Anticipated C/G change

[tom brightbill]

Is there any sort of guideline as to how much (%) the C/G will change between prior to and after finishing with dope? Rough estimate....?
Thanks

[Serge_Krauss]

Tom-

I'm pretty sure that someone will have something for you, although they'll need to know or choose themselves what it is that this position is compared to - like % of what? Also, you'll need to estimate how much your unfinished plane and its finish will weigh - separately - because heavier finishes will affect the outcome more than light ones to varing degrees as the unfinished weight varies (I've found that the finish weight will move the c.g. aft) on traditionally configured planes, especially flapless ones.

However, here's something you can do to get a rough approximation. It will work best on a profile model and worst on a fat-bodied model. For cambered surfaces like wings, it is striking how little the actual surface area varies from the wing area (projected) itself. They are close enough for this approximation. 'same for profile fuselages and especially for tail surfaces.

Last year the club created a .15-sized Goodyear/F-1 profile scale racing event that brought with it inherent nose heaviness problems. So for one, I tried to predict where the c.g. would fall on the finished model, so that we could decide how much ballast would be needed and whether it was necessary to shorten the nose. Working from photos of two partially finished models, I was able to make a little model of the model out of foam board. Note: All parts must be the same thickness and same material!The longitudinal c.g. or balance point of that model would be the same scaled location of the c.g. of the paint itself. Assuming your projected finish (of dope, whatever...) weight to be concentrated at that point, you can easily compute how much the model's c.g. will move aft with the application of your finish. The pictures below illustrate what I did:

1) Walt Elbrecht holding his "Shoestring" which had identical flying surfaces to the model in question.
2) Walt's and Bob Hudak's "Shoestrings", Bob's in foreground being the subject model in approximate side-view.
3) 2-V cut and paste on 8.5" x 11.0" paper of side and plan views to same scale at useful size.
4) Little model made by rubber cementing cut out photos as shown in #3 along side Walt's project.
5) Bob's finished model that flew well enough without chopping the nose.
6) Diagram of all the force estimates, since the hardware was not attached. All you need is the c.g. of your plane with engine/tank and your estimated finish weight. We guessed at 3.5 oz. for the small plane.

The little model balanced level at a straight pin pushed through the fuselage just slightly ahead of the wing root trailing edge, which is where the paint by itself would balance: c.g. of that model is the c.g. of the finish, within limits of other measurement accuracy.

What you do is choose a base line for comparison. It could conveniently be the foremost edge of the fuselage, the wing root leading edge, or anywhere else convenient. I like the root l.e.

1) Measure how far behind that point the unpainted plane balances, and multiply that distance by the unfinished plane's weight.

2) Make the little model and determine it's c.g. location. Compute how far back of your chosen base line that would be when scaled up to the size of the actual model. Multiply your estimated finish weight by that distance.

3) Add the two answers in #1 and #2. Then divide that sum by the sum of the two weights (unfinished model plus paint). That answer is the new c.g. distance behind your base line. You can then see how far the c.g. moves with added finish.

If you don't know your unfinished weight yet, a search on this or SSW forum will give you some sample weights for comparable planes, and there are definitely weights available in the SSWF archives and perhaps here too. I know a lot of these guys here do keep track of weight. From those data and any helpful member here who knows where his c.g. was before finishing, you can get a ballpark figure for your own plane.

My guess is that someone already knows your answer though. This has just been a DIY method, if you don't want to just trust the designers and build to their weights.

As always, FWIW.

SK

[Brett Buck]

Is there any sort of guideline as to how much (%) the C/G will change between prior to and after finishing with dope? Rough estimate....?
Thanks

   Impossible to predict because we don't know how much paint you will add, among other things. It will shift backwards, A LOT, like a few inches.

    Brett

[tom brightbill]

Serge, Thank you very much.  Now it's time to do my homework given the information you have shared.  FWIF, the plane is a SIG Chipmunk, with a LA 46.  I am getting close to covering, and thought that if there were a way to approximate the change, I may be able to add some initial ballast as appropriate, aiming for a conservative amount, and not have to cut into too large of an area later on.  Thanks again.

[tom brightbill]

Thanks Brett, I figured it would be a long shot, but thought it was at least worth asking.

[Tim Wescott]

I am getting close to covering, and thought that if there were a way to approximate the change, I may be able to add some initial ballast as appropriate, aiming for a conservative amount, and not have to cut into too large of an area later on.  Thanks again.

Is that it?  Heavens -- if you can't figure out how to add a good-looking weight box under the tail then bring it over here.  I'll sharpen up my hatchet and have something for you in a jiffy.

As an alternative (and one that I've never tried), I've seen someone on this group mention the idea of adding more and more fancy trim behind the CG until the plane balances out.  I dunno if he was being facetious, though.

[tom brightbill]

Ummm ya Tim, I've seen your hatchet.  Maybe I'll just use a good dull boulder instead.

[Dave_Trible]

Brett is correct, there are way too many variables like what weight of covering,  how many coats of dope and how much thinning, brush or spray, did you use filler-bought or brewed, etc.  Most likely scenario will be the need for nose weight-not tail weight.  Adding a box there may be exactly the wrong thing to do.  When you test fly the airplane take some clay or stick-on lead weights to find what you want at the field.  Then come home and bury the weight.  Sure it may add a step later but.......

Dave

[tom brightbill]

Thanks Dave, I just had to ask-.

[Rafael Gonzalez]

Most LHS sell weighted spinners in aluminum and brass. Hopefully you will not need a brass! In the end, you will adjust the C.G. to your preference.  As many in here have said, the plans give you a range of CG where the model will fly very stable.

 

[Tim Wescott]

If I recall correctly, that Chipmunk has a pretty long nose -- with a muffled 46 instead of an open-exhaust Fox 35, I'm not sure that you're going to come out needing nose weight.

But I always seem to get the balance thing wrong: in the past I've done some pretty elaborate calculations trying to determine just where the balance was going to come out on a plane, and I almost always end up sticking weights on someplace while wishing that I had made the nose longer or shorter.

And it isn't just me: I've seen mechanical engineers put in considerable sweat to assemblies that need to be balanced, and in the last few weeks before production release they're weighing all the wheel weights that have been epoxied onto the thing so they can make engineering drawings for threaded blocks of brass or tungsten to make everything come out right.

[Dave_Trible]

Tim,  what we do is pretty much equal parts science, experience, art, and dumb luck.  This topic is mostly covered by the last category.  Last year I built three airplanes as identical as I could make them.  They all weighed the same within two ounces. They all trimmed very different and the lightest one was the only one needing nose weight-go figure!  Or not. We can sure go crazy with calculations and bald when they don't work.  So....we build, we fly, we learn,  (oh yeah,  we have fun)

Dave

[Steve Helmick]

Tim,  what we do is pretty much equal parts science, experience, art, and dumb luck.  This topic is mostly covered by the last category.  Last year I built three airplanes as identical as I could make them.  They all weighed the same within two ounces. They all trimmed very different and the lightest one was the only one needing nose weight-go figure!  Or not. We can sure go crazy with calculations and bald when they don't work.  So....we build, we fly, we learn,  (oh yeah,  we have fun)

Dave

Dave...I'm wondering what trim parameters were different on the 3 'identical' planes?  I can envision 3 'identical' engines not using the exact same amount of fuel, which would then require a slight change in empty CG location. Different amount of tipweight would be very understandable. What else?    Steve

[Dave_Trible]

Steve all three obviously had different off-the-bench CGs. The lead out positions ended up a little different. This was due to minor alignment and a little warpage in the rudders and slight differences in engine offset. I'm sure unseen shaping issues in the fuse could be at work here. One ship holds the lines overhead better than the other two. Very small warp twisting in flaps caused a trim tab on one and flap replacement on another. The # 2 ship would occasionally stall on hard bottoms. I determined that I used an elevator horn that was a bit shorter than the rest by about 1/8" giving faster elevator than flap travel.  The new design I'm working with requires ALL the flaps on hard turns so the flaps weren't getting fully deployed in time.  Added a bit of flap chord to fix it. Understand I just refuse adjustable controls-can't stand wondering when the 'accident' will happen. I'll cut into the ship and correct something if I need too, just my choice. Anyway all three ships are getting face lifts right now so no doubt I will have to start over next spring.  I did get to fly the first one in new paint a few weeks ago and I think it flies better now even though its 5 ounces heavier.  Each airplane has its own lines and handle because those minute adjustments for each takes a while to walk in and are unique to each. Balsa, paper and paint are not as controllable or predictable as metal so I don't think perfect copies are possible.

Dave