wing weight

[cad]

do members weigh the wings of any model they are building prior to applying the covering, as at this stage it is still possible to sand away the weight difference. At what span does a gram or two difference become critical?

[001]

I would say that in the case of built-up Balsa wings for a sport/trainer type model of over 40" span, don't worry about a few ( say 3-5) grams. (Unless you have left something out).Before covering.

 

Balsa is so light, that a lot of sanding would be required to equalise.

Cover the wings, add servos etc., then balance the whole model laterally and add weight to one wingtip as required. You can usually get away with a small screw into the wingtip block (if it has one) or lift a small flap at the inside of the wingtip rib and glue a small piece of lead securely, towards the leading edge. Recover the hole with a small patch of the same colour.

 

I recently bought a 63" span ARTF and one wing weighed 20 grams more than the other!

Corrected by the 2nd method above. (Not 20 grams of lead, just enough to balance the model.)

You don't say if the model is I.C. powered, if so, definitely balance after everything is installed as the cylinder/silencer will probably be hanging out one side.

[cad]

Hi Richard

thanks for the comment. I am building a self designed glider two channel rc. Span 60". Wings are 48 and 54 grams respectively uncovered so that would not appear to an issue.

[Simon Chaddock]

cad

It will be easy to balance as 6g is not that much but when compared to their total weight one wing structure is 11% heavier than the other. This does raise the question - why?

 

Assuming the wings are physically identical then the weight difference is down to the wood used in their construction. How significant this is will rather depend on which bits of the wing structure are heavier.

If for example all the weight difference was solely due to the spars then the lighter wing is likely to be a bit weaker however if it was due simply to differences in solid balsa wing tips it would have no structural impact at all.

 

Apart from balancing there is nothing you can do about it now but it does show the importance of selecting equal and appropriate wood densities when building a wing, particularly for a glider.

[Gary Binnie]

Yep, I weigh all my larger glider panels. Much below 100" it's not too much of a problem. I have two 140" span gliders and one 100" glider that are carrying about 15 or 20 grammes of tip weight.

The 100" definitely needed rudder trim and I couldn't work it out, I checked the wing incidence angles, all good, then it dawned on me!!

I sit them on their towhooks and rear fuselage and see which side consistently falls then just tape thin sheet lead to the underside of the light tip till it falls either way.

I weigh every sheet of balsa with large built up wings, making sure that they are matched either side, despite that one glider came out with one wing 20 g heavier than the other and I could see no reason for it at all, I even used the same number of dope coats on each wing, mystified!!

This Hi-Phase is the offender!!

[cad]

Thanks Simon and Gary for the comments, I can see that balsa quality is more important than `i thought

[Gary Binnie]

On the other hand I've seen a full-size glider land with one wing full of water and the other empty, probably about 60 kg difference between sides!

 

Shows that ailerons can correct the problem, all of my thermal gliders (except one small one) are rudder only. The 100 incher with a heavy wing needed about 5 degrees of rudder to fly straight.

 

I've got a balsa weight chart pinned up on a cupboard door in my shed, the density range is amazing.

 

Using a 3" x 36" 1/8" sheet as an example ultra light grade is 14 grammes and ultra hard is 71 grammes, the stuff we use is around 35 grammes a sheet.

 

I do weigh uncovered panels like you asked in the first post and have a list of weights for five or six glider types if you're really interested!! The comparison with similar sized foam veneered panels is an eye-opener.

 

Some CAD programmes know the density of materials and can work out the weight of a part (or total of parts) by its volume, clever stuff!

 

GB

 

 

 

[PatMc]

CAD, at the worse your model might have a slight natural turn into the heavier wing that will be easily corrected by a smidgeon of rudder trim. But you probably won't want to fly in straight lines much of the time anyway so the imbalance is only likely to be a problem if you try to do big loops.

If you're determine to balance the wings a nail or two pushed into the light tip should do.

 

I often fly an glider with a Flycam either mounted under one wing or centrally on the nose hatch. The extra weight & drag on the camera wing requires a tiny amount of rudder trim to fly dead straight but I don't always bother as the model spends most of the flight turning anyway.

 

[Doug Ireland]

Although I don't fly gliders I do balance all my models in both directions. As all my models have engines I've always put any lateral imbalance down to the fact that there is usually an exhaust or a cylinder head sticking out one side, the thought that mismatched wood could play a significant part in the balance never occurred to me. About the only part of the wing construction I usually worry about is if the spars are the same and not warped. In future I'll be more selective over my choices of wood.