Such models are usually fixed-pitch (i.e., the blades cannot be rotated on their axes for different angles of attack), simplifying the model but eliminating the ability to compensate with collective input. Compensating for even the slightest breeze causes the model to climb rather than to fly forward even with full application of cyclic. Multirotor type Unmanned Aerial Vehicles exist in numerous configurations including duocopter, tricopter, quadcopter, hexacopter and octocopter. All of them can be upgraded to coaxial configuration in order to bring more stability and flight time while allowing carrying much more payload without gaining too much weight. Indeed, coaxial multirotors are made by having each arm carrying two motors facing in opposite directions (one up and one down). Therefore, it is possible to have an octorotor looking like a quadcopter thanks to coaxial configuration. Special Duocopters are characterised by two motors aligned in a vertical axis. The control is performed by the appropriate acceleration of a single rotor blade for targeted thrust generation during revolution. Having more lifting power for a greater payload explains why coaxial multirotors are preferred for all professional commercial application of UAS. The U.S. Department of Transportation has published a “Basic Helicopter Handbook”. One of the chapters in it is titled, “Some Hazards of Helicopter Flight'. Ten hazards have been listed to indicate what a typical single rotor helicopter has to deal with. The coaxial rotor design either reduces or completely eliminates these hazards. The following list indicates which:   The reduction and elimination of these hazards are the strong points for the safety of coaxial rotor design.