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NASA VAB Pilot Chute Test

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On Saturday March 21st 2015 a team of NASA Engineers from the Crew Section and representatives from central Florida area parachute manufactures (Strong, UPT, Parachute Labs and Mirage Systems) along with a plethora of videographers from the local skydiving community, assembled in the NASA Vehicle Assembly Building to test reserve pilot chutes. This was done as a lab project for a parachute engineering and physics class conducted by NASA’s Rocket University and presented by Parachute Labs. Inc.

Drop 4 was the Race. All 14 pilot chutes were released simultaneously from the crane catwalk with a profusion of color and dynamics.

https://www.youtube.com/watch?v=Ly5fTv_ZMYA
I Jumped with the guys who invented Skydiving.

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This test didn't simulate the conditions that those PCs were designed for.

There's no load,
No acceleration on that load
The missing load does not create a burble.

My conclusion - a hemphispherical apex (or subapex) vented PC should have been tested along side (with a small load)

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The test procedure required dropping the sample from a measurable height sufficient to get an accurate descent time. We used the 475 foot height of the overhead crane for this purpose. The height was divided by the descent time to obtain the Feet per Second Rate of Descent. The ROD in FPS was then squared and multiplied by half the density of air at the test altitude (.002378 Slugs per cubic foot at sea level/2). This equation produced the resistive or “Dynamic Pressure” in Pounds per Square foot experienced during the test. The Dynamic Pressure was then divided into the all up weight of the sample yielding the “Effective Square Footage” of the sample.
I Jumped with the guys who invented Skydiving.

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Quote

Are the participants the manufacturers of the PCs?



The participants were representatives from Strong, UPT, PLI, Mirage Sys. as well as 8 NASA Engineers and 4 NASA managers and some 6 or 8 photographers from the Skydiving world.
There were 14 pilot chutes tested 2 each from 2 of the represented companies and one military surplus. One each from the other two companies present which leaves 7 unrepresented. Some manufacturers were invited but could not make the trip as it was on short notice. There were additionally; 3 drogues and one rocket recovery parachute tested. They will not be reported on.

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Thanks for organizing the test, John. I haven't received a copy of the report. Oversight, or because I wasn't technically a "participant" (pilot chute tested, but not notified prior)? Regardless, I'd love to mull over the results:)
Also, what, if anything, was done to compensate for pilot chutes that descended more slowly WITH weight than WITHOUT?

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yoink

***Yes B|



So I have to ask - isn't sending the report out to them for comment a bit of a conflict of interest?
Don't get me wrong, I love the idea. I'm just not 100% convinced on your methodology. ;)


The NASA Engineers are convinced, you could take it up with them. We had at least ten with us doing the testing. :|
I Jumped with the guys who invented Skydiving.

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Maybe methodology is the wrong word.

If you're testing equipment provided by manufacturers and are then asking the manufacturers for comments on the results of those tests, I just doubt that any will accept negative comments...

I look forward to reading the report if it's made public.

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This is obviously an art project or silly project?

Dropping fluffy pilot chutes from a crane no more evaluates their performance under actual conditions than would rolling unmounted tires down a hill. So, if it is just a silly "doing this for fun" exercise, why even dignify it with mathematical calculations, etc... If it is just waste of time, why do it? The imprimaturs of "scientists or engineers" or "NASA", are as phony as having a guy on television with a lab coat saying that "leading experts agree" that this pill will make you lose weight" or something like that.

I will admit that watching it was a very cute moment.

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One thing I think about is how the orientation could affect the amount of drag. Most malfunctions involve enough drag to rotate the jumper into the upright position. In an RSL type activation the PC is first exposed to wind in a direction other than is being tested in this example. I feel like dropping dummies in each orientation and measuring the forces and times would be more realistic.

-Michael

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John sent me a copy of the report (thanks John!), and I've read through it, albeit pretty quickly. I noticed that some of the video links don't link to the correct pilot chutes, so that should be corrected (IMO) if it is intended to be distributed electronically.

Personally, I take the results with a grain of salt. In the original video posted, the last two pilot chutes to land were also the two last place finishers according to the test parameters. I've also heard of an incident where a friend of one of my employees had their reserve PC tow for 200' after an AAD fire while they were in a head high position, luckily they made it. That brand of pilot chute finished in the top 5 in this test.

I bring this up only to point out that the pilot chute is simply a single part of a system. Cd tells part of the story, as does bag extraction force. You can have a large, inefficient pilot chute that still creates enough force to extract a bagged canopy from it's container, you're just likely to pay a price in pack volume.

I also shared the report with an associate that knows a LOT more about parachutes and drag than I do, but doesn't work in the skydiving industry, so has no bias in this test. This is some of his feedback:
Quote

The report is interesting, but honestly not very scientific.
I think however for the students involved that it was a lot of fun and they learned some basic physics.

To answer your question about high speed performance, drag coefficient is the standard industry measure based on area.
Now where it gets tricky is what is area? Some use area based on nominal diameter (which was the basis of my dimensions to you) wherein if one calculated the total cloth area (less seams, hems, etc.) and assumed this area formed a circle then the diameter of this circle is the "nominal" diameter of the canopy. Others derive area from the max inflated diameter (typically the Brits do this), still others use other derived area measures.
Regarding high speed performance, if you're playing a hunch that it would be different, I think your hunch would be right. High speed (or rather dynamic pressure) does play a role in the inflated shape of the canopy. Think of it this way, the canopy is an elastic shell with elastic radials/suspension members attached to a payload. With increasing speed, the load on the shell and radials/suspension members also increases which in turn changes its shape as the canopy structure adjusts to accommodate the increased loading. There are many factors associated with this phenomena such as suspension line length, number of gores, fabric permeability and so on. But the general idea here is that since the overall structure is elastic, the structure will dimensionally change as the load on that structure changes.And since aerodynamics is a function of shape (among many other factors), so to will the canopy performance. How much change and whether there is a limit to that change depends upon several other factors, but change it will.

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I'm going to rant a little bit. This whole thing just seems so typically Sherman. He's got a perfectly valid issue. He's got an idea. He has an opportunity. And he still manages to bone it to some degree. He always seem to miss on that last 10% that would make it really well done.

Things I will say in his favor. It's a real issue. It's some thing that needs to be addressed. He found a really good place and opportunity to do some testing. And he went out and got a bunch of people together and did it.

Where he failed. I don't like the way he set up the test. I don't think they were well executed. And I think some of the data was at best suspect. They had a bunch of fun but I think he missed an opportunity or at least did not make the best use of one.

Issues. I don't think this is really good representative data for some of the pilot chutes. I don't think it good job of representing their behavior.

Examples. I don't think the goal of what they were testing was well defined. And I do not think the method used produced the best data for what I think should be examined. Several of the Pilot chutes were not well inflated and were permitted to behave in a manor not relevant to a deployment. When a pilot chute is only half inflated an is side slipping half on it's side it is not in the same mode that it would be in pulling at bridal stretch. When a pilot chute has a small weight on less then a foot long bridle and is allowed to oscillate 30+ deg spilling air it's not giving a good representation of it's behavior. In theory you can extrapolate data to different air speeds and loads but it requires that it be behaving in the same mode. And that can mean a lot of things. As an example, there are different ways that vortexes can shed off of some thing depending on the Rn involved. The point is that you don't extrapolate data unless you have to. And when you do resort to that you try to be very careful in how you do it. This is typical of Sherman. His back ground in this is not that strong. He has gone out and tryed to study which is more then most. He sees an equation in a book and grasp a hold of it not understanding the limitations and assumptions which go into it.

What should have been done. Rather then letting these pilot chutes flutter down like leaves and pretend that that represented their real performance and then project that rather questionable data to a higher load and airspeed. What should have been done. is to assemble the PC on an appropriate length bridle with a sand bad at the bottom. The sand bag should weigh as much as a typical canopy in a free bag. The pilot chute will have the opportunity to fully inflate. It will be locked down in orientation where it can not simple oscillate around it's CG. It can swing back and forth at bridle stretch but that is a very different behavior mode then a basically unloaded PC. This would give you a real idea of the terminal velocity of the PC and reserve canopy. All the "reserves" should weigh the same regardless of the weight of the PC. If it has a heavy metal cap on it, that it's problem. I would then do it again with a weight corresponding to a typical extraction force of the free bag on that container. This is a place where you could get a bit controversial. What number do you want to pick? But lets say you look at some data from repacks and pick a number for a container. Drop the PC at that weight. Now you have a direct measure of the terminal velocity necessary to extract the bag from that container. The jumper will fall and continue to tow the PC until it reaches that speed. Then based on the acceleration curve and the speed from the previous drop test you can start looking at the predicted time to line stretch. Basically the free bag will be decelerating to that speed and the jumper will be continuing to accelerate towards his terminal velocity. The only thing missing from this is the issue of a burble. I'm not dismissing this but it's worth noting that this is just throwing things off a catwalk. It does not involve the tens of thousands of dollars that the technical committy is spending on their PC in tow project. It's a compromise and their is nothing wrong with that as long as you recanise it.

All it would have taken to have gotten some really good data out of this is some 15 ft bridles and a few buckets of sand. That's all I'm saying. But this is Sherman... and in my opinion he kind of dropped the ball on this.

Lee
Lee
[email protected]
www.velocitysportswear.com

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That's one way. Personally I'd had attached each PC to a full length bridle and the bridle end to a Newton meter and clip that to the floor of a wind tunnel.

Choose a force for extraction and then measure the wind speed to generate that force.

Simple. Comparable.

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And rather than doing it in a wind tunnel tests are being sponsored by PIA measuring pull force and extraction force during live jumps. Force sensor on the bridle, PC being towed with burble of.live jumper. Project just underway and takes weeks per container brand.
I'm old for my age.
Terry Urban
D-8631
FAA DPRE

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RiggerLee

And that has been done. I don't think the chose the operating speed to correspond to the extraction force.



I see extraction force requirements as an entirely different experiment with many more variables.

This test seems to simply be to identify which PC produces the most drag. Of course, I've not seen the paper yet, so I could be well wrong.

How that then relates to the extraction force required for various container types and all of the reserve possibilities would be a much bigger task.


councilman24

And rather than doing it in a wind tunnel tests are being sponsored by PIA measuring pull force and extraction force during live jumps. Force sensor on the bridle, PC being towed with burble of.live jumper. Project just underway and takes weeks per container brand.



and that's the absolute best way of doing it. My suggestions is how I would have done it as a student. ;)

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RiggerLee

Things I will say in his favor. It's a real issue. It's some thing that needs to be addressed. He found a really good place and opportunity to do some testing. And he went out and got a bunch of people together and did it.


Agreed:)
I think my biggest issue is the measurement method. I think an inflated measurement (even if just done with the fan) would have given more accurate results. In the case of our pilot chute, part of the gores that are designed to be vertical or near vertical were added to the diameter because the measurement was taken either with the canopy flat on a table, or from equator to equator over the top (which is not how you measure diameter).

I haven't run the numbers to see how this would have affected the outcome, but it certainly would have reduced the spread between the measured and effective diameters of all of the PC's, maybe some more than others.

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