I have posted this information in another thread ("What do you think about the JetLegend L39" ) , but I am sure this is interesting enough to spend a separate thread of information on it.

Nicolas.

Dynamically balancing the wings
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This method is very similar to balancing heli blades.
In fact, most of the times pilots only check the CG, which is the balance in one axis only.
You should also check the balance in the other axis, which means : do both wings have the same weight ?
In 99% of the cases, builders don't check it, they simply correct it in the maiden flight with aileron trim and ignore the fact that the plane is, albeit flying straight after trim, not making nice loopings anymore or not rolling axial.
This has to do with balancing (as one of the reasons, next to unequal throws etc).
Not only the wings should have the same weight (= statical balancing), they should also have the CG on the same spot, while having the same weight as well.
I found that on my L-39, there was a difference between both wings of 23 grams (half the weight of a good servo). I found this by putting them on a very accurate digital balance. Where I had to add the lead of 23 grams, was determined by the method identical to dynamically balancing rotor blades. If not, the balance would have been statical but most likely not dynamical. Especially in high G-maneuvers, this should still result in necessary corrective stick movement on ailerons, rudder or elevator, for example to keep the loops in the right vertical plane.
Watch the very informative video below, and you will understand the method. Do not forget to create a user in Helifreak.com first, or you will not have access to the videos.

http://video.helifreak.com/?subpath=...lancing101.wmv

Nicolas.

This is how to apply this balancing method to our jet wings :

Of course, balancing rotor blades needs more precision than our wings, so here is the simplified method for our planes with separate wing halves :

1. Check the weight of each wing very accurately and calculate the difference. In my case, this was 23 grams. I should add 23 grams to the LIGHTEST wing, and at the same time make sure the CG of each wing is the same.

2. Take a tube which is large enough for the wings to tip back and forth when rolling the tube (this tube is replacing the role of the main shaft of the heli as in the video (= minutes 7.30 to 9.00 on the video)). This can be a plastic tube of about 2"-3" diameter. On wings without tip tanks, diameter can be less.

3. Make a mark on the wings approximately on where you think the CG will be of the wing halves. This reference length mark is measured from both the wing roots of the wings. This mark will serve to observe that both wings are at equal point rolling over the tube. Of course, this length mark must be measured very accurately so that they are identical on both wings.

4. Take the heaviest wing first, and roll it on the tube and check the location where the wing tips down and back. This is now the CG point of the heaviest wing. (= minute 8.30 on the video). Mark that point for easy reference as your CG balance point. (You'll have to put the wing upside down, as the oleo leg will tend to fall down on the table otherwise.)

5. Purpose is that you should add the lead (= the famous 23 grams) to the lightest wing, while making sure the CG of the lightest wing is exactly the same as the CG of the heavier wing. This is the vital point of DYNAMICALLY balancing the wings.

6. Put both wings on the tube now and roll the tube back and forth and move the 23 gram over the surface of the lightest wing until you'll find that both wings are tipping down at exactly the same point. This is where you should glue the lead inside the wing. Of course, if you use 5 grams of epoxy glue, only add 18 grams of lead to reach a total of 23 grams.


Small hint : I do not like to make marks on my planes either, but if you put some paper tape where you're going to mark with a pencil, you can remove this tape easily afterwards.


I hope this trick is very useful to you too, I have applied it to most of my planes now and some did show a major improvement in flight.
The champs ALWAYS balance their models on all axes, in particular in F3A competition.
This method is a bit more sophisticated than a simple statical balancing, and without doubt the way to go for high-G jets.

Too bad if we let the difference between a excellent jet and a bad jet over to some coincidence and good luck balancing ! Perfect CG is the key to superb flight characteristics.

Nicolas.

Am i right in saying that the problem is not only limited to the wings though? I have seen a jet go up and then when pulled up/down to it would always roll slightly to one particular side. The problem: batteries all on one side of the fuse. Wouldn't it be much better to laterally balance the whole plane in flight ready condition to take into account the overall weight of the whole plane, not just the wings? Even though the weights in the fuse are on much smaller moments they will probably still add up to be more than 23 grams on the wingtips (where the weight difference probably didn't come from anyway).

this is how i do it , AFTER one checkes the wings incidence , however on some jets it is not nessecary because they fly slow..

I agree, what should be done is, weightwise : try to build and assemble the parts as symmetrically as possible, then balance the wings as described on the above, then mount them again on the fuselage and balance the whole plane again. Any imbalance should NOT be coming from the wings anymore, but from the fuselage (and the stabs), and should be corrected there.

The practical difficulty is that it is VERY hard to balance a jet on two points on the fuselage. The nose is usually rather easy to hold, but how are we going to do that at the rear ? Holding your finger at the bottom of the fuselage under the thrust tube is in my eyes a very inaccurate point as this is too low under the CG and prone to very subjective observations and conclusions. Better would be to make a round insert with a ball bearing axle to fit in the thrust tube... But after all, we are modelers and such solutions and tools must be a bit reasonable too.

Note that a deviation of 23 grams, although not much at first glance, is easily the weight of a battery pack in 6G-force loops ! (And loopings are the typical maneuvre where you can see such imperfections, and to a lesser degree in fast rolls which are not perfectly axial, but this is more related to unequal throws of the ailerons etc)

And yes, by all means, you should start from a model wich is perfectly well shaped, is symmetrical in any respect, and has correct incidences. If not, all we try to do to correct mistakes afterwards, is like putting band aids instead of preventing the wound.

A former R/C world champion told me once that if trims are needed, there is something wrong with the model. The ultimate goal should be to avoid any trim at all. He showed me that his rudder had no trim at all, the TX rudder trim only corrected the straight track of the nose wheel. (TX settings : PMixer : Nose wheel --> rudder / trim : NO) Such a brilliant advise about the importance of proper balancing and building was a priceless lesson to me.

Technology allows us to throw in mixers, gyros, etcetera etcetera to correct the intrinsic flaws, but still you'll never manage to have it all mixed out due particularly to the wide speed range of jets causing a wide impact of different moments at different speeds.

N.