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This thread is privately moderated by mnowell129, who may elect to delete unwanted replies.
Jul 26, 2018, 12:56 AM
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tspeer's Avatar
Quote:
Originally Posted by PaulB
...An Auto Gyro does not accelerate the air downwards, the air approaching from the front flows upwards through the disk making it spin. The spinning blades create lift which then supports the weight of the aircraft....
That's not really true. An autogyro does accelerate the air downwards relative to the freestream. Yes, the air flows up through the rotor, but that's because the rotor is tilted back. That air comes out of the rotor disk with a vertical component that is moving slower than it goes in, which is equivalent to a wing deflecting the air down. if there was no net movement of the air downward in the wake, there would be no lift, either, and the autogyro wouldn't fly.

An autogyro really is a rotary winged airplane. There is a downward moving wake, and, like a fixed wing airplane, the autogyro flies in a downdraft of its own making. This lift-induced downdraft is reduced in ground effect. It's as though there was another autogyro flying upside down beneath it, so the gyro on top is buoyed by the draft from the imaginary upside down gyro underneath. The lower the autogyro flies to the ground, the closer the imaginary autogyro is underneath it and the stronger its influence.
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Jul 26, 2018, 07:26 AM
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PaulB's Avatar
Ok.....
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Jul 30, 2018, 12:58 PM
I'm not as bad as they say.
Thread OP
Not sure I buy post #691.
But there will be at least some ground effect, not sure how much.
The proximity to the ground will reduce the tip vortices on the blades provided they are within 1/2 span of the blade. So the efficiency will go up.
Also the rotor itself does create downwash behind it just like a wing does (otherwise, how does it fly? ), so will benefit from the increased pressure due to the rotors closeness to the ground and the ground's interference with the overall rotor downwash.
This is all mitigated by the fact that the rotor is usually pretty high and tough to get within 1/2 span of the ground, so I just assume the effect is pretty small compared to say a low wing fixed wing aircraft, or a sailplane where the 1/2 span is pretty big.
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Jul 30, 2018, 02:20 PM
Forever WTTM Pilot #7
Buzz's Avatar
So, what happens to the lift when you are literally hovering? Where is the lift coming from then?

I have a ciervo profile gyro and it will do loops. Where is the lift when its upside down?
Jul 30, 2018, 03:36 PM
I'm not as bad as they say.
Thread OP
Quote:
Originally Posted by Buzz
So, what happens to the lift when you are literally hovering? Where is the lift coming from then?

I have a ciervo profile gyro and it will do loops. Where is the lift when its upside down?
Hovering only with a headwind, thus not at zero airspeed, so all the factors are in play.
Question 2 I will take as a tongue in cheek question, as you can pull positive G's all around a loop. Push some down elevator at the top of your loop and try to flatten it out inverted and see what happens....
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Jul 30, 2018, 05:46 PM
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Buzz's Avatar
Quote:
Originally Posted by mnowell129
Hovering only with a headwind, thus not at zero airspeed, so all the factors are in play.
Question 2 I will take as a tongue in cheek question, as you can pull positive G's all around a loop. Push some down elevator at the top of your loop and try to flatten it out inverted and see what happens....
No thanks! Just passing through.
Dec 27, 2018, 04:26 AM
Registered User

Hinges


I try to understand what is explained in this thread. Yet, starting at the beginning I already got lost with image #3 in post #5 https://www.rcgroups.com/forums/show...58&postcount=5

I can understand your arguments, if the hub of the rotor blades were rotating around the blue, vertical axis. But if it rotates around the red axis, the centrifugal forces should bring the rotor blades into position NH-NH? Why should the 16 LB force pull horizontally and not perpendicular to the tilted red axis?
Dec 27, 2018, 01:28 PM
I'm not as bad as they say.
Thread OP
Quote:
Originally Posted by Ralf W.
I try to understand what is explained in this thread. Yet, starting at the beginning I already got lost with image #3 in post #5 https://www.rcgroups.com/forums/show...58&postcount=5

I can understand your arguments, if the hub of the rotor blades were rotating around the blue, vertical axis. But if it rotates around the red axis, the centrifugal forces should bring the rotor blades into position NH-NH? Why should the 16 LB force pull horizontally and not perpendicular to the tilted red axis?
Because tilting the axis does not instantaneously tilt the rotor disk; this takes time. During the time before the rotor tilts due to cyclic the centrifugal forces act in the rotor plane. Eventually the rotor will tilt perpendicular to the red axis and the forces will be as you suggest, but not instantaneously.
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Dec 27, 2018, 02:36 PM
Registered User
Thank you, Mickey!
Now I understand. Your explanations are awesome.
Jul 18, 2019, 03:45 AM
Fly low and slow
TrueBuld's Avatar

Rotor blade aerofoil shape (airfoil US )


Hand made rotor blades seem easier to make if leading edge is made sharp and then just lightly sanded round.
However a true Clark Y aerofoil has the centre of the rounded leading edge nearly a third of the depth up the leading edge, making production more difficult.

What is the approximate percentage loss in efficiency of a sharp flat bottom rotor blade?
Jul 18, 2019, 11:43 AM
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tspeer's Avatar
Quote:
Originally Posted by TrueBuld
Hand made rotor blades seem easier to make if leading edge is made sharp and then just lightly sanded round.
However a true Clark Y aerofoil has the centre of the rounded leading edge nearly a third of the depth up the leading edge, making production more difficult.

What is the approximate percentage loss in efficiency of a sharp flat bottom rotor blade?
A sharp leading edge produces laminar separation for angles of attack outside a narrow range. The separated laminar flow rapidly transitions to turbulent flow. Whether the turbulent boundary layer reattaches to the blade contour or not depends on how hard the flow would have to decelerate to follow the blade shape. You can probably estimate the effect using Xfoil. The gotcha is things might work just fine most of the time, but then suddenly go south without warning if the flow doesn't reattach.

There's also a drag penalty with the laminar separation bubble, and the autorotation of the rotor depends on the lift/drag ratio. I had a co-worker who was killed when the full-scale autogyro he was flying lost rotor rpm in the landing pattern. There was speculation that he'd been experimenting with blade pitch settings and had pushed things to the wrong side of the maximum lift/drag point.
Jul 28, 2019, 06:54 AM
Fly low and slow
TrueBuld's Avatar
Effect of airfoil camber on small autogyro rotors (3 min 49 sec)
Last edited by TrueBuld; Jul 28, 2019 at 08:11 AM.


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