Equilibrium in Flight
A plane is in equilibrium when lift/weight/thrust/drag are all balanced in unaccelerated flight. Shouldn't thrust be exceeding drag in this situation since the plane is moving forward?
2 Replies
If thrust and drag are equal, then there is no acceleration, but that doesn't mean there is no movement (i.e., velocity).  If you increase thrust, the airplane will accelerate, but drag will increase as it accelerates (drag being a function of velocity-squared).  The velocity (and drag) will continue to increase until drag equals the higher thrust level, and then the velocity will stabilize.  For a fuller discussion with graphics, I suggest Prof. John Denker's excellent "See How It Flies" web site:


Thanks, a great article which I think sums up the answer to my question in two quotes.

"It is ironic that according to convention, the total aerodynamic force is not listed among the four forces."

     edit - I see "Total Aerodynamic Force" - is actually the term for the relative wind so not actually the total of forces acting upon the aircraft.

"You may think lift, thrust, weight, and drag are defined in a crazy way, but the definitions aren’t going to change anytime soon."

Essentially drag is defined as that force which prevents further acceleration of the aircraft though the sum force moving it isn't actually in the definition - its thrust isn't the entire thrust produced by the engines only the thrust in excess of its current velocity.