Take a Moment Before You Decorate

I’ve been building and flying scratch-built foam planes for nearly three years now.  I like to build planes that both fly well and that look good.  As a result, I’ve spent a good deal of time determining what works for me in terms of painting, making decals, etc.  I am constantly amazed at how large an impact just painting a plane can have on the way it flies, and sometimes whether it will fly at all.

For example:  You build one of the Flite Test designs.  You follow the build video to the letter, get the plane to balance just where they tell you it should, and you take it out for a maiden flight just like they do in the build video, pristine white foam board glistening in the sun.  It flies like a dream.  You love it.  You love it so much that you decide to make it look better by adding some color with paint and make it last longer by adding some packing tape to the underside where it would get wet from the grass in the morning dew.   By the time you are done it looks pretty darn good.

You shove the battery back in just where you had it before and launch it into the wind.  Only this time, it’s everything you can do to get it down in one piece.

Why?

The simple answer is that you didn’t rebalance the plane before you tried to fly it.  You made the common mistake of thinking that adding a little paint and a couple inches of tape couldn’t make much difference.  And besides, you added it everywhere, not just in the back.

But depending on how much weight you added and where you added it, it can make a great deal of difference. This isn’t intuitive.  After all, you added paint everywhere, didn’t you?

Peter  Sripol’s Mini Corsair is an excellent example of how this works and how you can run into unforeseen trouble.  The plans say that the balance point should be put at 1 inch behind the leading edge of the wing.  (Peter flies with it much further back, but then Peter hovers cargo planes, too.)  If you look at this visually, you can see that most of the plane is behind the Center of Gravity.

In fact, about 75% of the paintable surface of the plane is behind the CG.

So on a very simple level; you can see that if you add weight equally, everywhere, you are adding 75% more of that weight toward the tail than toward the nose.  Your plane will be tail heavy when you are done.  You have to rebalance.

Yeah, ok.  But it’s not that much, right?  It’s just paint.  And a little very thin tape.  It can’t have a very big effect.

Paint is heavier than you might think.  I recently finished a scale SE5a with a 38” wingspan.  By the time I was done with the paint job and decals, I had added nearly 175 grams to the plane.  And then I ended up adding a huge amount of dead weight up front to “compensate” for all the weight I had just added.

As I said before, this isn’t intuitive.  If you want a better understanding of how it all works, read on.

As a pilot, when you balance a plane, you are essentially moving weight around until the plane balances where you want it to.  And that’s generally where the designer told you to balance it.  That usually means moving the battery around, or maybe using a bigger, heavier battery.  In a worse-case scenario, it could mean also adding dead weight up front.  If the plane is heavy in the back, you add weight in the front. 

This is all based on the concept of moments.  It’s something that’s talked about all the time and understood not so much.

A moment is simply weight acting at a specific point at a certain distance from a reference or a fulcrum.  It’s important to remember that it is weight times distance that is important, not just weight when you are dealing with balance.

The 5 grams on the right can be balanced by putting 5 grams at 5 inches on the left OR by putting 10 grams at 2-1/2 inches.  Obviously, if you are balancing a plane, you would rather put 5 grams at 5 inches and only add 5 grams to the plane.  But that might not be possible if the plane’s nose is too short.  Worst case, you have to add 50 grams at ½ inch.

But how do you apply that to the decoration of a plane?  And why bother?

When you add weight to the plane equally, everywhere – like painting or taping – that weight actually acts like it is all at one place for the purposes calculating balance.  That place is the geometric center of all the paintable surfaces on the plane.   It isn’t easy to calculate the geometric surface center unless you have a CAD program, but it can be done easily enough if you do.

In the case of the mini Corsair that point is 4.6 inches behind the leading edge of the wing.

So let’s say we got the plane to balance at 1” from the leading edge and then decorated it.  Paint is heavy.  Tape is much heavier yet.  When I added packing tape to my Mini Corsair, it added 43 grams of extra weight.  It is not unusual to add several ounces to a plane with a wingspan of over 30 inches just by decorating it.

Let’s say we only added 28 grams of paint to this little guy.  If you were to add several light coats of acrylic paint with an airbrush and a coat of sealer over that, you could easily add twice that amount, but let’s just use 28 grams (1 ounce) as an example. 

That means we added 28 grams X 4.6 inches or 128.8 gram-inches behind the leading edge.  But since the plane is already balanced at 1” behind the LE, we are adding 28 grams x 3.6 inches or 100.8 gram inches behind the balance point. In order to return the plane to balance at one inch, we will have to add 100.8 gram inches in front of balance point.

Normally we balance by moving the battery forward or backward.  We could add the 100.8 g-in by moving a 69 gram battery (800mA, 3S) forward by 1.46 inches.

Unfortunately, there isn’t any room at the front of this plane to do that.  The furthest forward you can get (the center of) a 800 mA, 3S battery in this plane is 1” behind the leading edge.  At that point it hits the power pod.

That’s right over the desired CG and would be great if you hadn’t added that extra 100 gram-inches of paint.

So you are stuck with adding dead weight up front.   If you add weight at the very front of the plane, as far forward as you can go, just behind the firewall at about 1.75 inches in from of the LE, you’ll have to add 37 grams.  So the 28 grams of decoration will end up being 65 grams of additional weight.

If you ignore all of this and just go flying, the CG will be over one quarter of an inch more toward the tail than it was.  On a wing this small, 1/4" is quite a change.  Peter can fly that, but not everyone can.

In order to get my Mini Corsair to fly with the additional 43 grams of weight, 32.25 grams of it behind the desired cg, I had to redesign and rebuild the front of the plane to get the battery right behind the firewall and then I still had to add 14 grams of dead weight.  It looked good, but it took some work to get it to fly.

When you design a plane, you can often design in the necessary room to move a battery around or to use a larger battery to get the plane to balance where you want it to.  But when you are working with a scale model like the Mini Corsair, you are limited to the shape of the original plane, and sometimes it’s not easy to design that in.  Peter had the further disadvantage of needing to use the mini power pod to make the plane compatible with the “Mighty Mini” series.

If you are going to build a plane and intend to decorate it, realize that paint, tape, and decals will not just add weight to the plane, but will add that weight in sometimes inconvenient places.  Look carefully at the plane’s design before you start to build and see if there is going to be enough room to compensate for the added weight behind the CG.  Depending on the plane, compensating for that may not be easy, and ignoring it can lead to unexpected results when you go to fly.  A tail-heavy plane does not fly well at all.