The manner in which your unmanned aircraft is loaded will greatly impact how it responds to controls and the manner in which it flies through the air. Before any flight, the Remote PIC should verify the aircraft is correctly loaded by determining the weight and balance (W&B) condition of the aircraft.
An aircraft’s W&B restrictions established by the manufacturer or the builder should be closely followed. Compliance with the manufacturer’s W&B limits is critical to flight safety. The remote PIC must consider the consequences of an overweight aircraft if an emergency condition arises.
It is important to understand that while in flight, there are always four forces acting on your unmanned aircraft:
Weight - the act of gravity on the aircraft, pulling it towards the ground
Lift - the upward force that counter acts gravity. This is created by the propellers pushing air downward (on a multi-rotor UAV), or by the high and low pressure areas created by air flowing over the wings (on a fixed wing UAV).
Thrust - what causes the aircraft to be propelled forward. This can either be from the propellers on the top of the aircraft being angled in one direction (on a multi-rotor UAV). In this instance, the propellers would be contributing to both lift and thrust. Thrust can also be created to a propeller mounted to the front or back of the unmanned aircraft that pushes air rearward and propels the aircraft forward.
Drag - the opposite of thrust. This is what causes the aircraft to slow down. This is generally created by different types of friction created by the aircraft structure itself. A very thin and aerodynamic aircraft will generate less drag than a large boxy aircraft, due to the different amounts of friction they create. Think of sticking your hand out the window of a car and the difference you feel between when you have your hand parallel with the ground or perpendicular to the ground.