Helicopter Urban Myths
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jellycopter,
There is no such thing. That is why it is called a myth. Ground effect does not rely on, nor does it make a high pressure under the helicopter.
There is no such thing. That is why it is called a myth. Ground effect does not rely on, nor does it make a high pressure under the helicopter.
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Dammit,
I would agree with Jelly.
I thought that the induced flow is forced toward the ground and cannot escape quick enough because of the ground. Therfore a higher local static air pressure is found (maybe 2mb hence the -60' indicated in the ALT)
Because of this the induced flow is reduced increasing the Angle of Attack and so on.
I would agree with Jelly.
I thought that the induced flow is forced toward the ground and cannot escape quick enough because of the ground. Therfore a higher local static air pressure is found (maybe 2mb hence the -60' indicated in the ALT)
Because of this the induced flow is reduced increasing the Angle of Attack and so on.
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At the risk of getting embroiled once again......
Ground effect is an aerodynamic effect caused by the placement of an obstruction underneath the downward flow from the rotor.
Aerodynamically saying, the rotational airflow remains constant, the induced airflow is slightly decreased by the added restriction placed in the induced flow, thus reducing the induced flow. The relative airflow, being taken from the top of the induced airflow vector is therefore also reduced slightly in angle. As the angle of attack is taken from a constant held pitch angle the angle between the chord/pitch angle and the relative airflow also increases slightly giving greater lift.
Whether or not a small localised pressure increase exists under the helo? yes probably but it has no/or possibly very little contributory effect to 'ground effect'.
This can probably best be seen when over a rig. The small landing pad offers the same ground effect with a small plan area therefore reducing the possibility of the ground effect 'bubble'.
Sigh..... Sit back and wait for flak.
P.s. The ROD is the increased flow from the rotor over the pitot staic system resulting in a small downward deflection caused by position error. Have a look at the beanie on the S-61N to see what mr Sikorsky tried to do about it!!!!
Ground effect is an aerodynamic effect caused by the placement of an obstruction underneath the downward flow from the rotor.
Aerodynamically saying, the rotational airflow remains constant, the induced airflow is slightly decreased by the added restriction placed in the induced flow, thus reducing the induced flow. The relative airflow, being taken from the top of the induced airflow vector is therefore also reduced slightly in angle. As the angle of attack is taken from a constant held pitch angle the angle between the chord/pitch angle and the relative airflow also increases slightly giving greater lift.
Whether or not a small localised pressure increase exists under the helo? yes probably but it has no/or possibly very little contributory effect to 'ground effect'.
This can probably best be seen when over a rig. The small landing pad offers the same ground effect with a small plan area therefore reducing the possibility of the ground effect 'bubble'.
Sigh..... Sit back and wait for flak.
P.s. The ROD is the increased flow from the rotor over the pitot staic system resulting in a small downward deflection caused by position error. Have a look at the beanie on the S-61N to see what mr Sikorsky tried to do about it!!!!
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Ground effect has nothing to do wiith pressure.
If the pressuure were higher under the helicopter, the power to hover would be greater IGE, because the pressure under the rotor would mean the engine would have to work harder to push the aiir into it!
Talk Turn, try to find one reference that is from a scientific source that discusses PRESSURE as an aspect of ground effect. (NOT a popcorn training site that repeats the myth).
If the pressuure were higher under the helicopter, the power to hover would be greater IGE, because the pressure under the rotor would mean the engine would have to work harder to push the aiir into it!
Talk Turn, try to find one reference that is from a scientific source that discusses PRESSURE as an aspect of ground effect. (NOT a popcorn training site that repeats the myth).
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To quote directly:
Ground Effect
7. In a free air hover the airflow through the rotor disc begins at zero velocity some distance above and accelerates through the disc and into the air below. There is little resistance to the downward movement of air. If the helicopter is hovered close to the ground the downwash meets the ground, is opposed, and escapes horizontally. A divergent duct is produced causing an increase in pressure...
Not exactly from a scientific reference, but I think the Royal Air Force Manual of Flying (AP3456) has been regarded as slightly more than a popcorn training site. It would appear that regard was misplaced.
If its not an increase in pressure, how is the induced flow reduced?
And if high pressure beneath the aircraft would mean the engine had to work harder to push air into it, does that mean hovering over lower pressure would reduce power required to hover?
Our heads hurt.
Ground Effect
7. In a free air hover the airflow through the rotor disc begins at zero velocity some distance above and accelerates through the disc and into the air below. There is little resistance to the downward movement of air. If the helicopter is hovered close to the ground the downwash meets the ground, is opposed, and escapes horizontally. A divergent duct is produced causing an increase in pressure...
Not exactly from a scientific reference, but I think the Royal Air Force Manual of Flying (AP3456) has been regarded as slightly more than a popcorn training site. It would appear that regard was misplaced.
If its not an increase in pressure, how is the induced flow reduced?
And if high pressure beneath the aircraft would mean the engine had to work harder to push air into it, does that mean hovering over lower pressure would reduce power required to hover?
Our heads hurt.
Clearly there is a pressure there, however none of the texts seem to reference it as anything to do with Ground Effect.
If we work it out,
Dynamic Pressure = 1/2 rho v squared.
V goes up Pressure goes up !
V goes down Pressure goes down.
Induced flow velocity decreases (because of the presence of the ground) pressure goes down (from the OGE case).
There is a method of calculating the vi reduction in Padfields book(helicopter flight dynamics I think) , depends on Disc area, the OGE vi and disc height above the ground, reducing vi more the closer the ground is.
There is a paper on it at,
http://www.adl.gatech.edu/archives/g...literature.pdf
I haven't had time to read it all, may answer some questions tho.
V.
If we work it out,
Dynamic Pressure = 1/2 rho v squared.
V goes up Pressure goes up !
V goes down Pressure goes down.
Induced flow velocity decreases (because of the presence of the ground) pressure goes down (from the OGE case).
There is a method of calculating the vi reduction in Padfields book(helicopter flight dynamics I think) , depends on Disc area, the OGE vi and disc height above the ground, reducing vi more the closer the ground is.
There is a paper on it at,
http://www.adl.gatech.edu/archives/g...literature.pdf
I haven't had time to read it all, may answer some questions tho.
V.
Last edited by VeeAny; 21st Jan 2005 at 16:45.
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The ground effect is not a pressure effect, in any sense. The reduction inpower required to hover is because the blade angle is reduced, so less power is wasted on induced drag.
The idea that pressure bubbles hold the airplane up is a third-grade pop-physics approach without any technical merit. When the FAA or the RAF repeat it, it is simply the unschooled drinking their own bathwater. That is why it is on the urban myths list!
Look at a 4 engined bomber airplane moving at 150 knots, within one wingspan of the ground. It stays up by using the ground to shape the flow around its wings, allowing the wing to produce the lift at a very much lower angle of attack, at very much lower drag. Lower drag, not high pressure, folks! Yes, the bent flow has a density, pressure and velocity relationship that someone in pprune land will assume is the "pressure bubble" but they are wrong, it is the flow direction change that brings on the reduction in angle of attack of the blade, and that angle reduction eats less power - That is ground effect.
Please, all you who will now produce some training manual somewhere that says pressure bubble, don't bother.
The idea that pressure bubbles hold the airplane up is a third-grade pop-physics approach without any technical merit. When the FAA or the RAF repeat it, it is simply the unschooled drinking their own bathwater. That is why it is on the urban myths list!
Look at a 4 engined bomber airplane moving at 150 knots, within one wingspan of the ground. It stays up by using the ground to shape the flow around its wings, allowing the wing to produce the lift at a very much lower angle of attack, at very much lower drag. Lower drag, not high pressure, folks! Yes, the bent flow has a density, pressure and velocity relationship that someone in pprune land will assume is the "pressure bubble" but they are wrong, it is the flow direction change that brings on the reduction in angle of attack of the blade, and that angle reduction eats less power - That is ground effect.
Please, all you who will now produce some training manual somewhere that says pressure bubble, don't bother.
Nick
Just in case you thought otherwise, my post about pressure calcs was to support your thoughts that it's nothing to do with it.
This is one of the best threads we've had in ages.
Here's another for the list, a wing acts like one half of a venturi !
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
Don't think anyone else has offered that one up yet.
V.
Just in case you thought otherwise, my post about pressure calcs was to support your thoughts that it's nothing to do with it.
This is one of the best threads we've had in ages.
Here's another for the list, a wing acts like one half of a venturi !
http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html
Don't think anyone else has offered that one up yet.
V.
Nick L,
If there's no such thing as Ground Effect (as a function of pressure increase) why do helicopter static instruments display such characteristics? Why, when I'm sat on the ground at min collective and I increase collective pitch do I observe, what appears to the uneducated like myself, to be a pressure increase? If it's not related to the proximity of the ground, why do helicopter static instruments not behave in the same manner when in a HOGE and collective pitch is increased?
These are genuine questions Nick; please educate me........ and try and keep it simple, I didn't do higher education.
J
If there's no such thing as Ground Effect (as a function of pressure increase) why do helicopter static instruments display such characteristics? Why, when I'm sat on the ground at min collective and I increase collective pitch do I observe, what appears to the uneducated like myself, to be a pressure increase? If it's not related to the proximity of the ground, why do helicopter static instruments not behave in the same manner when in a HOGE and collective pitch is increased?
These are genuine questions Nick; please educate me........ and try and keep it simple, I didn't do higher education.
J
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I must admit that I have never noticed a change in pressure instrument readings when lifting into the hover (maybe aircraft type, maybe because I am too busy looking out!). I suspect that these fluctuation are due to a fuselage affect ie some downwash is causing a dynamic pressure in the static system?
voicesfromthecreche (and others), the veracity of the text's you quote from are limited by the understanding of the subject by the author/s. By having been published, they do not of themselves create reality, they merely reflect that author's understanding at that point in time. We can see over history how scientific papers have been proven incomplete at latter dates by more advanced understandings, and helicopter technical explainations are no different. Whilst quoting technical texts is often our only way to understand a subject, sometimes they are just plain wrong (Lu, some even refer to centrifugal force!) That's why I propose another myth :
"helicopter technical texts are not limited by the understanding of their authors, rather they become factual and unquestionable explanations merely by being checked for spelling by an editor, and achieving print"
"helicopter technical texts are not limited by the understanding of their authors, rather they become factual and unquestionable explanations merely by being checked for spelling by an editor, and achieving print"
Does this count as a myth: the S-92 is 'all American'?
"Sikorsky is at the helm of a global partnership working together to design, build and support the S-92 to meet the needs of the worldwide helicopter industry. Sikorsky manufactures the advanced S-92 drivetrain and integrates all other parts and components at final assembly in the USA. Partners in the production of the S-92 include: Mitsubishi Heavy Industries in Japan; Aerospace Industrial Development Corporation in Taiwan; Gamesa in Spain; Jingdezhen Helicopter Group in the People’s Republic Of China and Embraer in Brazil."
"Sikorsky is at the helm of a global partnership working together to design, build and support the S-92 to meet the needs of the worldwide helicopter industry. Sikorsky manufactures the advanced S-92 drivetrain and integrates all other parts and components at final assembly in the USA. Partners in the production of the S-92 include: Mitsubishi Heavy Industries in Japan; Aerospace Industrial Development Corporation in Taiwan; Gamesa in Spain; Jingdezhen Helicopter Group in the People’s Republic Of China and Embraer in Brazil."
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212man,
It seems that you are mixing S-92 words about their partnership with VH-92 quotes about their partnership. Do you do that in the cockpit when you fly? With that sharp memory, you must really keep your co-pilots on their toes!
According to everything Sikorsky has put out (but obviously a bit too complex for you to grasp) the VH-92 uses Vought to do the airframe, making the VH-92 All American. Word is from the oil patch that the S-92 world-wide partners still make the airframe for the S-92, as you confused it with your snippy quote.
RJ
It seems that you are mixing S-92 words about their partnership with VH-92 quotes about their partnership. Do you do that in the cockpit when you fly? With that sharp memory, you must really keep your co-pilots on their toes!
According to everything Sikorsky has put out (but obviously a bit too complex for you to grasp) the VH-92 uses Vought to do the airframe, making the VH-92 All American. Word is from the oil patch that the S-92 world-wide partners still make the airframe for the S-92, as you confused it with your snippy quote.
RJ
RJS,
Ah, I see. So the oil workers and their pilots go second class in the slightly less safe version.
Thanks for clarifying that.
Ah, I see. So the oil workers and their pilots go second class in the slightly less safe version.
Thanks for clarifying that.
212man with rounded shoulders and upturned lip slinks off to the company bar to ruminate about his being several steps down from second class.....
Well, no intention of going into a downhill spiral on this one, so I guess that counts as giving it a break. However, you can expect some sniping when so much emphasis is made (check other threads plus press) on the VH-92 being safer than the US-101 by virtue of it being 'all American'. It must follow from that logic that if its sister ship is not 'all American' it must be less safe.
Personally, I would say they are more than likely equally safe and would be equally pleased to fly either.
The word 'class' was not meant to be inflammatory btw.
SASless; you should know we don't have a company bar anymore! Plus we travel Upper Class!
Personally, I would say they are more than likely equally safe and would be equally pleased to fly either.
The word 'class' was not meant to be inflammatory btw.
SASless; you should know we don't have a company bar anymore! Plus we travel Upper Class!
No company bar....now that is a first for a Bristow operation! There are more negative effects to being "americanized" than meets the eye.
I was trying to apply some left handed logic there....
Vxxx, 92,101,225,Tiger,155,365,Scyamore, 212 as to class.
Remind me....port side to Lagos....starboard side to London?
I was trying to apply some left handed logic there....
Vxxx, 92,101,225,Tiger,155,365,Scyamore, 212 as to class.
Remind me....port side to Lagos....starboard side to London?