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Author Topic: A question of weight and balance Guillow's Aeronca Champ build  (Read 323 times)
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Larry the Sailor
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« on: November 14, 2020, 11:35:51 AM »

I'm currently building a Guillow's Aeronca Champ that I plan to use for rubber powered FF. I don't expect too much from this thing but I would like it to be able to fly.
I'm new to this hobby so I may be overly concerned but I know weight and balance are important with these light weight models.
I have the major parts assembled and it's looking pretty OK so far.
I finished building the wings and decided to weigh them just to see where they came in at.
After shaping and sanding the left wing section weighs 6.01 grams and the right wing weighs 6.68 grams.
The scale I'm using is accurate and repeatable and I have checked it with know scale check weights.
I don't think I can sand off .6 grams and still have anything left but it would not be difficult to add to the light wing section in order to get them balanced better.
Any suggestions or am I getting too deep in the weeds with this?
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billdennis747
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« Reply #1 on: November 14, 2020, 12:32:47 PM »

In 60 years I have never weighed individual wings. I would stop worrying about it; if you can't, add some weight to the light one. Then you can start worrying about inertia effects of a weight at the tip!
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Larry the Sailor
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« Reply #2 on: November 14, 2020, 01:52:15 PM »

In 60 years I have never weighed individual wings. I would stop worrying about it; if you can't, add some weight to the light one. Then you can start worrying about inertia effects of a weight at the tip!

I think I'll add some weight by glueing stringer pieces spaced out along the ribs inside the wing to not have all the added weight in one spot.
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piecost
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« Reply #3 on: November 14, 2020, 06:44:25 PM »

Larry,

Models are generally trimmed to fly in circles. Often weight is added to a wing to aid this. So it follows that equal wing weights are nessesarily wanted. I would definately not add weight. See if you can tell the difference during trimming.
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kaintuck
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« Reply #4 on: November 14, 2020, 07:02:29 PM »

You might be able to hollow out ribs, or the tip.....and if you can't lighten her, then I agree, put more building material in the light wing.....it won't take much....600mg Grin
Or.....use heavy strut material on the light side!!!...some of that 'pine wood' in the kit...
Marc
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strat-o
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« Reply #5 on: November 14, 2020, 10:21:21 PM »

I agree with piecost here.  A 0.7 gram differential in weight really tells you nothing.  That's because you don't know what the weight distribution is.  If the heavier wing has higher density wood at the root and the lighter wing has higher density wood at the tip then the wings might, in reality, be perfectly balanced!  But as piecost pointed out, when you trim the model for flight for a nice gentle turn you will often add a small bit of clay to one wing tip to achieve it.

Marlin
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mescal1
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« Reply #6 on: November 15, 2020, 09:05:17 AM »

if you are building straight to the plan and using the typical heavy Guillows wood, then there are a lot of places you can remove wood.
the trailing edge is much wider than it needs.  you can scallop from the inside down to aprox 1/8".  There's plenty of spars and ribs, use
a dremel and cut holes in each wing rib.  You can even go further and build new wings with your own wood.  The plan has patterns to make
your own parts.  Personally, I'd finish it the way you've got it then build another using your own wood and modified to "add lightness".  focus
on the tail end of the plane.  You can look at other Aeronca Champion plans here on Hippocket to see how much lighter it can be built.  you
may also be surprised at how much lighter your plane can be by just using better wood.

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Larry the Sailor
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« Reply #7 on: November 18, 2020, 11:27:47 AM »

I added a little weight, about .5 gram, by adding small strips of balsa to the bottom of the ribs along the center of the length of the wing. When I get to the point of trying to fly it I'll update with how bad I may have screwed it up.
Thanks for the replies and tips.
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Indoorflyer
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« Reply #8 on: November 18, 2020, 12:13:52 PM »

Well, to answer your question, yes you are getting too deep in the woods.  I'd worry more about building the model "square", with no warps, and getting the two wing panels installed at the same incidence. If the components aren't even covered yet, you're definitely over-worrying this "problem."

Something to keep in mind, regarding  static balance:    A moment created by a weight W placed at 6 inches from the center can be "balanced"  on the opposite side by placing a weight W/2 at 12 inches!  As everyone has pointed out, unless you know the weight distribution along the length of the wing panel, simply adding weight is a bit of a crapshoot.  You may find that the light wing that you added weight to, will end up being the heavy side!
« Last Edit: November 18, 2020, 02:00:57 PM by Indoorflyer » Logged
USch
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« Reply #9 on: November 18, 2020, 03:02:02 PM »

As many said 0,7g difference is not something you have to worry about. Also it's on the right side, on the right wing and most probably you will trim the model to fly in right circles, at least during the climb.

If you really want to go down to the question (always a good idea in model building  Wink) you have to check the centre of gravity (CG) of each wing, his location spanwise and, why not, also chordwise. Once found the spanwise position you can exactly place the 0,7g so as to replicate the CG position of the right wing.

But as said before, I would not worry about, rather doing the CG check for curiosity.

Urs
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Jasco
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« Reply #10 on: November 18, 2020, 04:03:36 PM »

Everyone has said everything that needed to be said...except that the Champ is an excellent design to start with!

I've never built the Guillow's kit, but I have built other Champs and they flew like...champs.  I am not in the same strata with some of the master craftsmen on this forum, so just getting the thing relatively square goes a long way toward success.

One little 20" Champ I built had the vertical fin knocked loose unbeknownst to me. Every once in a while the rudder would apparently flop over and the plane would do a snap roll before continuing on as if nothing had happened.  At least that's what I think was happening.  Great fun!

I hope you can post some pix of your progress.
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60 years of wasting precious rainforest hardwood.
Larry the Sailor
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« Reply #11 on: November 19, 2020, 04:52:47 PM »

Coming along slowly. Still have the right arm in a sling after shoulder surgery so not as coordinated as I'd like but not looking too amateurish.
Still figuring the tissue covering but better than the results than the Javelin I built a couple of months ago.

https://flic.kr/p/2k8jstQ
A question of weight and balance Guillow's Aeronca Champ build
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OZPAF
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« Reply #12 on: November 19, 2020, 06:12:41 PM »

Nice neat work  - I doubt that you will have much trouble flying that.

John
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Larry the Sailor
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« Reply #13 on: November 22, 2020, 03:38:49 PM »

Mostly finished with this. Came it a 1.97 ounces with .32 oz of trim weight in the cowl to level it.
Made a few short test glides and a couple of flights with 100 winds on the prop and it seems like it has the capability to actually fly. It wants to fly straight and level so I guess I didn't screw up the balance to much with adding weight to one wing.
Too blustery to do much with it today, managed to crack a stringer in the nose right behind the cowl and decided to wait for a calmer day to try again.

https://flic.kr/p/2k9qwie
A question of weight and balance Guillow's Aeronca Champ build
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Jasco
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« Reply #14 on: November 23, 2020, 10:07:33 AM »

That looks very neat!.  I'm sure it will fly well.

I don't know about you, but I DO hate those Guillow's cowls. They all would fly better and last longer if the tailcone was vacuum-formed plastic and the nose was wood. Grin
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