Which Diameter Should Be Used in Parachute Design Calculations?

  • Thread starter Thread starter klokes
  • Start date Start date
  • Tags Tags
    Physics
AI Thread Summary
In parachute design calculations, there is a debate between using constructed diameter versus nominal diameter for determining the necessary area. While some sources advocate for the constructed diameter, a specialist professor suggests that nominal diameter is more commonly used due to its alignment with design constraints related to fabric weight and drag efficiency. The drag coefficient (C_d) for a hemispherical parachute with vents is estimated to be between 0.7 and 0.8. Ultimately, the choice between these diameters is a matter of convention, with most designers favoring nominal diameter for optimal drag per fabric used. Understanding these nuances is crucial for effective parachute design.
klokes
Messages
3
Reaction score
0

Homework Statement



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
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

Homework 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

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.

parachute.jpg
 
Last edited by a moderator:
Physics news on Phys.org
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.
 
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?
 
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
adding vents (on the side
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*
 

Thread 'Errors and significant figures'

I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
View full post »
Thread 'Collision of a bullet on a rod-string system: query'

Thread 'Collision of a bullet on a rod-string system: query'

In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
View full post »
Thread 'A cylinder connected to a hanging mass'

Thread 'A cylinder connected to a hanging mass'

Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
View full post »

Similar threads

Friction Hinge Design: Calculating Bolt Preload and Torque
Replies
2
Views
4K

Hot Threads

Recent Insights

Back
Top