Re: [Phys-l] stagnant air?

[John Denker <jsd@av8n.com>]

On 10/20/2011 08:39 AM, Bill Nettles wrote:
> I might create some tell-tales and stick them on the screen just to
> be sure that it wasn’t an extreme surface tension effect.

Good idea.  Tell-tales will indeed tell the tale.
I guarantee it will be interesting.

I recommend brightly colored yarn, held on with bits of tape.

Don't forget to check out the effect of a crosswind.

Tell-tales are standard equipment on the sails of a sailboat.
It is fun to add some tell-tales to the mast (not just the sails)
to show people that the leading edge of the sail does not simply 
"slice" into the wind.  There is in fact /circulation/ around 
the sail, and this requires a highly nontrivial amount of airflow 
flowing /around/ the mast.  In accordance with the Kutta-Zhukovsky 
theorem, there *must* be circulation, if and only if you are 
producing lift.
  http://www.av8n.com/how/htm/airfoils.html#sec-k-z

Tell-tales are also rather common on sailplanes (i.e. gliders).  
They come in quite handy in powered aircraft on occasion, e.g. 
when explaining to a student what the slip angle really is.
  In particular:  The so-called "slip/skid ball" is a misnomer.
  It is properly called the /inclinometer ball/ because it
  measures inclination, not slip or skid, as the yarn will
  demonstrate.  The distinction is important in a multi-engine
  aircraft with an engine out.
    http://www.av8n.com/how/htm/multi.html#sec-1out-coordination

A streamer attached to the trailing edge at the wingip allows 
you to visualize the wake vortex.

Et cetera.