Solid, Liquid, or Gas?

[Witch]

While moving a chicken coop onto the bed of a co-workers truck-all my chickens are gone, I had a thought about surfaces and how they affect ground effect.

 

The best surface for the least power use in a hover is concrete/asphalt obviously, and tall grass eats up power, but how about water? Does a river give the same affect as asphalt? How about a pond? Desert pavement? Snow? Gravel? Etc...

 

Another thought was about running takeoffs on these different surfaces-no river or pond.

 

Thoughts?

 

Later

[Sparker]

I bet ice is the best.... Like a big frozen lake.

 

What's desert pavement?

[Witch]

What's desert pavement?

 

Desert pavement is basically the hardpack in the desert. Mostly small, tightly-packed rock, like the gravel in the bottom of a fish tank. If you walk around the desert in theTucson area, that's it.

 

Later

[southernweyr]

There is a detailed highly technical discussion of ground effect here going on right now. It starts at the bottom of the page by me. One guy even says he got an extra 3 feet of hover height when he moved from concrete 11' to tall grass 14'. He says "the grass creates more ground effect because it slows down the downwash more than concrete does. I also remember that some pilots quoted that the bubble the downwash creates in water also produces the same effect (don't remember the thread anymore)"

[Goldy]

One guy even says he got an extra 3 feet of hover height when he moved from concrete 11' to tall grass 14'. He says "the grass creates more ground effect because it slows down the downwash more than concrete does.

 

 

huh ?

[romanweel]

That sounds like it ought to work, but it's never been my experience. I always lose altitude over grass (my RPM drops too, but that's probably just cause I'm not letting the correlator do its job ). Is this a placebo-type effect where whatever you were taught is what you think you're experiencing?

[Wally]

While moving a chicken coop onto the bed of a co-workers truck-all my chickens are gone, I had a thought about surfaces and how they affect ground effect.

 

The best surface for the least power use in a hover is concrete/asphalt obviously, and tall grass eats up power, but how about water? Does a river give the same affect as asphalt? How about a pond? Desert pavement? Snow? Gravel? Etc...

 

Another thought was about running takeoffs on these different surfaces-no river or pond.

 

Thoughts?

 

Later

 

Somebody's going to have to show me hard numbers before I buy any difference in ground effect from one surface to another. My opinion- free and worth every penny- is that the apparent change is caused by hover altitude variations due to poor/indifferent cues. Done a fair bit of overwater hover, and it's almost impossible to accurately judge altitude visually. Over vegetation, what might appear to be the surface plane will not be accurate, either.

[southernweyr]

After talking with the guy more he apparently was not saying that as fact, only speculating. He says, "I claimed the opposite, IF the grass slows down the outflow it may reduce induced speed, so I claimed a few feet higher hover when compaired to sleek concrete, but this is SPECULATIVE, as some pointed out I don't model this with CFD, so need to rely on experimental data. I observed this experimentally but not using a scientific set up."

[Goldy]

IF the grass slows down the outflow it may reduce induced speed, so I claimed a few feet higher hover when compaired to sleek concrete, but this is SPECULATIVE, as some pointed out I don't model this with CFD, so need to rely on experimental data. I observed this experimentally but not using a scientific set up."

 

 

huh ?

[FlyNHighNFast]

I second Goldy's "huh?"

 

Please explain your theory, in non scientific, non experimental, english.

[Optigirl]

Wing in ground effect' and helicopters.

 

Rotary wing aircraft experience performance changes associated with 'wing in ground effect' just like other aircraft. A helicopter hovering close to the ground will require less power than when hovering out of 'wing in ground effect'. To hover in 'wing in ground effect' you must be within one blade length of the surface and the surface must be a hard surface.

 

Following an engine failure (depending on weight and environmental conditions), a multi engine helicopter may be able to hover in 'wing in ground effect' but not outside it.

 

This effect can be described at three different levels of understanding: basic, intermediate and advanced.

 

 

Basic

As with a fixed wing surface a helicopter's blades produce a cushion of air when close to the ground which helps to support the aircraft.

 

Wind, surface slope and surface texture will all have an effect on how effectively the high pressure cushion is maintained under the rotor.

 

 

Intermediate

When hovering in free air a rotor disc produces a duct effect with an induced downwards movement of air above the rotor. Rather than being stationary, when the air hits the rotor it already has a downward vector as it attempts to fill the region of low pressure left by air below it which has already been forced downwards by the rotor. To hover, the rotor disc must produce a vertical force in the manner of Force = Mass x Acceleration where mass is dependent on number of rotors, RPM and rotor diameter and acceleration by the difference in the initial air vector and its resulting vector. Assuming mass moved is constant then a helicopter hovering in free air will be produce a force equivalent to V2 (speed after passing through the rotor) - V1 (starting speed of air) where V1 is some value above 0 given that the air is already moving downwards before reaching the rotor.

 

When close to the surface (generally considered 1/3-2/3 of the rotor diameter), air forced downwards through the rotor disc is restricted in its flow by the ground. This produces an area of high pressure below the disc and in turn, reduces the duct effect and hence the downwards velocity of the air above the disc. This reduces V1 while V2 remains relatively static, so the value of V2 - V1 increases. From the equation F = M x A we can see that the 'lift' of the rotor disc is greater when in ground effect.

 

 

Advanced

The relative air flow meeting the advancing face of a rotor blade in a hovering helicopter is determined by the speed of rotation (the blade moving forward into the air) and the induced flow (the induced downwards movement of air above the rotor). Rotational air flow is taken as horizontal, induced flow as vertically down and the relative airflow as the resulting vector. Through geometry the angle of attack(AoA) of the rotor blade will decrease as the inflow increases and the relative airflow direction moves closer to the chordline.

 

When the helicopter is in 'wing in ground effect' the induced flow is decreased as described above. This moves the relative airflow vector closer to the horizontal and increases the AoA for a given blade pitch. This increases the lift produced by the rotor disc and the helicopter will start to accelerate vertically. The vertical movement will induce its own inflow reducing the AoA again until a point of equilibrium is reached. In reality a pilot will lower the collective slightly -> reducing rotor pitch -> reducing AoA -> reducing lift and the helicopter will hover in ground effect(IGE) with a lower power setting than that required out of ground effect (OGE).

 

Reference: Wikipedia Ground effect in aircraft

[southernweyr]

I did not say that so I cannot explain it. Please click here to read the discussion. http://www.pprune.org/forums/showthread.ph...0955&page=2

[Goldy]

I did not say that so I cannot explain it. Please click here to read the discussion. http://www.pprune.org/forums/showthread.ph...0955&page=2

 

Southern- Thanks but no thanks. There is a reason why I am not a member of pprune, way too much BS and too little fact. I'll stick with my "huh" and move on. There is only one way that this guy hovers higher over tall grass than asphalt, its called raising the collective, or burning off 50 pounds of fuel, end of story.

 

Optigirl- nice explanation, lets go flying !

 

Goldy

[southernweyr]

I agree Goldy. I think he was saying that it might seem like you should get an increase in performance because the grass slows the air and that would slow the induced flow. I think that the grass slows the outward horizontal speed of the air and this increases the tip vortices. This is also how to tell what surfaces are worse, which ones have the most "skin friction?" Those are the ones that give you the worst perfromance.

[Wally]

Wing in ground effect' and helicopters.

 

Rotary wing aircraft experience performance changes associated with 'wing in ground effect' just like other aircraft. A helicopter hovering close to the ground will require less power than when hovering out of 'wing in ground effect'. To hover in 'wing in ground effect' you must be within one blade length of the surface and the surface must be a hard surface.

 

Following an engine failure (depending on weight and environmental conditions), a multi engine helicopter may be able to hover in 'wing in ground effect' but not outside it.

 

This effect can be described at three different levels of understanding: basic, intermediate and advanced.

Basic

As with a fixed wing surface a helicopter's blades produce a cushion of air when close to the ground which helps to support the aircraft.

 

Wind, surface slope and surface texture will all have an effect on how effectively the high pressure cushion is maintained under the rotor.

Intermediate

When hovering in free air a rotor disc produces a duct effect with an induced downwards movement of air above the rotor. Rather than being stationary, when the air hits the rotor it already has a downward vector as it attempts to fill the region of low pressure left by air below it which has already been forced downwards by the rotor. To hover, the rotor disc must produce a vertical force in the manner of Force = Mass x Acceleration where mass is dependent on number of rotors, RPM and rotor diameter and acceleration by the difference in the initial air vector and its resulting vector. Assuming mass moved is constant then a helicopter hovering in free air will be produce a force equivalent to V2 (speed after passing through the rotor) - V1 (starting speed of air) where V1 is some value above 0 given that the air is already moving downwards before reaching the rotor.

 

When close to the surface (generally considered 1/3-2/3 of the rotor diameter), air forced downwards through the rotor disc is restricted in its flow by the ground. This produces an area of high pressure below the disc and in turn, reduces the duct effect and hence the downwards velocity of the air above the disc. This reduces V1 while V2 remains relatively static, so the value of V2 - V1 increases. From the equation F = M x A we can see that the 'lift' of the rotor disc is greater when in ground effect.

Advanced

The relative air flow meeting the advancing face of a rotor blade in a hovering helicopter is determined by the speed of rotation (the blade moving forward into the air) and the induced flow (the induced downwards movement of air above the rotor). Rotational air flow is taken as horizontal, induced flow as vertically down and the relative airflow as the resulting vector. Through geometry the angle of attack(AoA) of the rotor blade will decrease as the inflow increases and the relative airflow direction moves closer to the chordline.

 

When the helicopter is in 'wing in ground effect' the induced flow is decreased as described above. This moves the relative airflow vector closer to the horizontal and increases the AoA for a given blade pitch. This increases the lift produced by the rotor disc and the helicopter will start to accelerate vertically. The vertical movement will induce its own inflow reducing the AoA again until a point of equilibrium is reached. In reality a pilot will lower the collective slightly -> reducing rotor pitch -> reducing AoA -> reducing lift and the helicopter will hover in ground effect(IGE) with a lower power setting than that required out of ground effect (OGE).

 

Reference: Wikipedia Ground effect in aircraft

 

Here's the obvious problem with that explanation- Ground effect is also recognizable at speed, when the airfoil's long gone before the any of that induced flow stuff could affect the AOA.

[Wally]

Wing in ground effect' and helicopters.

 

Rotary wing aircraft experience performance changes associated with 'wing in ground effect' just like other aircraft. A helicopter hovering close to the ground will require less power than when hovering out of 'wing in ground effect'. To hover in 'wing in ground effect' you must be within one blade length of the surface and the surface must be a hard surface.

 

Following an engine failure (depending on weight and environmental conditions), a multi engine helicopter may be able to hover in 'wing in ground effect' but not outside it.

 

This effect can be described at three different levels of understanding: basic, intermediate and advanced.

Basic

As with a fixed wing surface a helicopter's blades produce a cushion of air when close to the ground which helps to support the aircraft.

 

Wind, surface slope and surface texture will all have an effect on how effectively the high pressure cushion is maintained under the rotor.

Intermediate

When hovering in free air a rotor disc produces a duct effect with an induced downwards movement of air above the rotor. Rather than being stationary, when the air hits the rotor it already has a downward vector as it attempts to fill the region of low pressure left by air below it which has already been forced downwards by the rotor. To hover, the rotor disc must produce a vertical force in the manner of Force = Mass x Acceleration where mass is dependent on number of rotors, RPM and rotor diameter and acceleration by the difference in the initial air vector and its resulting vector. Assuming mass moved is constant then a helicopter hovering in free air will be produce a force equivalent to V2 (speed after passing through the rotor) - V1 (starting speed of air) where V1 is some value above 0 given that the air is already moving downwards before reaching the rotor.

 

When close to the surface (generally considered 1/3-2/3 of the rotor diameter), air forced downwards through the rotor disc is restricted in its flow by the ground. This produces an area of high pressure below the disc and in turn, reduces the duct effect and hence the downwards velocity of the air above the disc. This reduces V1 while V2 remains relatively static, so the value of V2 - V1 increases. From the equation F = M x A we can see that the 'lift' of the rotor disc is greater when in ground effect.

Advanced

The relative air flow meeting the advancing face of a rotor blade in a hovering helicopter is determined by the speed of rotation (the blade moving forward into the air) and the induced flow (the induced downwards movement of air above the rotor). Rotational air flow is taken as horizontal, induced flow as vertically down and the relative airflow as the resulting vector. Through geometry the angle of attack(AoA) of the rotor blade will decrease as the inflow increases and the relative airflow direction moves closer to the chordline.

 

When the helicopter is in 'wing in ground effect' the induced flow is decreased as described above. This moves the relative airflow vector closer to the horizontal and increases the AoA for a given blade pitch. This increases the lift produced by the rotor disc and the helicopter will start to accelerate vertically. The vertical movement will induce its own inflow reducing the AoA again until a point of equilibrium is reached. In reality a pilot will lower the collective slightly -> reducing rotor pitch -> reducing AoA -> reducing lift and the helicopter will hover in ground effect(IGE) with a lower power setting than that required out of ground effect (OGE).

 

Reference: Wikipedia Ground effect in aircraft

 

Here's the obvious problem with that explanation- Ground effect is also recognizable at speed, when the airfoil's long gone before the any of that induced flow stuff could affect the AOA.

[Vaqueroaero]

For a great visual of how ground effect works get a copy of Principles of Helicopter flight by W.J. Wagtendonk. Go to page 61. Right there are all the diagrams you will ever need to know about producing lift.

 

If I was really clever, which I'm not, I would post them here for all to see.

 

As for the rest of the stuff I'm going with Goldy and his 'huh?'