Seriously? Ya bought a poor toy.
http://www.hobbyworld3000.com/gh-308...-airplane.html
And that about sums it up. Sorry to be so negative but so many people but cheap airplanes like these and wonder WHY?

Well, this is one hobby where you do get what you pay for.

The other reason may have been you were flying too slow when you started the turn and/or attempted to turn too sharply.