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Apr 19, 2007, 05:00 PM
I'm not as bad as they say.
Joined Nov 2004
4,978 Posts
Let me see if I can make sense of the depron blades.
The basic diagram is shown.
The rotor makes lift because it is tilted back. The airflow from moving is combined with the airflow from turning. The result is that the airflow that the airflow "sees" is at an angle below 90 degrees from the shaft. The airfoil makes thrust 90 degrees (plus the L/D of the airfoil) from the wind which is shown. As long as the thrust is tilted forward of the shaft, the rotor makes lift and turns. A flat plate is just a lousy airfoil and works the same way. As long as the airflow comes from below the airfoil it will make lift. The rigging angle of the blade may look negative compared to the shaft but because of the tilt back and forward motion it is in fact at a positive angle of attack.
The reason that the flat plates have to be at really negative angles compared to airfoils has to do with L/D of the airfoil. With a good L/D the angle between the airflow and the airfoil thrust might be 95° so if the angle the air is coming in at is -10° the airfoil will have +5° real angle of attack if set to 0 with respect to the shaft. And the rotor thrust will be 5° ahead of the shaft so the rotor turns.
A flat plate has a lousy L/D so the angle between airflow and airfoil thrust might be 110°. So to get this airfoil in a positive +5° angle of attack AND have the airfoil thrust tilted forward with respect to the shaft the airflow has to be 115° from the shaft and the blades have to be set at -20° with respect to the shaft. Clearly to get the incoming airflow this low the whole rotor has to fly at a much higher angle (thus making more drag and hmmm... since they make more drag, they fly more slowly, (but not more efficiently!)).
The flat plates work on small models because at that size ALL airfoils have a lousy L/D, so it doesn't improve the performance of small models much to airfoil the blades and may create other problems since an airfoil has a twisting moment. The twisting moment may make the blade twist to a non-optimum angle and perform worse than the symmetrical, non twisting moment, flat plate.

This also explains how a full sized gyro can run a shallower mast angle and run the blades at higher angle of attack (although you can see now that the static angle the airfoil has with respect to the shaft is totally meaningless in terms of the actual angle of attack the blade "sees"). It has to do with the fact that a 12" airfoil has a much better L/D than a 1.5" airfoil. This is all the result of reynolds number effects. It's why models have to run negative pitch and greater mast angles. It's just the physics of air.
mickey

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 Apr 19, 2007, 10:52 PM FPV really is fun. Sedona, Az Joined Sep 2001 1,164 Posts Thank you very much. Makes sense now. It helps to realize that the smaller the blade gets, the less a true airfoil helps. Have a great weekend. Scott
 Apr 29, 2007, 03:57 AM Registered User Joined Jan 2007 5 Posts Hi Mickey, Excellent and interesting stuff. I have a question which I hope you can answer.... Which way should the blades rotate and why. Is there a convention on this i.e. clockwise is conventional when viewed from the overhead position. Thanks, John
 Apr 29, 2007, 03:11 PM Registered User Joined Nov 2006 111 Posts Perhaps it depends on whether you live in the Northern or Southern hemisphere! Here in the Southern our water goes down the plughole the opposite way to yours in the Northern. Seriously though it makes no difference. If you are right handed and want to hand launch it is convenient to have anticlockwise rotating blades - then you can pick up the model with your right hand and give the blades a spin with your left, It is easier to spin them anticlockwise. Colin
 Apr 29, 2007, 03:43 PM FPV really is fun. Sedona, Az Joined Sep 2001 1,164 Posts The leading edge should move forward. Its up to you to decide whether that leads to a clockwise, or counterclockwise spin.....
Apr 29, 2007, 05:21 PM
I'm not as bad as they say.
Joined Nov 2004
4,978 Posts
Quote:
 Originally Posted by Dangad Hi Mickey, Excellent and interesting stuff. I have a question which I hope you can answer.... Which way should the blades rotate and why. Is there a convention on this i.e. clockwise is conventional when viewed from the overhead position. Thanks, John
I think you want to balance motor torque with any rotor reaction. The rigid head doesn't seem to care. Mine turn CW as viewed from above. The flapping heads have more coning and tend to have some coning induced roll, hence a CW turner tends to roll left adding to torque. Seems that the flapping head would benefit from CCW rotation letting the coning induced roll counter motor torque. The advancing blade appearing to make more lift doesn't come into to play because this cause the rotor to pitch up, not roll over.
Perhaps someone with some additional flapping head experience will comment.
 May 07, 2007, 11:13 PM Registered User Kansas city MO Joined Jan 2004 40 Posts how do you determine the hinge point on a rotor blade. I'm using a flapping hinge head and aerobalsa blades?
 May 08, 2007, 06:43 AM I'm not as bad as they say. Joined Nov 2004 4,978 Posts The hinge point or the bolt hole position?
 May 08, 2007, 08:16 AM Registered User Kansas city MO Joined Jan 2004 40 Posts well I was thinking of the bolt hole but now I would like to know about the hinge point as well please.
 May 08, 2007, 08:33 AM I'm not as bad as they say. Joined Nov 2004 4,978 Posts The bolt hole needs to be around 25 to 30% of the chord. As to the hinge line, the closer to the shaft it is the lower the servo loads but also the the less control power. When the hinge line is farther out the servo loads go up but also the control power. You just have to experiment, generally.
 May 21, 2007, 06:02 PM Registered User Joined Jan 2005 1,286 Posts Mickey, a thought about dual rotor gyros like the Twirl or a coax. In those configurations, the coning effects of both rotors will obviously cancel each other out. But what about asymmetric lift? The effects of asymmetric lift from both rotors should be the sum of the effects of each single rotor, shouldn't it? If this is right, it seems to me that the use of flapping hinges is still indicated with dual rotor configurations. What do you think? Jochen
 May 21, 2007, 10:30 PM I'm not as bad as they say. Joined Nov 2004 4,978 Posts You'd think so wouldn't you, yet the duals seem to fly just fine without, even the ones with fairly rigid rotors. I think we have to keep in mind that with the rigid rotor the phase angle is < 90° so the duals, since they counter rotate, try to roll in on themselves, and this cancels in the lateral direction. As for the nose up part, these models all seem to have substantial elevators/stabs and can compensate. I supposed you might have to resort to flapping hinges if you ever wanted to lose or reduce the stab/elevator. I think this will apply to coax's as well as duals. All this applies to small models where the cyclic stresses on the hubs doesn't destroy them. I think if you made them bigger, you might see hub failures. Good practice says you should use flapping hinges of some variety (maybe elastomeric, or depend on blade flex) but the duals seem to get away with the brute force, keep it simple idea. Interesting question, I'll have to consider it some more. mickey
 Jun 01, 2007, 09:59 AM Winging it Ò> Joined May 2006 10,421 Posts Flybar phase angle Some rotor systems have flybars at 90 deg, others have a much different angle. My Blade CX has the flybar 45 deg ahead of the rotor blade. What are the issues and trade-offs with flybar phase angle?
Jun 01, 2007, 04:25 PM
Winging it Ò>
Joined May 2006
10,421 Posts
Flybar phase angle

Quote:
 Originally Posted by leadfeather Some rotor systems have flybars at 90 deg, others have a much different angle. My Blade CX has the flybar 45 deg ahead of the rotor blade. What are the issues and trade-offs with flybar phase angle?
I did some google searching and found this thread.

http://www.rcgroups.com/forums/showthread.php?t=491434

In this case, the author experimentally determined that 45 deg was best for his helicopter. I'm wondering if powered vs unpowered rotor has any bearing?
Jun 15, 2007, 11:39 AM
Keep it simple
Salisbury,England
Joined Jan 2005
1,013 Posts
Thrust line/stability

Hi,

A full-size accident report has been recently re-issued from my previous workplace, which I thought might be of interest to the group.

In particular there is a section on how the thrust line of a pusher autogyro relative to the vertical C of G affects static stability, and a recommendation on where this thrust line should be.

Al

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