This paper presents a new type of flying robot dedicated to grasping and manipulation of large size objects. The system can be basically presented as an aerial hand with four fingers actuated by four quadrotors whose arrangement permits the manipulation of the grasped object in the space. Each finger has two phalanges and is underactuated in order to adapt itself to the object size and shape. The opening/closing motion of each finger is actuated by the yaw motion of each quadrotor and transmitted through a non-backdrivable mechanism in order to enable the system to produce form-closed grasps. This stability criteria yields to secured grasps which do not rely on the capability of actuators nor on the contact friction between the phalanges and the object, and furthermore do not require any additional energy for gripping during the flight. The present paper gives guidelines to optimize the geometric parameters of a planar aerial robot in order to maximize the robot's manipulability and its capability to produce form-closed grasps.