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Jan 15, 2009, 04:32 PM
I'm not as bad as they say.
Joined Nov 2004
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Quote:
 Originally Posted by yak55x So you are usually at 0 degrees.
Geometrically they look negative, but aerodynamically they are around 0.

Quote:
 Originally Posted by yak55x Do your rotors autostart ?
Nope, they take a hand spin at the minimum. Most of mine have pre-rotators because I hate the appearance of having to hand spin and I can taxi out at my field and take off like any other kind of aircraft, rather than having to stand in the runway.

Quote:
 Originally Posted by yak55x Are your blades very rigid?
Not like my carbon heli blades, but the CG's are correct. I tried without and got goofy flight.
 Jan 15, 2009, 05:33 PM Registered User Chandler Arizona Joined Feb 2004 78 Posts [QUOTE=mnowell129]Geometrically they look negative, but aerodynamically they are around 0. I define pitch as in relation to the plane of rotation, I'm pretty sure I'm on the same page there. In maintenance they refer to the pitch rigging which to me is just setting up the min - max travels. I complety get the difference between AOA and pitch. What do you mean by geometically and aerodynamically? Do you have washout in the blades? How do they look negative?
Jan 15, 2009, 06:50 PM
I'm not as bad as they say.
Joined Nov 2004
4,981 Posts
Quote:
 Originally Posted by yak55x What do you mean by geometically and aerodynamically?
Most non-engineers consider the angle of attack measured from the chord line, the line drawn between the leading edge and trailing edge. But the angle of attack is against the zero lift line, the line where if parallel to the relative wind, the airfoil makes no lift. My blades look negative on the model but are close to 0 aerodynamically.
 Jan 15, 2009, 07:15 PM Registered User Chandler Arizona Joined Feb 2004 78 Posts My blades look negative on the model but are close to 0 aerodynamically.[/QUOTE] Mmmmm. I've never noticed the zero lift angle being used like that, just a zero lift line or zero lift axis. So for a cambered airfoil would be a negative angle from the chord line to the axis of rotation. Ok, will mill that over - feels like I'm getting somewhere. But the angle of attack will change quite a bit during flight. And I don't have a problem with AOA being positive - I'm looking specifically at the chord line in relation to the axis of rotation. If it looks negative on your model then what would a measurment of the chord line be in relation to the plane of rotation? A guess is ok. If I were to say a rotor or propeller is at a particilar rpm, and pitch, how would you know which direction the propwash or rotor wash was going? The pitch? It still seems like the pitch determines the direction of flow through the rotor disk to me. Unless in some extreme sitiuation of decelleration. So it still seems like to me you decide a few factors such as pitch, rpm and blade dimentions and the arrows get figured out from that. It seems like the popular explanation is getting the answer it want's by arbitrarily drawing the RW vector from below even though the pitch is positive. I just seems to go agains the grain - or airflow in this case. But I'll keep working on it........... Last edited by yak55x; Jan 15, 2009 at 08:03 PM.
Jan 15, 2009, 08:13 PM
I'm not as bad as they say.
Joined Nov 2004
4,981 Posts
Quote:
 Originally Posted by yak55x If it looks negative on your model then what would a measurment of the chord line be in relation to the plane of rotation?
It looks negative relative to the plane of rotation. How it looks to the model or the ground, I don't care.

Quote:
 Originally Posted by yak55x If I were to say a rotor or propeller is at a particilar rpm, and pitch, how would you know which direction the propwash or rotor wash was going? The pitch? It still seems like the pitch determines the direction of flow through the rotor disk to me. Unless in some extreme sitiuation of decelleration.
No, you have pick the flight condition you want to analyze. Say you have a rotor rigged at +1 degrees and you want to know how much power is required to hover. Then figure that the lift required per blade, etc, then given the rotor rpm and drag you can compute the power input required.
If you pick a 1500 fpm descent, then recompute based on a upflow of 1500 fpm, figure out the rpm, drag, etc and compute the power required is negative, meaning you can take power from the rotor. Or at a different sink rate you get power required = 0, i.e. autorotation, etc.
You can't just look at the pitch and RPM in a vacuum, you have to establish other conditions that you are evaluating for, then you get to draw vectors for that situation.
You're trying to make it simpler than it can be.
 Jan 15, 2009, 08:27 PM Registered User Chandler Arizona Joined Feb 2004 78 Posts [QUOTE=mnowell129]It looks negative relative to the plane of rotation. So what would a guess be as to what that negative angle is on your particular gyro? Thanks, this is what is important to me.
Jan 15, 2009, 09:37 PM
I'm not as bad as they say.
Joined Nov 2004
4,981 Posts
[QUOTE=yak55x]
Quote:
 Originally Posted by mnowell129 It looks negative relative to the plane of rotation. So what would a guess be as to what that negative angle is on your particular gyro? Thanks, this is what is important to me.
-4 -6 degrees with no pre-rotator, I've run -1 to -2 with.
 Jan 17, 2009, 09:40 PM Registered User Chandler Arizona Joined Feb 2004 78 Posts Thank you BTW Good info
Jan 18, 2009, 08:11 AM
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Following rate formula

Mike

writing down the explanations for the metric version of the following rate formula as a normal post was too complicated for me. I put it all in a Word document (Word 97 to 2003) and attached it to this post.

Jochen

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Last edited by JochenK; Jan 20, 2009 at 04:49 AM. Reason: new formula for factor a
Jan 18, 2009, 02:50 PM
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Mike,

here's an addition to my last post. It integrates tip weights into the formula for the moment of inertia.

Polars: I got the values for C-lift from a program called Profili v2.2. It’s shareware and costs something like 10 Euros.

Jochen

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 Jan 18, 2009, 11:42 PM Tannenzäpfle-Terminator Baden-Baden, Germany Joined Dec 2008 436 Posts Hello Jochen......... Good job. If I calculate the following rate of a given rotor, one time with adding a brass wire on the whole lenght of the leading edge and the other time with the same mass concentrated on the tip, I get some significant differences....... So i suppose that it would be best to use tungsten-wire for the tip weights. Tungsten has almost the double density of lead and more than double density than the soldering-wire commonly used. So it will help to concentrate the mass just a little bit more on the tip. But where to get some tungsten-wire? Go for welding electrodes from TIG-welding. Its tungsten-wire, diameter from 0.7mm up to 4.0 mm available, length about 200 mm, price: about two euros for diameter 1.5 mm. On your first calculation or the angular velocity you wrote ω = n / 60 • 2 • π / 360. Where does the 360 comes from? To my opinion it should be ω = n / 60 • 2 • π........... Bye Mike P.S.: Have you already tried the formula-editor included in word? Its the perfect tool for writing formulas.......
 Jan 19, 2009, 03:16 AM Registered User Joined Jan 2005 1,286 Posts Mike, you're right. I've corrected the mistake in the original document, can you delete the '360' in the copied post? Jochen