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Old Jan 06, 2009, 10:35 AM
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If you think about a teetering rotor, it makes the body move by pulling on the tip of the shaft, perpendicular to the tip path plane. It can never apply torque to the shaft directly. If it makes no thrust (0 g), it isn't pulling anywhere. So no matter what angle it is tilted to, it doesn't apply any force to the body of the aircraft. You can fly the rotor around wherever you like, it still applies no torque to the shaft. You can still control the rotor at 0 g with a teetering rotor, but as long as you remain at 0 g the rotor cannot apply any forces to the fuselage.
Add a stiff coupling, a spring, snubber or a flex plate (the misnamed Direct Control). Now when the rotor tilts it can apply a torque directly to the rotor shaft even in a 0 g condition. This is because the centrifugal force of each blade is tugging on the shaft through the stiff coupling. The teetering rotor has no such coupling in which to apply the torque to the shaft.
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Old Jan 06, 2009, 10:53 AM
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Mickey,

that's clear so far. But why should I want to move the body of a gyro in a 0g situation? I'd have thought the important part would be to get the rotor out of this condition.

My overall picture is still a bit diffuse.

Jochen
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Old Jan 06, 2009, 10:56 AM
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The earlier posts were about how full sized gyrocopters crash. They add quick throttle/thrust and get a nose over. The system goes into a 0 g situation and then since the rotor is teetering you can't get the nose back up. The rotor is limited by the teeter stops, can't apply any nose up moment, loses RPM because it's edge on to the airflow, etc. etc. then it hits the ground.
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Old Jan 06, 2009, 10:59 AM
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Quote:
Originally Posted by JochenK
I'd have thought the important part would be to get the rotor out of this condition.

Jochen
Yes it is, but in the pushover crash scenario the rotor goes 0 g or negative and then the rotor speed goes away. Once the rotor speed decays the ability to control it goes away as well.
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Old Jan 06, 2009, 11:05 AM
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Mickey,

I can move the rotor up, but since the body doesn't follow I'm running into trouble because of the limitations of the rotorhead. Did I get that right?

Jochen
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Old Jan 06, 2009, 11:27 AM
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Originally Posted by JochenK
Mickey,

I can move the rotor up, but since the body doesn't follow I'm running into trouble because of the limitations of the rotorhead. Did I get that right?

Jochen
I assume you mean nose up. Given that I think I agree with what you said. The aircraft is nose down, 0 g and falling. You pull the rotor to level, but so what? it's not making any lift..... But if the rotor has a stiff coupling then the level rotor, even at 0 lift, is applying nose up torque to the shaft and the nose comes up. Hopefully before all the rotor speed is gone and the ground comes into play.
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Old Jan 06, 2009, 01:09 PM
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Mickey,

I had to give this some thought, but now I think I understand. Thanks.

Jochen
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Old Jan 13, 2009, 12:14 AM
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The positive pitch autorotation unbeliever ;)

I have a tendency to be incredulous about some things untill I prove them to myself and I have to say I still don't buy the current vector diagram of autorotation. I understand it completely, it's in all the books - I just don't belive it. Many times the diagram is shown with a positive pitch of the blades and instictively I think this violates some principal of physics that I'm not smart enough to prove, yet observe daily Conservation of momentum comes to mind though. Why would the air go the hardest way to the trailing edge instead of taking the path of least resistance to the leading edge? I know it's spinning but it's seems that this condition would decay rotor speed - not keep it going wich is required to call it autorotation. And the angles in the diagrams are usually exagerated and shown above critical angles of attack for the airfoils depicted. Wouldn't a typical blade be at some average angle of attack of say 5-7 degress with rotational and forward speed/disk angle combined? I realize it varies throughout the blade but this would be the typical best L/D range for most airfoils. No? Not the 25-40 degress it looks like on the diagrams trying to make the forward thrust component noticable. And would that mean that the higher the angle of attack the more efficient the airfoil? Since lift is 90 degrees from the relative airstrem it seems in these descriptions that an airfoil is more efficient at extremely high angles of attack. Doesn't seem like reality to me even if the little arrows look pretty.


But I have over the years had some observations about different pitch rigging angles and types of material blades are made from that seem to support my theory that due to aeroelasticity of the blades twisting in flight they can be rigged to a positive pitch at the hub but they flex in flight to a negative pitch at the tips which drives the rotor in autorotation. My personal theory about models needing negative pitch at the hub is that due to scaling factors our blades are less flexable and the root of the blade is always set closer to the actuall negative autorotation angle that the tip has under load.

About 10 years ago I was following the RC gyro scene and everyone was making thier own blades out of thin balsa because they were the only type that they could get to work with a positve pitch and still autorotate. We're still using high AR balsa blades instead of the readily available, allready tip weighted and finished heli blades on the market. Why? Because balsa wood blades are really more efficient than molded composites? That doesn't make sense. They aren't all that much lighter after tip weighting. But balsa wood blades are more flexible. 10 years ago, they were really set that they had to have a positive pitch or it wasn't "real" autorotation. Obviously, those thin high aspect ratio balsa blades would be very flexible in twisting and coning, enough to go negative at the tips under load even if the root were at poistive 1 degree. Right?

Some first and second hand observations over the years seem to support this. For example, on full size gyrocopters it was said in PRA magazine some years back that fiberglass blades were more efficient but were harder to start due to being set at a higher pitch angle at the hub. It was noted that wood blades were easier to start but were less efficient evidenced by the fact that they had to be set at a lower pitch angle at the hub. There was never any mention of higher climb rate or any other flight performance. They seemed to only be judging the efficiency on the rigged hub angle of the blades. Wood, especially the spruce and other woods used in aviation are known for thier stiffness which is why they are still used as aerobatic airplane spars. They would not twist much under centrifugal (centripital? haha) and bending loads. The fiberglass blades in the picture were drooping significantly just under thier own weight. The wood blades - well....stiff as a board. Not to mention that the whole idea of fiberglass being more efficient is just silly. what does the air care what the material is? It only matters how smooth the surfaces are I would think and that can be done with wood and paint just as well as fiberglass.

I understand the effects that reynolds numbers may have on comparing small rotors to large but I can't find any example of any other lifting-turning /thrust device that can maintain a positive pitch angle to the direction of travel and produce thrust in that direction as well. Large wind turbines - negative pitch angles. Small wind turbines - negative pitch angles. Boat sails - negative pitch angles. A gliding aircraft - negative pitch angle. An airfoil in a windtunnel has drag. I've never seen net thrust reported. So this is why I still don't buy it.

My theory is that negative pitch is needed for autorataion and the only reason there is ever a positive static pitch rigging is because of blade twisting in flight.

The real reason not to use rc heli blades in my opinion is because they use thick, higher drag symetrical airfoils for the stiffness and +- consitancy necesary for aerobatics. A thinner, lower drag airfoil would be optimum with autogyros where aerobatics are not done. Very stiff blades should be desireable since they will be rigged at a negative angle and will start easier. It's notable that RC helis also require negative pitch for autorotation with those stiff blades. Some full size helis are also rigged for negative pitch at autorotation. I can't give a reference but an instructor and A&P told me that. I'd need the maintenance manuals for various helis for reference. I'm willing to bet it has more to do with the blade flexing than any major difference in the airfoil used or reynolds numbers.

I would love to find some facts about how much blades on full size autos and helis actually twist in flight. They have to twist some and the twist would have to be negative, relieving loads or the blades would not track in a stable manner. If they twisted positive pitch under increasing load the blades would self destruct. A few degress negative.....? That would make the difrerence. Could it be that the simplest explanation we all observe in windmills, sails and sycamore seeds is really is true?

So I guess hopefullly one of the engineers lurking out there can go over this in more detail to bring me into the fold on the whole positive pitch autorotation theory. Mabey I know I'm the only one holding out so I recognize my chances of being correct are slim. I'm asking for help. Please... just don't show me any exagerated angled post stall AOA diagrams -it's a pet peeve of mine. Truthfull angles in a vector sum diagram would be fine. Thanks.
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Old Jan 13, 2009, 08:42 PM
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Quote:
Originally Posted by yak55x
A gliding aircraft - negative pitch angle.
How do you figure it's a negative pitch angle.
I think what you are overlooking is that the wing force (combined lift and drag) is perpendicular to the airflow plus the L/D of the airfoil, not the chord line of the wing. So an airfoil with a L/D of 10:1 has a resultant thrust vector of 90 + atan(.1) = 96 degrees, relative to the local airflow, not the chord line.
So a glider (or rotating wing) with a +1 rigging angle, with a descent angle of -8 degrees (or rotor tiltback angle of the same) has a resultant force vector of 88 degrees for the glider or 98 degrees for the rotor blade. In either case the force vector is forward of vertical (90 degrees) (glider) or forward of the rotor axis (110 degrees). Thus positive rigging yields forward thrust.
No doubt twisting blades confuses things because the tip angle isn't always the hub angle. But this doesn't mean that the section can't be pulling forward.

BTW wind turbines use a positive angle of attack with respect to the oncoming wind, and they turn the right direction.
So do sailboats, if you measure with respect to the relative wind.

All you have to achieve is a relative wind that is slightly more than 90 degrees plus the L/D angle away from the line that is perpendicular to the direction you want to go.

For some real proof, go check out a real Huey sometimes, they autorotate just fine and the blades won't go negative.
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Old Jan 14, 2009, 12:23 AM
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Thank you for your resonse I am a big fan of this thread I've been studying and pondering for a couple years. Great stuff - Thanks.

I'm not an engineer so I struggle getting the correct terminology. I'm hung up on the angle of the blades measured from the chord line and the plane of rotation being positive. I know I'm the slowest guy in the class here but I really want to tear this apart.

I understand that the angle of attack can be positive with a pitch angle of negative or positve and so the rotor can create lift either way.

I do understand the lift is perpendicuar to the realative wind and drag is going the direction ahe RW. So far so good. Thanks for the LD of 10 because I've been drawing some diagrams and I didn't know what a blades L/D was likely to be. I understand the the way you figured that out and it makes sense. I can see how it's figured out anyway.

It seeme to me that just like any inclined plane or in this case a partial helix I guess would have to be at a negative pitch with relation to the plane of rotation to have the air molecules impart energy into the blade in a forward direction as well as upward. The diagram looks right I admit - but when we look at the diagram we are allredy assuming some things like the air is moving upward through the disk and the blades are moving forward. These preconditions to the diagram don't seem possible to me if the blades are steady state at a positive angle to rotation.

So take me back to the back basic way an inclined plane or helix works and why in this case the air is moving upward towards the disk, then for a brief moment while the positive pitched blades pass is moving downward again (which seems like it would require energy from the blade) and then the air would change directions again and have a net movement going back up through the disk. So the upward movement of air would have to be only in-between blades and I don't see where the energy transfer to the rotor can take place when it doesn't have the blade to work on then. That's what I don't get and why in the case of autorotation the blades are not just acting in the basic machine principals of a helix. So that's I'm trying to reconcile. Why do basic machine pricipals of the inclined plane seem to be thrown out here? I do generally subscribe to the simplest answer being true so that's where I'm hung up I think.



So let me ponder.....

I'm working this out with polars from a naca 0012 which is pretty typical tight?. Are the blades on average going to be near best LD for autorotation? And I forget how I figure out the angle of attack from the lift coeficient but I'll find it and draw a vector diagram. mabey there is still hope for me.

The diagram looks good but then so did all the theories of describing lift I'd been taught untill I read a page NASA. http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html It makes gurney flaps and underwing pylons and stores esier to accept. We never talk about the air being slowed down UNDER the wing. And also seems like the simplest form of thinking of lift forces. I like simple.
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Old Jan 14, 2009, 12:00 PM
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Quote:
Originally Posted by yak55x
The diagram looks right I admit - but when we look at the diagram we are allredy assuming some things like the air is moving upward through the disk and the blades are moving forward. These preconditions to the diagram don't seem possible to me if the blades are steady state at a positive angle to rotation.
This is why autorotation doesn't self start. You need a hand flip or something in the right direction. You are trying to analyze the startup condition which isn't autorotation.

Quote:
Originally Posted by yak55x

So take me back to the back basic way an inclined plane or helix works and why in this case the air is moving upward towards the disk, then for a brief moment while the positive pitched blades pass is moving downward again (which seems like it would require energy from the blade) and then the air would change directions again and have a net movement going back up through the disk. So the upward movement of air would have to be only in-between blades and I don't see where the energy transfer to the rotor can take place when it doesn't have the blade to work on then. That's what
The airflow from the blades doesn't have to reverse. All that is required is that the airfoil turn the oncoming air some, not completely reverse it. A glider wing or a rotor disk do the same thing, they just turn the mass of air down a little bit. Then net flow through the disk is upward everywhere, it's just that each blade turns the air a little bit and when doing so creates lift.
A glider won't wing won't readily fly forward when you drop it either, it needs some initial forward motion to work right as well. A glider only moves forward when you drop it because it is trimmed to drop the nose when stalled and this builds up forward airspeed.
I think you've got yourself all wound around the axle over the condition of windmilling which is a self starting condition where you intend to extract shaft horsepower out of the turbine. Autorotation is not self starting and you can extract very little torque out of a autorotating rotor. Small amounts of shaft drag will stop it.
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Old Jan 14, 2009, 09:08 PM
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That's how I've always heard it described, kind of a sweet spot or something... One article called it a "curious phenomina". I guess I understand the explanations without completly buying in yet to the explanations. I still think the preconditions of the vector graphs are incompatible with a positive pitch angle. No big deal, I will keep working at it. The popular explanation of autorotation seems like a quantum probability to me - only at the wrong scale. Haha Air being forced down through the disk plane and going upward at the same time..... phys geeks get it.

So will autorotation be close to best L/D, at least in the driving region?

From 2 hand observation it seems most small models are excluded from being in true autorotation due to reynolds numbers/scaling efects. Did I understand that correctly? Or at least I've read quite a bit of models using negative pitch rigging angles or washout which would be windmilling then and not autorotation? Auto-windmillers. How bout gyromillers, autowindthingy...... Now i'm I just being silly

Really though thanks for the help. This thread is great and I appreciate that you are willing to share your knowedge and methods to get these rc gyros flying. The rotor control explanations have been very enlightening. At least I get that part

I would still like to find out how must twisting goes on in flight if there is anything out there. I need a novel means of measuring max twisting at the tip without resorting to costly strain guages. Small dvr video camera at the hub possibly focused on a wire or something. I've seen some blade flapping videos on youtube but they were hardly anyway to measure it.
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Old Jan 14, 2009, 10:01 PM
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I have found a few videos of blades in flight.

Rotor Blade in flight (0 min 25 sec)


I can't see any twisting just flapping in general. Wish I could slow it down.

Leading Edge, Rotorblade In Flight, Rotorhead View eDVR (2 min 41 sec)


http://www.youtube.com/watch?v=SjFSt...eature=related

shows a basic helix action.

Unsteady wake behind a climbing helicopter rotor blade (0 min 28 sec)
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Old Jan 15, 2009, 06:00 AM
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Haha Air being forced down through the disk plane and going upward at the same time..... phys geeks get it.
Not really, not sure why it has to to that. Just because the airfoil turns the air doesn't mean it has to turn it backwards. Remember it just has to turn it WRT to the relative wind.

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So will autorotation be close to best L/D, at least in the driving region?
That's the idea for best performance.

Quote:
Originally Posted by yak55x

From 2 hand observation it seems most small models are excluded from being in true autorotation due to reynolds numbers/scaling efects. Did I understand that correctly? Or at least I've read quite a bit of models using negative pitch rigging angles or washout which would be windmilling then and not autorotation? Auto-windmillers. How bout gyromillers, autowindthingy...... Now i'm I just being silly
The micro guys are likely windmilling. But I'm certain mine are in autorotation as are most models rigged somewhere around 0 degrees (true). Negative pitch isn't the binary criteria for windmilling. It's about how much rotor thrust can be turned into power (to grind the wheat).
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Old Jan 15, 2009, 04:14 PM
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Well I'm still interested in collecting information like that. So you are usually at 0 degrees. Do your rotors autostart ? Are your blades very rigid?
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