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Old Feb 07, 2007, 09:04 AM
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Working with 3-bladed flybarless head......

In verse 141, I mentioned a 3-bladed, flybarless set up. www.mscompositusa.com has a 3-blade, flybarless system for their Hornet. The size is a bit smaller than the Blade or GWS heli parts.

QUESTION: Mickey, I'm interested in your opinion about how you feel this type of rotor system might fly. In particular, this system will be responding to a head speed of perhaps half that of a helicopter.

I was considering 1" cord blades, but AeroBalsa is developing 1-1/8" x 12" all balsa autogyro blades which may work with this head. I'll be CG correcting these blades.

One thing about this head is that the blade holders are rigidly mounted to the hub, as in no hinge. MS Composit says flexible rotor blades make this posible. YOUR COMMENT PLEASE.
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Old Feb 07, 2007, 12:24 PM
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David,

I too am looking forward to the new AeroBalsa 1 1/8" blades, and have been developing a (rigid) LC head for them. Until Mike starts manufactuing them though, if you'd like to experiement with smaller, more flexible blades, I'll be happy to send you some Slow-G blades. They are 20x245mm or .79"x9.6" and the lighter weights are quite flexible.

How much does the MSC setup in your photo cost? Kinda makes me want to try it myself :=)

Ari.
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Old Feb 07, 2007, 01:34 PM
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Ari, I'll post info for this on the thread I started on 3-bladed, flybarless autogyros.



http://www.rcgroups.com/forums/showt...39#post6885308
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Old Feb 07, 2007, 07:37 PM
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Quote:
Originally Posted by David A Ramsey
In verse 141,
Funny..verse 141, I suppose your quoting the BOM.

Quote:
Originally Posted by David A Ramsey
QUESTION: Mickey, I'm interested in your opinion about how you feel this type of rotor system might fly. In particular, this system will be responding to a head speed of perhaps half that of a helicopter.
Don't know that the head speed will be that much lower. G3PO shows 1500 hundred on the skytach and 1200 on the pre-rotator which isn't quite flight RPM.

Quote:
Originally Posted by David A Ramsey
YOUR COMMENT PLEASE.
I still think it's going to be touchy. All multi-bladed head models are touchy. MSComposit knows this, hence the flexible blades. I'd definitely make provisions for tip weights. Clearly the following rate can be made manageable with 3 blades in a model so the prognosis is good. I'd just be ready to make blade changes until it flies like you want.
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Old Feb 08, 2007, 09:07 AM
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Quote:
Originally Posted by mnowell129
Funny..verse 141, I suppose your quoting the BOM.
Of course!


Quote:
Originally Posted by mnowell129
Don't know that the head speed will be that much lower. G3PO shows 1500 hundred on the skytach and 1200 on the pre-rotator which isn't quite flight RPM.
Didn't think my G3PO, or Begi ran that high. Perhaps I'm not using enough negative pitch.



Quote:
Originally Posted by mnowell129
I still think it's going to be touchy. All multi-bladed head models are touchy. MSComposit knows this, hence the flexible blades. I'd definitely make provisions for tip weights. Clearly the following rate can be made manageable with 3 blades in a model so the prognosis is good. I'd just be ready to make blade changes until it flies like you want.
Agree on "touchyyy".

I'm gonna have to check the cord and pivot hole dimensions of the 3-bld Hornet rotor blade because in the MSC 3-bld instructions they recommend that the CG should be "5-10mm in front of the blade axis of the rotor". That seems more nose heavy than we've been thinking about. Wondering if the blades might lag a lot? (Lag a lot/ LAL: a high tech. aerodynamics definition???)
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Old Feb 08, 2007, 09:43 AM
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Originally Posted by David A Ramsey
Didn't think my G3PO, or Begi ran that high. Perhaps I'm not using enough negative pitch.
I would have guessed far lower, but the tach tells the tale.
When you do the math, it agrees with the RPM.
Quote:
Originally Posted by David A Ramsey
Wondering if the blades might lag a lot? (Lag a lot/ LAL: a high tech. aerodynamics definition???)
This all makes sense. If they spec nose heavy, flexible blades then they are depending on a lagging blade opposing control input. Further if the blades are flexible they will try to spin flat due to centrifugal force. It all adds up to applying a full court press to get the following rate down.

Note (see following rate calculations from the book earlier in the thread) that the following rate is linear, that is proportional, to RPM, but is proportional to the inverse of the radius of gyration squared. Translation : lowering the rotor speed by 20% lowers the following rate 20%, but increasing the radius of gyration by 20% lowers the following rate by 44% (1.2 * 1.2 = 1.44). And this ignores the additional lowering of the following rate by raising the mass of the blade. Conclusion : Tip weight is the best bang for the buck in lowering the following rate. A uniform rod on the leading edge increases the overall mass but doesn't move the radius of gyration out, but tip weight does. The bottom line is that tip weight is still the best way to lower the following rate.
I would CG correct the blades (or make them nose heavy) you are going to use with lead in the leading edge tip, and then be prepared to add tip weight on the blade CG until it flies like you want.

Seems like deja vu all over again... the heli guys already went through this. Remember the horizon two bladed flybarless heli? It had hockey pucks of lead in the tips.
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Old Feb 08, 2007, 09:44 AM
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Quote:
Originally Posted by David A Ramsey
or Begi
Didn't know that you finally flew your BEGi.
Did Michael fly his?
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Old Feb 08, 2007, 11:51 AM
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Originally Posted by mnowell129
Translation : lowering the rotor speed by 20% lowers the following rate 20%, but increasing the radius of gyration by 20% lowers the following rate by 44% (1.2 * 1.2 = 1.44). And this ignores the additional lowering of the following rate by raising the mass of the blade. Conclusion : Tip weight is the best bang for the buck in lowering the following rate. A uniform rod on the leading edge increases the overall mass but doesn't move the radius of gyration out, but tip weight does. The bottom line is that tip weight is still the best way to lower the following rate.
I would CG correct the blades (or make them nose heavy) you are going to use with lead in the leading edge tip, and then be prepared to add tip weight on the blade CG until it flies like you want.

Seems like deja vu all over again... the heli guys already went through this. Remember the horizon two bladed flybarless heli? It had hockey pucks of lead in the tips.
Translation works for me. I never saw the Horizon set-up. I only know Schluter. Blades had a smaller cord with 1/8" steel wire forming the length of the leading edge. The Whopper is the same, but Clark-Y airfoil.

Thanks.
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Old Feb 08, 2007, 12:09 PM
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Quote:
Originally Posted by mnowell129
Didn't know that you finally flew your BEGi.
Did Michael fly his?
God this is embarassing. 3 attempts with BEGi were hand launches with immediate low lift landings with blade strikes. I changed the head and is now same as G3PO, but I've been gun shy to try it again, plus life gets in the way of fun sometimes. My excuse now is it's too cold. But not too cold to build something.

Don't think Michael has the time to build much as the magazine and family eats up much of his time. It might end up that I build BEGi and he flys it.
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Old Feb 08, 2007, 12:35 PM
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The horizon:
http://www.runryder.net/rrpw.htm?i=/...5/100_0145.jpg
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Old Feb 08, 2007, 01:58 PM
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That was a while ago. Had the same muffler on my Mini Boy.
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Old Feb 09, 2007, 08:47 AM
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Originally Posted by geo18
I have a book about autogyro that say : with a pusher the cg
must be aft the mast of rotor , then the lift create a pitch up moment counterbalancing the pitch down moment of motor thrusline.
There are no hard and fast rules about the CG on the pusher, its that generally the thrustline on a pusher is higher than the tractor, which you must correct for in some way. An aft CG would help correct this but you could also use an elevator or just nose up trim depending on the amount of nose down moment caused by the thrust. The bottom line is that you have to balance all the forces if you want a trim condition.
In the diagram are some of the possible forces. The thrust of the rotor can be treated as a thrust or broken down into lift and drag parts. The way to analyze is to extend the force in parallel (red lines) then find the 90 lever arm that that force has at the CG. These forces create a moment (or torque) (purple arrows) that urge the nose up or down depending on their direction. For trim you have to have all the moments sum up to 0.
You can move the CG around and see that the rotor can create a nose up or nose down moment. A horizontal tail can do the same, but usually creates a nose up moment for stability. Also see that if you move the motor up and down you can change whether or not the moment from thrust is up or down. I think the big difference in the tractor and pusher is not the front to back difference but the up and down difference. It's up to the designer to figure out what is needed for each design, that's what makes it a challenge.
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Old Feb 10, 2007, 09:01 PM
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Hi Mickey,

I thought a lesson on controllability vs stability might be beneficial. They are closely related and inferred sometimes, but a detailed lesson may help clear up some thinking.

Great lessons, don't let the silence fool you, people are reading and learning. It's helped me a lot personally.

Cheers,
Dan
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Old Feb 14, 2007, 03:39 PM
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Hmm.. controllability vs stability.
I'm going to be out for a week, just wanted to let you know I'm not ignoring this question.
Will take some further thought, but here are some opening ideas.
Stability is the property of something such that when you disturb it, that thing returns to its initial state.
A marble resting in the bottom of a bowl is stable, for when you bump it it rolls up the side and then settles back down in the bottom. It generally overshoots to the other side back and forth a few times before settling down. This is then a stable system with an oscillatory (back and forth overshooting) response. If we filled the bowl with syrup and then bumped the marble it might come back to rest without overshooting, this would be a non-oscillatory response due to the damping of the syrup. If we used water, the marble might overshoot once, showing a different amount of damping.
If we turn the bowl over, the situation changes for once we bump the marble it rolls off the bowl, never to return, regardless of whether it's in air, water or syrup. This is an unstable system.
If we put the marble on a flat piece of glass and bump it it rolls away and stops, but doesn't return. Instead of just racing away like the upside down bowl it just comes to rest somewhere. This is a neutrally stable system.
What does this have to do with aircraft?
An aircraft that is stable in one measurement direction, say pitch, will return to the trimmed condition when disturbed. This aircraft will be easy to fly in pitch because it self corrects. As we make the aircraft more and more unstable eventually we can't fly it because it pitches so quickly we can't control it. So an airplane like this is similar to the marble on the upside bowl in air. Once it leaves the trim condition it pitches so quickly there is nothing a human can do (but a computer can, this is where stability augmentation systems on jet fighters can get more performance out of plane than if a human was flying it).
Lets go back to the upside down bowl and put our system in syrup in a cube that we hold in our hands. Now we disturb the marble and it slowly starts to slide down
the bowl, but we tilt the cube so that the marble runs back to the center. Because the syrup slows things down we can easily balance the marble on the upside down bowl, something we couldn't do if it was in air.
What we have here is an unstable system with damping. If we don't correct the marble's slide it will eventually roll off the bowl, But the damping is high enough that the system response is slow enough that a human can keep up with it and apply the proper correction to keep it balanced.
This describes the pitch stability of rotorcraft.
All rotor craft are unstable in pitch, but they are damped enough that the pilot can make a timely correction and not have the aircraft go full nose up or nose down. This is why all rotor craft are harder to fly than fixed wing airplanes. A fixed wing airplane can be made stable in pitch, that is it will return to the trimmed condition. A rotor craft doesn't do this, unless you put a horizontal tail on it. This is why helicopters have horizontal tails, it reduces the pilots workload some in pitch trim. However the rotor is also unstable in the roll direction just like the pitch direction, so the rotorcraft will always tend to continue rolling from a disturbance, but at a slow rate. This is why you have to fly a rotor craft 100% of the time.
So the first real point here is that rotorcraft are unstable in pitch and roll, but heavily damped so the pilot can make corrections in a timely manner. This requires more attention on the part of the pilot.
As an aside, the little two rotor helicopters like the blade CX have a stabilizer bar that likes to level itself and is arranged to provide control feed back to the rotor such that it drives the model back to level. This provides stability that makes them easy to fly. It's also why when you let go of the controls they stop moving. This is unlike regular helicopters that must be stopped with control input (because they are unstable). Their success probably has something to do with the fact that you can learn to work the throttle and rudder without having to worry about the constant corrections needed on roll and pitch on a regular heli.......
more soon.
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Old Feb 14, 2007, 06:00 PM
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Good start, thanks!

Dan
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