Hello I just got my first real arf setup, a SU-31 ep by black horse models and it says to balence the airplane 80 mm behind the leading edge of the wing so i hold it upside down and add weights to the front and i finally reach ballence but then take the weights out and see that i had added 4 1/2 ounces to a plane where the box says that it should weigh 2.1 pounds is this normal?

you may need to move things around inside to make it balance better. like try to move the batt, or the receiver alittle

is 4 ounces normal for a little electric plane like this? or is it way to much. the battery is already all the way forward.

I'm a rank newbie....but....why hold the plane upside down?? I realize that logic says it should weigh the same either way...but.??? Did you try it without turning it upside down? It does sound like a LOT of weight to add. Rich

the instructions say to balence upside down

Depending on the motor, battery and what not, you can end up adding quite a bit of weight to balance. One example is the Sig 4*60. There are three of them at our field. All kit built which will add a bit more weight variable. I'll throw in a fourth which was my first one. Mine use a Magnum 91 four stroke. One is using a Saito four stroke, around the same size and the last is using an larger two stroke. My first plane used the stock motor mounts and I had to add 5 1/2 oz in steel behind the motor mounts which also pushed the weight of the motor forward. My second used the same motor and Dubro mounts and I find I am about right by just moving the battery back. I may need to add an ounce on the tail. The one with the saito has 1 lb of lead under the engine. THe one with the 2 stroke has the rudder and elevator servos moved way back and it still needed a couple ounces of tail weight to balance out. Quiet a spread on the same model.

Your 4 1/2 oz could be a bit much on a 2 lb plane. Have you tried moving weight forward, IE the batteries for one. If you can't balance by moving the radio equipment, the your option is to move the motor forward (screws up a cowl tough) or adding weight. One other thing to look at, if you have to add weight, a larger motor will some times solve that problem and give you higher performance in the process. IE adding usefull weight vs dead weight. Bottom line though is you do what you have to do to get it to balance. Remember that removing an ounce from the tail is about the same as adding five ounces to the nose. You may look aft to see if there is some weight you can shead. Last, box weight claims are not very accurate. Some weigh an empty plane, no radio or motor, some flight ready and then the motor and batterys used make a big difference. Just use that weight as a big ball park number.

Don

Read this:

[link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=601]Why balance a low-wing plane upside down?[/link]

The only other question is where are you putting the weight? The farther forward you get it the less you will need.
I have been known to glue lead shot into the front of the cowl.