Thread: Discussion Ducted Fan & Velocity of the air in tailpipe. View Single Post
Oct 15, 2012, 11:58 AM
Sink stinks
United States, GA, Atlanta
Joined Apr 2005
4,648 Posts
Quote:
 Originally Posted by TangoKilo Montag, first law, the velocity of a body is constant if there is no net force acting on it. A body (toy aeroplane) moving through the air has a force acting on it called aerodynamic drag. In order for the net force to be zero there must be a force of the same magnitude acting in opposition.That force is thrust. That is Newton's Law, not mine. Second law, the acceleration of a body is proportional to the net force acting on it. If there were no thrust, the only force (therefore the net force) acting on the body would be drag. What keeps the model moving at a constant speed? Those are the basic applications of Newton's 1st. and 2nd. Laws. You can not tell anyone who has a basic understanding of physics that a model moving at a constant maximum speed is producing no thrust and if the maths tells you that is so, then the maths are wrong.
You're arguing a strawman. I never said the plane was moving at constant speed, only that if the model's speed is equal to the efflux velocity, no net thrust is produced.

The fact that it is not moving at a constant speed in this situation should have been obvious when I said that the plane would slow down until thrust increases enough to counter drag.