the lower flap vent is a primary flap, it works also in conjunction with the radiator vent on top. On many landings the upper vent will come downwards when the flaps are deployed for landings. This is a good video to see flaps fully deployed.

http://www.youtube.com/watch?v=HCRnQTsAPw4


This model looks awesome on landing approach with the flaps down and gear down.