title_body: Can something be mounted underneath a quadcopter rotor?
upvoted_answer: If you block the airflow into or out of a rotor, you reduce the lift it can generate (approximately) in proportion to the area you block. That means the motors will have to work harder, and your flight time will be reduced.
  Work out what percentage of the total rotor area your camera will block. If it's more than about 20% you should probably look for a larger frame.
  Obstructions above the rotors are less critical when hovering (as the rotors are sucking in stationary air from all sides) but in forward flight the airflow is more linear - and rotors don't like working in disturbed air so it's worse for high speed flight.