There should be a trim tab which you can adjust for straight flying.I can't see your transmitter very close,but all flying RC's have a trim tab,simply marked 'trim' usually,and could be printed very small.It may have one for proper centering during straight,level flight also when letting off the pitch control(up/down).It is quite sensitive,so just a little movement will make a big difference.Also applies to RC cars,at least the better made ones.
Good luck,Greg