Tapered Wood Spars for Large Scale Aircraft

[Tom]

I have an on going project of building a 1/3 rd scale Pratt Reid to replace my 1/4 scale that went "ker-platt" a couple of years back. I'm at a point where I need to build the wing spars...... so I can build the wings ......so I can figure out how the wings are going to mount to the fuse....scratch building can be fun...but time consuming!

First up .....have a plan! My spars are double tapered....96" long with the root end starting at 1" x 3/4" that goes out to 34" and then from that point they taper in both width and thickness to 3/8" x 3/8"



There are a basically two ways to do this....hand tools...or power tools.....I have the power tools....so power tools it is!

1/2hp router with a spiral down cutting bit with a guide bearing on the bottom of it....



I cut my spar blanks from straight grain fir that I bought at my local lumber yard....it was about $15 for the wood for these 4 spars.
I cut them to 1" x 3/4" to start...



And then picked 4 from the 6 I cut and laid them out so the straightest grain was at the root and any run out was toward the tips....these "sticks" where real good with very little run out...

I then labeled them as to what they where and which ends is which...



I then marked where the cut line needed to be on each set of spars....

[Tom]

Next up is the "Jig" that I use to taper the spars in.....I made this jig from some left over 1 x 4 fir that I had....first I drilled holes down the center every 10" and screwed the both of them down to the bench with the skinnier one on the bottom...I then routed the top board to match the bottom board.

I then back out the screws and put the spars in the jig with the areas that need to be cut hanging out side of the edges of the jig....





Then I made a bunch of saw dust....I took the wood down in layers rather then try to just "plow" thru it to make sure it didn't chip out...the spiral bit works wonders for that but you still run the risk of chip out if you feed to fast and if the bit is not super sharp...

Next up you turn the spars on their side and mark the other taper line and clamp them back in the jig....this time you need to be careful with the bit....you can only cut so far before it hits the table and you need to reset the depth of the bit....



Once both sides are done you end up with nicely tapered spars in a fraction of the time it would have taken to do it with a hand plane....

The root ends.....



The tips.....



Here are the joiner brackets laid on top of where they will end up.....



The back half of the wing that goes on the spars.....



And what was left of the parts I cut off!!