Was: Recommended specs on reserve exit weight, do you go over max?

[980 0]

You are so out of line it’s not even funny anymore.

Willing to argue math with people who obviously understand it better than you, but also willing to contradict empirical evidence gathered over man hundreds and thousands of test jumps just because you do not understand the physics involved.

Here is a quote from the article that you linked:

“Here the reference parachute (labeled "chute 1") is characterized by the parameter set characterizing a Sabre 150 used by a 205lbs jumper falling at 176 ft/sec at deployment time and at 125 ft/sec prior to slider descent (deployment altitude was 4000ft MSL). The figure shows that for most values of the scale factor, smaller parachutes actually open harder than the larger ones, a trend which is opposite to that of instant openings. For example, if all the dimensions of this particular chute are doubled, the resulting 600 ft2 parachute would feature a maximum deceleration that is 40% smaller than the original 150 ft2 canopy (with same jumper!).”

Since they do not scale for suspended weight anywhere, we could say considering the case of a smaller parachute for the same weight jumper should have the same effect as a heavier jumper for the same size parachute.

This will tell us that at a higher wingloading parachutes open harder (both higher peak force and higher stress on parachute). This is a known fact supported by both theory and empirical evidence. Extrapolating this effect will get you to the point where the parachute fails during opening due to overload.

If you still refuse to believe the truth, do us all a favour and provide a reasonable physics theory on why or some empirical evidence to support your statement, rather than being so reactionary on this forum.

Thank you. Maybe next time link an article that supports your statements, not those of the people you are contradicting…

[matthewcline 0]

Except it has happened.

I have watched this thread with a bit of a chuckle all the way. Mathematically maybe it can't happen. Realistically it does. Parachuutes have failed, "blown up", period. It happened becuase they where used out side the parameters the ywhere certified for.

The one I had fail was inside all parameters but deployment speed. It could not take 220 lbs exit weight at 160mph. The openning did not hurt more than any other brisk openning, the canopy failed before full openning shock could be transfered to my body.

There have been reserve failures, "blow ups", documented in Parachutist and Skydiving magazine.

Based on my personnal experience and the written reports I have seen I do not feel it is a myth.
But like you I could be wrong, I am Human after all.

An Instructors first concern is student safety.
So, start being safe, first!!!

[SethInMI 145]

980:
you are correct, I see that article as further validation of the model I created, and what everyone is telling phoenix.

phoenix:
The spreadsheet I created matches well with the data measured in the article you posted. For example, the data in "Figure 2" shows shock loads of 4k-5k Newtons for a 200lb skydiver deploying at 110 mph. Using that data and a 3 sec opening time in my spreadsheet, you get 4.5k Newtons (1000 lbs) for peak loads.

The article just shows that model I am using is valid. And the model shows that increasing suspended weight increases peak shock. There is a reason the model matches the measured data, and that is it is based on basic physics, Fd=Cv^2 and F=ma. Thanks for posting the link!

Seth

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[phoenixlpr 0]

Quote

Since they do not scale for suspended weight anywhere, we could say considering the case of a smaller parachute for the same weight jumper should have the same effect as a heavier jumper for the same size parachute.

This will tell us that at a higher wingloading parachutes open harder (both higher peak force and higher stress on parachute). This is a known fact supported by both theory and empirical evidence. Extrapolating this effect will get you to the point where the parachute fails during opening due to overload.

Check peak opening forces on Figure2 (I've attached).
How does it support your point of you?

[phoenixlpr 0]

I was wrong on the basic equation.

Fd=Cv^2 and F=ma could be terminal speed...

Forces on the risers: m*g - C * v ^ 2

Check figure 2 again: peak forces are higher on Sabre230 than Sabre 120 and opening takes longer on a 120 than on a 230.

[mjosparky 3]

Quote

This will tell us that at a higher wingloading parachutes open harder (both higher peak force and higher stress on parachute). This is a known fact supported by both theory and empirical evidence. Extrapolating this effect will get you to the point where the parachute fails during opening due to overload.

This is a quote from your own reference. Read it and tell me what it says.

And just so you know it is not a myth, I have blown up canopies during drop tests and during live test jumps. Believe me it can happen.

You need to face the fact you are in over your head on this and "Your opinion is wrong".

My idea of a fair fight is clubbing baby seals

[980 0]

sparky

actually that part is not from the article, it is my interpretation of the data that I quoted from the article

the quote from the article is in quote marks

I have changed the formatting to make it clearer.

[SethInMI 145]

Quote

I was wrong on the basic equation.
Fd=Cv^2 and F=ma could be terminal speed...
Forces on the risers: m*g - C * v ^ 2

You are now on the right track. However, the equation you have is the net force on the system, which determines how fast it will decelerate. If there is no deceleration, like under a fully open canopy, then m*g - C*v^2 = 0. But at that point there is still m*g force on the risers.

The correct equation is:
Riser Force = Drag Force on canopy (initially low, spikes up then settles on value = m*g)

Think of it this way, the canopy weighs a lot less than a skydiver, so almost all the force exerted on the canopy as it opens travels down the risers and exerts itself on the suspended load.

To be very precise, in the system equation m*g = Cv^2 , C is the sum of drag on the canopy and the skydiver. Initially all drag is skydiver, but very quickly canopy drag becomes dominant.

Seth

Regarding your comment for Figure 2, with the same suspended weight, a longer opening will have lower peak forces. I am talking about different weights.

It's flare not flair, brakes not breaks, bridle not bridal, "could NOT care less" not "could care less".

[Scoop 0]

Sorry if I'm pissing on anyones bonfire, but do you find it funny how forces can supposedly be calculated using equations. I think its all bullshit. You can have an idea but ultimately real world testing is the only thing I trust and believe. No amount of theory will change my mind. People used to say the world was a cube too, after all, they were wrong about that.

If a manufacturer states a figure because they've done some rough calculations based on experience and then tested their canopy to confirm this rating, thats good enough for me and most people I'd say. If you decide to exceed that then you are in the caution territory and anything you do is at your own risk.

[phoenixlpr 0]

Quote

Regarding your comment for Figure 2, with the same suspended weight, a longer opening will have lower peak forces. I am talking about different weights.

Does maximum peak force depending on suspended weight? It does not seems so.

[mjosparky 3]

Quote

Quote

Regarding your comment for Figure 2, with the same suspended weight, a longer opening will have lower peak forces. I am talking about different weights.

Does maximum peak force depending on suspended weight? It does not seems so.

The forces generated during deployment are the result of speed at deployment, load (weight) on deployment and fill time of the canopy. The final design of any canopy is a compromise of opening characteristics, performance characteristics, landing characteristics and numerous other factors.

You can design a canopy to open very softly with a load of 1000 pounds but there are trade offs. For one it will pack up the size of a refrigerator and take several hundred feet to open. A small elliptical will open much faster, (harder) then a larger rectangle canopy given the same load. The fill time on the elliptical is shorter so the load slows down faster.

The variables are endless and one action causes a reaction……. That is why testing is done on a new design. What looks good on paper may in fact be for shit in the real world.

In the real world the manufactures of your 145 sq. ft. Cricket should not be used with an exit weight more then 160 pounds. Why do you think they would set this limit?

My idea of a fair fight is clubbing baby seals

[ZigZagMarquis 8]

Its okay... if PxLpr wants to think his reserve couldn't blow-up if overloaded, let him...

... can I get $20 on PxLpr on the Bounce Bingo Board?

[mjosparky 3]

Quote

Its okay... if PxLpr wants to think his reserve couldn't blow-up if overloaded, let him...

... can I get $20 on PxLpr on the Bounce Bingo Board?

Sorry all the slots were taken 30 posts ago. I guess people know a good thing when they see it.

My idea of a fair fight is clubbing baby seals

[phoenixlpr 0]

Quote

In the real world the manufactures of your 145 sq. ft. Cricket should not be used with an exit weight more then 160 pounds. Why do you think they would set this limit?Smile

I do know that overload does not improve flight characteristics.

[phoenixlpr 0]

Quote

Quote

Its okay... if PxLpr wants to think his reserve couldn't blow-up if overloaded, let him...

... can I get $20 on PxLpr on the Bounce Bingo Board?

Sorry all the slots were taken 30 posts ago. I guess people know a good thing when they see it.

It's so nice to see that you are taking any debate so positive, so personal....

[riggermick 6]

It's so nice to see that you are taking any debate so positive, so personal....

[phoenixlpr 0]

Quote

There are MANY more people who know much more than you can currently understand!

It was not a question.

[mjosparky 3]

Quote

Quote

There are MANY more people who know much more than you can currently understand!

It was not a question.

No it was not a question, I think he meant it as statement of fact. But then what do I know?

Quote

I do know that overload does not improve flight characteristics.

I think you are starting to see the light.

My idea of a fair fight is clubbing baby seals

[Pendragon 1]

Let's look at it this way:

It is not necessarily true that the additional force to slow a heavier jumper is directly proportional to the mass difference. Force is a function of acceleration (negative in this case) - so stopping an object with only a small amount of kinetic energy but within a short distance can require the same force as a much larger object brought to a stop more slowly.

If a heavier jumper's deployment consumed more altitude, then the forces would be lessened. Similarly, a lighter jumper having a premature whilst head-down is going to subject their canopy to much greater stresses than someone just overloading it and going through a belly-to-earth delployment, especially since the excess energy is only proportional to additional mass but to the square of the velocity.

Body shapes may dictate that 2 jumpers of different exit weights fall at the same speed at terminal due to drag differences. If this is the case, and the loss of height during deployement was identical, only then would the braking forces have a ratio identical to that of the exit weights.

In reality, I'm not sure whether anyone can say for certain how the canopy will open under a given loading: this is a function of how the canopy pressurises, stages and presents drag to the relative wind, and may not correlate linearly with the speed of the relative wind. This in turn determines how quickly the body is slowed and the forces involved.

--
BASE #1182
Muff #3573
PFI #52; UK WSI #13

[mjosparky 3]

Quote

In reality, I'm not sure whether anyone can say for certain how the canopy will open under a given loading:

There are things called Somats and load links that will give you very exact data on openings.

My idea of a fair fight is clubbing baby seals

[riggermick 6]

Quote

Quote

In reality, I'm not sure whether anyone can say for certain how the canopy will open under a given loading:

There are things called Somats and load links that will give you very exact data on openings.

Brinnell blocks work well for measuring peak opening force, one on each riser. That's how we did it for TSO testing.



Mick.

[mjosparky 3]

Quote

Brinnell blocks work well for measuring peak opening force, one on each riser. That's how we did it for TSO testing.

The Somats can give you at least 8 data points. You can get peak forces on each riser, deployment speed and duration and the latest interest rate from BofA.

My idea of a fair fight is clubbing baby seals

[crewkeith 0]

have you had a terminal reserve ride yet?? and what is this stuff about smaller reserves opening slower? i think i still got bruises from my last one under a pd 113 6 months ago. go jump and take 30 second delay then pull silver handle. see what opening force is like. then when you get down.... if you get down push your thumb thru your reserve fabric. it aint that strong. shit happens cheif. even if your math says it cant.
bsbd
keith
p.s. nobody really wants you to bounce but this is kinda a silly argument.

The skies are no longer safe

I'm back

[mjosparky 3]

Quote

have you had a terminal reserve ride yet?

Kettle meet pot. You are 65 pounds over the maximum recommended weight for an expert canopy pilot on your PD-113.

My idea of a fair fight is clubbing baby seals

[phoenixlpr 0]

Quote

have you had a terminal reserve ride yet??

Yes. I had.