When Should I balance a Kit?
 

I am finishing up my Something Extra and was wondering if it is best to try to get it 'partially' balanced before I cover the fuse? I was thinking that it would provide me with access to the rear of the fuse if it is nose heavy. Is there any technique or common practice about balancing that makes it better if I do it before the plane is entirely covered? I would like to avoid putting lead weights on the outside.

RE: When Should I balance a Kit?
 

Other opinions may vary, but I would hold off until it's covered. I had the same idea and balanced my EasySport before covering and added about half an ounce in the very back to get at the CG. After covering, I balanced again and had to add an ounce and a half to the cheeks to counter the weight which was now buried in the frame. It seems that that the covering adds more weight behind the CG than in front of it.

RE: When Should I balance a Kit?
 

I like to balance, and weigh, the plane's frame before installing that tank, radio, engine, push rods, etc.

Then I measure the weight of the individual components, and measure their prospective location on the plane.

Then I make a little spread sheet to estimate where the CG is going to fall. The spread sheet looks like this:

Weight (oz) Component CG location (in) Mass Moment (oz-in)
bare plane xxx yyy xxx * yyy
engine xxx yyy xxx * yyy
servo 1
servo 2
battery
receiver
tank
spinner
prop
pushrod 1
pushrod 2
etc
balance
-------- --------------------------
Totals W oz XY oz-in

Overall CG location = XY / W (in)
To measure the component locations you pick a single point on the plane and measure
everything relative to that point. Components behind your reference have positive
location values, in front they have negative values. The overall location is relative
to your reference.

The simplest reference point is the leading edge of the wing since the desired CG on the plan will be referenced to that location. But, I prefer to use the firewall simply because it is a good place to hook a tape measure.

The last entry in the list above, !QUOT!balance!QUOT!, is the balance weight. Enter zero to begin with.
If the overall CG is no good, then you either move some components around on the spread
sheet to get it to work out, or you add some balance to the mix.

I've done this for a UCD46 because I read they often come out very tail heavy. After sorting it all out with the spread sheet, the actual CG came out nearly exactly were I estimated it would fall. I choose moved the rudder servo up from the tail to under the wing, shifted the throttle servo forward a bit, choose a battery location under the tank, and selected a more powerful and heavier engine - all based on the spread sheet.

I've done the same for GP Tracer with similar results. I first tried to balance it by simple putting everything in and seeing what happened. It was way out of wack and I had to pull the gear out. Then I made the spread sheet and decided to move a servo back to the tail, and put in a small compartment for the receiver and battery behind the wing. It I had used the spread sheet to begin with it would have been simpler.

If you don't care too much about the total weight it is not worth the trouble, however.

RE: When Should I balance a Kit?
 

No offense intended here, but I would rather put lead weights on the outside, or leave an uncovered bay at the rear to put weight in. Any other ideas out there? I know my SE will probably be nose heavy since I put a heavier landing gear on it. I will do the usual, move the battery back as far as possible, etc. I have thought about putting a 1/2 oz of lead in the tail as a preliminary adjustment. I was just wondering if there was an easier method.

RE: When Should I balance a Kit?
 

I usually build a small hatch into the rear of each plane. Then I finish it completely with everything installed. I then balance it. Put weight in the nose if needed there, or in the tail if needed there. The hatch allows me to change engines, etc and rebalance without tearing the plane a part to put weights in our out. Learned this from an old timer in the sixties and been doing it ever since.

RE: When Should I balance a Kit?
 

I do somewhat like staggerwing with a compartment at the rear sometimes. I also do test balancing throughout the build. Prepositioning and rearranging equipment as I go. I usually always never have to add led.
Edwin

RE: When Should I balance a Kit?
 

Wait till it's done. Unless you want to do math that would take a genius to figure out (You can take that as a compliment Jim) Just cover it first, then worry about the balance.

RE: When Should I balance a Kit?
 

Measuring and weighing all the bits and pieces if more tedius than the math - assuming you can use a spread sheet. I will post a spread sheet (if that is possible).

I guess I tend to enjoy the fiddling and figuring part of it.

RE: When Should I balance a Kit?
 

Hooked,

Staggerwing has got it right. If you have the room, a hatch is the way to go. Who knows how much that future tuned pipe will throw the CG off by;)

RE: When Should I balance a Kit?
 

I don't know how you could finish the plane w/o balancing it. How do you decide where to put the servos? Repeat after me, "Lead is evil, weight is bad..." Say that five times. With some planning you can almost always avoid needing to add weight. Once you have everything built, but before the radio is installed and the model covered, assemble everything including the tank, engine, prop, spinner, muffler and so on. Then select a servo placement that will put the cg as close as possible. Then place the receiver and battery to bring the cg the rest of the way in, or if it was already balanced, to keep it there. Here's the key, though. Don't permanently affix the receiver and battery if you can help it. That way you can move them around to make any necessary adjustments once the model is covered. Unless you are covering the model with sheet metal the covering shouldn't change the cg very much.

RE: When Should I balance a Kit?
 

Don't add lead weights whenever possible, add gussets and strengthening trusses and cross braces around fin/rudder/rudder (not the fuse at the tail). In this way, you convert the additional weight needed to good use while attempting to balance the plane. Convert all these lead weights into balsa gussets and you have plentiful of gussets to put to good use.

RE: When Should I balance a Kit?
 

Actually, the covering always moves the CG back, for two reasons. First, the tail is about twice as long as the nose, and second, there's a lot more material back there to cover the rudder and stab. A kit usually has things worked out well enough that you'll be safe installing pushrods and covering and doing final balance with radio position. If you're scratchbuilding, it always pays to check balance as you go. You have to be able to make it just a little nose heavy without covering. If it's coming out too nose heavy you can do things like building a (robust) batterey box behind the wing compartment or moving servos to the rear of the plane. If it's coming out too tail heavy you probably need to do more sanding!
Regarding the Somethin' Extra....almost everyone ends up moving the balance point back after the first few flights, sometimes a lot, but because of that 2:1 tail-to-nose ratio, it doesn't take a whole lot of lead to make a significant move, so I wouldn't worry about it too much. I bolted two chunks of 1/8" lead about 1" square to the bottom of the rudder right behind the hinge line to get mine where I liked it.

RE: When Should I balance a Kit?
 

I wouldn't say most kit manufacturers have things all worked out.

When I built my Tracer I asked a few questions here and found that most of them come out nose heavy. In the stock config I would have needed to add 5 ounces in the tail - which would have pushed to total weight over 6 lb, which I found awful. And it didnt' have a heavy engine, just a 46fx.

The GP UCD46 was the opposite. They come out tail heavy - there are long threads on the subject. If you used a 46 2-stroke, and the CG that Great Planes originally suggested, you'd find yourself adding 8 ounces (or more!) of lead to the nose. Re-arrange a few things to avoid the ballast and you get a much lighter plane. GP have since posted a tech notice suggesting a CG further aft which will alleviate the problem to a certain extent.

I have an LT-40 where I am not too concerned about the total weight. In that case, it has a hatch made from the rear most die (err laser) cut out. I'll add what ever is necessary when it is all together. Same goes for a Cub that is in progress.

RE: When Should I balance a Kit?
 

Thanks for all of the suggestions. It looks like I might try a combination of things that are suggested here. One thing I did consider was to weigh the original landing gear, and the new one that I added, and see exacly how much I have added in overall weight. Since the gear is not clear out front, but close to the CG, my thought was the it might not affect it too much anyway. Lastly, I will check the weight of the sullivan tailwheel assembly that I used, and account for that while comparing the different weight between the landing gears. Since one went in front of the CG, and the other behind, I may be concerned over nothing, or at least just a small change in the CG. I am using the common OS46FX engine combination for the SE since it is a pretty common match up to the plane.

Thanks again for all of your suggestions.

BTW: JimTrainor, my next plane is the UCD46, but I will be using the Saito 72 for power. I hope that it will add some of the weight I might need to get a better balance.

RE: When Should I balance a Kit?
 

You should read the tech notice on GP's site re the UCD46.

I have a Saito 80, which is a bunch heavier than the 72, That balances okay.

RE: When Should I balance a Kit?
 

Sorry. It's not a tech notice. It is an addendum to the UCD46 manual. Here:

http://www.greatplanes.com/manuals/g...-addendum2.pdf

RE: When Should I balance a Kit?
 

Thanks Jim,
I will make sure I read through it.

RE: When Should I balance a Kit?
 

As a general rule of thumb, I prefer to balance my airplanes before their first flight. :-)

RE: When Should I balance a Kit?
 

Wow, you and I have the same idea. I too balance before my first flight.:D:);)