I am working on a pattern wing design it will be a foam wing. My question while cutting the foam wing is will the main root rib & tip rib be at same height i mean 0 dehideral.. OR i was thinking to put the tip rib at the level of root rib to get some dehideral on the wing. please help me on this... what is the best way to do it.

This is a 40 size pattern plane design i am working on.

Thanks..
High

you will  normally need some dehiedral , but if the design is almost like everyone else make the wing flat on the toop , make the highest point off root and tipp template at same height on the table when you cut the wing ...remeber to make one left and one right wing!
This will also make the root easy to join or fit to the fuse if your using a wing tube.

The location of the wing in the fuselage plays a role here.
The lower the wing in the fuselage the more dihedral is needed to avoid a rudder roll coupling.
For finding the exact amount you need to finetune. Look at current designs for a startingpoint.

A 40 sized pattern model will most likely have a one piece wing.

Don't fret the foam cut so much. A simple way to do it is to mark a center line around the foam block and place the templates on the line, root and tip. Make the cut as needed.

After the sheeting is done, simply sand the root of both cores to get the dihedral you want. In general we join wing halves up side down on a flat surface. If you want about a degree or two (depending on your span) just build the top side flat on the surface. This works well if the wing is to be located within 1/2" or so of your fuselage reference line

If the wing will be located lower on the fuselage, then block the center of the wing halves some, as much as 1/4"-3/8".

If the wing will be on the bottom of the fuse, block up the center of the panels more, to as much as 1/2" or maybe more

A pattern plane flies best when there is zero roll coupling when you apply rudder. Luckily, this is the easiest thing to fix when building a wing.

Good luck

Matt has it right, except that I disagree with this.

The first foam wing I did was with a core that was cut by a guy using this method. He used pins and double sided tape to attach the templates to the center line of the foam block. The weight of the hot wire pushed down slightly on the template as he cut them, and the result was that the thickness of the two wings varied by over 1/32". The next set I cut myself using the method shown in the picture below. The trick is to insure that the top and bottom of the foam blank are perfectly square to each other. I use a couple of 2"x2" aluminum angles to skim cut the blocks if they are not.

This method works well for symmetrical wings. Since the block and the template are resting on the work surface, there is no distortion. Once one side is cut, turn the block over and cut from the other side.