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Ground effect


hooked4life
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So I know that ground effect is caused by the fact that the ground disrupts the airflow around the rotor, thus decreasing induced flow and vortices and since a reduction in iduced flow increases the AOA more lift is produced. I also know that hovering over rough, sloping, or grassy terrain can decrease ground effect. I was just a little confused as to why that is. I mean grass and rocks and stuff should be disturbing the airflow even more than smooth pavement, right? I was wondering if someone could give me some insight into this phenomenon that is confusing me at the moment.

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So I know that ground effect is caused by the fact that the ground disrupts the airflow around the rotor, thus decreasing induced flow and vortices and since a reduction in iduced flow increases the AOA more lift is produced. I also know that hovering over rough, sloping, or grassy terrain can decrease ground effect. I was just a little confused as to why that is. I mean grass and rocks and stuff should be disturbing the airflow even more than smooth pavement, right? I was wondering if someone could give me some insight into this phenomenon that is confusing me at the moment.

 

Think for a second how a hovercraft works. A cushion of air supporting the weight of an aircraft. Now you are producing a rather large cushion of air under you in a hover. Even without a skirt to hold the air in, a portion of the descending air bounces back up at you and forms a similar cushion. Without a hard surface more of that air dissipates creating less of a cushion, and requiring more power. The closer to the ground you are, the stronger that cushion is.

 

Only way I could think of describing it, I hope it helps.

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All of my aerodynamics instructors denounced the cushion of air explanation as incorrect. It was explained to me rather to be a reduction of wing tip vortices.

 

Since the benefit increases with proximity, obviously sloping terrain requires a higher hover, but the grass/water factor always puzzled me.

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I believe this is more widely-accepted misinformation. Where are the flight test results of decreased ground effect due to the type of surface? I flew seaplanes for years and can say for a fact that ground effect is just as pronounced over water as it is over hard ground.

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Ground effect increases performance by reducing wing tip vortices and reducing induced flow. When in ground effect, induced flow is reduced by the fact that rotor downwash is subject to surface friction and also has to change direction. As mentioned, these two factors reduce induced flow which in turn increases AOA.

 

The way I look at is similar to the critical angle of attack. Consider the CAOA - more lift is produced as the AOA is increased, up until the CAOA. At the CAOA, the airflow separates from the airfoil and becomes turbulent causing a loss of lift and large increase in drag. I look at the surface friction caused by surface type and texture in the same way. Smooth concrete causes enough surface friction to reduce induced flow and increase AOA. Short grass causes a little more surface friction to occur than smooth concrete, causing a slightly greater reduction in induced flow, and so slightly "better" ground effect is achieved. Long grass, extremely rough surface texture, or water, however, cause not only surface friction (which is good), but a turbulent airflow disruption similar to the CAOA (which is bad). Obviously, I'm not suggesting that hovering over these surfaces will have the same effect as exceeding the CAOA, it's just an analogy.

 

I'm no aerodynamics engineer, but that's how I understand it...

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So I know that ground effect is caused by the fact that the ground disrupts the airflow around the rotor, thus decreasing induced flow and vortices and since a reduction in iduced flow increases the AOA more lift is produced.

 

I also know that hovering over rough, sloping, or grassy terrain can decrease ground effect.

 

I was just a little confused as to why that is. I mean grass and rocks and stuff should be disturbing the airflow even more than smooth pavement, right?

 

I was wondering if someone could give me some insight into this phenomenon that is confusing me at the moment.

 

You answered your own question: Grass and rocks and stuff disturb the airflow even more than smooth pavement.

 

The presence of the ground acts as a hindrance to the advancement of the wake and since the wake is constrained in the downward axial direction, it expands radially outward as it approaches the ground. This in turn changes the aerodynamic characteristics of the overall flow, including the strength of the tip-vortex.

 

So, we have an induced flow moving vertically downward and horizontally outward as it contacts the ground. Remember, we need to take into account both flow paths, (1) the vertical flow and (2) the radial flow moving along the ground and expanding outward away from the rotor. Tall grass, rough terrain, or water causes a partial breakdown, absorption, and cancellation of that outward flow and the return of large vortex patterns. You need to look at the total flow dynamics not just the fact that induce flow is reduced.

 

Most helicopter textbooks don't go that deep into aerodynamics. If you want more of the WHY, click the link below (pdf document University of Maryland 9.6MB).

 

Fluid Dynamics of Interacting Blade Tip. Vortices With a Ground Plane

Edited by iChris
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I believe this is more widely-accepted misinformation. Where are the flight test results of decreased ground effect due to the type of surface?

 

It most definitely is not mis-information. Who needs flight test results when you can verify it yourself? Hover on a taxiway, look at what your MAP is, move over to the grass and hover there. Check your MAP and it will be higher.

 

I can't speak for water as I've never been in a hover over water but I imagine it's the same.

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I believe this is more widely-accepted misinformation. Where are the flight test results of decreased ground effect due to the type of surface? I flew seaplanes for years and can say for a fact that ground effect is just as pronounced over water as it is over hard ground.

 

Many pilots have reported that hovering over tall grass or water provides less ground effect.

 

Cal Poly University at San Luis Obispo made ground-effect measurements over astroturf (not tall grass) and over water. The astroturf results tend to refute the pilot's observations. The ground effect at half a rotor diameter was roughly 30% stronger than over the smooth solid surface. However, over water there was little measurable difference (Ref. More Helicopter Aerodynamics, pg. 6, R.W. Prouty).

 

I've not seen enough real test data to support or refute the full magnitude of the ground-effect. However, it's apparent the type surface dose effect the overall dynamics of ground-effect.

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Many pilots have reported that hovering over tall grass or water provides less ground effect.

 

Cal Poly University at San Luis Obispo made ground-effect measurements over astroturf (not tall grass) and over water. The astroturf results tend to refute the pilot's observations. The ground effect at half a rotor diameter was roughly 30% stronger than over the smooth solid surface. However, over water there was little measurable difference (Ref. More Helicopter Aerodynamics, pg. 6, R.W. Prouty).

 

.

My point exactly. Measurement results tend to refute the pilot's observations, as you quoted from Prouty.

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My point exactly. Measurement results tend to refute the pilot's observations, as you quoted from Prouty.

 

I don't need a study to confirm or refute my observations. If I'm in an R-22 hovering over the hard taxiway surface at, say 20". I move just a short distance away over the grass and now I'm pulling 21", maybe a little more. Move back over the taxiway, back to 20". There is clearly a difference in ground effect when hovering over a taxiway and hovering over grass. It's even more pronounced the longer the grass is (to a point I imagine).

 

As for that study that refutes it? Astroturf? Well, it's good to know that if I'm hovering over an astroturf football field or on a golf-course green that I'll be good to go. Seriously? How can you compare astroturf to grass? I would expect little difference in astroturf over pavement.

 

Note, I'm referring only to (real) grass. I make it a habit to not test IGE hovers over water :)

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As for that study that refutes it? Astroturf? Well, it's good to know that if I'm hovering over an astroturf football field or on a golf-course green that I'll be good to go. Seriously? How can you compare astroturf to grass? I would expect little difference in astroturf over pavement.

 

Note, I'm referring only to (real) grass. I make it a habit to not test IGE hovers over water :)

 

I was referring to the water. Still open on the grass issue.

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Hovering over water is not the same as flying an amphibian over water, or flying a helicopter at speed over water. There is certainly ground effect over water, but if you hover over water of any depth, the surface quickly becomes very rough, and the longer you hover, the rougher it gets. In a heavy helicopter the waves can get fairly high, and the spray goes well above the rotor system. Ground effect can be noticeably reduced, although it still exists.

Edited by Gomer Pylot
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great video.. thats very interesting.

 

I love their videos. So let me get this straight. It's the reduction in vortices which is pushing down on the water and leaving an impression, a wake, in this ship's path. Or gee, maybe it's also air that is being squeezed between the wing and the surface which is causing it.

 

just a guess. Maybe that's why they still call it a "phenomenon".

Edited by Goldy
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Wow, thanks for all the input guys. I wasn't expecting this much discussion. The article and video really help. I really like the particle imagery in the article. I always find it interesting when you can "see" the air.

 

Also,

Tall grass, rough terrain, or water causes a partial breakdown, absorption, and cancellation of that outward flow and the return of large vortex patterns. You need to look at the total flow dynamics not just the fact that induce flow is reduced.[/font]

clears things up a lot.

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According to Wagtendonk, one of my favorites: "Any surface that absorbs downwash energy and causes molecular movement reduces the benefit of ground effect. For example, hovering over water causes surface ripples which absorb energy, taking away from the potential energy contained within the rotor downwash. Similarly, hovering over tall grass, dry sand, or snow lowers the hover in-ground effect ceiling. ... If you intend to rely on ground effect when operating at altitude and/or at high gross weight it is important to remember that surface disturbances can seriously reduce the vvenefits of ground effect."

 

So it sounds like Prouty and Cal Poly differ from Wagtendonk with regards to water affecting the IGE hover. I guess certain variables could make it difficult to measure precisely. Either way though, excellent informative posts here instead of the easy, simple explanations I was expecting. I think I just get excited when Prouty and Leishman are cited in the same thread.

Edited by rjl2001
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Interesting conversation, so I figured I would chime in with a point that I did not see discussed. At the height above water to benefit from ground effect wouldn't the spray or water droplets and gaseous/vapor state of water created by the downwash also explain the extra power required over water? All that moisture must at least slightly lessen the rotor effectiveness and add drag.

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Good point TimW68 being a Turbine mechanic I would have to say that the biggest factor with the moisture would be the fact that moisture displaces the oxygen in the air making it less dense. This means the engine produces less power and the rotors provide less lift. The concept that high humidity caused these problems confused me and seemed backwards at first, since people often refer to high humidity situations as the air being thick. I started to get it when it was explained that the more water in the air the less room there is for oxygen, which is what fuels our fire. Therefore our reference to humid air is what is backwards.

 

As far as drag, I don't remember for sure but I think that may have been mentioned.

 

Of course you could expect to experience the same thing when dealing with snow and dust.

 

As far as how much effect from this you will have....I dunno, I hope to find out soon.

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I think I just get excited when Prouty and Leishman are cited in the same thread.

 

You know Ray (Prouty) is still around. I run into him 3 or 4 times a year at events here in LA. He still does some live events around the country.

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