Drag calculation

Parachutes are used to slow objects down as they fall through an atmosphere. They increase the area resisting the flow of air and so offer higher “drag”. For instance, a human may fall at 120 miles per hour without a parachute. This is pretty fast and would be splatsville when s/he hit. With a parachute a person may descend at 15 miles per hour or even slower.

How much “drag” does a parachute have? You would like to know this since that tells you how much weight you can put on a parachute for any descent speed. If you got in a car as a passenger and you had a fish scale attached to a small parachute (a big one would take you right out of the car!), as the driver was holding at a certain speed, you could extend the scale out the window, the parachute would open and the fish scale would read a certain value. A 13-1/2 inch diameter parachute would read 1 pound at 30 feet per second (20 miles per hour). That means that if you put a 1 pound object on a 13-1/2 inch diameter parachute, it would fall with a descent rate of 30 feet per second. Incidentally, if you try this it will have to be on private property and “off-road” since the Highway Patrol probably would not be fond of your doing this. Also, under no circumstances would you drive the car and try to do this at the same time! Another way you can test a parachute is by dropping it from a high place and timing its descent. Again, be careful from where you drop it and make sure no spectators are below!

Yet another way to test a parachute (small ones) is with a blower or fan. You could make a science fair project out of this idea.

Here is the equation for calculating the drag of a parachute:

D = 1/2 * p * V^2 * S * Cd

Where:

D=Drag; Cd=Coefficient of Drag (approx. .8 to 1.0); p(or rho)=Air Density; V= Velocity, S = surface area of parachute

Alternatively you can determine the proper parachute sizing by using this formula:

D = 1/2 * p * V^2 * Cd

which gives drag per square foot. Plug in a parachute area times the result to get the correct parachute.

Air density http://en.wikipedia.org/wiki/Density_of_air

http://www.aeroconsystems.com/chutes/drag_calculator.htm

Categorised in: Math