# Homework Help: Parachuting Physics Inquires

1. Apr 14, 2010

### klokes

1. The problem statement, all variables and given/known data

In designing a hemispherical parachute, i´ve come across conflicting information regarding whatever to use a constructed (ortographical projection) vs. a nominal diameter in calculating the necessary area required in the parachute.

Sources using constructed diameter in equations:
http://en.wikipedia.org/wiki/Drag_(physics)
http://my.execpc.com/~culp/rockets/descent.html [Broken]
http://physics.info/drag/
Plus a few more.

The main source promoting a nominal diameter is a specialist professor in the area, making me doubt my previous decision to use a constructed diameter (which seems physically reasonable):
http://www.pcprg.com/rounddes.pdf

2. Relevant equations

The equation for drag, computed from a mix of bernoullis equation and pressure over an area, with a coefficient for drag resistance added, C.

F= 1/2×c×p×v^2×A

3. The attempt at a solution

Choosing the constructed diameter in my further equations, because of the assumption that an inclined surface should, because of the steep angle of approach relative the air-particles, affect them (summarized) in an ortographical manner.

The paper is due friday, and thus i would very much appreciate a quick answer.

Last edited by a moderator: May 4, 2017
2. Apr 14, 2010

### PhanthomJay

Your equation is missing the drag factor commonly referred to as C_d. Typically, one uses the projected diameter in the area calculation, with the appropraite C_d for the drag (or shape) factor. For example, a flat surface has a shape factor that is 1.6 times greater than the shape factor for a cylindrical (convex) surface. I am not sure what the C_d is for a concave shaped hemispherical parachute.

3. Apr 15, 2010

### klokes

Sorry, the drag factor was supposed to be included in the equation. (fixed)

The drag factor for a hemispherical parachute with vents is somewhere about 0,7-0,8.

My question still remains; does the aerodynamics-professor provide the wrong answer?

4. Apr 15, 2010

### klokes

Just received a mail regarding this question from the author of the discussed article, effectively solving the problem;

*QUOTE*

Jacob

Using constructed vs nominal surface area is a matter on convention.
You can use either.

But most parachute designers use the nominal area (ie S0) b/c the
weight of the parachute,
and therefore the total amount of fabric used for its construction, is
a design constraint.
And so designers will want to build a canopy that yields the most drag
per amount of
fabric used. They do this by going to more complicated shapes and by
that shoots the air out partially downwards), with the same total
amount of fabric material.
Thus for the same S0, some shape/vent combinations will have a larger
Cd than others.

Hope this explanation helps.

Sincerely

Jean Potvin

*QUOTE*