The Pathfinder is a good design for stunt competition, and conversely, a great sport plane. It's designed to make it easy for a low time builder to construct. The trike gear is there for a reason. It will handle a grass field well, and take offs and landings are much easier, therefore will score higher in competitions.
The wing is designed to be built on a flat surface, without the need for jigging or levelling blocks, You'll notice that the airfoil is flat from the high point back to the trailing edge, Though the instructions call out the leveling blocks, it's simpler to not use them. Your choice though.
First, with a soft pencil, draw a centerline on each rib.
Make up your spars and trailing edge sheeting. Cover the plans with the plastic from the back of your favorite plastic coating, or wax paper.
Pin the trailing edge sheeting to the plans in the correct position. Set the spar over the correct position, and install the ribs. Be sure to place the center ribs in the center and that you have the Ribs with the doubler and cutout for the gear blocks in the right place. The cutouts for the gear blocks should be closest to the building surface.
Make sure the ribs are correctly aligned and install the top spar. Pin as needed, and make sure the bottom spar is all the way into place on each rib. Then using thin Ca and an applicator tip, glue every thing in place starting at the trailing edge. Once the assembly is glued, use some Ambroid, or thick slow Ca and glue the top trailing edge into place.
Install the leading edge next. Make sure that it's straight and that any curves are not oriented up or down, rather they should be either fore or aft. If it's not possible to orient them correctly, either make up a new one without any twists or bowing, or draw a center line and line that up with the center line of each rib. With the leading edge Glued into place, use a small block plane to true up the shape similar to what is shown on the fuse side view. It should look more like a triangle, you will bring it to final shape after the sheeting and cap strips are installed.
Install the controls as shown.
Install the top L.E. sheeting starting at the spar. Roll the sheeting towards the leading edge, and tack glue it to the prepared leading edge. At this point, the glue is only on at the spar and the leading edge. Install the center sheeting. don't leave off the radii at the leading and trailing edge sheeting. Clear the slot for the flap push rod where shown. Install the cap strips on the top of the ribs as shown.
Now, turn the wing over, block up the trailig edge so the flap pushrod clears,lay some weights across the ribs to make sure the Top, (now on the bottom) leading edge sheeting is flat and tight to your building board and install the leading edge sheeting in the same way you did the top. Remove the weights. Install the gear blocks and tip weight box. Now install the cap strips and bottom center sheeting.
Remove the wing and holding it so you can use thin Ca and an applicator tip, glue the leading edge sheeting to the ribs.
All that is left are the wing tips and adjustable leadouts. They install as per instructions.
Sand the leading edge shape true to what is shown on the plans.
Set up a sanding block with 220 or 320 sand paper and give the entire wing a good sanding. Pay attention to where the cap strips butt into the leading and trailing edge sheeting. You want the transition to be smooth. I also sand a slight radius on all surfaces where the covering will contact wood. Not much, just enough to soften the outline that will show when it's covered.
On the fuse, the recessed tank mount is worth the time it takes to set ip up. Also, if you can, use the optional "corrected" flap horn shown at the top of the plans. All demensions are provided and it's not too hard to make, but you will have dead on symetrical throws on the controlling surfaces if you use it. The RC style horn on the elevator will work fine.
Use at least 1/8" music wire for the bellcrank to flap link and a CF arrowshaft for the elevator link. Ball links work great when connecting the bellcrank to the flap, but 4-40 clevises, secured with a ring of fuel tubing, will work fine for the elevator link. You could use a Z bend at the flap end without a problem. But you want to be able to adjust the elevator, so use a clevis on that end at least.
Some feel that clevises are an accident waiting to happen, and they can be if you don't use the right ones. Never ever use the common 2-56 RC clevises. they will fail. Always use 4-40 clevises. I prefer the type that has a securing hook for the fuel tubing safety. I've used them for years without a failure.
Use Sullivan light weight wheels. they are a little bit heavier than the Dave Brown units, but have a hard rubber surface that won't wear out so fast.
The Pathfinder is designed to use Monokote or other iron on coverings on the flying surfaces to help control weight buildup, but any good covering system can be used. We've always used silkspan and dope to prep our fuselages. I've finished my last few planes using Top Flights Lusterkote. It matches the Monokote and is a darned good paint that is very fuel proof. Of course if you use something other than plastic covering on the flight surfaces, you can choose the finish you want. I'm partial to Brodaks myself.
Balance where shown to start. Trim to suit, and you'll be surprized at how well it flies.
Power? We have almost always used a modified OS 40fp. I've seen nice flying examples powered by ST .46's and I'm sure that there are many engines in that size range that are acceptable. One of the possible drawbacks to larger than a .40 engine might be having to use .018 lines. I know some topfliers who prefer them though.
Avoid heavy engines that will require additional weight to balance as it's just dead weight. The plane should come out at 44 ounces for a light one, and about 52 ounces for one approaching the upper limits.
When the balance is correct, the plane will just barely set proper without fuel in the tank. When you push the tail down, it will set that way. Gear placement, for trikes, is important and the way this one is set up allows for almost effortless take offs and landings.
Enjoy your Pathfinder, It won't hold you back.