I have been running antenna inside the airplane fuselage for about 10 years now, in planes ranging from 2M Pattern to 40% Scale Aerobatics stuff. Never a hint of a problem. Provided that the fuselage is not made of or covered with anything that will reflect or absorb RF energy, you should have no problems. Now think about this for a moment Suppose you do run the antenna straight outside of the airplane. For some aircraft attitudes and orientations from the transmitter, the antenna will be completely in the "shadow" of the airframe and you will have succeeded in effectively doubling the amount of airfame material in the way. So why does it work? Because RF goes through paint, monokote, balsa, ply, fiberglass, Nomex, Kevlar really well, that's why. Rumor has it that metallic colors, especially Monokote chrome can be a problem. Don't know if that is true, so duoble check if using something like that. You might have some loss of range with alot of Carbon Fiber surrounding the antenna.

Anyway, no matter what the model is made of, do a series of range checks with the antenna inside and out and see what you get. Makes sure you do this from a variety of orientations, but be 100% consistent and repeatable with the way you do the tests for each method, otherwise, your results won't mean much. Besides the neat asthetics of having the antenna inside, you also provide an extra measure of security because it can't get damaged as easily, plus you don't have to clean it. Another thing, as the antenna wire whips and vibrates outside, the wire strands will work harden, making them more brittle. Also, you aren't protecting the insulation from UV and it will harden and eventually crack. So run it inside unless the range check shows poor results!