Thorp Air Command - T18.net

Hi,

I am hoping that someone can help with the answer to a question on T-18 spar construction.

With reference to Drawing 537 of the Thorp T-18 plan set (centre section wing spar).

The question concerns the 1/4" thick 2014-T6 extruded channel / angle spar caps -2 and -3 (channel trimmed to an angle at the tips). At the tip (in the angle portion) the top cap -2 has a angle where it meets the skin of 1 deg 30 min (i.e. 1.5 deg) and the lower -3 cap has an angle of 1 deg. At the root (in the channel region) these angles are 1 deg and 0.5 deg respectively. The vertical leg of the channel / angle is vertical so this angle must be put into the horizontal leg so it sits against the wing skin.

Is the extrusion bent slightly to achieve these angles and if so what is the technique used to form it ?

Or is the angle simply filed into the spar cap ?

The outer spar cap is similar but the angle only 3/32" thick.

I am hoping that someone who has built the spars can help ....

Thanks,

Stephen

No, you do not bend the extrusion. That would be exceedingly difficult to do.

After I completed my wings, I used a quality filler to smooth the bump along the spar on the top and bottom of the wing skins. Other builders just ignor this bump and their planes fly just fine. So, I am not sure if the effort was worth it, or not, but I carefully filled and then sanded along the spar so that the airfoil is as smooth as I could make it.

Thanks ... so it sounds like you are saying that you just left the spar caps as a right angle, riveted the skin in place and filled the resulting bump ?

By my calculation .... the cap is 2" wide at the root and with 1 deg of angle this is a step of 0.035" .... at the tip the step is 0.026". I suppose not too bad but not ideal for the polished (no filler) finish I was planning.

Has anyone actually formed the angle specified in the drawings and if so, how was it done ? Looking at the drawings closely it appears as though the intent was not to bend the cap but rather cut the angle.

Use a jointer or large disc sander set at the desired angle, remember some woods are harder than aluminum.