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Old Jan 14, 2010, 07:04 PM
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Motor Thrust angle in relation to the Keel?

Motor Thrust angle in relation to the Keel?


The attached photo shows a drawing of a gyro I am testing. I had the motor screwed straight onto the mast which is titled back 10 degrees to the Keel. The makes my prop sit at a 10 degree angle as well. Top of the prop is further back then the bottom.

The interesting thing is this setup makes the gyro unstable. It flies like its tail heavy and has a tendency to want to do a back flip when power is added. Even a very large nose down hang angle didnít help it much. The a hang angle was so bad the gyro would barely flair and would drop the nose rapidly as the motor was cut.

I moved the motor and servos around to try to readjust the TL but nothing would really help it out.

So I finally set the motor to zero degrees in relation to the keel and the 1st thing I had to do was add in a bit of aft rotor trim but the gyro fly nicely after this minor change.

I would have though this would more or less just make the thrust line higher but it seems to give the opposite effect. With a 10 degree hang angle the gyro would fly nose high and was very tail heavy acting by trying to do back flips!

So why does the motor angle have the much effect on the flight characteristics of this model?
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Old Jan 15, 2010, 03:17 AM
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Grant,

this is quite astonishing and I have no explanation for this behaviour. Like you, I'd have thought that the blue arrangement is just another way of raising the thrust line - at least on one of the usual autogyros with a horizontal stab.

Naturally, stabless gyros are more sensitive to thrust line changes, but not normally in the way you described.

Does your gyro fly faster with the blue arrangement than with the red one?

Jochen
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Old Jan 15, 2010, 06:13 AM
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Is there an horizontal stabiliser ?
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Old Jan 15, 2010, 08:10 AM
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Yes it has a H stab. 3" X 10" on top of the V stab in the prop wash. With the blue line the gyro is much slower and I have to baby the throttle or she will do a back flip! I can not fly it fast becase throttle just makes it shoot nose up. Red line the gyro flys faster and much more stable and pitches nose down with strong increases in power. Also 10o hang angle works fine for red line and not for the blue. I had to trim the rotor head very far foward to fly the blue line.
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Old Jan 15, 2010, 09:52 AM
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Quote:
Originally Posted by grant31781 View Post
Yes it has a H stab. 3" X 10" on top of the V stab in the prop wash.
There's your problem then (I think ....) The horizontal stab is at an angle to the airflow in the prop wash. With the motor in the blue position, imagine holding the model so that the prop is vertical, and the whole of the stab becomes a big elevator with the stick pulled back, which will push the tail down and the nose up.

PeterO
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Old Jan 15, 2010, 10:11 AM
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Grant,

I agree with Peter.

Jochen
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Old Jan 15, 2010, 10:33 AM
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Grant ,
Your gyro must look like this
http://www.rcgroups.com/forums/attac...mentid=1471296
PeterO
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Old Jan 15, 2010, 01:23 PM
It was a GLITCH! Honestly!
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Peter,
I also 100% agree


I have seen a few RPG's built now and sometimes they pitch up or down under power. Everyone blames the thrust line in relationship to the CG being 'to high'...its just a load of rubbish but knowone seems to take a blind bit of notice of me.
If using a horizontal stab you can get away with a very high thrustline, the most important thing is that the stab and thrust line are built true and are parrallel.
Without the stab the thrustline is then critical in relationship to the CG, with a stab you can get away with murder...almost!


Regards Rich
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Old Jan 15, 2010, 02:41 PM
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It makes sense to me! I guess as the gyro is moving forward the prop rediverts the airflow in a downward angle increasing the H stabs down force. I even moved the tail back 1.5 inches more and it was a tad bit more stable but not much.


So more forward speed equals more thrust plus more oncoming air = stronger nose up pitch.

Peter,

No this one is my own design. I will post pictures when I finish it. I am going to rebuild it again because I want to make a few changes. It only takes me about 3 hours to build one!
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Old Jan 15, 2010, 02:52 PM
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Originally Posted by grant31781 View Post
Peter,
No this one is my own design. I will post pictures when I finish it. I am going to rebuild it again because I want to make a few changes. It only takes me about 3 hours to build one!
I was only joking about it looking like a Minimum, but the layout is proven and it shows the motor and prop is arranged "true" to the horizontal stab.
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