Air density

I live close to the sea and when I move to a fly site in the interior of the country at an altitude of 4500 feet above the sea level I can see a lot of pilots taking off with a wind speed of 30 kmh They say 30 kmh at 4500 feet are not the same as 30 kmh at the sea level.

Air density depends on temperature, pressure and humidity. The expression for the air density is:

$$D=\frac{P}{R \cdot T}$$

Where:

D = density, kg/m3
P = pressure, Pascals (multiply mb by 100 to get Pascals)
R = gas constant, J/(kg*ºK) = 287.05 for dry air
T = temperature, ºK = ºC + 273.15

Which gives us an air density of D₀ = (101325) / (287.05 * (15 + 273.15)) = 1.2250 kg/m³ for the dry air in standard sea level conditions.

Air density decreases with altitude. That means less number of particles in the same volume and less impact of the air on our glider for the same airspeed. Air density affects lift and drag through the term DV² where D is the air density, and V is the airspeed.

In standard atmospheric conditions airspeeds and sink rates of the polar increase with altitude by the quantity \( \sqrt{\frac{D_0}D} \) where D₀ is the air density in standard sea level conditions (1.225 kg/m³). When altitude increases the polar graph stretches along both axis in the same amount so the best glide ration doesn’t change but occurs at a higher airspeed and airsink. Altitude does affect calculus of the better glide speed as stated in article Glide farthest. The minimum and maximum speed at 4500 feet are 7% greater than those at level sea and so altitude affects calculus of the speed to flight as stated in article ….

Do you think your fly instrument has into account the air density to calculate the optimum speed to fly? The polar curve point’s entered by the pilot in the wing specs screen are only valid in standard sea level conditions. Airspace app makes the necessary corrections to adjust it to the air densitiy at all altitudes.

You surely ask yourself why the anemometer measures aren’t affected by the decrease in air density with the altitude. If the interaction between the air and the anemometer blades is weaker the blades speed should decrease and so the wind speed measured by the anemometer. We’ve jus seen that the weaker push is countered by the less drag and so the blades speed is exactly the same.