Re: [RCSE] Does Altitude cause bad air?

[Don Stackhouse @ DJ Aerotech]

Glenn R. Whitcomb asks:

>...Does high altitude affect lift and the way gliders
>fly?

In addition to the increase in true airspeed caused by the lower air
density, as discussed by Oliver Wilson, and the associated increase in sink
rate and turning radius, there is also a decrease in Reynolds number. This
usually reduces the max lift coefficient, and increases drag.

The reduction in air density causes an increase in speed, but since lift is
proportional to the square of the velocity, that speed increase is
inversely proportional to the SQUARE ROOT of the air density change.
Meanwhile, since Reynolds number is LINEARLY proportional to density (and
because air viscosity at our altitudes is approximately constant with
altitude), the change in Reynolds number follows a linear function.
Therefore, the increase in airspeed due to the thinner air at higher
altitude cannot compensate completely for the loss of air density, and the
Reynolds numbers go lower. This usually hurts performance over the entire
operating envelope, althoiugh the exact extent of that loss depends on how
well the design of the specific model in question handles the decrease in
Reynolds numbers.


Don Stackhouse @ DJ Aerotech
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http://www.bright.net/~djwerks/
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