The Forces at Play:


                When an object is moving through air with a constant velocity, the total force acting on it is zero. The forces that come into play during any flight, are :


1) Gravity - always acting downward.


2) Drag - another force opposing the motion. Drags can have a variety of causes, and they are discussed in detail later. The net backward force, opposite to the direction of motion, can be referred to as the drag.


3) Lift - a force generated by the wings, and it acts perpendicular to the direction of motion of the wings. The upward component of the lift is the upward force that cancels the effect of the gravitational pull.


4) Thrust - another force generated by the flying object, in its direction of motion, to counteract drag.




The forces acting on the flyer (Adapted from Alexander, D. E, 2002. Nature's Flyers & http://www.freefoto.com)









One primary difference between typical man-made aircrafts, and natural flyers (like birds and insects), is that, for the former, two different mechanisms are employed to generate lift and thrust, while for the latter, both lift and thrust are generated simultaneously.