thanks for the reply.

I think I understand about the battery now. I've found out how useful the ESC is already, in 1st place the shut down of the motor when the voltage is low, but also this : I was going to fly and turned on the receiver + radio...nothing happened and I heard a beep coming from the ESC like when the battery is empty. Since I charged it before, that wasn't the problem and then I saw what had happened : I accidentaly moved the controls so that it was at full power. Luckily this ESC has got the safety that it's impossible to turn it on at full power..it could have been very dangerous since I wasn't paying attention at the prop.

I'm gonna buy a NIMH battery pack since I heard it's better for longer flights or maybe a nicad pack with higher mAh.
The problem is the wind around here, for me the plane doesnt have to move at topspeed all the time, but with the wind I cant but give it full power. Else it wont move a bit. I try to glide as much as possible along the wind when it's possible, thus saving some energy.