This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
First check for a warp or misalignment in the wing or
stab. These are the most common cause of boost problems, and
the reason that accurate building is so critical. If anything
is found, fix it.
Most gliders will have some pitch down at ignition and early
boost, and gradually change to a pitch up condition near
burnout. This results in an "S" shaped flight profile. If the
deviation is minor, don't worry about it. A slight roll
during boost will keep your glider headed in the right
Models that have boost problems can often be helped with a
longer and/or heavier pod. Extending the fuselage to put the
motor farther in front of the wing also helps. A longer rod
may help boost also, as will avoiding high winds when
If the model pitches down severely under thrust, the pylon may
be too tall or the thrust may be misaligned. If the model
pitches up under thrust, the pylon may be too low, or the
thrust misaligned. If the model starts straight, then starts
pitching up, the wing lift is causing the problem.