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Canopy operning forces

 


mark  (D 6108)

Sep 24, 2012, 6:30 AM
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Canopy operning forces Can't Post

Since the previous thread got locked before I got a response:

MEL -

How could I find out the amount of internal pressure created inside the tail during inflation, and how that compares with the internal pressure in the rest of the canopy?

What is the source for the fact that line burn is the #1 cause of blowouts?

Do you think the tightness of the center "C" line larksheads is a result more of forces during internal pressurization than because of, say, lower-surface inflation earlier in the process?

If the "C" lines take more force, then is it a myth that the lines most likely to break during deployment are the center "A" lines? Should Precision be switching to continuous "C" and "D" center lines instead of just continuous "A" and "B" lines on their reserves?

Thanks,
Mark


JohnSherman  (D 2105)

Sep 24, 2012, 10:47 AM
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Re: [mark] Canopy operning forces [In reply to] Can't Post

Quote:
How could I find out the amount of internal pressure created inside the tail during inflation, and how that compares with the internal pressure in the rest of the canopy?

The INTERNAL pressure of a ram air never exceed ambient! Neither during opening or full inflation. It is the external pressure which reduces and because the ambient is greater we have a differential which causes the inflated ridgity.

Quote:
What is the source for the fact that line burn is the #1 cause of blowouts?

Line burn does not cause blowouts. In My total experience with Ram air canopies I have never seen a line burn except from a accidentally pre-packed malfunction.

The greatest force occure at "Snatch". The line attachment knot tightnest will relate to the location of the bridle attachment and the amount of brakes to a lesser extent.


RiggerLee

Sep 24, 2012, 11:58 AM
Post #3 of 9 (1170 views)
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Re: [mark] Canopy operning forces [In reply to] Can't Post

Another interesting question. I think what you're actually asking about is internal pressurization, the pressure differental between the inside and the out side durring opening. It's my oppionion that most of the dammage you see is not from positive pressure building up on the inside but from the pressure/ force on the bottom skin durring opening. In other words I don't think it blows out wards but inwards. One possable exception might be some damage they saw when they tryed to put airlocks on some base canopies. That was about the only damage I think I've ever seen from over pressurazation in a canopy.

Here are some of the things I've seen and my thoughts on it.

Brake line attachments, perticuarly the center ones. I've seen them tear off and I've seen tears in the fabric around them. There is deffinently a lot of load on them but most of the tears I've seen appear to be the result of a slightly messy roll of the tail creating stress points in eather the top or the bottom skin. Or the bartack slightly off creating a point load. Once the fabric tears slightly releaving the point load the stress is released and it genterally does not tear further. I've taken tails apart and patched the pannels and rebuilt it. You're going to laugh at this but I've also just sewen the tail togather around it in a cresant to stableize it. Guy wanted to make the next load and I'm not fast enough to do the full repare on a ten min call. Funny thing was they guy didn't bring it back that evening. He seemed to think that it was fine and last I heard the canopy had another 500 jumps on it when I lost track of it. So althoue the break line are a stress point most of the problems I've seen have related to the construction of the seam.

C and D lines. I have in fact seen some line attachment points tear lose here. I've even seen the seam tear out of the bottom of the canopy. I'm trying to recall the locations but I recall most of them as being off center, not neccesarily being the center C and D. I think this is the product of the bottom skin inflation of the canopy trapped by the curraling down of the tail cupping the canopy and trappin the air. As to which lines take the most stress that seems to depend on the break setting. With some canopies after opening there is actually a slight bow in the D lines on eather side implying that the C and break lines are takeing the majority of the load.

Nose. I can set here and teorise all day about the highest load being at the high point of the bottom skin under the tail but the truth is that I have repared far more dammage from the nose of the canopy, A and B lines then any where else. Where does a canopy blow up on a hard opening? It breaks it's A and B lines. Where did all of the Ravins with type 3 fail, A and B. Where did the Flick fail? A and B. All the damage that we saw on our heave drops? A and B lines. I just replaced the first four feet of a rib that we tore out at the leading edge of the nose. It snaped a 1 inch type 3 tape. As to where all of this comes from? YOu got me. The opening of a canopy is caose incarnate. There are probable a lot of variables. The cut of the nose, lip, break setting, trim. Some of it seems to relate to how much the canopy surges forwards durring opening, trim, angle of slider, break setting. all of this seems to relate to how much air you catch in the nose and how much load it sees. As an example we had a break come unstowed on the last opening. that was the side that we had the rib damage on.

Center ribs. I have seen ribs tear internally. You might say that internal preasure played a part in it but I think that the damage was from the bridal attachment point. I think it broke the tape and tore the rib. All the damage I've seen of this kind has been on the center three ribs. I don't think it was an "inflation" issue.

Top skins. I have seen top skins tear, tandoms, cargo canopies, but it was in the front half of the canopy, or it appears to have started there. I still think this was load from air being caught in the nose during opening. I don't think it was an over inflation/presureazation issue.

"Blow outs." The relatively few times that I can point to a peace of fabric that "blew out" with no obveous point of stress concentation that started the tear there have often been signs of line burn which damaged the fabric and then split of some other damage. Cat piss turn nylon into tissue.

I went looking for a picture of a pressure diagram for a wing. I found this:

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Put it on an air foil and play with the angle a bit. It will at least give you an idea of the distrobution during flight. It also gives you an idea of how serious leakage could be at diffrent areas. Look at the graph. Basicaly the highth of the lines is the presure relitive to the free stream preasure. The peak on top at the front is the stagnation pressure inside the airfoil. Any thing less then that is pressure inside the canopy trying to push out. In other words if you had a hole in the canopy there air would leak out of it. Note that the pressure on the top skin is much lower then on the bottom skin, hens why it flys. But that also implies that a hole in the top skin will lose a lot more air then one in the bottom skin. Also note that the pressure at the front of the top skin is much lower then the presure at the back of the top skin. In other words a hole farther forward in the top skin will lose much more air then one farther back on the tail. The bottom skin is the opisit. A hole towards the front is not as bad as one towards the tail in terms of air loss.

What happions when you have a hole? Well, air leaks out of it. In a sence it changes the shape of the airfoil. Think of it as a bump that the out side air that is makeing your lift must flow over. It pushes the air flow away from the canopy. If can cause flow seperation of premature stalling, at least localy, of that section of the wing. You get more drag when that happions and that could affect your pitching and flare. Trying to keep this symple. I don't want to get too mathy here. but from a sence of the sevarity of a leak and also airodynamicly a hole towards the front of the top skin is far worse then in the tail.

Stability. Canopies used to have big open conservitive noses. The stagnation point was almose garentead to be in the opening of the nose no matter what the angle of attack was. Now with lips on our canopies and openings that are almose on the bottom of the canopy you can't always say that the skin will be positivly pressureised. In fact when you look up at your nose and see that dimple in the lip that is on allmost every canopy today you know exactly where that point is. If you front riser or add breaks you'll see that dimple move or go away as the angle of attack changes. The canopies are a lot high in performance today but it also putes them closer to the edge. They like the airflow to be stagnent. They really don't like air flow into the nose. Whether it's cross flow, leakage through the seams, the fabric or a hole. It's all the same. It will make that dimple in the lip less stable. Ultomently it can roll backwards causing a leading edge collaps. Depending on a number of factores this can be any thing from a small hickup to a catistofic event, but the point is leaks are bad. Some canopies can tolerate them some cant. I don't bother patching holes in my crw rig till they're big enough to stick your head through. At that point I concider them to become a danger because some one might get there head caught in it. On the other hand I once had an icerous that would colaps just from turning the canopy, cross flow and posable seams.

Have I confused the issue enough? Well I got to go. Cant wait to see how I get slamed on this one.

Lee


RiggerLee

Sep 24, 2012, 12:05 PM
Post #4 of 9 (1160 views)
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Re: [JohnSherman] Canopy operning forces [In reply to] Can't Post

It would be better to say dynamic pressure not ambient.

More acceratly it probably relates closly to the minimun preasure at the nose opening. The nose is larg enough that you can't just look at some teoretical stagnation preasure at one point. It's tall enough that there is probable some flow in and out. Hmmm, what would it be? perhaps an adverage across the nose area? I'd have to think about it. But Q is close enough for goverment work.

Lee


JohnSherman  (D 2105)

Sep 25, 2012, 8:47 PM
Post #5 of 9 (1055 views)
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Re: [RiggerLee] Canopy operning forces [In reply to] Can't Post

Quote:
It would be better to say dynamic pressure not ambient.

I mean Ambient not Dynamic pressure. Dynamic pressure "Q" is the force on the bottom skin. Which is the product of speed and density. The internal pressure is equal to the surrounding atmospheric pressure which has no increase due to speed. Its whatever the barometer reads at that point. The top skin has a reduced pressure because of the circulation of flow. Think of it as the inside being neutral with the outside being a battle between Newton and Bernoulli. Newton on the bottom and Bernoulli on the top.
"Q" never gets past the bow wake.


(This post was edited by JohnSherman on Sep 25, 2012, 8:51 PM)


RiggerLee

Sep 26, 2012, 10:09 AM
Post #6 of 9 (994 views)
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Re: [JohnSherman] Canopy operning forces [In reply to] Can't Post

Ummm... no. Your wrong, but I should actually be a bit more formal my self. Q, dynamic pressure, .5rv^2 is the kinetic energy stored in the free stream velocity. In theory it would be the diffrence between the static and dynamic ports on an ideal peto tube. Now it never works out quite that cleanly. There's always a certin amout of calibration. You can't really talk about the exact, still relitive, number with out examining the fluid dynamics of the situration. Way more then we want to go into here. But for all practical purpouses the leading edge of the airfoil, the stagnation point where the edge splits, expereances the full dynamic preasure, Q, plus free stream. The pressures on the top and bottom skin are complex questions determind by the fluid flow. Look at the little toy model. Presure on the bottom skin reduces quickly and even drops below freestream, ambeant, then it climbs again towards the tail till it is again over ambeant. The preasure on the top skin is much lower but even there the preasure towards the tail can climb above ambeant. But you'll notice that the canopy bulges out wards, excluding the lip on the nose, every where including the bottom skin. You are positivly preasurized. And if you were to put a preasure transduicer any where inside the canopy looking at absolute preasure you would find it very close to the total preasure, dynamic + static, that you see at the stagnation point at the leading edge of the airfoil.

Lee


CrazyL  (D 17699)

Sep 26, 2012, 11:23 AM
Post #7 of 9 (968 views)
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Re: [mark] Canopy operning forces [In reply to] Can't Post

About the C-line larkshead taking more force. On a main, here's a little field experiment to observe. With main open and ready to pack. Pull on the main bridle and see which lines are loaded due to the bridle pull. I think you'll find another reason the center C's larkshead is so tight. Typically the center B's and C's take load from the bridle.
In reply to:


mark  (D 6108)

Sep 26, 2012, 11:45 AM
Post #8 of 9 (960 views)
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Re: [CrazyL] Canopy operning forces [In reply to] Can't Post

In reply to:
About the C-line larkshead taking more force. On a main, here's a little field experiment to observe. With main open and ready to pack. Pull on the main bridle and see which lines are loaded due to the bridle pull. I think you'll find another reason the center C's larkshead is so tight. Typically the center B's and C's take load from the bridle.
MEL's claim, I think, was that the center "C" lines take more force than the center "A" or "B" lines. I understand that the center "C" lines take more of the load from snatch force, but that's different than claiming they take more load generally. My question for him still stands: if the center "C" lines are taking more force, why is it that the center "A" lines are more likely to break?

Mark


CrazyL  (D 17699)

Sep 28, 2012, 11:26 AM
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Re: [mark] Canopy operning forces [In reply to] Can't Post

Well Mark, as you hunt for answers. The center C-lines position on the canopy possibly allows more directions for load to spread throughout the canopy. The center A-lines have near a third less directions for load to spread. With the varying ways for a canopy to inflate, bottom skin, nose, crossports, and varying combinations of both bottom skin and nose inflation. It seems one type of inflation could overcome the other. So, another question to find an answer to is concerning the lesser ideal inflation like mostly nose or mostly bottom skin inflation. Take a BASE canopy no slider for instance. Observe the harsh bottom skin inflation, inner canopy pressure would be low inflation pressure after bottom skin inflation. Add a slider, then the inflation is shared by both bottom skin and nose. Loads on lines are less than bottom skin inflation alone.
Opening main in high upper winds facing upwind or downwind will produce 2 different opening speeds. The nose just loves to inhale wind blowing into it.
Been in a crew wrap. Because the jumpers weight is mostly being held by the center C's, while in a stack, a canopy can deflate it's wingtips, distort the center A's, and still fly in a stack. Disturb the center C's. All hell breaks loose. So, why do A-lines break more so than center C's. Excessive shock load, with not enough places for stress to be relieved is one answer of many.



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