Ground Effects on the rotor

dabkb2

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Did not want to high jack Vance's thread, so I'll start this here.
Chuck said

Today, 07:56 AM
C. Beaty
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The story is different when performing slow flight in a nose high attitude at high power.

Then, the propeller thrust line is no longer parallel to the flight path.

In level flight with a nose high attitude, the vertical component of propeller thrust unloads the rotor. It becomes the product of propeller thrust and the sin of the angle.

For example, with a noseup angle of 30º and 400 lb. of propeller thrust acting on a machine of 500 lb. AUW, the vertical component of propeller thrust is 200 lb. (sin 30º = 0.5) and the remaining rotor load drops to 300 lb. Rotor RPM drops to (300/500)^0.5 = 77%.

That’s dangerous.
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In ground effect I can fly slow, 10knt, nose high for extended periods of time with no problem. I tryied that at 500' and will not do that again. It is obvious to me that Ground Effect does effect the rotor, or have I missed something? Just trying to keep myself out of trouble.
 

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bryancobb

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Within 1/2 the rotor span and the ground, there is significant interaction between the ground and the air. Noticeably less lift is needed from the rotor, or the rotor makes noticeably more lift...(depending on whether you are a cup half full, or cup half empty person)
 

Vance

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Hello Stan,

Do you feel this ground effect for gyroplanes will reduces the rotor rpm?

Thank you, Vance
 

bryancobb

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In the same attitude and airspeed at 100'AGL, the rotor must make more lift than when in ground effect. Here's my guess.

COMPARE UNACCELERATED S&L
.................SPD........Nose........Rot AOA......Horiz Prop Thrust......RPM
100'..........20.........20d UP.......35d UP...............200#................400

IGE...........20.........18d UP.......33d UP...............190#................385
 

Vance

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Try to stay on track Brian.

Try to stay on track Brian.

In the same attitude and airspeed at 100'AGL, the rotor must make more lift than when in ground effect. Here's my guess.

COMPARE UNACCELERATED S&L
.................SPD........Nose........Rot AOA......Horiz Prop Thrust......RPM
100'..........20.........20d UP.......35d UP...............200#................400

IGE...........20.........18d UP.......33d UP...............190#................385
David’s question was about gyroplanes in ground effect not helicopters.

Cutting and pasting something about helicopters hovering in and out of ground effect to answer a question about gyroplanes is a poor substitute for knowledge and understanding.

In my opinion gyroplanes cannot hover without wind.

Thank you, Vance
 

bryancobb

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Mr. Honorable Vance,

May I humbly present YOUR question?
"Do you feel this ground effect for gyroplanes will reduces the rotor rpm?"

I'm flattered that you feel my examples of a gyro in low speed level flight to IGE level flight was pasted. It was actually impromptu.

IGE means in ground effect as David bacon's picture depicts. You asked about RPM IGE. I showed you that it decreases.
 

Vance

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I stand corrected Brian.

I stand corrected Brian.

I didn’t realize you just made it up.

Did you calculate anything?

It appears unrelated to my gyroplane experience.

I suspect David’s gyroplane has more than 200 pounds of thrust.

I was asking Stan the question because he had described his experience with ground effect in a gyroplane earlier on the telephone.

You have not shown me anything.

No source, no math and no methodology.

Please give it a rest Brian.
 

bryancobb

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I did make it up just to illustrate the origins of my guess.

David's gyro LOOKS like maybe a Benson type ? that probably weighs 250-400 lbs.
I would think 200# of thrust in slow level flight would be in the ballpark?

I'm sure your 1200# Lycoming job has a LOT more? All just guesses.

Now a personal comment. I put a lot of effort in trying to only make posts that pertain to what is being discussed. I will continue to do that. Someone on a forum I participate in, described a forum as hanging out in the forum owner's living and talking with friends. I had never thought of it that way. Since, I have behaved differently.
 

Vance

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Not in the ball park

Not in the ball park

I did make it up just to illustrate the origins of my guess.

David's gyro LOOKS like maybe a Benson type ? that probably weighs 250-400 lbs.
I would think 200# of thrust in slow level flight would be in the ballpark?

I'm sure your 1200# Lycoming job has a LOT more? All just guesses.

No; in my opinion 200 pounds of thrust would not be in the ballpark Bryan.

What Dave is describing is wide open throttle and minimum air speed.

If he can fly level at 10kts in my opinion he has a lot more than 200 pounds of thrust.

I cannot fly level at 10kts IAS in The Predator at 1,100 pounds particularly at the typical density altitude of El Mirage.

You do not appear to be making progress in your understanding of how a gyroplane flies.

Please find something else to do.
 

bryancobb

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How much thrust did one of the old Mac's produce?

Just found it.
A McCulloch 72 HP ( 68 actual) two-stroke in a gyrocopter swinging a 48" or 50" diameter prop delivers about 260 lbs of thrust at around 4000 rpm.

I would think a fairly slow nose up pass, in ground effect, could be made at 77% power. I'd say for illustration purposes, my 200# number was realistic.
 
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dabkb2

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If I could break in here, my poorlly worded question was, Chuck said that hanging on your prop was very dangerous, and I agree. At 500' I tryed it and got some very disturbing stick bumpping and will not be doing that again. I was wondering why in ground effect the rotors do not seem to slow?
 

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There is no ground effect in a gyroplanes rotorsystem as compared to that of a helicopter.

At a nose high attitude near the ground you can fly slower due to two main reasons;

1. Your thrust of the propeller is angled down and helping to provide lift.

2. Your propeller is compressing the air between the ground and the backside of the rotor-disk making that part of your rotor create more lift.
 
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dabkb2

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Thanks Dennis, is there a danger in doing this?
 

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I recall a take off near El Paso at over 4000' MSL and 113 Farenheit when my J-2 would break ground and fly close to the surface but couldn't climb away from it (I had to burn off fuel and catch what thermal lift I could to get to a decent cruise altitude). It was not in a high pitch angle / low speed situation, but zipping nearly 70 mph at the time, within a rotor diameter of the ground.

If that's not ground effect, one must wonder what it was.
 

DennisFetters

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Dennis- You just described that there is indeed ground effect in a gyro, although not near as an effect as with a helicopter going much slower and with a more level rotor .


Stan
Yes, but as I said its not like the ground effect that is created from the compression of air between the rotor and the ground like with a powered rotorsystem, as some people seem to think.
 
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bryancobb

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Thanks Dennis, is there a danger in doing this?
I wouldn't do it any higher than I'd be willing to fall. And even then, if the engine hiccups once you'll take a tumble and maybe a blade won't hit your body before it stops.
 

DennisFetters

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I recall a take off near El Paso at over 4000' MSL and 113 Farenheit when my J-2 would break ground and fly close to the surface but couldn't climb away from it (I had to burn off fuel and catch what thermal lift I could to get to a decent cruise altitude). It was not in a high pitch angle / low speed situation, but zipping nearly 70 mph at the time, within a rotor diameter of the ground.

If that's not ground effect, one must wonder what it was.
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.
 

bryancobb

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Wasp,

You are right, It's within one rotor-span, not 1/2 as I said in my incorrect post.
 

bryancobb

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Agreed

Agreed

Yes, but as I said its not like the ground effect that is created from the compression of air between the rotor and the ground like with a powered rotorsystem, as some people seem to think.
I agree Dennis. The helicopter in ground effect is experiencing almost 100% of the benefit of the slower induced flow, since the rotor disc is relatively parallel to the ground.

In a gyro, the rotor disc is tilted back significantly so that doesn't slow inflow as much, and that slower number is reduced even further by the sine of the disc AOA.

But there IS some benefit from it.
 
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