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Old Dec 25, 2007, 01:42 PM
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Thank you Mickey. I completely missed the physical refresh rate issues :=) Though the servo doesn't have to go lock-to-lock, .09 is clearly not enough.

On the other hand, cooling may not be much of a problem if you put servos on blade tips, all for blade balancing of course, and get them into the slipstream :=)

Ari.
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Old Dec 25, 2007, 02:04 PM
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Quote:
Originally Posted by iter
On the other hand, cooling may not be much of a problem if you put servos on blade tips, all for blade balancing of course, and get them into the slipstream :=)

Ari.
I like this plan.
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Old Dec 25, 2007, 11:13 PM
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Blade airfoils

Mickey, you said more than once that at this size the actual cross-section doesn't matter much, but you also repeat that symmetrical airfoils' center of pressure doesn't move as the pitch changes. Why not use symmetrical blades for autogyros, the kind available off-the-shelf for CP helicopters?

Ari.
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Old Dec 26, 2007, 06:05 AM
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Quote:
Originally Posted by iter
Mickey, you said more than once that at this size the actual cross-section doesn't matter much, but you also repeat that symmetrical airfoils' center of pressure doesn't move as the pitch changes. Why not use symmetrical blades for autogyros, the kind available off-the-shelf for CP helicopters?

Ari.
I guess I should be more specific. Cierva used symmetrical airfoils because of this fact, but modern helicopter airfoils aren't generally symmetric.
The difference between say a Clark Y and a Selig aren't that much on a small model but there are small differences between these two and a fully symmetrical. But the answer is frustrating because at very small size (Twirl, etc.) the plank airfoil works and is symmetrical. Only as the model gets bigger and the Reynolds number gets large to airfoil differences become significant. The only reason you would ever prefer the symmetrical is for structural reasons to keep the blades from twisting under load, but this just isn't an issue for models generally since the blades are very stiff relatively speaking.
What I was trying to say was that for typical model sizes, once you have chosen a typical cambered 10-12% thick airfoil section the differences between individual airfoils in the this whole class of airfoils is not that important (like the difference between a Selig 6042 and 6043).
So the bottom line is that for model sized craft there is a difference between symmetrical and cambered airfoils but not a significant reason to resort to the symmetrical for structural reasons.
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Old Dec 27, 2007, 11:52 AM
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Could we go back to the servo and flapping stuff for a moment, please? I would need that in a more verbose form because I didn't quite get what you were talking about ...
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Old Dec 27, 2007, 12:32 PM
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Originally Posted by edi
Could we go back to the servo and flapping stuff for a moment, please? I would need that in a more verbose form because I didn't quite get what you were talking about ...
Could you make this into a specific question? I'm not certain what you are asking about specifically.
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Old Dec 27, 2007, 04:33 PM
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I was referring to this one:

Quote:
Originally Posted by mnowell129
The only thing I know is the Kaman full sized stuff that uses a servo flap to implement cyclic. It is still mechanical in nature however and depends on blade flex to work.

I've considered this approach but even the fastest of servos isn't fast enough for a small model.
For example at 500 rpm, this is 8 revs/sec or .12 seconds/rev. The servo would have to be 4 times that fast or .03 seconds. I don't know of any that fast, let me know if you know of some. It is an interesting approach however.
What is this approach?
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Old Dec 27, 2007, 05:11 PM
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Originally Posted by edi
What is this approach?
The kaman helicopters use a swashplate for cyclic, but instead of letting the swashplate twist the blades on feathering bearings, the linkage moves a servo flap at about 75% of the blade radius. This makes the flexible blade twist and change it's pitch for both cyclic and collective.
It depends on the blades being torsionally flexible.
Put "Kaman Servo Flap" in google to see lots of info.

Ari wanted to replace the swashplate and linkage with a servo and electronics to drive the servo flap electronically, a technique that has been studied or perhaps tried on full sized aircraft. But the response of the servo has to be fast, probably 10 times faster than the time it takes the blade to go around. For a full sized aircraft this might be a long time at a few hundred RPM, but for a model at 1000 RPM the servo would have to be extremely fast, much faster than any currently available servo. Also model blades are probably too rigid for this approach. So the approach is possible but not practical right now.
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Old Dec 29, 2007, 04:18 AM
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question about coning, see #26

Hi,

I'm sorry for the late reply but I just started reading this thread.

I have red the post over and over and think the comparison with dihydral is a bit confusing but that's because I have to translate the text in my mind and I'm not that familiar with planes.

In forward flight the forward blade is pushed up by the headwind while the aft blade is pushed down. Due to precession the plane will roll to the advancing blade instead of pitching up.

If there is a gust from the side (slip) the forward blade (with respect to the direction of gust) is pushed up and the aft blade is pushed down, like above.
Depending on the forward blade being advancing or retreating the result is a pitch up (when the advancing blade is forward) or or a pitch down.

Did I understand the post?

Leo
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Old Dec 29, 2007, 05:50 AM
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Did I understand the post?
Leo
Yes. Hopefully you got the point, that is, coning does not make the proper correction to a disturbance, but makes a correction 90 degrees later where it is not helpful.
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Old Dec 29, 2007, 07:48 AM
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Disk loading

A long time ago it was stated that helicopters typically have higher disk loadings than do autogyros.

Can you explain why this is so?
Does this change with size/scale?
Would the ideal or typical disk loading on a PPR craft be somewhere in between?
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Old Dec 29, 2007, 08:41 AM
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Quote:
Originally Posted by leadfeather
A long time ago it was stated that helicopters typically have higher disk loadings than do autogyros.

Can you explain why this is so?
The autorotative state is not the maximum lift state. When the rotor is powered you can operate it at higher lift, thus the disk loading can be higher. As the helicopter has collective pitch an autorotative landing can be at a higher disk loading since you can pull pitch at the end and cannot with fixed pitch.
Also when the rotor is powered it can be run at a higher rpm, this increases lift proportional to the RPM squared and lowers the coning and reduces all the side effects therein.

Quote:
Originally Posted by leadfeather
Does this change with size/scale?
probably not, none of the effects are scale related.
Quote:
Originally Posted by leadfeather
Would the ideal or typical disk loading on a PPR craft be somewhere in between?
I'm not certain. I'd guess so. It probably depends on whether or not you have collective pitch. I don't know how effective the PPR is on model scale since PPR is primarily used to increase the steady cruise efficiency and models don't spend to much time at steady cruise.
I had a partially powered rotor while testing my pre-rotator and it didn't make any noticable difference, although I didn't test very long.
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Old Dec 29, 2007, 10:29 AM
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Thanks Mickey, great explanations as always!
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Old Dec 29, 2007, 11:13 AM
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You're welcome. Just trying to do my part.
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Old Dec 31, 2007, 01:24 PM
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Here's some reference material I ran across.
http://www.internetage.com/cartercop...s6.htm#Answer1
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