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180 degree front riser requirement for "A"

 

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kallend  (D 23151)

Sep 19, 2007, 8:57 PM
Post #51 of 99 (1018 views)
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Re: [d123] 180 degree front riser requirement for "A" [In reply to] Can't Post

Take a look at http://www.pilotsweb.com/...iple/art/press02.jpg and http://www.pilotsweb.com/...iple/art/press03.jpg

The center of pressure (lift) indeed moves forward as the AOA is increased on a typical airfoil.


(This post was edited by kallend on Sep 19, 2007, 9:01 PM)


bob.dino  (E 2185)

Sep 19, 2007, 9:22 PM
Post #52 of 99 (1008 views)
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Re: [JohnMitchell] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
So, for the same suspended weight, should some canopies be easier to front riser than others? Would the difference be significant?

Borrow a Katana and a VX in the same size. The VX is renowned for high front-riser pressure. I have at least one friend that passed on it because of that characteristic. He was performing his turns at the limit of his strength. That's not ideal, so now he's on a XAOS-27.

The Katana has (comparatively) light front riser pressure. Almost scarily so. Check the early reviews of the canopy on this forum for more...


JohnMitchell  (D 6462)

Sep 19, 2007, 9:42 PM
Post #53 of 99 (1004 views)
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Re: [bob.dino] 180 degree front riser requirement for "A" [In reply to] Can't Post

Ahhh, that's the info I was looking for. Thanks.Smile


Zenister  (A 42)

Sep 19, 2007, 10:07 PM
Post #54 of 99 (1003 views)
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Re: [rehmwa] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In reply to:
I almost fly my entire base leg in deep brakes (with regard to traffic).

I'd like to thank everyone that does this, especially when it's crowded. AND especially when you are one of the first down. It's just great to see the pattern get all crushed and crowded and dangerous above you all.

some times you are far to subtle for DZ.com...Laugh


phoenixlpr  (D 3049)

Sep 19, 2007, 10:40 PM
Post #55 of 99 (992 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
During the turn you are effectively increasing your weight through the centripetal force you are applying on the lines. The real trick is figuring out what affects the amount of force being applied to a single front riser at different amounts of deflection.

-Michael
How about this: you increase you speed with your turn, than increased speed makes more life on your wing and that is the force what wants to rip the risers out of your hand.


d123  (B 6134)

Sep 19, 2007, 10:40 PM
Post #56 of 99 (992 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

Ok, I see your point. The center of lift can be also changed by the AoA.

If I read those pictures correctly on lower AoA the center of presure is "well" distributed (at lest for that specific profile but we can "extrapolate"). This translates to lower front riser presure.

The current parachute profiles are still close to Domina Jalbert original profile design with the bump near the nose?
http://www.kite.com/kite/jalbertp/jfig2.gif

Great pictures btw! Made everything clear really fast!


Andrewwhyte  (C 1988)

Sep 19, 2007, 11:05 PM
Post #57 of 99 (983 views)
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

I think you are thinking of Vector/sigma & Icarus tandem canopies; the only strong canopy to be normally outfitted with flare toggles were the Master 525s.


Premier billvon  (D 16479)
Moderator
Sep 19, 2007, 11:21 PM
Post #58 of 99 (982 views)
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Re: [mnskydiver688] 180 degree front riser requirement for "A" [In reply to] Can't Post

> If the canopy already has a fairly steep angle of attack wouldn't
>that make front riser input easier because you are not changing the angle
>of attack as much as you would on a canopy with a flatter angle of attack
>in full glide?

No. 90% of the rigidity of a canopy comes from lift acting against the normal force of the lines. Very little of it comes from the 'structure' of the canopy, what some call "the air mattress effect." So deflecting it doesn't require much force.

Neither does changing the AOA. On an aircraft, all you have to do is slow down and the AOA changes. There's no 'resistance' other than the aircraft's inherent pitch stability. (Parachutes have pitch stability as well, which is why they return to trim airspeed after a manuever.)


Premier billvon  (D 16479)
Moderator
Sep 19, 2007, 11:22 PM
Post #59 of 99 (981 views)
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Re: [Andrewwhyte] 180 degree front riser requirement for "A" [In reply to] Can't Post

>the only strong canopy to be normally outfitted with flare toggles were the Master 525s.

Yep, although you don't see them much any more (fortunately!)


phoenixlpr  (D 3049)

Sep 20, 2007, 12:05 AM
Post #60 of 99 (976 views)
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
>the only strong canopy to be normally outfitted with flare toggles were the Master 525s.

Yep, although you don't see them much any more (fortunately!)
How can you say that? That is an 11 cell beauty! Pirate


hackish  (No License)

Sep 20, 2007, 4:54 AM
Post #61 of 99 (962 views)
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Re: [d123] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Ok, I see your point. The center of lift can be also changed by the AoA.

If I read those pictures correctly on lower AoA the center of presure is "well" distributed (at lest for that specific profile but we can "extrapolate"). This translates to lower front riser presure.

The pics do seem to make sense to what I've felt pulling the front riser. First few inches always seemed to be the toughest and after that a bit easier. It may have been due to my arm position and the geometry of that but if distorting the wing (it's AOA) has the effect shown in some of those photos then it will be moving the lift back to the rear riser on that side. Can't wait for this weekend to try it some more...

-Michael


d123  (B 6134)

Sep 20, 2007, 7:32 AM
Post #62 of 99 (942 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

I I've understood correctly those pictures the main question is: are we increasing the AoA when we symmetrically load the AB lines(put our weight on both front risers)?
If yes Center of Lift moves closer to the nose.
If no Center of Lift moves away from the nose.

Just to clarify. AoA is the angle between the wing (chord line) and the relative wind and not the angle between the wing and horizontal axis nor the angle between the flight path and a horizontal axis.
Attachments: Parafoil.jpg (57.7 KB)


hackish  (No License)

Sep 20, 2007, 9:05 AM
Post #63 of 99 (926 views)
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Re: [d123] 180 degree front riser requirement for "A" [In reply to] Can't Post

What I was incorrectly calling AoA is actually the RAI. If you can imagine deforming that canopy such that you're pulling on the A & B lines I believe it will increase the cross sectional symmetry of the wing. This should result in the decrease in lift but I'm not sure what it does with the airflow vector.

-Michael


mnskydiver688  (D 30125)

Sep 20, 2007, 10:16 AM
Post #64 of 99 (916 views)
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Re: [d123] 180 degree front riser requirement for "A" [In reply to] Can't Post

Since we are on this subject of front riser physics... Why when I do front riser turns on my Sabre 150 about 50% of the time it bucks and shakes and wants to pull the riser out of my hand? The steering lines are proper length. Some have said it is just the amount that I am loading the canopy. I am wondering why it happens sometimes and sometimes it doesn't? I have tried to do double front riser approaches and had to stop because the canopy was bucking so bad it might have collapsed. I never experienced these situations on my Sabre 170.


phoenixlpr  (D 3049)

Sep 20, 2007, 10:22 AM
Post #65 of 99 (915 views)
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Re: [mnskydiver688] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I have tried to do double front riser approaches and had to stop because the canopy was bucking so bad it might have collapsed.
You might pull some breaks too and/or your canopy is out of trim.


mnskydiver688  (D 30125)

Sep 20, 2007, 10:30 AM
Post #66 of 99 (912 views)
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Re: [phoenixlpr] 180 degree front riser requirement for "A" [In reply to] Can't Post

The canopy has about 75 jumps on it and I have watched when I pull on a front riser and the steering lines are not pulling down the tail.


Premier skybytch  (D License)

Sep 20, 2007, 10:44 AM
Post #67 of 99 (910 views)
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Re: [mnskydiver688] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I am wondering why it happens sometimes and sometimes it doesn't?

What are the winds doing when it happens? Does it only happen when you land in a particular area?


rehmwa  (D 12816)

Sep 20, 2007, 11:25 AM
Post #68 of 99 (902 views)
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Re: [mnskydiver688] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I do front riser turns on my Sabre 150 about 50% of the time it bucks and shakes

sounds like your lines are already out of trim

except if that was the case, it would buck and shake every time


mnskydiver688  (D 30125)

Sep 20, 2007, 12:38 PM
Post #69 of 99 (885 views)
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Re: [skybytch] 180 degree front riser requirement for "A" [In reply to] Can't Post

It will do it at any point in the decent so it isn't being caused by turbulence off of buildings or trees. It is frustrating because there are times where I can crank on the front risers and get a nice smooth response and then there are times where I pull the riser down and inch and it starts to buck and try to pull the riser out of my hand. I plan on going to FL sometime in the winter so I might have PD take a look at it. I use front risers quite a bit if it looks like I am going to overshoot the target so hopefully I can resolve the issue because braked approaches aren't fun at all!


relyon  (D 18973)

Sep 20, 2007, 12:41 PM
Post #70 of 99 (883 views)
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Re: [mnskydiver688] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Since we are on this subject of front riser physics... Why when I do front riser turns on my Sabre 150 about 50% of the time it bucks and shakes and wants to pull the riser out of my hand? The steering lines are proper length. Some have said it is just the amount that I am loading the canopy. I am wondering why it happens sometimes and sometimes it doesn't? I have tried to do double front riser approaches and had to stop because the canopy was bucking so bad it might have collapsed. I never experienced these situations on my Sabre 170.

This sounds like a case of the the stagnation point moving onto the top skin.

The stagnation point is the point where an air molecule comes to a stop. Those above it go over the topskin; those below under the bottom skin. Usually, it is in front of the nose, but if it moves onto the topskin, it will [partially] collapse the canopy. When that occurs, lift is lost, the canopy drops/dives, the stagnation point moves back to a lower point, the canopy reinflates, lift is restored, then the process starts again. The result feels like the bucking you describe.

It is entirely possible to induce this by pulling one or both front risers far enough such that the stagnation point moves significantly higher. I've done it many times myself. Line trim may be the problem, but I doubt that given the number on jumps on the canopy. Wind/turbulence may also be part of the problem, but it's more likely from pulling too much front riser for the particular canopy and trim.

Bob


mnskydiver688  (D 30125)

Sep 20, 2007, 1:05 PM
Post #71 of 99 (885 views)
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Re: [relyon] 180 degree front riser requirement for "A" [In reply to] Can't Post

Would the length of the risers make a difference? The ones on the 150 are quite a bit longer than the ones on the 170. Also would the placement of the 3 ring on the harness make a difference? The rig with the 170 has the three ring assembly on top of the shoulders and even a little further back while the rig with the 150 has the 3 ring on the front of the shoulders.


d123  (B 6134)

Sep 20, 2007, 2:12 PM
Post #72 of 99 (874 views)
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Re: [mnskydiver688] 180 degree front riser requirement for "A" [In reply to] Can't Post

I can only guess and it might be a bad guess but anyway here is my guess.

I'm guessing that the front risers pressure on Sabre is lower since you can "pull" them. This might suggest that the "center of lift" is located a bit farther away from the nose of wing and it's offering a more uniform load distribution between AB and CD lines. With that distribution in mind I'm guessing that maybe by putting all your weight on AB lines you are deforming the wing so much that the flow gets detached sooner than it suppose to be. This means that you have a borderline airfoil. Was the tail bouncing or did you feel any bouncing of the wing? That might be the air flow attaching and detaching. I can really feel that happening on few of my parafoil kites when I play with trimming (Beamer I 2.5 meters for instance).

Spin turns might be trickier to analyze because of the Centrifugal force. With the centrifugal force you can load the left AB lines (if you are doing a left spin) a lot higher than your own weight. This might mean even a bigger deformation than in double risers case.

Why is changing from day to day? Maybe air temperature/density is changing enough to make a difference. I've wrote a few months ago to PD and ask them if I can get the glide ratio graph for different toggle input on Navigator and they told me that glide ratio change from hour to hour due to harness position, air pressure/temperature/density so it's quite hard to do a lot of flights for each wing to get an average for my graphs. I should share that e-mail.

In the end I can only speculate about the physics involved and just like you I'll like to know more about what are we dealing with. Maybe someone will like to bring some new ideas, new food for the mind about the front risers physics.

One thing that I like about kiteboarding and paragliding magazines is the fact that different companies talks about their wings designs with under the hood details about the wing profile, planform, bridle system and describes what everything is doing. Not a lot of info but just a bit. I would love to see the same thing in skydiving.


phoenixlpr  (D 3049)

Sep 20, 2007, 2:44 PM
Post #73 of 99 (857 views)
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Re: [d123] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
One thing that I like about kiteboarding and paragliding magazines is the fact that different companies talks about their wings designs with under the hood details about the wing profile, planform, bridle system and describes what everything is doing. Not a lot of info but just a bit. I would love to see the same thing in skydiving.

Do birds know physics, aerodynamics, solving differential equations? Do they fly all weather and all year around? I would not bother too much about those numbers just feel, watch and fly.


d123  (B 6134)

Sep 20, 2007, 3:02 PM
Post #74 of 99 (852 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
First few inches always seemed to be the toughest and after that a bit easier.
-Michael
Anyway, I think you were right about it. When you "pull" (shift your weight to) AB lines you decrease the wing AoA.

For a "gliding" wing at equilibrium the lift is equal with the load. If the air speed increase (and it does) this means that the AoA decrease. In other words if you increase the air speed in order to have the same lift (your weight) the AoA needs to decrease.

So I really think you were right. As you pull on AB the center of lift moves a bit backwards(away from the nose) as Kallen pictures suggest.


d123  (B 6134)

Sep 20, 2007, 3:16 PM
Post #75 of 99 (851 views)
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Re: [phoenixlpr] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Do birds know physics, aerodynamics, solving differential equations?
No they don't. These discussions are only for the curiosity. I'll doubt that they have any real application.
The airfoil got invented a long time ago and same thing for parafoil and I doubt that there are going to be changes in the way we fly in 21 century.

I'll still like to know more about it not as a monkey that tries out anything that reads on the internet but rather out of my (and some other people) curiosity.

These are only pass time talks and maybe they should be placed in a different thread as you suggest it.

Edit to add: Anyway, me personally I'll love to be able to read somewhere here on dz.com what other people like Kallend, Billvon or wing designers are thinking about the physics of flight. Not for the practical reason just to know about it. I don't know if this is bad or good or is offending you or other people from here but I would really love to understand more and I think that this should be the place.


(This post was edited by d123 on Sep 20, 2007, 3:35 PM)


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