Speed is your friend.

For any straight flight in balance, the lift of the wing must equal the weight of the model.
In the same way, for any turning flight in balance, the lift of the wing must equal the virtual weight of the model (actual weight plus centrifugal force).
In other words, the wings of a model turning level while banking at 60 degrees must lift twice as much as they would lift when flying straight and level.

For either straight or turning flight in balance, when air speed decreases, the AOA must increase.
But the limit to this rule is the stall or critical AOA.
The only way to sustain flight just before reaching the AOA is by increasing the air speed.

For a light model, that necessary air speed is not much (remember how lift depends on AOA and square of air speed).
However, for a heavy model, that air speed needs to be high, or simply, the model will descend because the weight will be higher that the lift produced by that air speed.

Read more about stall prevention here:

http://www.rcuniverse.com/forum/m_89...tm.htm#8947745