Side view

But the side view on the plans seems to indicate the elevators are 3/8" thick. Maybe the solution is two 3/8" rods front and back of the 1/2" elevator and sand then down to 3/8". With my luck they will warp.

  At the time this was designed, it was common practice to have the elevators slightly thinner than the stabilizer. This is supposed to help eliminate hunting, I think, in place of having slop in the elevator control horn hole. It , in theory, makes a sort of "dead" area of airflow over the elevators. If you start with 1/2" thick wood, by the time you finish you should have a precise 3/8" thick elevator leading edge. If you start with 3/8", You have to be careful you don't inadvertently make the leading edge too thin. That's my take on it.
  Type at you later,
   Dan McEntee

I cant figure out what to do now. I read the elevator Tapering instructions and it calls out for a 3/8" rod for 3/8" elevators. But both the stab and elevators included in this Vector 40 kit are a 1/2" thick. So how do i properly  taper that? Do i need different rods??

Kevin, see Al Resinger’s post and mine that follows it in the following thread. In short, you can not make proper tapers where the planform of the flap or elevator tapers using the rod method without a jig.

https://stunthanger.com/smf/building-techniques/sanding-a-taper-in-flaps-stab-ect/msg568720/#msg568720

Kim

   You would be correct, assuming the flap is completely rigid. Fortunately for this purpose, it isn't, and also fortunately, sanding a thick balsa sheet to a taper releases enough other stresses that it swamps the exceedingly low stress induced by sanding these two "out-of-plane" surfaces on a flat tabletop. So the "two rods" method, with or without a jig, is a perfectly satisfactory way of achieving the goal.

     
       Brett

Yes, you can manage with 2 rods , but you will have to glue the t.e. rod temporarily to very centreline of t.e. Cutting a well-fitting groove in the c/l helps a lot with that.
If you want to have both rods against the table, you'll need 3 rods;

-1st t.e. rod with ø= (flap total thickness :2) + (final t.e. thickness : 2)
-2st t.e. rod with the ø=final t.e. thickness.
-And the front  rod with same height/ø than flap l.e. thickness, of course.

That is naturally more complex and has a bigger risk of introducing an error to the flap, so in my opinion gluing the rod in flap c/l is better.
But even better, more stable and often with a better stiffness/weight ratio, is a built-up flap with 1/20..1/16" skins and some ribs inside.
Using same kind of highly complex calculations as above, it is very easy to sand or hot wire-cut a foam building jig with correct surface twist to produce a straight flap.
I'm just about to build such flaps for my new model, so I can document the process in here. But don't hold your breath, it'll take a while. L

Oh sorry, I saw the picture too late.
I think you'll introduce more (permanent) stress by sanding with too high pressure/dull sandpaper. L

Another way which I use is to glue a basswood strip (3/32 sq.) down the center of the trailing edge, then sand down to it.  The strip can be removed or just left on to help prevent dings on the trailing edge . 

Dave

I do the sme thin

Another way which I use is to glue a basswood strip (3/32 sq.) down the center of the trailing edge, then sand down to it.  The strip can be removed or just left on to help prevent dings on the trailing edge . 

Dave

I use a 3mm x .5mm strip to do the same thing.  Not a single warp in stab or flaps where I have used the cap.

Ken