Parachuting Physics Inquires

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In summary, a specialist professor promotes a nominal diameter for hemispherical parachutes, while other sources using a constructed diameter. Hemispherical parachutes with vents have a drag factor of somewhere about 0,7-0,8.
  • #1
klokes
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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 [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

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
 
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  • #2
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
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
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*
 

1. How does gravity affect parachuting?

Gravity plays a crucial role in parachuting. It is the force that pulls the parachutist towards the ground, causing them to accelerate. The larger the mass of the parachutist, the greater the force of gravity and the faster they will accelerate towards the ground. This is why parachutists with heavier equipment or clothing will have a faster descent compared to those with lighter equipment.

2. What is the ideal shape for a parachute?

The ideal shape for a parachute is a round or dome shape with a large surface area. This shape allows for maximum air resistance, which helps to slow down the descent of the parachutist. Additionally, the round shape allows for equal distribution of air pressure, preventing any uneven collapses of the parachute.

3. How does air resistance affect a parachute?

Air resistance, also known as drag, is a force that opposes the motion of an object through the air. In parachuting, air resistance plays a crucial role in slowing down the descent of the parachutist. As the parachute opens, it creates a larger surface area, which increases air resistance. This, in turn, helps to slow down the descent of the parachutist and allows for a safe landing.

4. What is the difference between terminal velocity and freefall?

Terminal velocity is the maximum speed that an object can reach while falling, due to the balance between the force of gravity and air resistance. In parachuting, this is the speed at which the parachutist will continue to fall until they reach the ground. Freefall, on the other hand, is the state in which an object is falling under the sole influence of gravity, with no air resistance. Parachutists experience freefall for a brief period before their parachute opens and they reach terminal velocity.

5. How does the size of the parachute affect the descent?

The size of the parachute directly affects the descent of the parachutist. A larger parachute will have a greater surface area, which means more air resistance and a slower descent. On the other hand, a smaller parachute will have less air resistance and a faster descent. The size of the parachute must be carefully chosen based on the weight and size of the parachutist, as well as the desired speed of descent.

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