That's really the part that scares me the most :P
How to guess the location of the engine and opening and valves?
Unfortuantely I don't a second transparent dummy supplied with my model...

Also, what tools do you use? how to proceed?
Lots of pictures are very appreciated

Refer to the attached diagram.

I usually use a Dremel to cut the cowl openings out. One trick, cover the area that you are going to cut with masking tape to keep the finish from chipping. [link]http://www.rcuniverse.com/magazine/reviews/814/100_5367.jpg[/link]

Ken

RCKEN;

That is the simplest method that I have ever seen!!

And It will Work!

Ken is showing the best way to locate opening.

I rough cut them with the method shown, but use a small sanding drum in the Dremel for final shaping. This allows a very smooth edge, and you can blend the shapes. Use a good dust mask and eye protection, and do it outdoors.

For the big cut-outs I use a hole saw in the small drill press, or you can use a hand drill. This
will rough-out a nice big hole, and get you started.

Then use a drum sander to do all the rest of the opening for the engine.

Then use a small drum sander in the drill press, or the Dremel with a small drun sander
fot the detail work. If you keep the edges and corners of the cowl round, it will lessen the
chances of it cracking from vibration later.

Using the drill press works best because it holds the cutting part for you. You can use both
hands on the cowl, lessen the danger of flying parts, and have better control of the cutting process.

Use a mask, and do not breathe the fiberglass dust....not good.

FBD.

I have found your tips very useful, such as covering the area that you are going to cut with masking tape, but about where to cut the cowl I am in trouble here.
The fact is that I have to assemble an IMPULSE (www.bnmodels.com) and in this ARF the fuselage and the engine cowl are one single piece of fiber glass, so that you can't remove the cowl.
The engine can be mounted through a large opening, however I need to cut the cowl to let the muffler protrude off one side.
The plane deserves a neat job here and I really wonder how I will make it.

Franco....

....you have to plan your cuts. Look at some other planes and see how they are done.
You will have to cut your plane by yourself. This is the learning process, and it is fun.
Don't be afraid to make a mistake. You would do well to invest in a Dremel tool, or
something simular if you are going to hobby.

First you must cut the opening so the engine can fit in. Then you must cut for the muffler to fit.
Finally, you must cut for the needle valve access, and anythig else that needs trimmed.

Here are some ideas on cuts....[sm=thumbup.gif]....just go for it.

FBD.