Hi I just installed a new Rx-Tx system on a plane (http://www.nitroplanes.com/skytrainer400-red-arf.htmlif it's relevant)

<span class="Apple-style-span" style="font-family: arial; ">But the thing is that whenever i pull down on the right stick (pitch control), instead of just having the elevators move up, the ailerons go down as well.

To the best of my knowledge, you only need to raise the elevators to make a plane climb. However i can't stop the ailerons from moving when I pull down on the right stick.

So here's the question: is it OK if the ailerons also play a part in making the plane climb and descend (as opposed to just the elevators). If so, should they go tilt up or down to make the plane climb?</span><br type="_moz" />

No, that's not right at all. Your first assumption that only the elevator moves is the correct one. Some flyers do choose to incorporate flaperons that are coupled to the elevator function but this is only found on some serious aerobatic models.

If the ailerons are moving at all due to you inadvertently moving the stick while moving in the elevator direction then the ailerons should be moving one up and one down. Not both down together or both up together. Also if you carefully move the stick to move in the elevator only axis then at most you should just get a slight nervousness or very small wiggle of the ailerons where they hardly move at all. If it's more than that I'm going to guess that you have a fancy radio set that has some mixing functions. If so then the first thing to check is to plod through the programming manual and find out how to disable ALL of the mixing functions. By the sounds of things I'm betting that your Tx is set up with a mix or two in the "on" setting. For a trainer you definetly want to disable these.

The other thought is that if this only happens when you assemble the model that the elevator servo or pushrod is jamming into the aileron servo and pushing it. If this is the case you MUST correct this issue as the mechanical jamming of this sort will overload the servos. At best it would just result in using up your airborne battery sooner. At worse it'll result in the servo(s) overloading and failing due to the extra heat.

Did you get this plane already built and programmed by someone else? Now, some have programmed the action you described as it will tighten up the loops. This was often used in U-control systems to make smaller loops but seldom done with radio controlled planes. Now, if the plane was programmed for flapperons, where the ailerons are also used as flaps, there may have been some elevator mix programmed in as well for that but it is usually in the opposite direction; i.e. the elevator goes slightly down as the flaps (ailerons) are deflected down to prevent the usual ballooning as you lower the flaps. However, if the radio you are using is not programmable there would have to be some other explanation as it is not a normally desired characteristic. PS, make sure that the elevon switch is not ON as that will mix elevator and aileron functions.

Yea, my controller has proportional mixing, and it's impossible to disable the mixing...

Rodney, the plane came ARF, I installed the electronics. The Rx-Tx in question are the ones taken from an E-Flite Blade CP Pro Helicopter. Flaps are optional on my plane and I don't intend to use them.

Thanks a lot for the fast informative and speedy answers Rodney and Matthews.

So here's another question: assuming that it IS my 6ch mixing Tx that's causing the problem, will another 4ch or 5ch Tx work or must it be a 6ch? Feel free to post a link to a specific Tx you you would recommend (price over performance).
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If you're using the transmitter that comes with the Blade CP, it is the CCPM mixing in the transmitter that causes the aileron and elevator servos to move with the pitch stick with the receiver from the CP.
It isn't possible to open the back of the transmitter and check the settings on the panel for the DIP switches to disable the CCPM function, as there doesn't appear to be such a panel easily available.
A non-CPPM transmitter with my CP receiver has no cross-activities between pitch and roll. (Hitec Optic 6)
Changing programming on a non-Blade transmitter, take it out of helicopter mode back to airplane.
Or, change the swash plate selection from 120 degrees to 90 degrees.
Any negative-shift (Futaba, Hitec) FM 4-channel transmitter will work your receiver in the airplane mode.