L = Lift, in pounds CL = Coefficient of Lift (varies with airfoil shape and Angle of Attack) S = Square feet of wing area Rho = Air density in slugs per cubic foot (Rho = P/1716TR) P = Pressure, in pounds per square foot 1716 = is a constant number derived from the product of the constant of air (53.3) and the acceleration of gravity (32.2) TR = Temperature, in Rankine (Fahrenheit plus 460) V = True airspeed
Please note that airspeed simply does NOT equal lift! It’s –way- more complex than that!
Air density decreases as altitude increases. A rough rule of thumb is that with about every 18,000 to 20,000 ft of altitude gained, there is about half as much air density. So at about 40,000 ft, there is about one quarter the amount of molecules per cubic centimeter of air as found at sea level.
Air density varies with temperature. For every ±15°F or ±8.5°C variation from the standard temperature at your pressure altitude, the density-altitude is increased or decreased by 1000 ft.
To create the same amount of lift, for each 1000 ft increase in density-altitude the true airspeed will increase by 2 percent.
Since kinetic energy increases at the square of the velocity, for each 1000 ft increase in density-altitude, the landing rollout (or the energy required to be absorbed by your legs) will increase by 4 percent.
Putting this all together, we can see that if on a cool, no wind day in winter you could bring your canopy to a stop within 30 ft, on a day where the temperatures were 60°F higher, you’d need about 35 ft to stop. And, your 100 ft long swoop during that cool winter day -could- be about 116 ft long during the hotter summer day.
Going from a low altitude DZ like Monterey in the winter to a high DZ like Mile-Hi in summer, be aware that your canopy will land quite differently!
Do your Pro-Rating accuracy landings in the cool of winter and your long distance swoops in the summer.
If this board supported sub-scripting, the L, in CL, would be sub-scripted. If it supported greek letters, Rho would be . . . a greek letter. And If I could have done the "velocity squared" part correctly I wouldn't have used the "^".
Oh well. ;^)
(So, is that a wink raised to the power of a smile?)
Rho = Air density in slugs per cubic foot (Rho = P/1716TR)
Just an addendum.
Humidity -is- figured into the Rho calculation, but it's variation is not a significant factor for Lift. Going from a very dry condition to 100 percent humidity at 100°F, reduces the air density by about 2 to 3 percent.
It -is- a significant factor for take-offs in piston engines. As under these same conditions horsepower would be reduced by about 12 percent. Turbojet engines are not affected so much by increased moisture.
Very aware of the true definition. That's what makes her reply all the more funny! Exactly what I thought when I read it. Nice little interlude from the 'serious' technical stuff.
