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Author Topic: Question about CG  (Read 1809 times)
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TWWARCH
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« on: July 05, 2018, 07:59:56 PM »

This is my square bird and I'm very proud of it. It's all done except for adding keys to the wing and setting the CG. Currently with the wing all the way forward the model balances a scant 25% of the chord length back from the leading edge. This might be fine for all I know except that I won't be able to adjust wing position to play with flight characteristics. To get the wing further back - somewhere in the middle of the adjustment range - it looks like I'll need to add tail weight.

What would you do?

WW
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Question about CG
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OZPAF
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« Reply #1 on: July 05, 2018, 08:28:49 PM »

I notice that you have a plastic prop and my suggestion would be to replace it with a balsa prop or considerably lighten the plastic one. Use a single edge rasor blade and scrape span wise along the front and back of both blades. It's surprising how much weight you can take off doing this.

It's still likely even with the lightened plastic prop  that you may need to add some weight at the tail, but it will be much less.

I suspect that the balsa prop will be around 1/3 weight of the plastic one and may solve the CG problem by it self.

John

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FF Bruce
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« Reply #2 on: July 05, 2018, 08:48:20 PM »

I just thought I'd ask is the rubber in? Most of the time the CG comes out close to the center of the rubber motor.
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TWWARCH
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« Reply #3 on: July 05, 2018, 09:24:44 PM »

Motor's not in! Duh. I've already made it but it's not in the plane. I'll load it and try again.

John, the prop is great advice too. Making a prop is on my to-do list but I was hoping to put it off through at least a couple of builds while I focus on the basics of good craftsmanship.

Thanks for your help.
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packardpursuit
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« Reply #4 on: July 07, 2018, 10:59:02 AM »

Seems like an appropriate place to ask:  Does the CG change by adding more,less rubber to an existing/established distance between the prop hook and motor peg?
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calgoddard
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« Reply #5 on: July 07, 2018, 12:32:07 PM »

Yes - except when the CG is at the midpoint of the length of the rubber motor.  Some builders design and/or build their models so that they can change the weight of the rubber motor without changing the CG.
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Bigbandito
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« Reply #6 on: July 07, 2018, 02:09:32 PM »

Nice looking plane WW. Well done. Hope it flies as good as it looks.


Sent from my iPhone using Tapatalk
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"If you love something, set it free...  Just make sure you've got a DT on it."  - Richard Bach (paraphrased)
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« Reply #7 on: July 07, 2018, 07:29:44 PM »

In Reply #5 I should have said:

"Yes - except when the CG is located midway between the end of the motor hook and the motor peg."
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TWWARCH
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« Reply #8 on: July 09, 2018, 01:30:02 PM »

Took it out Saturday for testing. Flies well but now it needs repairing Shocked

Seems I'm systematically working through all the mistakes one can make in this hobby. Need to put the wheels back on and reinforce the mounting better, relocated the motor peg slightly, and crank in a tad more downthrust but...

it flies great. Turns a nice right circle under power and glide. When I cranked up the power it climbed rapidly until the torque abated some, hung on the prop for a second, but then instead of stalling it mushed down and started a long easy glide. With the repairs made and flown on full winds it ought to really get up there.
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