Ground Effects on the rotor

birdy

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the leaves were actually going out in front of us.
The only time the leaves will blow out in frunt in a gyro is AFTER youv unloaded the rotor [ landed] or if your landn with a tail wind.

That sounds good, but why would it not do that in ground effect?
It will, but not to the same extent, coz the disc is flatter wen in ground effect.
 

phantom

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Yes Bell 206L had a tail rotor and a CH 46 hovers in one large donut of swirling snow and not two small ones from each rotor, all very interesting to watch or at least it was for me but then I like watching wrinkle paint dry.

Norm
 

RotoPlane

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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.
I can wrap my head around Juergen's explanation of ground effect....and it can stand firm with the ground effects I've seen and felt.

There is a drawing that makes this more visual...but I've never been able to bring them along with the text....post #32.
 

Vance

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I learned something today.

I learned something today.

Let me start by saying I did not know if gyroplanes were affected by ground effect and I know this does not answer Dave’s original question. I do not feel comfortable flying at 10kts close to the ground in The Predator so I did my tests at 30 kts indicated air speed.

Both SMX and SBP were busy so I only made one run to try to get a feel for ground effect.

The Predator flew straight and level at 30kts at 2,050 engine RPM at 1,300 feet MSL. I feel I was able to get a good consistent reading over 3 runs. There was a steady 11kt tail wind over most of the Santa Maria Valley. The air was cool (52 F) with a -159 foot density altitude at SMX’s 261 foot MSL runway. This is a 160 propeller RPM below the norm for 30kts straight and level.

To put that in perspective I was almost 190 propeller rpm below the norm for straight and level at 50kts.

I asked ATC at SMX to make a low pass and land at taxiway Echo. It was approved as requested. I only did one pass so I don’t know how good a run it was although I feel there was no question there was ground effect.

I held the wheels off the runway between 12 inches and 18 inches at 30 kts indicated air speed. I almost got her to stay put at 1,675 propeller RPM but it wasn’t quite enough. I advanced the throttle and made it a half mile at 1,700 propeller RPM very close to 12 inches. This is a rotor head height of approximately 10’ 8” on the Predator with 30 foot eight and a half inch Sport Copter blades. My IAS probably was as much as 35 kts but not much below 30kts IAS. I found maintaining airspeed and altitude precisely a challenge.

I will do more testing when the airport is less busy and calm wind.

Regards, Vance
 

kolibri282

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Vance,

thank you for your effort to make experiment the base upon which argument should be built, I think this is the only approach that will ultimately get us anywhere. I have filed your "Understanding Rotor RPM" thread because I will hopefully be able during the Christmas break to feed the data you provided into my simulation program and try how far I get with reproducing the results you have recorded. I have not taken part in this very interesting thread because I feel that without the support of a validated numerical model I could (as a non pilot) not contribute anything that would advance understanding of your data. One small point, just while I'm at it, naca always recorded air pressure at the height at which full scale flight tests were conducted, since air density enters in many of the equations describing the physics of flight. Keep up the good work, Vance.

Ed,

it seems that my intention to make it easier to understand the text by providing a drawing has given quite the opposite outcome. What I tried to explain is that we see a complex interaction between the rotor and the surrounding global flow field. The rotor creates this flow field but rotor state also depends on the nature of the global flow. If global flow is now changed from outside, e.g. by proximity to the ground, this in turn influences the rotor. I will be glad to improve my description if you point me towards where the gap between my text and the drawing is.

Thanks for your feed back,

Juergen
 
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RotoPlane

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Juergen:
I was in a rush and apparently didn't explain myself well….I wanted to bring your drawing along with your post 32 quoted text, but I did not know how to do that.

I understood and agree with what you wrote…..including possible variable effects on the rotor from any changes in the surrounding global flow ;).
 

Jean Claude

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Near the ground, the air can not escape from below as it would in the sky. Therefore ground effect only acts by decreasing the induced speed and the induced power.
Helicopters requires a great induced power, due to their low forward speed and their high load per square foot. Then the ground effect provides an important aid in flight.
But gyroplane is
1 - much less charge per square foot,
2 - he can not fly so slowly as helicopter
3 - when it does, then a component of its induced speed is horizontal is not affected by the ground.
My opinion is the gyroplane is less sensitive to the ground effect as an helicopter.
 
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kolibri282

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I fully agree, Jean-Claude if you are referring to modern helicopters. Let me try to explain the sentence below a bit further:
therefor ground effect is the same for helicopter or gyro and only depends on the
thrust coefficient/disk loading.
Early Helicopters like the S-58 were unable to hover in ground effect at maximum load, lacking the required power. These aircraft had wheels fitted which enabled them to accelerate on the ground since the forward speed would add to rotor inflow until the combined flow would lift the aircraft. What I wanted to point out is that a gyro with the same disk loading as the S-58 would benefit in exactly the same way from ground effect if it were flying at the forward speed the helicopter needed to break ground contact. Of course the ratio of rotor diameter to ground clearance and the inflow angles of the rotors would also have to be the same, but given these three factors mentioned previously there is no difference in ground effect whatsoever between helicopter and autogyro.
I remember another interesting quote from a film featuring UH-1 helicopters in Vietnam. A pilot recounted that his helo was sometimes so overloaded that he accelerated the aircraft forward on it's skids over suitable ground until he was able to lift off.
 
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birdy

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Strange, it seemed so real.
Reality usualy seems strange, if you dont understand wot your see'n. ;)
An autorotating rotor has no down wash through the disc in steady flight, except at the outside tips. If it did, you go negative and loose rrpm.
This small amount of down wash isnt decending fast enuf to create the 'mushroom' when near the ground.
Coz a gyro cant stop, its gon past the wash before there is any effect on the ground, even if your dragn your tail wheel.
An inertia spot landing is the only time its go'n to have wash like a heli, and im pretty sure you didnt do one of them.
 

kolibri282

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Lift is generated all the way along the blade of an autogyro rotor and since lift can only be generated with the proper circulation about the section of blade profile it means that induced velocity distribution in a rotor of equal disk loading is the same for helo and gyro. The only difference is that for the driving region of the rotor the resultant force is inclined forward but this alters the circulation negligibly so that the induced velocity distribution is for all practical purposes the same for a helicopter and an autogyro at the same disk loading. As can be seen below the induced velocity is almost constant over the rotordisk for lateral cuts especially over the rear of the rotor disk and by no means restricted to the blade tips. The picture below is from this report by Peters and He. Incidently the report always speaks about rotorcraft and does not make any restriction of applicability of the numerical model to helicopters. Note though that the picture shows results of measurements.

http://www.rotaryforum.com/forum/showthread.php?p=504829
 

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DennisFetters

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I have been traveling, so it was just too hard to continue participation on this topic until now.

Back in the good old days when I was doing a lot of experimenting with gyroplanes, I had to find ways to make the gyroplane more efficient to use the smaller Rotax engines. As many of you old-boys will remember, I demonstrated some of these experiments at the air shows, like using smoke in front of the gyroplane to demonstrate how the airflow fed the propeller, and its exit points.

I also did rotorsystem experiments. I took about 2 foot of red yarn and taped it to my wheel-pants on my 532 Commander Elite and allowed it to stream behind as I observed it while fly close to the ground. Now, if there were any ground effect, you could see the yarn pulsating up and down to the beat of the rotors. No matter how slow and low I flew, the yarn didn’t deviate from the airstream whatsoever, until I landed and the airflow revirced through the rotors.

I continued to experiment further, and I took another 1.5 foot length of red yarn and tied it to the end of a thin bamboo stick, where I could reach out to inside the outer ¾ of the disk. I would take off with the bamboo sticking forward in a small tube to hold it, and then as flying at different speeds, I could remove the bamboo in my throttle hand and hold it up to the rotors. Only until I got the yarn about a foot from the rotor disk could I see any pulsation. Even while landing there was absolutely no pulsation below about a foot from the disk, until again the air reversed and then there was the downwash.

I’m sure everyone remembers my 447 Commander promotion video. You remember me hovering into the wind and flying sideways over the tall vegetation. I continued to experiment in my heavier 532 Commander Elite by flying as close to the tall grass as possible, and as slow as possible while maintaining level flight so not to point the nose into the air and hang in the prop. There was never any grass being disturbed under my disk. But, there was disturbance behind me, so there are vortices reaching the ground, but not under the disk.

Then, I would fly over the grass low, slow and at a nose high attitude. The prop-blast disturbed the grass, and the closer I got to the ground, the wider the pattern of disturbance became, but always well behind my axles, in fact about even with my tail wheel.

Those were my personal observations on the matter.
 

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Dennis, you use "except during landing" a couple of times in your post.
Do you remember any conclusions you came to during the flare and landing, that you came to after watching your yarn strings?

It sounds like your testing indicates no, or almost no benefit from ground effect in a low pass at speed.

An airplane making a low pass at speed does benefit from ground effect.
What is the difference in a rotorcraft and a fixed wing?

My guess is it has something to do with ratio of the chord of an airplane wing (in ft) to the airplane's speed (in ft/sec), as compared to a rotorcraft's ratio of the chord of a rotor blade (in ft) to the aircraft's speed (in ft/sec)??????

Economy Of Scale (of the airfoil) seems to have something to do with how much being close to the ground effects flow?

If an 8" chord rotor blade at 400 MPH takes 1/800 sec. to pass one spot on the ground,
and the 48" chord of an airplane wing at 150 MPH takes 1/55 sec. to pass the same spot, that seems to me why one could benefit from ground effect much more than the other?
 

DennisFetters

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Dennis, you use "except during landing" a couple of times in your post.
Do you remember any conclusions you came to during the flare and landing, that you came to after watching your yarn strings?

It sounds like your testing indicates no, or almost no benefit from ground effect in a low pass at speed.

An airplane making a low pass at speed does benefit from ground effect.
What is the difference in a rotorcraft and a fixed wing?

My guess is it has something to do with ratio of the chord of an airplane wing (in ft) to the airplane's speed (in ft/sec), as compared to a rotorcraft's ratio of the chord of a rotor blade (in ft) to the aircraft's speed (in ft/sec)??????

Economy Of Scale (of the airfoil) seems to have something to do with how much being close to the ground effects flow?

If an 8" chord rotor blade at 400 MPH takes 1/800 sec. to pass one spot on the ground,
and the 48" chord of an airplane wing at 150 MPH takes 1/55 sec. to pass the same spot, that seems to me why one could benefit from ground effect much more than the other?
Exactly correct. A gyroplane rotor is simply a wing, more like that on an airplane than a helicopter.

As we all know, the helicopter rotor is pushing the air through the rotor, compressing it between the ground and the disk.

A gyroplane always has the air move through the bottom of the rotor, so there is never a pumping of air between the ground and the disk, except when forward speed is stopped.

As in a helicopter, I can do a hard flair in my gyroplane a few feet off the ground to zero airspeed. The airflow will reverse and I can sit down on a cushion of air while using my blade inertia. When doing so, you could see the yarn abruptly blow down and the dust blow to the sides.

A wings effect by ground effect is directly affected by its width ratio. The larger the wing, the more it can create a cushion of ground effect. But, it has to be near enough to the ground, and under a certain speed, or it will pass over the cushion before it can affect the wing.

The wing of a gyroplane is simply too narrow and too high to create ground effect, nor does it pump airflow through it between the ground and the disk, so it can’t create the increased pressure to benefit from ground effect.
 
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bryancobb

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You are right, Dennis.

A low wing airplane with a 48" chord, in a 5' high low pass is 1.25 chords above the ground.

A 8' tall gyro with an 8" chord, in a 5' high low pass is 19.5 chords above the ground.

What i am learning is...A gyro being flared for landing or being flown nose-high, behind the power curve, experiences ground effect similar to a helicopter and it comes into play when less than 1 rotor to the ground.

A gyro flying "up on the step" experiences ground effect like an airplane, and since the rotor is above, and the chord is so small, it's almost nonexistent.
 

bryancobb

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That's good Stan...I'm getting a pretty good understanding of gyros from you guys, even though I have never sat in one.
 

WaspAir

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A wings effect by ground effect is directly affected by its width ratio. The larger the wing, the more it can create a cushion of ground effect. But, it has to be near enough to the ground, and under a certain speed, or it will pass over the cushion before it can affect the wing.
A low wing airplane with a 48" chord, in a 5' high low pass is 1.25 chords above the ground.

A 8' tall gyro with an 8" chord, in a 5' high low pass is 19.5 chords above the ground.
So you guys are telling me that an ASG-29 sailplane, with a 59 foot span and a 30.4 : 1 aspect ratio (average chord less than two feet) will not experience ground effect on a fast pass at 30 feet (1/2 span above the ground), because it's more than 15 chords above the ground and/or your "cushion" can't keep up with you?
 

WaspAir

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A gyro flying "up on the step" experiences ground effect like an airplane, and since the rotor is above, and the chord is so small, it's almost nonexistent.
There is no "step". But if you mean out of the landing flare, not behind the power curve, all you need to do to experience the "nonexistent" effect for yourself is to try to climb out from a field elevation near your ceiling.
 
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