Parachutes
01 December 2003, 10:12 PM
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A friend's 10 yr old has some homework with a question about parachutes. There's a graph that shows that as the area of the parachute increases so does the time to reach the ground but there comes a point when the increases in area decrease the time to reach the ground.
Can anyone provide a scientific answer why this happens? I have theories such as:
There's a point where the mass of the parachute and the object and the effect of gravity out do the air resistance provided by the parachute.
There's a point at which the area of the parachute will be such that the canopy collapses or loses efficiency as the area increases.
Could well be a load of rubbish
The kid is still at junior school FFS
Mark
Can anyone provide a scientific answer why this happens? I have theories such as:
There's a point where the mass of the parachute and the object and the effect of gravity out do the air resistance provided by the parachute.
There's a point at which the area of the parachute will be such that the canopy collapses or loses efficiency as the area increases.
Could well be a load of rubbish
The kid is still at junior school FFS
Mark
01 December 2003, 10:46 PM
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As the area increases so does the weight of the total package, i.e. an increase in parachute material = increase in weight = increase in descent speed.
Cheers
Ian
Cheers
Ian
01 December 2003, 10:53 PM
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If you want to demonstrate it to your child, make a parachute out of a hankie and some string and put washers on the bottom.
You will be able to show that there is an optimum weight. Much heavier or much lighter and the parachute will fall faster. This is the same effect as having a larger canopy for the same weight, but easier to replicate.
It's because the canopy relies on having a certain volume of air pushed into it at a specific speed to inflate and to retain its shape. When the volume of air (which is decided by the area of the canopy) falls below that which is required (and this will happen if the canopy is too large for the weight it is supporting) then the canopy will collapse and spill air out of the sides, reducing it's efficiency still further. As an aside, this will sometimes make the parachute more efficient again as it falls faster and reinflates. It will also sometimes turn the canopy inside out, twist it and plummet to the ground...
Larger canopies are more vulnerable to this - that's why things like cargo drops or Apollo capsules have several medium size canopies rather than one big one.
Hope that helps...
SB
You will be able to show that there is an optimum weight. Much heavier or much lighter and the parachute will fall faster. This is the same effect as having a larger canopy for the same weight, but easier to replicate.
It's because the canopy relies on having a certain volume of air pushed into it at a specific speed to inflate and to retain its shape. When the volume of air (which is decided by the area of the canopy) falls below that which is required (and this will happen if the canopy is too large for the weight it is supporting) then the canopy will collapse and spill air out of the sides, reducing it's efficiency still further. As an aside, this will sometimes make the parachute more efficient again as it falls faster and reinflates. It will also sometimes turn the canopy inside out, twist it and plummet to the ground...
Larger canopies are more vulnerable to this - that's why things like cargo drops or Apollo capsules have several medium size canopies rather than one big one.
Hope that helps...
SB
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