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HelicopterScale Model RC Helicopters › Need help with CG
12-24-2008 04:45 PM  8 years agoPost 1
pvande

rrNovice

FL - USA

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I'm having a problem getting my CG correct for a hover wth a new scale project and what's happening isn't making sense to me.

I have the Swift 16 mechs in a Century Agusta fuselage - had to add weights to the nose as it was tail heavy.

Here is the problem: my swashplate is set up so that it is exactly perpendicular to the main shaft when I'm at 2/3rd stick for hovering...

So if I get the CG such that the main shaft is exactly vertical when the heli hangs from the rotor it should be in a stable hover right?

Well the heli keeps exhibiting tail heavy behaviour, i.e. it moves backwards when I take my hands of the stick. This even at the point where I have made the nose so heavy that when hanging from it's rotors the main shaft is at a sharp angle to the horizon from the nose...

I've double check the trim settings on my radio - no trim in any of the modes I fly in. This doesn't make sense to me - what am I missing?

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12-24-2008 05:07 PM  8 years agoPost 2
copterklutz

rrApprentice

Western​Massachusetts

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From the pictures, it would appear that the mechanics are tilted too much....the mechanics are probably level in a hover, but the fuselage won't be....






" Well, Um.... it's fixable......"

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12-24-2008 05:20 PM  8 years agoPost 3
pvande

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FL - USA

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If the mechs were level in hover then the helicopter won't have a tendency to move towards it's tail.

The fuselage is supposed to be nose up in hover - that is not the problem...

The problem is that the heli has a tendency to move towards it's tail when I give no cyclic stick input... when the current CG makes me believe that it should be moving forward...

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12-24-2008 05:58 PM  8 years agoPost 4
v22chap

rrKey Veteran

Granger, Indiana

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does putting in more forward cyclic trim keep it in a hover ??? or does it still drift back ??

Larry C ----------Bergen R/C helicopters

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12-24-2008 06:12 PM  8 years agoPost 5
Helipilot01

rrApprentice

ColliervilleTn

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This is a problem we often ran into when flying FAI. Even with the mechanics setup and the controls all neutral and swashplate level, the helicopter would still sometimes back up in hover. This is because there is more exposure of the fuse and horz. fin aft of the main shaft than there is forward of the main shaft. The downward force or the rotorwash is actually pushing the tail down. The fix for this is to shim the mechanics so they have a forward tilt. This keeps your mechanics pure without running a bunch of trim (bad) or holding the stick forward all the time.

One other thing that can cause this is that the flybar is too heavy and the flybar is not correcting for wind gust.

Hope this helps,

Mike

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12-24-2008 06:33 PM  8 years agoPost 6
pvande

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FL - USA

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Mike

Thanks for the response - that has to be it - this fuselage has a set of stub wings in the back which would definitely create the problem you described...

Paul

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12-24-2008 08:20 PM  8 years agoPost 7
J_Cunny

rrVeteran

Austin, Texas

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Paul,

Hope you have more room to tilt yours forward that I did with my 450 sized Augusta. I finally just had to live with a little forward stick for mine. Nothing major, just a slight bit of pressure. I guess I could trim it out, but I'm lazy.

JC

I used to be normal, but then I started flying heli's....

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12-25-2008 09:02 PM  8 years agoPost 8
imnxtc

rrVeteran

BC.Canada

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The problem may be one of visual perception.

Many of the European manufactures helis (of which Augusta is one) have the mast at a slightly forward angle to the fuselage. This is so that in forward flight the fuselage sits level instead of tilting forward...more pleasant from a pasengers point of view. Testing on passengers show that they are comfortable siiting level or leaned back slightly, but not so if leaned forward. A level masted heli will always lean forward in forward flight.

So when you model one that is manufactured with a forard tilt, it to should have the forward tilt. That will mean that in the hover (when the rotor is level) the fuselage will be in a slightly tilted back attitude.

This may make you think it has a rearward C/G. But if it is trimmed to hover with the blade (disc) level fore/aft, then all is well, even if the fuselage itself is tilted back a bit.

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