Someone gave me a very large / heavy trainer and while in flight the spar broke at dihedral angle of the wing. I was not pulling hardly any g's so in retrospect, it must have had a crack from a previous crash. I have been able to remove the spar and am now trying to figure out how to build a new spar. The old one measures 27-28 mm tall, 10 mm thickness, and about 17 inches in length. The spar appears to be two pieces of plywood (5 mm each) glued together to equal the 10 mm thickness.
I have never done this before so need advice: 1. what kind of plywood (spruce, fir, basswood ???) and where can i buy it.
Also any other advice is more than welcome
thanks
Irvin

You'll need the aircraft plywood that hobby shops sell. It's comprised on this layers of good quality wood and is really strong for its weight.

Do not use basswood for a spar or anykind of a spar doubler. If the original type was not plywood (pretty common) then Spruce is a preferred type of spar material. Most model airplane spars are not plywood however any kind spar doubler or joiner the preferred material would be Birch plywood that is multi layer and often sold in shops. A common ply used in many airplanes especially arfs is called LitePly and it has a minimum layer count usually not more than three ply whereas Birch Ply in the same sizes are up to seven ply. Lite Ply as well as basswood is a really bad idea for spars or doublers as it is very, very weak.

John

Just a mite confused but isn't this a spar. I understand not using bass wood for a joiner but every kit I have ever built has either a bass wood spar or a balsa spar. I just finished a wing for an Up-Roar and it used bass wood spars. In my case this isn't a kit but one of my rip off scratch builds and I cut my own wood. In this case this wing has a top and bottom 1/4 sq. bass wood spar. Is the OP asking about a spar or joiner?? I keep two big blocks of bass wood on hand just for cutting spars as needed.

Well GB in this hugh world of modeling spars have been made of just about everything from paper tubes all the way up to carbon fibre but since the question was asked directly as to the preferred between spruce, fir and basswood I indeed offered my opinion that basswood is the poorer choice and yes I did offer up my opinion that that birch ply would definitely be my choice and far superior to LitePly in that application.

John

In regards to plywood, Fact: Basswood plywood meets mill spec. 6070 aircraft standards and is just about as strong as birch plywood and almost as light as light ply. With that said it a much better choice then either birch ply or light ply because you get the best of both worlds in a properly designed airframe. I have been using primarily 3/32" thick basswood plywood from Aircraft Spruce & Specialty on all my giant scale stuff like motor boxes wings, tail feathers and fuselage components without a single failure to date so I don't see anything wrong with using it as a doubler in a wing.

P.S. I do use 1/4" thick basswood plywood for my firewalls and landing gear plates though, anyway just my point of view.



Bob

Thanks for help. Ordered birch plywood and will now have to cut to length and shape when it arrives. this will be a first for me on an RC. Biggest challenge I foresee will be duplicating the dihedral
Irvin