Ground Effects on the rotor

birdy

Newbie
Joined
Mar 19, 2004
Messages
7,052
Location
Alice Springs-central Oz.
Aircraft
open frame single seat & a 'wasa' RAF, among other types.
Total Flight Time
7000 odd, bout 5000 gyro
air pressure of a wing only effects the air 2.25 time the cord width downward

So you say.
Dennis, the gyro hasa 26' rotor DISC.

2.25x26'=???
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
air pressure of a wing only effects the air 2.25 time the cord width downward

So you say.
Dennis, the gyro hasa 26' rotor DISC.

2.25x26'=???
No David, I said the cord of the rotor, or actually its width, not span.

With a helicopter, you have ground effect due to the column of air being displaced through the top of the disk to the bottom that will change the tip vortices making for a more efficient hover with less power needed. This will start to occur approximately ½ the rotor diameter.

A rotorcraft in autorotation will not produce that same column of air as a helicopter in hover, and there is simply not enough airflow downward to effect the tip vortices, so no efficiency is gained when close to the ground.

Therefore, we should not measure using the rotor disk for a rotorcraft in autorotation, but we should measure the same as a fixed wing, by wing width.

Let me explain in this manner; lets imagine we take a rotorblade and mount it on the side of a car making it adjustable up and down and flat to the ground, and then go drive the rig down the road at an appropriate speed (pretending the blade will not flap all over hell). Now begin to lower the blade closer to the ground. At about 2.25 times the width of the blade or about 18” for an 8” blade (actually it would be greater number because the blade is subjected to the same airspeed this way, while spinning it is not), you will start to measure a difference in lift. This will vary by speed and angle of attack.

That is why there is no downwash from a rotorcraft in autorotation that will help in ground effect. The disk of the rotors in autorotation has far too little solidity ratio and mounted too high to make a difference in pressure between the ground and disk for any added lift, also the width of the rotor is too small and mounted too high to gain performance from increased pressure between the ground as a fixed-wing would, and the tip vortices are too high from the ground to be influenced to flow out or decrease in size to provide extra performance.
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
Dennis, You confuse "Down wash" with "crossing from above to below the disk.
Watch this:

Jean, no I'm not confused, and you and Chuck are both correct in your statements and diagrams, "at same AUW, rotor diameter and airspeed, downwash of gyro will be the same".

But, you have changed the subject from what we have been discussing in this thread.

As discussed, will a gyroplane fly in ground effect? No.

In your diagram, the helicopter has crossed through translational lift, so it too will not experience ground effect until it transitions back to hover within 1/2 the rotor diameter height.
 

WaspAir

Supreme Allied Gyro CFI
Joined
Oct 21, 2006
Messages
4,661
Location
Colorado front range
Aircraft
Bell 47G-3B-1 / A&S 18A / Phoebus C, etc.
Total Flight Time
stopped caring at 1000
No David, I said the cord of the rotor, or actually its width, not span.

With a helicopter, you have ground effect due to the column of air being displaced through the top of the disk to the bottom that will change the tip vortices making for a more efficient hover with less power needed. This will start to occur approximately ½ the rotor diameter.

A rotorcraft in autorotation will not produce that same column of air as a helicopter in hover, and there is simply not enough airflow downward to effect the tip vortices, so no efficiency is gained when close to the ground.

Therefore, we should not measure using the rotor disk for a rotorcraft in autorotation, but we should measure the same as a fixed wing, by wing width.

Let me explain in this manner; lets imagine we take a rotorblade and mount it on the side of a car making it adjustable up and down and flat to the ground, and then go drive the rig down the road at an appropriate speed (pretending the blade will not flap all over hell). Now begin to lower the blade closer to the ground. At about 2.25 times the width of the blade or about 18” for an 8” blade (actually it would be greater number because the blade is subjected to the same airspeed this way, while spinning it is not), you will start to measure a difference in lift. This will vary by speed and angle of attack.

That is why there is no downwash from a rotorcraft in autorotation that will help in ground effect. The disk of the rotors in autorotation has far too little solidity ratio and mounted too high to make a difference in pressure between the ground and disk for any added lift, also the width of the rotor is too small and mounted too high to gain performance from increased pressure between the ground as a fixed-wing would, and the tip vortices are too high from the ground to be influenced to flow out or decrease in size to provide extra performance.
If I attach a wing from my glider to that car (instead of a rotor blade), it will have a chord of only about three feet at the root, tapering down to less than one foot at the tip, and a span (from car to tip, for just one of the two wings) of 28 feet (17 meters tip to tip). That's still not much of a "solidity ratio, considering a 56 foot diameter "disc". Your analysis suggests that I would have to get that wing within about 7 feet of the ground to get a performance change. There is no difference in practice or principle between mounting it on a car, and mounting it on the glider itself, and I have plenty of data on that. Every aviation reference book I have ever seen suggests that I should start to see the effect by at least half span (28 feet), not 2.25 times chord (2 to 7 feet), and my own personal piloting experience in that aircraft agrees with the books, not you. Ground effect is huge at 20 feet.
Jean, no I'm not confused, and you and Chuck are both correct in your statements and diagrams, "at same AUW, rotor diameter and airspeed, downwash of gyro will be the same".

But, you have changed the subject from what we have been discussing in this thread.

As discussed, will a gyroplane fly in ground effect? No.

In your diagram, the helicopter has crossed through translational lift, so it too will not experience ground effect until it transitions back to hover within 1/2 the rotor diameter height.
Sailplanes have extremely high aspect ratios (low "solidity"), and get ONLY translation lift, but still get ground effect in spades.

What is the source of your two claims that ground effect depends upon chord, not span, and that translational lift destroys ground effect? I'm beginning to think you're making this stuff up.

Your declaration that gyros don't experience ground effect seems supported only by your inability to see disturbed ground cover (which one doesn't see in fixed wings, either, where ground effect is unquestioned). Along the way, you have also offered no explanation for the ground effect I encountered in my J-2, when I could break ground, fly low, fast (70), and level, but not climb out when attempting a take-off at a density altitude corresponding to the aircraft's ceiling (4000+ foot elevation and temperature of 113 degrees F). If not seeing blowing dust is adequate proof for you, why shouldn't inability to climb out be adequate proof for me?
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
--Cut-- Every aviation reference book I have ever seen suggests that I should start to see the effect by at least half span (28 feet), not 2.25 times chord (2 to 7 feet), and my own personal piloting experience in that aircraft agrees with the books, not you. Ground effect is huge at 20 feet.
ONLY

Your declaration that gyros don't experience ground effect seems supported only by your inability to see disturbed ground cover (which one doesn't see in fixed wings, either, where ground effect is unquestioned). Along the way, you have also offered no explanation for the ground effect I encountered in my J-2, when I could break ground, fly low, fast (70), and level, but not climb out when attempting a take-off at a density altitude corresponding to the aircraft's ceiling (4000+ foot elevation and temperature of 113 degrees F). If not seeing blowing dust is adequate proof for you, why shouldn't inability to climb out be adequate proof for me?
I agree completely with you about ground effect on a fixed wing and glider. I was only trying give an idea for the results on an experiment to explain the width of an airfoil verses ground effect "for a rotor", and I did defiantly say "(actually it would be greater number because the blade is subjected to the same airspeed this way, while spinning it is not)". However, since the airspeed is not the same over the entire rotorblades surface, nether is the lift produced on any particular section as it would on a fixed-wing aircraft.

And yes, I have previously answered and provided explanations as to why a gyroplane would not be able to climb out. You just need to go back and read them.
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
I had a few minutes free today, so let’s do an experiment to see what our gyroplane rotorblades are doing in forward flight;

If a gyroplane is flying at only 40 Miles/Hour, then that is equal to 0.6666667 Minutes/Hour, which is also equal to a straight line distance of 3,520 Feet flown over 1 minutes time.

Now, let’s designate one blade of our test vehicle “A blade”, and the other blade “B blade”. Let’s also designate a clock angle on our gyroplane saying the nose is 12:00, the right side is 3:00, and the left side is 9:00. Ok, Ok J.D., the ass-end is 6:00 so we won’t get confused ;-).

The average gyroplanes rotor RPM is about 350, so we will use 350 in our calculations.

After stable flight is achieved at only 40 Miles/Hour airspeed (in zero wind conditions) we begin our experiment when the “A” blade reaches the 3:00 position on our disk. Now, that means in a straight line distance when we fly 3,520 Feet from the starting point at 40 Miles/Hour with a 350 rotor RPM, our “A” blade will reach the 3:00 position every 10 Feet, and do so 350 times in that distance!

Yes, at such a low airspeed the “A” blade will not reach the same clock angle until 10 Feet later. This also means that the “B” blade will not reach that same 3:00 clock angle until 5 Feet later following the “A” blade.

How is this significant?

When a gyroplane is in forward flight, the rotor disk is tilted back at about 10 degrees angle of attack. This means as it flies forward, the rotors are spiraling through undisturbed air at all times, so long as it is in autorotation. Due to this fact that the combined forward speed, disk angle of attack and the spiraling flight paths, keeps the rotorblades far enough apart from each other that the disturbances of the proceeding rotorblade pressure wave nor tip vortices will NOT affect the following rotorblade whatsoever. Each blade has simply flown in front of the disturbances created by the previous revolutions of the system.

Therefore, due to these facts, a gyroplane will not benefit from ground effect, because it will not generate a downwash that would influence the pressure between the rotorsystem and the ground, nor effect the tip vortices causing less drag and more lifting area on the blade tips..
 
Last edited:

ckurz7000

Senior Member
Joined
Nov 7, 2006
Messages
3,438
Location
Vienna
Aircraft
ArrowCopter
Dennis, can you explain to me in short words what, to you, is the cause of geound effect? Because whether or not a wing flies through clean air has nothing to do with ground efect. It is like saying that the pilot's legs are about 4 feet long, and since the pilot flies the plane, it will experience ground effect up to a height of 4 feet.

-- Chris.
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
Dennis, can you explain to me in short words what, to you, is the cause of geound effect? Because whether or not a wing flies through clean air has nothing to do with ground efect. It is like saying that the pilot's legs are about 4 feet long, and since the pilot flies the plane, it will experience ground effect up to a height of 4 feet.

-- Chris.
In what type of aircraft, Chris? There are many clips on YouTube that would be much easier for you to watch and understand better than me typing it.
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000

ckurz7000

Senior Member
Joined
Nov 7, 2006
Messages
3,438
Location
Vienna
Aircraft
ArrowCopter
The definition of ground effect does not depend on the type of aircraft. Just tell me what, in your opinion, causes it. Videos don't do that. If you really do understand a physical phenomenon you ought to be able to explain its cause. So can you or can you not?

-- Chris.
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
The definition of ground effect does not depend on the type of aircraft. Just tell me what, in your opinion, causes it. Videos don't do that. If you really do understand a physical phenomenon you ought to be able to explain its cause. So can you or can you not?

-- Chris.
Chris, I'm not going to take an hour typing an explanation that you will pick apart no matter how correct I am. I have a job to do and I'm not much for games. The video below shows you my understanding of fixed-wing ground effect;

www.youtube.com/watch?v=v5jU1mADgRo

For helicopter ground effect, I agree with the following print, or go back and read what I already explained in previous posts.;

http://www.copters.com/aero/ground_effect.html
 

JAL

Member
Joined
Apr 2, 2009
Messages
745
Location
Perth
Aircraft
MT03sport/C172
Total Flight Time
250
The theory I read is that if the wing is flying close enough to the ground then the wing tip vortices will be disrupted by the ground which results in reducing drag. The rule of thumb is that this starts to occur at height that is half the wingspan or less.

So it would seem that low wing aircraft would get the most benefit but I can see that a gyrocopter may get some benefit as well. A gyro with a 8m diameter rotor would be in ground effect with the rotor around 4m height from the ground, as most gyros are say around 2m in height the gyro would probably need to be at or below 2m height to be in ground effect.

I assume that the closer the wing can get to the ground the greater the effect.

One thing though I am not sure about is if the disk is tilted back then the vortices of the 12 o'clock blade is higher off the ground than the 6:00 o'clock blade so the reduction in drag across the rotor disk may not be equal? Would this induce a roll or negate any benefits of ground effect?
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
The theory I read is that if the wing is flying close enough to the ground then the wing tip vortices will be disrupted by the ground which results in reducing drag. The rule of thumb is that this starts to occur at height that is half the wingspan or less.

So it would seem that low wing aircraft would get the most benefit but I can see that a gyrocopter may get some benefit as well. A gyro with a 8m diameter rotor would be in ground effect with the rotor around 4m height from the ground, as most gyros are say around 2m in height the gyro would probably need to be at or below 2m height to be in ground effect.

I assume that the closer the wing can get to the ground the greater the effect.

One thing though I am not sure about is if the disk is tilted back then the vortices of the 12 o'clock blade is higher off the ground than the 6:00 o'clock blade so the reduction in drag across the rotor disk may not be equal? Would this induce a roll or negate any benefits of ground effect?
But Jorden, you cannot calculate the distance from the ground as 1/2 the diameter of the disk, because a gyroplane will not produce the downward column of air like that of a helicopter where that measurement applies.
 

ckurz7000

Senior Member
Joined
Nov 7, 2006
Messages
3,438
Location
Vienna
Aircraft
ArrowCopter
Dennis, should I surmise, then, that your understanding of this matter derives from youtube videos? It would have been so simple: Ground effect is the modification of the flow field around a lifting surface by the proximity of the ground.

Nowhere in that definition does it say that two lifting surfaces influence each other or that a rotor blade seeing only "clean air" does not produce ground effect as you implied in a previous post.

-- Chris.
 

DennisFetters

Gold Supporter
Joined
Jul 18, 2005
Messages
3,182
Location
Changsha, China
Aircraft
582 Commander Elite
Total Flight Time
Stopped counting years ago after 5,000
Dennis, should I surmise, then, that your understanding of this matter derives from youtube videos? It would have been so simple: Ground effect is the modification of the flow field around a lifting surface by the proximity of the ground.

Nowhere in that definition does it say that two lifting surfaces influence each other or that a rotor blade seeing only "clean air" does not produce ground effect as you implied in a previous post.

-- Chris.
Well, you ought to make a YouTube video and explain it better!! I'm going home now. Enjoy your day.
 

ckurz7000

Senior Member
Joined
Nov 7, 2006
Messages
3,438
Location
Vienna
Aircraft
ArrowCopter
It is not an explanation, it is a DEFINITION. And people working in the field understand it perfectly well. Do you prefer to learn physics by watching youtube? That would explain a thing or two.

-- Chris.
 

birdy

Newbie
Joined
Mar 19, 2004
Messages
7,052
Location
Alice Springs-central Oz.
Aircraft
open frame single seat & a 'wasa' RAF, among other types.
Total Flight Time
7000 odd, bout 5000 gyro
No David, I said the cord of the rotor, or actually its width, not span.
I know.
Thats why i asked.
GE depth is understood by 99.9% of aviaters as be,n approx 1/2 SPAN.
Not chord.
If you could accept that the 99.9% just mite be rite, then your understandn of why you dont understand would be much clearer.
A FW, from a GE perspective wouldnt know if its 1' or 100' behind anatha wing.
 

thomasant

Member
Joined
Mar 20, 2011
Messages
1,010
Location
Katy, Texas
Aircraft
AR1, Aviomania Genesis Sport
Total Flight Time
3400+
I heard this a long time ago:

I heard this a long time ago:

“He who knows not and knows not he knows not: he is a fool - shun him. He who knows not and knows he knows not: he is simple - teach him. He who knows and knows not he knows: he is asleep - wake him. He who knows and knows he knows: he is wise - follow him.”
 

birdy

Newbie
Joined
Mar 19, 2004
Messages
7,052
Location
Alice Springs-central Oz.
Aircraft
open frame single seat & a 'wasa' RAF, among other types.
Total Flight Time
7000 odd, bout 5000 gyro
You dont know wot you dont know.
A knowall dont know wot he dont know, but it dont matter to him, coz he knows it all. ;)
 

All_In

Gold Supporter
Joined
Apr 21, 2008
Messages
14,794
Location
San Diego, CA. USA
Aircraft
Piper Archer, Aviomania G1sb
Total Flight Time
Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
But Jorden, you cannot calculate the distance from the ground as 1/2 the diameter of the disk, because a gyroplane will not produce the downward column of air like that of a helicopter where that measurement applies.
Good morning Dennis!

It is all about improved laminar flow of the air over the surface of the wing as the column of air is compressed by the ground it straightens out some of the vortices on all wings.
The closer to the ground the wing gets the more compression of the column of air and the more vortice affected areas are removed as they are actually straighten out and the more lift the same blade or wing will create.
The compression is a little like dropping a sheet of plywood on a flat concrete floor. It does not go Bang but swish as the air is compressed between the wing in this case a sheet of plywood and the floor. The closer to the floor the sheet gets the more compression.

The blades are only a spinning WING.

When ANY wing gets close to the ground the vortices (disturbed air) are straightened out increasing the laminar flow = LESS DRAG on all wings and it starts within 1/2 the wing span to the surface as the column of air under the spinning wing is now being compressed between the wing and the ground.

This means the laminar flow over all wings is improved and the actual lifting surface area is increased on all wings because there are less vortices/drag!

It has NOTHING to do with DOWN-WASH!!!!

It occurs because all wings have areas where vortices/DRAG exists until many of them are straighten out by the proximity of the ground!!!!

For us fling winger I suspect it is generating lift for a longer distance in towards the rotor head too where it could not at altitude.

It is exactly like putting a longer wider wing (or rotor blade) on any aircraft when it is in ground effect! And the closer to the floor you get the wider and longer your wings GROW.


They may not look like it but our blades are ONLY SPINNING WINGS! Forget the back-wash completely if you ever wish to know the truth and understand ground effect.

It's acts like a different wider and longer blade/wing when in ground effect because it has less drag, fewer vortices, (less disturbed air) and more of the blades surface area is now generating lift than the wing could with the vortices present.
 
Last edited:
Top