On Tue, 20 May 2008 13:55:43 -0500, "Danny Deger"
wrote:

Look at the link for a picture of my manual aviation computer. It has two
different places to put in pressure altitude and temp to get density
altitude and convert airspeeds. Can anyone explain the correct use of these
two similar functions?

http://flickr.com/photos/26467772@N05/2508589733/


Density altitude converts the current altitude to a "performance"
altitude, based on temperature and pressure.

For instance, if my local field (250 MSL) has the correct conditions
to create a 1250 MSL density altitude, I would use the performance
charts for 1250 ft. to predict my airplane's behavior and limits.

Here is the calculation:
http://en.wikipedia.org/wiki/Density_altitude

The airspeed conversion is used to convert indicated airspeed to true
airspeed by compensating for lower air density and temperature at
altitude.

http://en.wikipedia.org/wiki/True_airspeed


The instructions for the unit in the photo are actually printed on it.
Here's a link to a manual for a similar unit that may help better
explain:
http://www.asa2fly.com/files/support/E6B_Manual.pdf
Some aircraft have such converters built into the airspeed indicator.
http://en.wikipedia.org/wiki/Airspeed_indicator

The top photo has an instrument with rings allowing the pilot to set
the temperature and altitude, then the TAS is read from bottom of the
outside ring.