Patience and perseverance may be needed to get some models to fly. There are a number of common causes for a model failing to fly, that beginners may not be aware of.
The first is that the thin flying wire has resistance and although you may have 12 volts at the pole end, the voltage is lost as it passes down the wire. You can end up with as little as 9 volts at the motor terminals. Ideally you should have a power supply greater than 12 volts, but the small motors (4550 & 4551) will fly with a 12 volt supply provided that the line length does not exceed 2 metres. Furthermore, it is essential that the transformer has an adequate current output. The current output must be at least the same as the motor you are using.
If the model whizzes around the ground without taking off, then more up elevator should help (see also C of G below). If too much up elevator is given, then the model will fly much slower. This is where it climbs into the air but has not the power to continue to climb and then suddenly drops usually ending in a crash.
It might also be that the centre of gravity (C of G) is too far forward. In R.T.P. the C of G is much further forward than usual free flight models, it should be between the leading edge and one third chord back from the leading edge.
Flying outdoors modellers do not appreciate the effect of the wind. When the model is flying into the wind the air speed increases causing the model to climb. It then comes downwind flying with the wind, its air speed is considerably reduced causing the model to dive. As it comes round again into the wind it starts to climb again but higher than before, it then dives on the downwind until eventually crashing. To overcome this, you must ease up on the power as it comes into wind and then give it full power as it goes downwind.