Path: utzoo!attcan!uunet!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!rpi!turing.cs.rpi.edu!deweeset
From: deweeset@turing.cs.rpi.edu (Thomas DeWeese)
Newsgroups: comp.arch
Subject: Why not evacuate Disk drives.
Message-ID: <V%6$_&|@rpi.edu>
Date: 31 Jul 90 14:17:58 GMT
Reply-To: deweeset@turing.cs.rpi.edu (Thomas DeWeese)
Organization: RPI CS Dept.
Lines: 24

The problem with evacuating disk drives is that the fact that they are not
evacuated is the major reason why the heads are able to fly so close to the
disk media.  Yes fluid mechanics is a *bitch* to deal with but it does
several nice things for you.  The air over a platter is moving with the platter
(at a relatively high speed) also the speed of the air also drops off quickly
as one moves away from the platter.  So what is done it that the head is 
shaped so that it will be blown upward by the air
eg.      |   |
         \   |
    ->    \  |
->  ->  -> \_|<- read-write head
->->->->->________________________Platter
^^^^^^^Air

So that if it tries to come in contact with the platter it is blown upward
but if it goes to high there will not be enough lift to hold it up and it will
come back down.  Without the air to act as a sperator you would have to do
a lot of fancy foot work like 
  1. Guess where the platter is (impossable to do with reasonable accuracy).
  2. Use range finding  hardware (expensive if possable).

and with both of these meathods you loose head crash protection.

Thomas DeWeese