I probably shouldn't mention "True Airspeed" and how THAT relates to all this as well. But suffice to say, when flying a model airplane at a high density altitude, and then later flying at a low density altitude, one can almost "feel" the difference. It is like you are flying against a "cushion" of air at the lower altitudes, and when you reduce the power, it is like you "put on the brakes!" VERY noticeable difference! Same thing in full size planes....

It is because airpeed is, if you will, a "quantity of air molecules hitting the airplane". In thin air, the airplane has to go faster across the ground to "feel" the same number of molecules that it does in thicker air and hence indicate the same airspeed. Therefore, an indicated airspeed of "x" miles per hour is a much higher speed across the ground at higher density altitudes than that at lower. It quickly makes short airstrips really short at high density altitudes, and longer ones "just right" because what is important in take offs and landings as far as distance needed is your actual groundspeed.

This all of course does not take into account the effect of wind, which changes groundspeed as well...

AV8TOR