Selecting the right balsa for your models is not as straightforward as it might appear….
There are probably 5 phases of Balsa selection that you will go through as you learn the art of flying indoor models:-
1) Use what is to hand and feels light in your balsa box.
2) Calculate the density of your balsa and try to select/buy some light stuff from the model shop.
3) Buy some expensive light stuff from the specialist suppliers.
4) Realise that stiffness counts even more than weight and start testing every piece you possess.
5) Realise that every piece you possess is not stiff enough so try to find some that is!
……welcome to the madness that is indoor duration flying!!
Bob Bailey’s article in the BMFA Indoor Forum Report describes, amongst many other fine articles, how to find and test good wood.
This report can be obtained from me at £10 + P&P
In the meantime below is a link to a lovely web page that allows you to enter the details of the wood you wish to test then calculates the Density and Stiffness Coefficient; I’ve checked the results against my own spreadsheet and can confirm that it is spot on.
To find the bending load of thin wood you may need to use 0-10 or 0-20 gm scales otherwise for 1/16″ sheet and thicker normal digital kitchen scales will be ok. Simply hold the wood vertically and press against the scale pan until it bends, you will notice that once it bends the load stays pretty much the same. Note the reading then flex the sheet in the other direction, if not the same then take the average. I normally repeat until I feel the results are consistent. Don’t overbend the sheet – an inch flex in an 18″ sheet is fine. You can test strip or sheet using this method. 1/4″ thick 3″ sheet is not going to bend much at all so you will have to strip it down, about 3/4″ wide will work on the sort of densities we would be interested in.
NB. This testing method does not work for C grain wood, for this use the droop test over the side of your bench and choose the piece that droops least………hmmmmmm.
There are various similar articles in the INAV archives, Larry Coslick’s Hobby Shopper article (issue 107) sets the initial scene well too.
This is a photo of my deflection gauge which I made from the Coslick article, cut a selection of spars and choose the pairs that are stiffest ie. bend the least! I generally scarf join tapered wing/tail spars in the centre to give a close bending match.