Hi crab and TET, at the risk of thread creep, the BlackHawk accident was not a VRS event, and is very unlikely to have been "insufficient power". It is most likely to have been the S70A's tendency to suffer significant RRPM droop following a high NR manoeuvre (flare, dropping lever, etc) followed immediately by a rapid application of collective. The donks do not respond quickly and the result is significant RRPM droop until they catch up. This droop can be induced to the extent of dropping the generators offline, and has caused several other incidents in the S70A, though those others impacted the ground and became airborne again. Regardless of OGE hover power margin!
This one unfortunately impacted the ship's edge and was thus unable to become airborne again. Bingers (the pilot) came very, very close to lining up with the ship and recovering the situation, and this proximity no doubt enabled enough of the airframe to stay together and make it survivable to most on board.
Another example: US Army Demo
Of course, as I know all too well, I was not there and may have missed significant elements of the sequence, but I am concerned that we would point at VRS or SWP whilst excluding what I consider is the real lesson for UH60 pilots.
This one unfortunately impacted the ship's edge and was thus unable to become airborne again. Bingers (the pilot) came very, very close to lining up with the ship and recovering the situation, and this proximity no doubt enabled enough of the airframe to stay together and make it survivable to most on board.
Another example: US Army Demo
Of course, as I know all too well, I was not there and may have missed significant elements of the sequence, but I am concerned that we would point at VRS or SWP whilst excluding what I consider is the real lesson for UH60 pilots.
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Aucky,
The myth that ground cushion is a pressure bubble that supports you is picturesque and actually helpful, but really physically inaccurate. At some point, it is useful to transition from the basic explanations that tend to simplify (but may work) to the actual physical relationship, which is deeper and more meaningful.
Power goes down as speed goes up beacuse the power "savings" of IGE acts only on the induced power, which drops to insignificance at mid speeds, so the effect of IGE at speed is small. This is not because the bubble is somehow left behind.
As some kind of proof, look at an airplane achieving significant ground effect at 150 knots, where the bubble is far behind the aircraft, as seen by the disturbance on the water behind the aircraft. The ground effect works even so, because the induced drag at that speed is high, so the 15% savings is significant.
An Ekranoplane showing the "bubble" behind the aircraft.
The myth that ground cushion is a pressure bubble that supports you is picturesque and actually helpful, but really physically inaccurate. At some point, it is useful to transition from the basic explanations that tend to simplify (but may work) to the actual physical relationship, which is deeper and more meaningful.
Power goes down as speed goes up beacuse the power "savings" of IGE acts only on the induced power, which drops to insignificance at mid speeds, so the effect of IGE at speed is small. This is not because the bubble is somehow left behind.
As some kind of proof, look at an airplane achieving significant ground effect at 150 knots, where the bubble is far behind the aircraft, as seen by the disturbance on the water behind the aircraft. The ground effect works even so, because the induced drag at that speed is high, so the 15% savings is significant.
An Ekranoplane showing the "bubble" behind the aircraft.
Nick doesn't like my resistance to his take on ground effect, but I still feel it is relevant:
In Ground effect - the air approaching the aeroplane (which is close to the surface) is forced up over the aircraft and has little or no effect...and it is also forced between the fuselage and the surface. The latter causes the air pressure to build which induces a ground effect...similar to the Ekranoplane in the picture. This 'effect' is utilised by the designer to assist the motion of the vehicle over the surface.
In addition there is Ground cushion. This develops when the downwash from rotors impacts the surface and bounces outwards and inwards. The inwards vectors induce addtional air pressure, or a 'bubble' which to some extent, supports the airframe, reduces the induced airflow and increases lift which allows the operator to reduce the collective to maintain position.
It also causes the static tube to register a downward indication in the VSI.
There are 2 effects consequently, each being sourced differently.
WRT VRS Vs SWP:
My take for what it's worth:
SWP: you arrive into the hover at your listed weight and respective approach speed and these two factors are too much for the remaining power margin you have left in the collective. Hence you run out of arresting power to stop the momentum of the arriving vehicle. I liken it to driving too fast and the brakes being too ineffective to stop you.
VRS: Is disc related and is to do with placing the vehicle in an aerodynamically unstable situation w.r.t.: RoD / Speed and disc angle.
Once again, my observations suggest that the North Americans tend to confuse the two, whereas the Europenas gravitate more towards VRS and its effects and barely talk about SWP.
two cnets worth
In Ground effect - the air approaching the aeroplane (which is close to the surface) is forced up over the aircraft and has little or no effect...and it is also forced between the fuselage and the surface. The latter causes the air pressure to build which induces a ground effect...similar to the Ekranoplane in the picture. This 'effect' is utilised by the designer to assist the motion of the vehicle over the surface.
In addition there is Ground cushion. This develops when the downwash from rotors impacts the surface and bounces outwards and inwards. The inwards vectors induce addtional air pressure, or a 'bubble' which to some extent, supports the airframe, reduces the induced airflow and increases lift which allows the operator to reduce the collective to maintain position.
It also causes the static tube to register a downward indication in the VSI.
There are 2 effects consequently, each being sourced differently.
WRT VRS Vs SWP:
My take for what it's worth:
SWP: you arrive into the hover at your listed weight and respective approach speed and these two factors are too much for the remaining power margin you have left in the collective. Hence you run out of arresting power to stop the momentum of the arriving vehicle. I liken it to driving too fast and the brakes being too ineffective to stop you.
VRS: Is disc related and is to do with placing the vehicle in an aerodynamically unstable situation w.r.t.: RoD / Speed and disc angle.
Once again, my observations suggest that the North Americans tend to confuse the two, whereas the Europenas gravitate more towards VRS and its effects and barely talk about SWP.
two cnets worth
TC - on the ekranoplane, the air between the fuselage and the ground/sea will speed up under the wing and reduce pressure rather than being squeezed to increase pressure. Once it has exhausted from under the wing it will slow down again and increase pressure back to static which will provide a barrier to the accelerated air coming off the top of the wing and reduce the downwash - if the downwash is reduced there is less drag and therefore more power available for thrust.
You can't 'squash' air unless it is constrained on all sides.
The CFS explanation of the ground cushion has always been a convenient way to explain what happens but never scientifically accurate - all that boll8cks about how a rough surface 'scatters' the air and reduces the 'cushion' never made sense.
You can't 'squash' air unless it is constrained on all sides.
The CFS explanation of the ground cushion has always been a convenient way to explain what happens but never scientifically accurate - all that boll8cks about how a rough surface 'scatters' the air and reduces the 'cushion' never made sense.
Crab: nooooooo
.
The pressure between the wing and the sea actually INCREASES
Try this:
Air apporaching a normal aerofoil goes up and over, speeds up and the pressure reduces. The air going underneath carries on at normal pressure. Hence lift is produced overall.
However with Wing in Ground Effect (WIG) vehicles what happens in reality is that the sea/ground partially blocks the trailing vortices and decreases the amount of downwash generated by the wing. This reduction in downwash increases the effective angle of attack of the wing (up to 75% in some examples) so that it creates more lift and less drag than it would otherwise. This is ground effect.
An additional bonus is ram pressure. As the distance between the wing and sea/ground decreases, the incoming air is "rammed" in between the two surfaces and becomes more compressed. This effect increases the pressure on the lower surface of the wing to create additional lift.
A2 brutus?
.The pressure between the wing and the sea actually INCREASES
Try this:
Air apporaching a normal aerofoil goes up and over, speeds up and the pressure reduces. The air going underneath carries on at normal pressure. Hence lift is produced overall.
However with Wing in Ground Effect (WIG) vehicles what happens in reality is that the sea/ground partially blocks the trailing vortices and decreases the amount of downwash generated by the wing. This reduction in downwash increases the effective angle of attack of the wing (up to 75% in some examples) so that it creates more lift and less drag than it would otherwise. This is ground effect.
An additional bonus is ram pressure. As the distance between the wing and sea/ground decreases, the incoming air is "rammed" in between the two surfaces and becomes more compressed. This effect increases the pressure on the lower surface of the wing to create additional lift.
A2 brutus?
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Crab. My understanding is that a rough surface/long grass etc slows the outflow which is then picked up and enhances the tip vortices. This leads to loss of rotor thrust at the outboard parts of the blade thus reducing the benefit of ground effect.
Rotorfossil:
Correct - for the same reasons as above. Anything which obstructs the exit path of tip edge vortices, causes the vortices to retain their integrity as they move onto the upper surface. Lift is lost outboard and the ground cushion weakens.
Correct - for the same reasons as above. Anything which obstructs the exit path of tip edge vortices, causes the vortices to retain their integrity as they move onto the upper surface. Lift is lost outboard and the ground cushion weakens.
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RVDT,
Brilliant site - were I smart enough, I would have written it. I have argued for decades that if pressure where the thing, the balsa gliders that I built as a kid didn't fly, since their wings were flat pieces of wood. Newton Si, Bernouilli Non.
As K.D. Wood said, "If it looks like a wing, it will fly almost as good as the best wing."
Brilliant site - were I smart enough, I would have written it. I have argued for decades that if pressure where the thing, the balsa gliders that I built as a kid didn't fly, since their wings were flat pieces of wood. Newton Si, Bernouilli Non.
As K.D. Wood said, "If it looks like a wing, it will fly almost as good as the best wing."
TC - that link would suggest some more bedtime reading required - and not just AP3456
It's all about downwash not high pressure
It's all about downwash not high pressure
Many pilots (and the FAA VFR Exam-O-Gram No. 47) mistakenly believe that ground effect is the result of air being compressed between the wing and the ground.
RVDT: Bernoulli Vs Newton - each describes principles of equal transit and also conservation of energy, both allude to there being a pressure reduction on the aerofoils upper surface. This forms PART of the resultant lift. Coanda effect and airflow deflection also plays a big part. However there is definitely a speed increase over the upper surface.
-----------------------------------------
When air flows past an airplane wing, it breaks into two airstreams. The one that goes under the wing encounters the wing's surface, which acts as a ramp and pushes the air downward and forward. 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.
But the airstream that goes over the wing has a complicated trip. First it encounters the leading edge of the wing and is pushed upward and forward. This air slows somewhat and its pressure increases. So far, this upper airstream isn't helpful to the plane because it pushes the plane backward. But the airstream then follows the curving upper surface of the wing because of a phenomenon known as the Coanda effect. The Coanda effect is a common behavior in fluids—viscosity and friction keep them flowing along surfaces as long as they don't have to turn too quickly. (The next time your coffee dribbles down the side of the pitcher when you poured too slowly, blame it on the Coanda effect.)
Because of the Coanda effect, the upper airstream now has to bend inward to follow the wing's upper surface. This inward bending involves an inward acceleration that requires an inward force. That force appears as the result of a pressure imbalance between the ambient pressure far above the wing and a reduced pressure at the top surface of the wing. The Coanda effect is the result (i.e. air follows the wing's top surface) but air pressure is the means to achieve that result (i.e. a low pressure region must form above the wing in order for the airstream to arc inward and follow the plane's top surface).
The low pressure region above the wing helps to support the plane because it allows air pressure below the wing to be more effective at lifting the wing. But this low pressure also causes the upper airstream to accelerate. With more pressure behind it than in front of it, the airstream accelerates—it's pushed forward by the pressure imbalance. Of course, the low pressure region doesn't last forever and the upper airstream has to decelerate as it approaches the wing's trailing edge—a complicated process that produces a small amount of turbulence on even the most carefully designed wing.
In short, the curvature of the upper airstream gives rise to a drop in air pressure above the wing and the drop in air pressure above the wing causes a temporary increase in the speed of the upper airstream as it passes over much of the wing.
--------------------------------------------
Crab: Look into WIG more closely, old boy. There is definitely a pressure increase caused by ram effect of incoming air between hull and sea surface. And because the outriders slug the wing tip vortices, it prevents the IAF from increasing.
Thats how WIG's work.................
-----------------------------------------
When air flows past an airplane wing, it breaks into two airstreams. The one that goes under the wing encounters the wing's surface, which acts as a ramp and pushes the air downward and forward. 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.
But the airstream that goes over the wing has a complicated trip. First it encounters the leading edge of the wing and is pushed upward and forward. This air slows somewhat and its pressure increases. So far, this upper airstream isn't helpful to the plane because it pushes the plane backward. But the airstream then follows the curving upper surface of the wing because of a phenomenon known as the Coanda effect. The Coanda effect is a common behavior in fluids—viscosity and friction keep them flowing along surfaces as long as they don't have to turn too quickly. (The next time your coffee dribbles down the side of the pitcher when you poured too slowly, blame it on the Coanda effect.)
Because of the Coanda effect, the upper airstream now has to bend inward to follow the wing's upper surface. This inward bending involves an inward acceleration that requires an inward force. That force appears as the result of a pressure imbalance between the ambient pressure far above the wing and a reduced pressure at the top surface of the wing. The Coanda effect is the result (i.e. air follows the wing's top surface) but air pressure is the means to achieve that result (i.e. a low pressure region must form above the wing in order for the airstream to arc inward and follow the plane's top surface).
The low pressure region above the wing helps to support the plane because it allows air pressure below the wing to be more effective at lifting the wing. But this low pressure also causes the upper airstream to accelerate. With more pressure behind it than in front of it, the airstream accelerates—it's pushed forward by the pressure imbalance. Of course, the low pressure region doesn't last forever and the upper airstream has to decelerate as it approaches the wing's trailing edge—a complicated process that produces a small amount of turbulence on even the most carefully designed wing.
In short, the curvature of the upper airstream gives rise to a drop in air pressure above the wing and the drop in air pressure above the wing causes a temporary increase in the speed of the upper airstream as it passes over much of the wing.
--------------------------------------------
Crab: Look into WIG more closely, old boy. There is definitely a pressure increase caused by ram effect of incoming air between hull and sea surface. And because the outriders slug the wing tip vortices, it prevents the IAF from increasing.
Thats how WIG's work.................