Re: [Phys-L] Dirigible Flight Question

[Paul Nord <paul.nord@valpo.edu>]

Thanks.  And that question is really the crux of what I am trying to understand.

If I were able to plot "drag" vs. velocity what would I get?

Is the function simply parabolic?  (or rather abs(v) * v )
Will that fit most low velocities?
Is there also a linear term?

I don't see how there could be a non-zero offset.  But there is a non-zero force required to accelerate the mass through zero velocity.

Can a separate the variables at zero velocity?

What I want to do is make a measurement of the additional mass at the point where the balloon's vertical wind speed goes to zero.

Paul


On Nov 5, 2012, at 1:07 PM, John Denker <jsd@av8n.com> wrote:

> > If it use the mass of the displaced fluid in the calculation above,
> > am I already accounting for some of the "drag" force which is
> > normally accounted for in this velocity squared term?
>
> That's a smart question.  In this business, there are very often multiple 
> ways of explaining the same thing, such that when you try to quantify it 
> you run the risk of counting something and its synonym as two things when 
> really there is only one thing.
>
> In this case it comes down to a question of terminology:  By convention the
> unadorned term "drag" includes everything that acts like drag.
>
> Also note that everything is /indirectly/ dependent on everything else.  For
> example, even if you think the drag does not directly depend on the displaced
> mass, it indirectly depends, because it is related to the overall size of the
> aircraft, and drag certainly depends on size.  At the end of the day the most
> you can achieve is consistency, i.e. a consistent set of numbers for the size, 
> mass, acceleration, velocity-field, pressure-field, et cetera.  You might wish 
> for some nice simple separation of variables, but you're not going to get it.