Ground Effects on the rotor

WaspAir

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Actually that sounds more like you were overloaded and flying behind the power curve. Of course, I was not in the cockpit with you so it is just speculation, because that happened to me as well in a SxS Air Command when flying in Venezuela many moons ago.
No way. I was not "behind the power curve". I was at best rate of climb speed (around 70 mph), and solo in the aircraft with just my skinny 140 lb body. But I was at/just above the aircraft's absolute ceiling with that combination of load and density altitude (converting in my head, I'm pretty sure that well over 4000 ft + 113 degrees is substantially over 8000 DA). Without ground effect to help, the J-2 would not have flown at all.
The terrain was free of obstructions for many miles ahead (just some tumbleweed), and I continued in runway direction skimming above the ground. I could hold ten feet AGL, but where the terrain went slightly lower, so did I, and similarly for little rises. If there had been a drop off down to 3500 feet, I would not have been able to hold my MSL altitude above it because the ground effect would have been lost.

I understand reluctance to compare helicopter and gyro ground effect because of the confusion of hovering flight with translational motion. But in practical terms, a gyro in my situation benefits from ground effect just as a fixed wing would.
 

DennisFetters

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No way. I was not "behind the power curve". I was at best rate of climb speed (around 70 mph), and solo in the aircraft with just my skinny 140 lb body. But I was at/just above the aircraft's absolute ceiling with that combination of load and density altitude (converting in my head, I'm pretty sure that well over 4000 ft + 113 degrees is substantially over 8000 DA). Without ground effect to help, the J-2 would not have flown at all.
The terrain was free of obstructions for many miles ahead (just some tumbleweed), and I continued in runway direction skimming above the ground. I could hold ten feet AGL, but where the terrain went slightly lower, so did I, and similarly for little rises. If there had been a drop off down to 3500 feet, I would not have been able to hold my MSL altitude above it because the ground effect would have been lost.

I understand reluctance to compare helicopter and gyro ground effect because of the confusion of hovering flight with translational motion. But in practical terms, a gyro in my situation benefits from ground effect just as a fixed wing would.
Like I said, I was not there.

Also thinking out loud, I have been in situations where there was a layer of colder air hugging the ground in the mornings where I could feel a significant increase of lift while skimming on top of it. That lift would go away after the temperature normalized, not saying that was it, I was not there with you to know.

A gyroplane simply does not have a significant amount of ground effect to make a difference, so another logical answer has to be other than that.
 

WaspAir

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I can assure you that there was no layer of cold air at 113 degrees in west Texas in August.

It was ground effect, pure and simple, and with no doubt whatsoever.

My prior comment, "If that's not ground effect, one must wonder what it was" was purely rhetorical., and not meant to indicate that some other mysterious force was at work. The explanation is obvious.

While you might not think that ground effect can be significant for a gyro, my gyro disagrees with you, and it is the ultimate authority. The power required to skim the ground at 70 mph is less than the power required to fly at 500 agl at 70 mph, like it or not.
 

DennisFetters

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I can assure you that there was no layer of cold air at 113 degrees in west Texas in August.

It was ground effect, pure and simple, and with no doubt whatsoever.

My prior comment, "If that's not ground effect, one must wonder what it was" was purely rhetorical., and not meant to indicate that some other mysterious force was at work. The explanation is obvious.

While you might not think that ground effect can be significant for a gyro, my gyro disagrees with you, and it is the ultimate authority. The power required to skim the ground at 70 mph is less than the power required to fly at 500 agl at 70 mph, like it or not.
Like I said, I was not there with you, so I can't explain what you say happened.

I can only assure you that ground effect from rotor down-wash has Little to no effect on a gyroplane, unless it is close to the ground, slow airspeed and nose pointed into the sky, and then thats not ground effect off the main-rotor but from the propellers thrust and some compressibility from that thrust between the ground and rear portion of the rotor disk, if you are low enough.
 

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I never completely understood the explanations that I've seen for ground effect. Some say the ground is reducing wing-tip vortices, thus improving wing efficiency. Some say the ground is creating a "high pressure cushion", again improving the wing efficiency. In any case, my own experience is that ground effect is noticeable in gyros. I feel it mostly during roundout and flare in a lightly loaded gyro (both single and tandem) using a Dragon Wings rotor. I feel a noticeable change in the descent attitude in the last 10-15 feet - more than what I would have expected for the amount of back stick used for the roundout. To me it feels as if I am hitting a cushion of air, and I have always attributed that feel to ground effect.

Udi
 

DennisFetters

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I never completely understood the explanations that I've seen for ground effect. Some say the ground is reducing wing-tip vortices, thus improving wing efficiency. Some say the ground is creating a "high pressure cushion", again improving the wing efficiency. In any case, my own experience is that ground effect is noticeable in gyros. I feel it mostly during roundout and flare in a lightly loaded gyro (both single and tandem) using a Dragon Wings rotor. I feel a noticeable change in the descent attitude in the last 10-15 feet - more than what I would have expected for the amount of back stick used for the roundout. To me it feels as if I am hitting a cushion of air, and I have always attributed that feel to ground effect.

Udi
When you are landing a gyroplane again two things will happen;

You flair the rotorsystem exposing the disk to more air, creating more lift and rpm. You will
fill that boost of lift as the add lift decays.

Also, as your forward speed is gone, the blades will reverse airflow and you will get some ground effect similar to that of a helicopter rotorsystem for a moment.

Sorry, I'm limited in typing with my PDA, I'm at the bullet train station going to HongKong.
 
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kolibri282

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I would like to propose this explanation for ground effect:

In naca 73254 ground effect for a rotor is modeled by introducing
a reduction factor for induced velocity of :

Kr = 1 -exp(z/D/G)

where z is distance from rotor hub to ground, D is rotor diameter
and G is some factor to match test data.

My interpretation goes like this (see attached drawing):

Out of ground effect the circulation needed to generate thrust to support
the weight of the aircraft induces velocity vi which inclines the flow
leaving the rotor downwards. To maintain sufficient inflow to keep up
autorotation the rotor disk has to be inclined backwards until it makes
the proper angle of attack with the resultant flow (airspeed plus induced
velocity)

In ground effect the flow near the ground is parallel to the ground since the
air can not penetrate the surface. The streamlines near the rotor are forced
to be more flat since the flow changes gradually with height. The rotor can
thus be flown at a smaller backward inclination with the same angle of
attack as before needed to maintain inflow for autorotation. The reduction in
induced velocity leads to a very small overpressure below the rotor but the
decisive point is the difference in angle of attack of the rotor disk. The rotor
in ground effect has a better L/D and thus the aircraft as a whole has a better
glide angle.

This reduction does not depend on the way the induced flow is created,
whether by a rotor driven by an engine or a rotor in autorotation, therefor
ground effect is the same for helicopter or gyro and only depends on the
thrust coefficient/disk loading.
 

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bryancobb

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Heli-Gyro

Heli-Gyro

...You flair the rotorsystem exposing the disk to more air, creating more lift and rpm...Also, as your forward speed is gone, the blades will reverse airflow and you will get some ground effect similar to that of a helicopter rotorsystem for a moment.
...
As you describe :) gyros landing and helicopters doing autos to touchdown experience the same thing. For a brief few seconds.

The same added lift from ground effect lasts the whole time (in a gyro) if you make a low, slow, high power pass, the full length of the runway. I can't prove it without drawing myself a bunch of different free-body diagrams so I understand it better, but my intuition tells me that in that condition, you are right at the threshold where airflow is about to reverse. I would bet that if your grass runway had just been mowed and you did this low-pass, you'd see grass clippings blown forward in front of you.
 

birdy

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At 500' I tryed it and got some very disturbing stick bumpping and will not be doing that again.
Im pretty sure that this stick bumping you felt is only the rotor flyn through the prop blast, maken the stick shake.
 

bryancobb

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I just had a "visual" pop into my head to illustrate what the air is doing in the vicinity of a gyro, at low altitude, steep attitude, high power, level flight...

A water-skier at about 5MPH before "coming up" is like a bulldozer, plowing through the water, pushing up a swell of water in front of him. He has massive amounts of drag. As the water escapes the leading swell, it finds the path of least resistance and rushes around wherever it can find an exit, and creates eddies and turbulence as it does.

Does anyone agree this is a good illustration or am I F.O.S.?
 

neutrax

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Hi Bryan

While landing in the sport copter trainer I realized our intended touchdown point was covered with leaves and as we came in; the leaves were actually going out in front of us. To see that while feeling the braking lift of the high angle and the loss of flight speed gave me some good insight into just what was going on.

It is a really cool image to recall.

Gilbert
 

dabkb2

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At 500' I tryed it and got some very disturbing stick bumpping and will not be doing that again.
Im pretty sure that this stick bumping you felt is only the rotor flyn through the prop blast, maken the stick shake.
That sounds good, but why would it not do that in ground effect?
 

bryancobb

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Hi Bryan

While landing in the sport copter trainer I realized our intended touchdown point was covered with leaves and as we came in; the leaves were actually going out in front of us. To see that while feeling the braking lift of the high angle and the loss of flight speed gave me some good insight into just what was going on.

It is a really cool image to recall.

Gilbert
Thanks for that. That confirmed, to me at least?, that I'm not F.O.S.
 

WaspAir

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The same added lift from ground effect lasts the whole time (in a gyro) if you make a low, slow, high power pass, the full length of the runway.
You get a benefit in a low, not-slow, moderate power pass the full length of the runway, too. You don't need to be standing on the prop to get a ground effect advantage.
 

bryancobb

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You may be right Wasp,

It's just not easy to intuitively see that. It appears that at low rotor AOA's the gyro would be more like a water skier that is smoothly gliding on top of the water rather than plowing through it.

But you have felt and experienced it, and I haven't.
 

phantom

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when I fly over light snow at about 5 feet the snow is pushed out from under the rotor and the slower the flight the wider the path of desturbed snow, when I look back there is a cloud of drifting snow behind me that looks like the drifting snow behind my fixed wing, there is what appears to be wing tip vortices of swirling snow on each side and when ever someone ask about it I have always told them it is because I am in ground effect and the air compressed between the rotor and the surface rushes out to the lower pressure air outside the rotor.

Norm
 

bryancobb

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Seeing that would open my eyes to what's going on. You had your own private wind tunnel.
 

phantom

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it also is surprizing at first to see that a helicopter in a hover will not sit in the center of the swirling donut of snowdrift but a little off center.

Norm
 

brett s

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it also is surprizing at first to see that a helicopter in a hover will not sit in the center of the swirling donut of snowdrift but a little off center.
Only it if has a tail rotor.
 
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