Rotary Wing Forum Basic Autogyro Theory 101
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#1
02-14-2012, 09:30 PM
 kolibri282 Platinum Member Join Date: Aug 2009 Location: Duesseldorf Posts: 1,701
Basic Autogyro Theory 101

Dear all,

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Cheers,

Juergen

..Il semble que la perfection soit atteinte..
....non quand il n'y a plus rien à ajouter,...
...mais quand il n'y a plus rien à retrancher...
- Antoine de Saint-Exupéry -

Last edited by kolibri282; 08-30-2012 at 02:44 PM.
#2
05-14-2012, 12:52 PM
 kolibri282 Platinum Member Join Date: Aug 2009 Location: Duesseldorf Posts: 1,701

Below to the right is the best diagram explaining auto rotation I have seen so far, from here:
https://faasafety.gov/gslac/ALC/cour...ID=104&sID=452

The picture to the left from here:
http://aerofoilengineering.com/
makes it easy to understand why the resultant force on the profile is inclined forward with respect to the plane of rotation of the rotor. If you sum up all the blue arrows on the upper surface (and deduce the brown ones from below) you are left with a net force inclined forward (to the left) with respect to a horizontal line ( the plane of rotation of the rotor ). The angle of attack shown here is 5.5°. ( This by the way is exactly the same physics that allows a sailing boat to sail upwind )
Attached Thumbnails

Last edited by kolibri282; 05-18-2012 at 10:01 PM.
#3
05-18-2012, 09:26 AM
 okikuma Member Join Date: May 2006 Location: Santa Clarita, CA Posts: 1,405

Hi Jeurgen,

One of the best explanation and presentations I have seen. Thank you for sharing.

It is also the same theory how the Horten Brothers flying wings created "proverse yaw" thus the elimination of vertical stabilizers.

Wayne

http://yarchive.net/mil/flying_wing.html

TEDxNASA@SiliconValley - Al Bowers - Toward More Bird-Like Flight: Thinking Outside the Box - YouTube

Last edited by okikuma; 05-18-2012 at 09:42 AM.
#4
05-18-2012, 03:13 PM
 kolibri282 Platinum Member Join Date: Aug 2009 Location: Duesseldorf Posts: 1,701

Wayne,

thanks for this very interesting link. I've so far only thought of the famous Horton "bell shaped" lift distribution to cope with pitch stability in flying wings. Quite amazing to learn they dealt with adverse yaw at the same time!
#5
05-24-2012, 05:44 PM
 RotoPlane Gold Supporter Join Date: Jan 2008 Location: Ohio USA Posts: 3,097
Height Velocity Curve for Gyroplanes

Very informative report on the Power and H/V curve for gyroplanes.

- Title: Height Velocity Curve for Gyroplanes
- Author: Greg Gremminger

Includes illustrations, information on "Behind the Power Curve" and shows the H/V Curve.
#6
05-25-2012, 03:10 AM
 All_In Platinum Member Join Date: Apr 2008 Location: San Diego, CA. USA Posts: 9,677

Hi guys.

I've read Greg's but thanks for posting all of them!!!!
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#7
08-29-2012, 09:16 PM
 kolibri282 Platinum Member Join Date: Aug 2009 Location: Duesseldorf Posts: 1,701

A great article by Chuck Beaty on blade motion can be found here:
#8
08-29-2012, 10:14 PM
 SGK Member Join Date: Sep 2009 Location: Malmoe, Sweden Posts: 61

A question, once again, based on this very basic diagram:
What‘s the physics behind the statements found here and there, that negative G lowers the rrpm considerably and even stops (besides chopping the tail) and that holding the stick into the wind after landing helps you to lower your rrpm faster?
Just trying to find a good, easy to understand, pedagogical formulation of explanation.

Last edited by SGK; 08-29-2012 at 10:53 PM.
#9
08-30-2012, 12:20 AM
 Vance vance breese Join Date: Oct 2003 Location: Nipomo,California Posts: 9,785
Just the basics.

Quote:
 Originally Posted by SGK A question, once again, based on this very basic diagram: What‘s the physics behind the statements found here and there, that negative G lowers the rrpm considerably and even stops (besides chopping the tail) and that holding the stick into the wind after landing helps you to lower your rrpm faster? Just trying to find a good, easy to understand, pedagogical formulation of explanation.
Hello RT, it would help me to understand you question if you filled out you profile so I had a better idea of your experience and at least knew your name.

I don’t know what diagram you are referring to.

In my opinion with an autogiro the air going up through the rotor blades is what drives them; less air drives them less.

In my opinion as the blades move through the air there is aerodynamic drag that tries to slow the blades down.

When I fly with a passenger at a higher gross weight the rotor apears to turn faster.

When I fly at a higher density altitude the blades apear to turn faster.

My rotor tachometer confirms this impression.

I find that a low G maneuver makes for less air going up through the rotor and the drag slows the rotor quickly.

When I land and the rotor is not supporting the weight of the gyroplane it slows quickly.

I feel at that point the rotor sees zero Gs.

I don’t understand your second question.

I suspect it is the same answer. Less air going up through the rotor and the rotor slows.

Thank you, Vance
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Vance Breese

Last edited by Vance; 08-30-2012 at 12:28 AM.
#10
08-30-2012, 01:30 AM
 SGK Member Join Date: Sep 2009 Location: Malmoe, Sweden Posts: 61

Hi Vance,
Thank you for trying to help me. My name is Roman, i‘m 48, live and fly in Sweden. I‘m still learning.
My questions are general and any try to answer them on any level would help me to try to find right way to explain my thoughts to my students.
I reffered to the picture of wing profile showing generated forces published in this thread.
In my world, there are no forces that could force rotor to stop when relative wind comes from above. The profile is not so effective when so, but significant braking force is something I can‘t see. Also, the fact that most rotors have blades with positive pitched profiles makes me believe that rotor can autorotate in same direction even when it "blows from above".
Both my questios are related to the same phenomena.
What I‘m trying to do is to modernize our theory material and erase the statements based on myths.
Regards,
Roman
#11
08-30-2012, 01:49 AM
 Vance vance breese Join Date: Oct 2003 Location: Nipomo,California Posts: 9,785

Hello Roman,

What I am describing is about a low G maneuver that causes less air to come up through the rotor driving it and my observations as to the response of the rotor RPM.

Air flowing in the opposite direction is another thing entirely.

I am not an educated man so I am not be able to help you with theory and correct descriptive terms.

I am just offering my opinions and observations and they are often flawed.

Hopefully one of our more technically oriented forum members will help you.

Thank you, Vance
__________________
Vance Breese
#12
08-30-2012, 03:11 AM
 kolibri282 Platinum Member Join Date: Aug 2009 Location: Duesseldorf Posts: 1,701

Quote:
 Also, the fact that most rotors have blades with positive pitched profiles makes me believe that rotor can autorotate in same direction even when it "blows from above".
It would probably autorotate alright but the rotor force would then be pointing downwards so the hole you leave in the ground on impact would be much bigger due to the added acceleration from the rotor force...;-)
The serious answer though is that your gyro would become highly unstable with the rotorforce pointing the wrong way round so you would tumble end over end in no time after flow direction has been reversed.

Last edited by kolibri282; 08-30-2012 at 03:47 AM.
#13
08-30-2012, 03:26 AM
 SGK Member Join Date: Sep 2009 Location: Malmoe, Sweden Posts: 61

Hi Kolibri 282
Hehe, yes, The hole would be deeper.
I never said that lift would point upwards in negative G. I meant that there are no forces forcing the rotor to stop when negative loaded (besides usual drag).
Still, you can find statements here and there that negative G stops the rotor, as well as recommendations to hold the stick into the wind after landing in order to stop it faster.
The only test I have done was comapring times needed to stop the rotor. Horizontal in calm conditions vs. with stick induced flapping. The difference is considerable. Meaning of doing it was to get a feeling about how much increased flapping causes rotor to decrease in speed, the idea that apparently not all forum members share.

Roman

Last edited by SGK; 08-30-2012 at 07:12 AM.
#14
08-30-2012, 03:45 AM
 XXavier Member Join Date: Nov 2006 Location: Madrid, Spain Posts: 262

Quote:
 Originally Posted by SGK A question, once again, based on this very basic diagram: What‘s the physics behind the statements found here and there, that negative G lowers the rrpm considerably and even stops (besides chopping the tail) and that holding the stick into the wind after landing helps you to lower your rrpm faster? Just trying to find a good, easy to understand, pedagogical formulation of explanation.
IMHO, only drag is responsible for the deceleration of the rotor in both situations. Under negative Gs, and not taking into account very real problems of blades losing their 'rigidity', striking the fin and the prop; besides that, and for obvious reasons, you're not going to have enough time to build up enough airspeed 'in reverse' to aid the rotor to pull you to the hard ground...

And when you hold the stick into the wind, after landing, the disk is usually parallel to the flow of the wind, and thus only drag is at work...
#15
08-30-2012, 05:18 AM
 bryancobb Platinum Member Join Date: Aug 2010 Location: Cartersville, GA Posts: 3,053

Hey Fellas,

My 2 cents...

Any positioning of the blades, by pilot input, aircraft maneuvering, rigging, or any other way, THAT CREATES A TOTAL AERODYNAMIC FORCE VECTOR,TILTED REARWARD FROM THE ROTATIONAL AXIS WILL SLOW THE RPM.

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