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How to find CG on unknown plane - 2/8/2013 12:00 AM   
mikes68charger


 

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Status: offline 		 I got a Spacewalker 2 from a friend who passed away. I can't figure out who makes it. I think it may be a Sig Kit but not sure

Been reading about CG calculator. But I'm scared of getting it wrong.

its got a very simple rectangle wing. 

Its 13.5 in wide and 82in long. 

I know. CG is 25-30% of the with

so I'm I right in thinking my CG is 3.5-4in?

thanks


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RE: How to find CG on unknown plane - 2/8/2013 12:54 AM   
flyinwalenda


 

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Status: online 		Looking at the manual for a Seagull Spacewalker2 120 (wing is 82.7 X 13.15) the recommended CG is 4.3 inches back from the LE.
Yes , for a constant cord wing you take the cord measurement time 25 -30 percent.   I normally balance at 27% and go from there.



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RE: How to find CG on unknown plane - 2/8/2013 4:50 AM   
Lnewqban



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Status: offline 		Yes, 3.5" will be a safe start.

For more precise calculation, the on-line calculator listed in the sticky thread above includes the tail.

Maiden a little nose heavy and remove nose weight at taste a test flight at a time.

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RE: How to find CG on unknown plane - 2/8/2013 2:38 PM   
charlie111


 

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Status: offline 		I am Old School But, you can always get close on a staight wing by balancing the plane from about 1/3 back from leading edge.A bit more complex with a swept wing.Try balancing it from a point that bysects the wing in the middle when a line is drawn connecting wingtips.See what happens when you hold the plane at the front of wingtips on swept wing?AS you would never add water to a Habitat Soup.Never make your plane (Tail Heavy) It will try to stall in lavel flight !

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RE: How to find CG on unknown plane - 2/9/2013 12:38 AM   
GaryHarris



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RE: How to find CG on unknown plane - 2/9/2013 2:29 AM   
da Rock



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quote:

ORIGINAL: GaryHarris

Main spar?



Really should mention the main spar isn't a good location if it's not straight. (Wing isn't swept)



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RE: How to find CG on unknown plane - 2/9/2013 2:35 AM  2 votes
da Rock



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Status: offline 		The really simple suggestions usually leave out a whole bunch of details.

It's really simple to make the measurements online applications ask for. It's dead simple and takes almost no time to do it.

Measure the wing's tip chord, root chord, half span, and sweep.
Measure the same things for the horizontal stab.
Measure from the LE of the wing to the LE of the stab.

How hard is that?

The applications deal with wing sweep, taper, etc and consider all the things that matter for pitch stability. The real simple techniques don't.

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RE: How to find CG on unknown plane - 2/10/2013 2:42 AM   
karolh


 

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Status: offline 		It can be quite confusing for a first timer knowing just what to use as the CG as there is calculated CG and there is also the suggested %f MAC to arrive at a balance point which is significantly different from the CG dimension, all of which can really muddle the water.



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RE: How to find CG on unknown plane - 2/10/2013 11:59 AM   
JohnBuckner



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quote:

ORIGINAL: mikes68charger

Its 13.5 in wide and 82in long. 




Your Spacewalker will fly best at 25% mac. Divide the 13.5 inch chord by four and measure back from the leading edge by that amount and that is where you balance the airplane at.

John

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RE: How to find CG on unknown plane - 2/10/2013 8:24 PM   
da Rock



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quote:

ORIGINAL: karolh

It can be quite confusing for a first timer knowing just what to use as the CG as there is calculated CG and there is also the suggested %f MAC to arrive at a balance point which is significantly different from the CG dimension, all of which can really muddle the water.





With the online calculators, if they can work a yardstick, that's all they need be able to do.

They don't do anything except measure some things, type in the measurements, then hit a key.

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RE: How to find CG on unknown plane - 2/11/2013 12:01 PM   
AA5BY


 

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Status: online 		I can understand the trepidation of trusting a calculator but maybe can help lay it to rest. Last winter I designed & built a 35cc model and used the calculator for the CG. It nailed the position. I've played both fore and aft and came back to the position, which provided a neutral elevator, no pitch coupling in knife edge and common power on/off trim.

btw... 4.3" is a good number for the Seagull Spacewalker II 1.20. Upgraded recently to LiFe batteries and rebalanced and flew mine last week and it flew great with that number.

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RE: How to find CG on unknown plane - 2/12/2013 7:23 PM   
buzzard bait


 

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Status: offline 		I've been using the formula the calculators use for about 30 years with great success. The neutral point depends on the wing area, stab area, length of tail arm, and the average chord of the wing. Once you know where it is you can decide how stable you want your plane to be and calculate a CG based on that.

People always recommend rules of thumb when this question comes up. They work a lot of the time because designers do things in similar ways. But you can do much better than just balancing on the spar or 25% back from the LE or 30%. Yes, those rules will usually allow the plane to fly, but you can do much better with a formula or calculator.

Jim

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RE: How to find CG on unknown plane - 2/12/2013 7:28 PM   
mikes68charger


 

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Status: offline 		Thanks guys,

I flew it 8 times yesterday. I put the CG a little ahead of 4 in, and it flew great, but it was hard to get a smooth landing, it seemed to fall out the sky the last foot or so.

So I guess I need to more the cg more to the rear.

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RE: How to find CG on unknown plane - 2/12/2013 9:21 PM   
karolh


 

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RE: How to find CG on unknown plane - 2/12/2013 11:21 PM   
AA5BY


 

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Status: online 		I second the Yup... losing elevator authority to hold positive angle of attack.

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RE: How to find CG on unknown plane - 2/16/2013 4:11 PM   
da Rock



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quote:

ORIGINAL: mikes68charger

Thanks guys,

I flew it 8 times yesterday. I put the CG a little ahead of 4 in, and it flew great, but it was hard to get a smooth landing, it seemed to fall out the sky the last foot or so.

So I guess I need to more the cg more to the rear.



Or come in with a click or two more throttle. Either that or don't pull off the throttle at the last foot or so. No matter what the CG, they fall out of the sky because they don't have enough airspeed or we have too much elevator input.

Sometimes we don't need another trim change, we need a slightly different flying technique.

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RE: How to find CG on unknown plane - 2/16/2013 4:44 PM   
AA5BY


 

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Status: online 		To some extent, the Spacewalker is finicky regarding landing. It enjoys the balance point being close. Too far forward and the mains hit too hard and bounce.

Too far aft and when the plane touches down it drives the tail down and if enough speed exist, it lifts back off and awaits a second landing. When the balance is good, it lands sweet.



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RE: How to find CG on unknown plane - 2/18/2013 6:06 PM   
Lnewqban



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quote:

ORIGINAL: mikes68charger

Thanks guys,

I flew it 8 times yesterday. I put the CG a little ahead of 4 in, and it flew great, but it was hard to get a smooth landing, it seemed to fall out the sky the last foot or so.

So I guess I need to more the cg more to the rear.


You are welcome

Nose heavy models must land hot.

You can safely move the CG aft 1/8" at a time until the elevator inputs start feeling twitchy.

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