Thread: Discussion Ducted Fan & Velocity of the air in tailpipe. View Single Post
Oct 15, 2012, 05:16 PM
Sink stinks
United States, GA, Atlanta
Joined Apr 2005
4,648 Posts
Quote:
 Originally Posted by TangoKilo Sorry, I have no idea what a strawman is. Are we not talking about a model moving at its maximum speed which would be (more or less) constant? Of course there is no net thrust, the sum of drag and thrust is zero.
Arguing a strawman means you are arguing something you have constructed yourself, instead of the actual point made by the other person in the debate.

We are talking about what happens when flight speed = efflux velocity. The flight speed is not constant in this case, if the ducted fan jet is the only thing producing thrust. My argument is that the maximum speed attainable by the aircraft under its own power must be less than the efflux velocity, because in that condition the propeller is not producing any thrust, which, as you pointed out, would be required to overcome drag. If the aircraft has reached the same speed as the efflux velocity (how that occurred is not important; we could just say it got dropped from a full-scale aircraft at that speed, for example), then it will quickly decelerate until thrust = drag.

The maximum velocity of the aircraft will be when drag equals thrust, as I'm sure you agree with. However, this maximum speed is not necessarily easy to predict because:
1) Thrust produced by the motor depends on forward flight speed
2) Drag depends on forward flight speed

Based on your posts, you seem to agree with #2 but take issue with #1, for some reason that I can't figure out. Is that correct? If so, could you explain why you do not agree with #1?
Last edited by Montag DP; Oct 15, 2012 at 05:23 PM.