Forums: Skydiving: Safety and Training:
180 degree front riser requirement for "A"

 


hackish  (No License)

Sep 18, 2007, 7:48 PM
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180 degree front riser requirement for "A" Can't Post

From another thread someone pointed out via PM that it's nearly impossible under a large canopy to successfully perform a 180 degree front riser turn.

I thought for sure that I had done it and satisfied the requirement for the CSPA "A". Upon further discussion it would appear that on a 1:1 wing-loading it should be nearly impossible for me to perform this task because of the G-forces involved.

So this weekend I'm going to grill the instructor for more details and try to re-do that portion of the requirements just to see if I am able to satisfy it.

This all begs the question of why the license would ask you to do a task that should be more or less physically impossible? Or are some people working on their "A" license flying smaller faster sport canopies? At my DZ the student gear is in the 280 range PD280/Parafab 284/Tutor 9 cell). At the DZ where I did my FJC they flew Mantas which were also around 280ish.

-Michael


Chris-Ottawa  (A License)

Sep 18, 2007, 7:54 PM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

Hey Mike,

Not sure who told you it's impossible....but I did it. Now if I was asked to do more than a 360, yeah, it might get really tough.

Imagine me, 5'8", 120 lbs, on the front riser of a Manta 288.

Trust me, it is fully possible.

In my opinion, if you "can't" do at least a 180...you're too weak to be jumping. That may sound harsh...but seriously.

Chris

Edited to add:
By "you're too weak to jump", I'm not indicating that this is you specifically, but jumpers in general.


(This post was edited by Chris-Ottawa on Sep 18, 2007, 7:57 PM)


Premier DSE  (D 29060)

Sep 18, 2007, 8:14 PM
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Re: [Chris-Ottawa] 180 degree front riser requirement for "A" [In reply to] Can't Post

I'm one of the ones that said it's very difficult to perform a front riser full 180 at his wingloading.
Hackish has indicated that his instructors (or someone) is requiring that he perform a full 180 degree turn via front risers in order to gain his A CoP. Front riser turns (Cat H) for an A license/CoP in the States may be waived in the event of insufficient strength (again demonstrating that it ain't so easy). I couldn't find any waiver, but that doesn't mean it can be waived, only that front/rear riser maneuvers are required. I'd find it very difficult to accept that a 180 on fronts is required of a student, depending on the wingloading. Then again, maybe it's different in Canada?


monkycndo  (D License)

Sep 18, 2007, 8:16 PM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

If you want to make the front riser pressure lighter, hit the brakes first. As soon as you let off the brakes, pull down one riser. It will be much easier. As taught by Brian Germain.


bob.dino  (E 2185)

Sep 18, 2007, 8:36 PM
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Re: [monkycndo] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
If you want to make the front riser pressure lighter, hit the brakes first. As soon as you let off the brakes, pull down one riser. It will be much easier.

This is the answer. Go to half brakes and hold for 5+sec. Smoothly let up the toggles and pull down the front riser. Easy!


loumeinhart  (D 30065)

Sep 18, 2007, 8:39 PM
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Re: [monkycndo] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
hit the brakes first. As soon as you let off the brakes, pull down one riser

I also use this as a final traffic check before riser input, I almost fly my entire base leg in deep brakes (with regard to traffic).


kelpdiver  (B 7)

Sep 18, 2007, 8:39 PM
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Re: [monkycndo] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
If you want to make the front riser pressure lighter, hit the brakes first. As soon as you let off the brakes, pull down one riser. It will be much easier. As taught by Brian Germain.

but begs the question, what was the point then?


bob.dino  (E 2185)

Sep 18, 2007, 8:49 PM
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Re: [kelpdiver] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In reply to:
If you want to make the front riser pressure lighter, hit the brakes first. As soon as you let off the brakes, pull down one riser. It will be much easier. As taught by Brian Germain.

but begs the question, what was the point then?

So the jumper can see & feel how a canopy reacts to front riser input?


Many swoopers start from brakes.


hackish  (No License)

Sep 18, 2007, 9:06 PM
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Re: [monkycndo] 180 degree front riser requirement for "A" [In reply to] Can't Post

Spent the drive home considering a post I read about the force vectors of landing and flare some of it commented on by Brian Germain. Just trying to work out the physics of what happens and if it's possible on the gear a student would fly...

Turns out I'm wrong on the CSPA requirements as they ask for:
Under canopy, demonstrated a 360 front riser spiral (above 2000 ft)

So in pondering things I've got a bit of an incomplete picture of what happens when you pull down on a front riser. I assume on a 9 cell configuration the front riser is attached to the A and B lines of 4 1/2 of the cells?

In full glide I assume they're going to be supporting about 1/4 of your weight which you need the strength to "lift". So if you pull down on the front riser you're not only going to change the angle of attack for one side of the canopy but your weight may pivot toward the front riser you're pulling increasing the angle of attack on the other side because the opposite force is applied to the C&D lines on that side...

I think this is going to be a more consistent or "aggressive" change on a more elliptical canopy and more abrupt on one with a different aspect ratio (smaller from front to back). I assume this is why the sport canopies respond faster. Gets complicated really fast. I'm assuming here that an elliptical canopy is shaped such that the length of the lines are equal rather than something that flies "flat" above your head. If the canopy is tapered it should even be more aggressive as the AOA should change more toward the outsides...

So that covers how I understand the canopy will then begin the spiral turn but as the turn goes there will be centrifugal force applied from your mass to the lines plus the riser you're holding down. This may as DSE pointed out via PM overcome your strength. I wonder if the force isn't applied more to the unpulled toggles as in theory the changed angle of attack should mean less lift on the corner of the canopy you're pulling the riser on?

If you put on the brakes as monkycndo says then your inertia should swing you forward a bit causing more slack in the front risers and thus making it easier to pull down but will that make you do the front spiral better? I'm not convinced because I assume the spiral comes from the deformation of the wing - something not yet accomplished by taking up the slack of swinging your mass forward from applying brakes.

Ok I worked a 16h shift and now my head hurts... Anyone have comments on the physics here?

-Michael


murps2000  (D 23114)

Sep 18, 2007, 9:36 PM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

Check out Brian Germain's book "The Parachute and it's Pilot". It will answer many of your questions. Among other things, he discusses in depth the effects of various control inputs on the CG and angle of attack of your canopy.


Premier billvon  (D 16479)
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Sep 18, 2007, 10:04 PM
Post #11 of 99 (3192 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

>If you put on the brakes as monkycndo says then your inertia
>should swing you forward a bit causing more slack in the front risers and
>thus making it easier to pull down but will that make you do the front
>spiral better?

It makes the turn easier to initiate by putting the canopy in its "recovery" regime; that unloads the lines momentarily. The forces will still increase as the spiral steepens.


dragon2  (D 101989)

Sep 19, 2007, 1:00 AM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

Quit whining Wink over here we need to do 3x360 one way followed by 3x 360 the other way on frontrisers for B Crazy

Luckily I was exempt because of all my crw jumps, but man Crazy


speedy

Sep 19, 2007, 2:02 AM
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Re: [dragon2] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Quit whining Wink over here we need to do 3x360 one way followed by 3x 360 the other way on frontrisers for B Crazy

Luckily I was exempt because of all my crw jumps, but man Crazy

That is no problem with a R.A.G.E from Paratec.
You can steer that thing with front risers all the time. There is very little pressure on the fronts, which makes for a lot more pressure on the rears.


phoenixlpr  (D 3049)

Sep 19, 2007, 2:43 AM
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Re: [speedy] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
That is no problem with a R.A.G.E from Paratec.
You can steer that thing with front risers all the time. There is very little pressure on the fronts, which makes for a lot more pressure on the rears.
AKAIK R.A.G.E. is not a student canopy... Wink

I've heard that you even have to use fronts to inflate your canopy. So they should be universal...Pirate


kallend  (D 23151)

Sep 19, 2007, 7:13 AM
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Re: [dragon2] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Quit whining Wink over here we need to do 3x360 one way followed by 3x 360 the other way on frontrisers for B Crazy

Luckily I was exempt because of all my crw jumps, but man Crazy

And the purpose of that demonstration is...?


rehmwa  (D 12816)

Sep 19, 2007, 7:23 AM
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Re: [loumeinhart] 180 degree front riser requirement for "A" [In reply to] Can't Post

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.


illusioneer  (A 13037)

Sep 19, 2007, 7:29 AM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

On a navigator 280 I have tried a few front riser turns...the canopy did not respond much at all...I would have a hard time doing any front riser turns.


atsaubrey  (A 44961)

Sep 19, 2007, 7:50 AM
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Post deleted by atsaubrey [In reply to]

 


kallend  (D 23151)

Sep 19, 2007, 8:26 AM
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Re: [atsaubrey] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In reply to:
Hey Mike,

Not sure who told you it's impossible....but I did it. Now if I was asked to do more than a 360, yeah, it might get really tough.

Imagine me, 5'8", 120 lbs, on the front riser of a Manta 288.

Trust me, it is fully possible.

In my opinion, if you "can't" do at least a 180...you're too weak to be jumping. That may sound harsh...but seriously.

Chris

Edited to add:
By "you're too weak to jump", I'm not indicating that this is you specifically, but jumpers in general.

I am a pretty big boy and can bench more than my weight which is over 300lbs. I CANNOT pull down my front risers so I guess I should quit jumping. When I can see a tandem master do front rider turns then I will quit jumping. Tell you what you come jump my rig and I will consider it if you can pull a front 180 on my rig.

Forget the bench presses, do more pull-ups.Wink


rehmwa  (D 12816)

Sep 19, 2007, 8:57 AM
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Re: [atsaubrey] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
When I can see a tandem master do front rider turns then I will quit jumping.

I think most people can do a single pull up and hold their own weight.

I doubt many can pullup twice their weight.


atsaubrey  (A 44961)

Sep 19, 2007, 9:07 AM
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Re: [rehmwa] 180 degree front riser requirement for "A" [In reply to] Can't Post

Sorry certain things get under my skin, and I hadn'e had my cup of coffee this morning. Thought it is better to delete than start a bitch. sorry for my attitude.


Premier billvon  (D 16479)
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Sep 19, 2007, 9:07 AM
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

>Forget the bench presses, do more pull-ups.

Yep. To pull a riser down you need the other set of muscles in your arms. But if you can do a pullup you can pull down a riser.


atsaubrey  (A 44961)

Sep 19, 2007, 9:16 AM
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

I will admit I can do pullups (not many..lol), but can't get enough of a grip on a front riser to move it more than an inch or two. But thats me.


Premier billvon  (D 16479)
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Sep 19, 2007, 9:29 AM
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Re: [atsaubrey] 180 degree front riser requirement for "A" [In reply to] Can't Post

>but can't get enough of a grip on a front riser to move it more than
>an inch or two. But thats me.

Oh, OK! That's a different problem. One way to solve it is to grab the riser at the link where there's more "stuff" to hang onto. If you can't reach that, dive loops or dive blocks can help, and are fairly easy to add.


atsaubrey  (A 44961)

Sep 19, 2007, 10:15 AM
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

Not to take away from the original post, but no way I can reach the link. Heck my slider usually hangs out above the links about 4-5 inches and I can't reach it unless I grab a bunch of rear riser with one hand to be able to reach it. I will admit a 330 canopy is a mofo to move with front risers and I have yet to need to use them.


Andrewwhyte  (C 1988)

Sep 19, 2007, 10:16 AM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

Getting back to the OP. I have never heard a novice complain that they were unable to complete the riser spiral for the A CoP.
Your post said that you believed you had done it, but because of something somebody said you now know you didn't. When observations don't fit the theory I suggest you re-examine the theory. If you still have doubts go for a skydive and repeat the experiment; go try another 360 front riser spiral.
You talk about being on a 1:1 wing-loading, but then about being on student gear; are you in the 250lb range?
You asked if some people working on their A are jumping smaller, faster canopies. The answer is yes they are. Most dZs have some smaller transition canopies that people start to use around the time they get their SOLO certificates. If however you do weigh 250 lbs, I suspect you will be staying on the 280s for a while.


(This post was edited by Andrewwhyte on Sep 19, 2007, 2:34 PM)


hackish  (No License)

Sep 19, 2007, 10:44 AM
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Re: [Andrewwhyte] 180 degree front riser requirement for "A" [In reply to] Can't Post

Well I just honked on the front riser and it turned around. I sometimes do this if there is a tandem coming down after me so I've landed and am well clear of that.

However, someone with a respectable amount of experience pointed out that the physics of pulling a front riser on a larger canopy make it impossible. For that reason I'm exploring the possibility. He also pointed out that a body builder friend wasn't strong enough. If he isn't then I am probably not!

I'm flying a sabre 210 as I got my solo license about 10 jumps ago. Body weight is 180 so my exit weight should put me around 1:1.

-Michael


mnskydiver688  (D 30125)

Sep 19, 2007, 10:55 AM
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Re: [rehmwa] 180 degree front riser requirement for "A" [In reply to] Can't Post

Agreed!


riggerrob  (D 14840)

Sep 19, 2007, 11:08 AM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

I am not sure who is feeing you that foolish theory.

Any canopy can be turned with front risers.

However, big student canopies tend to turn very slowly - with front risers.
That is why I usually wait until students transition down to 230 or 190 square foot canopies to teach them front riser turns.

There is also the issue of grip strength. Front riser turns require a lot of grip strength, more than some students have, ergo we wait until they down-size to 190s (with front riser dive loops) before teaching front riser maneuvers.

In conclusion, you need to skydive more and listen to less theory.


rehmwa  (D 12816)

Sep 19, 2007, 11:12 AM
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Re: [] 180 degree front riser requirement for "A" [In reply to] Can't Post

Not trying to flip here, but I think the point is, no one is 'pulling the canopy down', they are actually just pulling their body weight up.

So, since people keep talking about the 'physics' here:

I don't think it matters what size the canopy is, the most force one can apply to the start of a turn is their body weight almost on one riser. (And no, I don't think this is a simplification in any way)

In fact, for those that can do a pullup (and gripping and pulling isn't always easy on a smooth riser - understood) centrifugal force being the point that limits the pilot, it is likely harder to stay on a front riser with a small 'fast turning' canopy, than a big boat that turns slow. As the spec will load up in the turn much quicker. On the big boat - the issue is you have to hold on Longer, but the force is likely less - so stamina in holding on the extra few seconds matters.

So if a student can't do a front riser turn, then they really should work on two things, their grip strength, and pull ups. The canopy size is not the issue.

IMHO.


(This post was edited by rehmwa on Sep 19, 2007, 11:14 AM)


rehmwa  (D 12816)

Sep 19, 2007, 11:14 AM
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Re: [riggerrob] 180 degree front riser requirement for "A" [In reply to] Can't Post

Once again, riggerrob says is better and more briefly than I could.


Chris-Ottawa  (A License)

Sep 19, 2007, 11:23 AM
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Re: [riggerrob] 180 degree front riser requirement for "A" [In reply to] Can't Post

Ok, for clarification.

I still stand by my post. Everyone should be able to lift at least their own weight to "bend" the canopy and get it to turn. I load my Spectre 135 at 1:1 and after a 360, it is pulling damn hard, but I can still hold it. After a 720, it is pretty near impossible for me. Now, introduce a second hand and....sprials away...

However.....

I had not considered the dive loops. Every canopy I've jumped, other than Mantas have had dive loops. On the Mantas, I was able to grab the top of the riser and thread my fingers through the lines then pull down.

Sorry about that, but I wouldn't consider that the inability to do a front riser spiral, but the inability to get a good grip on the riser.


hackish  (No License)

Sep 19, 2007, 11:24 AM
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Re: [rehmwa] 180 degree front riser requirement for "A" [In reply to] Can't Post

I always like to hear more than one opinion. Now I'm anxiously awaiting the theory books I ordered from Paragear and the weekend so I can try this stuff out again. Since my instructor is more than willing to sign me off he's obviously satisfied that the tasks were complete but I'm considering this a learning exercise.

-Michael


Premier skybytch  (D License)

Sep 19, 2007, 11:35 AM
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Re: [Chris-Ottawa] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I still stand by my post. Everyone should be able to lift at least their own weight to "bend" the canopy and get it to turn.

Why?


rehmwa  (D 12816)

Sep 19, 2007, 11:57 AM
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Re: [skybytch] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In reply to:
I still stand by my post. Everyone should be able to lift at least their own weight to "bend" the canopy and get it to turn.

Why?

I think in terms of 'strength' in skydiving it's less important to be able to do 'that' pullup (though being strong enough to fully use your canopy inputs is a "good thing") than the following safety issues:

1 - Being strong enough to be able to close your container vs having a loose closing loop because you can't...

2 - Being strong enough to be able to push on the air enough in freefall to establish control (some people just aren't strong enough to fly well, surprising but true. I know one woman who just isn't strong enough to push down during dearching to slow down, the freefall pretty well puts her into one body position and that's it. She will be very less effective in the event of a total and that slowness may affect her safety.)

3 - Strong enough to be able to run it out and not collapse on landing. I've seen people with problems in this area too. injuries could result.

Being strong enough to effectively use all the inputs for canopy piloting? I think that it is rare that the inability to confidently do front riser pulls results in a safety issue. But it's possible. Just not likely. I'd rather see someone with a good tight closing pin. (I'd rather see both, but if I had to choose....)

{{{I think in most cases, the whole pullup strength isn't needed for riser turns anyway}}}}

skydiving doesn't require much strength, but it's essential to be able to do certain things well. But it only 'really' matters like in the above in terms of safety

so, "everyone SHOULD be able to to lift...." I agree with Chris, but -too bad- not everyone CAN - and they'll still be able to skydive fine, they just won't have all the tools they could.. Let's hope it doesn't result in them colliding with someone someday because they couldn't take the proper evasive actions due to being strong enough to use toggles only.


Edit: The bigger issue isn't if someone can't do 1 of those things above. It's that most that can't do 1 of them can't also do ANY of them. We watch over those people carefully.


(This post was edited by rehmwa on Sep 19, 2007, 11:59 AM)


Premier billvon  (D 16479)
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Sep 19, 2007, 1:45 PM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

>However, someone with a respectable amount of experience pointed
> out that the physics of pulling a front riser on a larger canopy make it
>impossible.

Experience in skydiving does not indicate competence in physics.

If I jump a 300 square foot canopy, the load on the lines is my weight plus my harness/reserve - about 190 pounds. I will have to deflect a percentage of that (usually around 1/4 to 1/3, depending on design) to pull down a front riser to start a front riser turn.

If I jump my 108 Nitro, the load on the lines will be . . . 190 pounds. Jumping a large canopy does not change the load on the lines, and no canopy out there is rigid enough to present much opposition to deflection (compared to the load on the lines that is.)

There is no difference (other than canopy design and front/rear loading breakdowns) in starting a front riser turn with a big or a small canopy. However, it's harder to HOLD a turn with a small canopy because your speed increases rapidly.


riggerrob  (D 14840)

Sep 19, 2007, 2:28 PM
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

IOW, the lighter the wing-loading, the less muscle required to do riser turns.
For example, if I take a 100 pound girl and a 250 pound guy for tandem jumps - with the same 400 square foot canopy - I can easily do rear riser turns and rear riser flares with the light girl, but those same maneuvers are a struggle with the heavy male student.


hackish  (No License)

Sep 19, 2007, 2:43 PM
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

It is the physics I was trying to decode. RiggerRob and others suggested some books covering rigging and canopy flight so maybe those will be able to fill in some of the technical details. My concern in this thread dealt with whether I had "done it right" especially if the physics (which I do not yet fully understand) say otherwise.

-Michael


Premier DSE  (D 29060)

Sep 19, 2007, 3:00 PM
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

Rightfully or wrongfully, at least you're thinking about it, and IMO, that's the most important aspect.
Putting it into practice may be an entirely different matter, however.Smile


JohnMitchell  (D 6462)

Sep 19, 2007, 3:18 PM
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

[replyThere is no difference (other than canopy design and front/rear loading breakdowns) in starting a front riser turn with a big or a small canopy. However, it's harder to HOLD a turn with a small canopy because your speed increases rapidly.Why are toggle pressures so different on different canopies, if my suspended weight never changes? Are there some additonal factors we're overlooking?


kelpdiver  (B 7)

Sep 19, 2007, 3:23 PM
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Re: [Chris-Ottawa] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I still stand by my post. Everyone should be able to lift at least their own weight to "bend" the canopy and get it to turn. I load my Spectre 135 at 1:1 and after a 360, it is pulling damn hard, but I can still hold it.

well, it's a lot easier to turn a 135 then it is a 200+, even at the same wing load. That's why the warnings apply to 150s and under.

With a dive loop, I can force the triathlon 210 to turn, but it doesn't do it in any hurry. I think the distinction of doing a pullup rather than trying to pull down the riser is helpful.


kelpdiver  (B 7)

Sep 19, 2007, 3:26 PM
Post #42 of 99 (908 views)
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Re: [bob.dino] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In reply to:
but begs the question, what was the point then?

So the jumper can see & feel how a canopy reacts to front riser input?

Many swoopers start from brakes.

ah, good to know. If I put into deep brakes I can then do a double front riser dive, though it doesn't take very long before the riser pressure wins again.


kschilk

Sep 19, 2007, 4:14 PM
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
>but can't get enough of a grip on a front riser to move it more than
>an inch or two. But thats me.

Oh, OK! That's a different problem. One way to solve it is to grab the riser at the link where there's more "stuff" to hang onto. If you can't reach that, dive loops or dive blocks can help, and are fairly easy to add.

A redneck jumper'd just use comealongs.


kallend  (D 23151)

Sep 19, 2007, 4:31 PM
Post #44 of 99 (892 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:

However, someone with a respectable amount of experience pointed out that the physics of pulling a front riser on a larger canopy make it impossible.

Maybe you should talk to a REAL physicist.


kallend  (D 23151)

Sep 19, 2007, 4:37 PM
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Re: [JohnMitchell] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
[replyThere is no difference (other than canopy design and front/rear loading breakdowns) in starting a front riser turn with a big or a small canopy. However, it's harder to HOLD a turn with a small canopy because your speed increases rapidly.
Why are toggle pressures so different on different canopies, if my suspended weight never changes? Are there some additonal factors we're overlooking?
Load distribution between front and rear.


Premier billvon  (D 16479)
Moderator
Sep 19, 2007, 5:51 PM
Post #46 of 99 (870 views)
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Re: [JohnMitchell] 180 degree front riser requirement for "A" [In reply to] Can't Post

>Why are toggle pressures so different on different canopies,
>if my suspended weight never changes?

Canopy design.

A Strong tandem main is a good example. There are "turn" toggles and "flare" toggles. The pressure on the 'turn' toggles is fairly light (well, for a tandem anyway) and it's how you steer the canopy. When you flare you bring all four toggles down. The toggle pressure then becomes much heavier, because you're deflecting more of the canopy.

Think about the rear of the canopy and where the brake lines are attached. If you deflect a small part of the canopy, you're changing the line loading very slightly, and thus there's not too much pressure. If you deflect a lot of the tail, then you're changing line loading more significantly, and there is more pressure.

Likewise, many steeply-trimmed canopies have heavy front riser pressure because the canopy has more area suspended from the A-B lines as opposed to the C-D-brake lines, and more of the lift is happening over those lines. Heck, on Katanas (and many other elliptical canopies) they omit some D lines because they're just not needed! The result can be heavier front riser pressure.

However, in all cases, no matter what the design or loading, 100% of your weight is suspended from the risers - no more and no less. (Unless you're turning of course.)


mnskydiver688  (D 30125)

Sep 19, 2007, 7:03 PM
Post #47 of 99 (851 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

Now I am not a physicist so I am merely thinking out loud. 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? On a canopy with a flatter angle of attack you are having to deflect more of the canopy in order to get that increased angle of attack. Also if more weight is distributed to the nose wouldn't that also make front riser input easier because you are not having to shift that much weight to the front risers? I could be and probably am way off, but I like this thinking out loud stuff.


JohnMitchell  (D 6462)

Sep 19, 2007, 7:31 PM
Post #48 of 99 (844 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:

Load distribution between front and rear.
So, for the same suspended weight, should some canopies be easier to front riser than others? Would the difference be significant?

BTW, I've never even tried to front riser a tandem canopy.CrazyLaugh Just didn't see the point. Hope I'm still qualified to do tandems.Smile


hackish  (No License)

Sep 19, 2007, 7:55 PM
Post #49 of 99 (832 views)
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Re: [JohnMitchell] 180 degree front riser requirement for "A" [In reply to] Can't Post

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


d123  (B 6134)

Sep 19, 2007, 8:48 PM
Post #50 of 99 (818 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Load distribution between front and rear.
Hi Kallend,

I have a small theory about why the front riser pressure is high on some wings and maybe you can take a look over it and let me know what you think about it. I'll try to explain it as clear as I can but I doubt it that I'll do a good job since English is not my first language. I'm guessing that the weight distribution between AB and CD lines is controlled by the parafoil profile and not by trimming (RiggerAngleOfIncidence). Off course that I might be wrong also but anyway here goes:

If front riser pressure is high this means that the AB lines sustains most of your weight and CD lines just a small fraction of your weight. In that case, I believe that most of the lift is generated near the nose of the canopy where the AB lines are connected to the wing (but on the upper skin). Probably somewhere in the CD area the flow gets detached and that's why there's only litle lift created in the CD area. Also somewhere between the AB lines the wing camber has the biggest thickness and from that "bump" on we have the "down wash" of the attached flow that "creates" the lift.

By changing the trimming (Rigger Angle Of Incidence) I don't know if you can change the place where the main lift is generated on the wing and I'm still expecting to have strong front riser pressure.

If we move the area where the wing camber has the biggest thickness (that "bump") somewhere closer to the center of the wing this might change the place where the main lift is generated and change the weight distribution between AB and CD lines = lower the front riser pressure and increase the back riser pressure.

I know 2 other things that might suggest that the main lift is generated in the front of the wing if I'm interpreting them right.

1. Some wings have ZP only on the 1st half of the upper skin. I've seen one like that somewhere but I forget where exactly.

2.In paragliding only on "some" wings you have something called B Lines stall. ABCD lines are not cascaded. By pulling on the B lines you can lower your glide ratio BIG time. I'm thinking that you actually mess around with the area where the main lift is generated and that's close to the B lines for those wings.

If I'm right and most of the lift is generated in the AB area for the wings that have high AB presure then maybe by pulling on left AB is similar with pulling on left ABCD. So when you pull on left AB you basically change the weight distribution between the left and right just like in a harness turn. The difference might come from drag because the wing is changing shape when you put most of your weight on left AB.

Let me know if this could make sense.

Just to politically correct:
Apparently "B lines stall" is a DANGEROUS thing to do and it doesn't work on all the wings. I never try it and I heard from my instructors that recovery from B lines stall gift wrapped few people.


kallend  (D 23151)

Sep 19, 2007, 8:57 PM
Post #51 of 99 (986 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 (976 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 (972 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 (971 views)
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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 (960 views)
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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 (960 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 (951 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 (950 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 (949 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 (944 views)
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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 (930 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 (910 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 (894 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 (884 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 (883 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 (880 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 (878 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 (870 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
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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 (851 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
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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 (842 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 (825 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 (820 views)
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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 (819 views)
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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)


hackish  (No License)

Sep 20, 2007, 4:04 PM
Post #76 of 99 (1415 views)
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In reply to:
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.

They do not. Some people need to analyze and understand the technical details behind a concept before it is truly understood. Others are happy to just accept what they're told and are able to regurgitate the factoids on command.

-Michael


UDSkyJunkie  (D 25746)

Sep 20, 2007, 6:25 PM
Post #77 of 99 (1399 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In full glide I assume they're going to be supporting about 1/4 of your weight which you need the strength to "lift". So if you pull down on the front riser you're not only going to change the angle of attack for one side of the canopy but your weight may pivot toward the front riser you're pulling increasing the angle of attack on the other side because the opposite force is applied to the C&D lines on that side...

I think this is going to be a more consistent or "aggressive" change on a more elliptical canopy and more abrupt on one with a different aspect ratio (smaller from front to back). I assume this is why the sport canopies respond faster. Gets complicated really fast. I'm assuming here that an elliptical canopy is shaped such that the length of the lines are equal rather than something that flies "flat" above your head. If the canopy is tapered it should even be more aggressive as the AOA should change more toward the outsides...

So that covers how I understand the canopy will then begin the spiral turn but as the turn goes there will be centrifugal force applied from your mass to the lines plus the riser you're holding down. This may as DSE pointed out via PM overcome your strength. I wonder if the force isn't applied more to the unpulled toggles as in theory the changed angle of attack should mean less lift on the corner of the canopy you're pulling the riser on?

If you put on the brakes as monkycndo says then your inertia should swing you forward a bit causing more slack in the front risers and thus making it easier to pull down but will that make you do the front spiral better? I'm not convinced because I assume the spiral comes from the deformation of the wing - something not yet accomplished by taking up the slack of swinging your mass forward from applying brakes.

Dude... you're thinking waaay to hard. I'm an aerospace engineer and I didn't follow all that.

you are correct that the front riser connects to A and B lines on 4 1/2 cells

aspect ratio is for glide. higher aspect ratio wings have less drag and more lift. But parachutes don't really have wildly different aspect ratio's anyway. Typical 9-cell square main (Sabre or PD 9-cell) has an AR of 2.5. The katana's and velocities of the world are more like 2.7. 7-cells are lower, like 2.1. AR by itself probably doesn't have much effect on riser pressure.

sport canopies respond faster because they are smaller, more heavily loaded, and lower drag. Same reason an F-16 responds more quickly than a 747.

when you let up on the brakes, your glide flattens and speed slows. Result is less air pressure in your canopy. Less pressure = easier to deform (think of a nearly flat vs. very tight air mattress. Centrifugal forces and such come into play, but not that much, 'cause you aren't spinning super fast like a toggle spiral. Your airspeed increases, and the pressure increases, so it's harder to deform. This is why even in double-front risers it gets harder to hold them down once your speed picks up, even though you aren't turning.

Even if you're super-strong, it might not matter... on my old Sabre 135 I could make my harness go slack on double-fronts... I didn't weigh enough to deform it any more.


denete  (B 33880)

Sep 20, 2007, 6:32 PM
Post #78 of 99 (1399 views)
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Re: [atsaubrey] 180 degree front riser requirement for "A" [In reply to] Can't Post

It seems like a lot of people think of this as though they are pulling the front riser down. I just think of pulling my body up. If you really can't do it...I guess you "could" use two hands. Nah, you can do it with one. Once the turn starts, just maintain it.


denete  (B 33880)

Sep 20, 2007, 6:46 PM
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

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

Theoretical thoughts: Speaking of pressure...apply brakes, as you swing forward under / in front of the canopy, the nose comes up. This angle along with the slowing of the canopy would decrease the compression of air in front of the nose. That should decrease the internal pressure of the canopy. If so, the canopy should be easier to "warp" with the front riser (we are just twisting the canopy into a bit of a propeller shape, right?).

Am I far off on this one?


hackish  (No License)

Sep 20, 2007, 7:20 PM
Post #80 of 99 (1386 views)
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Re: [UDSkyJunkie] 180 degree front riser requirement for "A" [In reply to] Can't Post

Very informative - learned a lot from reading your response actually. I was basing my assumptions mostly on the physics of where the AR if a canopy would then place each line set. I was counting on the "thickness" of the wing to have a lot of effect on how it "handled" not simply how small it was.

The speed versus inflation pressure thing does make sense but if you can pull a riser down far enough to initiate a turn and the forces from the turn do not make a big difference then you should be able to hold that during the entire spiral which some are saying is nearly impossible.

-Michael


d123  (B 6134)

Sep 20, 2007, 7:32 PM
Post #81 of 99 (1389 views)
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Re: [UDSkyJunkie] 180 degree front riser requirement for "A" [In reply to] Can't Post

Hi UDSSkyJunkie,

You might have a point with the internal pressure adding something to the "equation" but in full flight we have the same internal pressure inside the entire wing (from nose to tail) and we can still deform the wing a lot more easier with the CD or break lines than with the AB lines.

I'm sure that internal presure might play a role on the riser presure when the air speed is higher than full flight.


Premier billvon  (D 16479)
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Sep 20, 2007, 9:51 PM
Post #82 of 99 (1378 views)
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Re: [UDSkyJunkie] 180 degree front riser requirement for "A" [In reply to] Can't Post

> Result is less air pressure in your canopy. Less pressure = easier
> to deform (think of a nearly flat vs. very tight air mattress.

This has almost zero effect. The "air mattress" effect simply can't resist you much, and is negligible compared to the stabilizing force of the lines opposing the lift the canopy is generating. Consider a tight air mattress and then consider that the pressure in a canopy is many times less.


dgw  (C License)

Sep 21, 2007, 12:25 PM
Post #83 of 99 (1330 views)
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

I have a theory.

Please see attached crude diagram.

The 'steady state' riser tension is always 'F', for a given suspended weight.

In order to pull a riser downwards, one must apply a force of F/cos(a).

With larger canopies, 'L' increases, which increases angle 'a'. This reduces cos(a), and increases the force required to pull a riser down.

The wider the canopy, the higher the downward force required to pull a riser 'down'. As you will appreciate, the riser tension remains constant as 'F'.

I don't have data on L, but for the sake of comparison, if F=100kg, and a = 45 degrees, then the downward riser load required on the riser is about 141kg. If a = 50 degrees, riser downward pull is 156kg.

I'm sure there are other factors. This simple view offers a 'fundamental' explanation for rising downward pull forces with increasing canopy size, I think.


(This post was edited by dgw on Sep 21, 2007, 12:39 PM)
Attachments: Loading.BMP (17.4 KB)


kallend  (D 23151)

Sep 21, 2007, 12:50 PM
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Re: [dgw] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I have a theory.

Please see attached crude diagram.

The 'steady state' riser tension is always 'F', for a given suspended weight.

In order to pull a riser downwards, one must apply a force of F/cos(a).

With larger canopies, 'L' increases, which increases angle 'a'. This reduces cos(a), and increases the force required to pull a riser down.

The wider the canopy, the higher the downward force required to pull a riser 'down'. As you will appreciate, the riser tension remains constant as 'F'.

I don't have data on L, but for the sake of comparison, if F=100kg, and a = 45 degrees, then the downward riser load required on the riser is about 141kg. If a = 50 degrees, riser downward pull is 156kg.

I'm sure there are other factors. This simple view offers a 'fundamental' explanation for rising downward pull forces with increasing canopy size, I think.

The sum of the "downward" components of force on the four risers cannot exceed the load due to the mass of the skydiver and any centripetal forces. The airfoil characteristics and trim can affect the distribution between front and rear, by moving the center of lift around.


Premier billvon  (D 16479)
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Sep 21, 2007, 1:22 PM
Post #85 of 99 (1312 views)
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Re: [dgw] 180 degree front riser requirement for "A" [In reply to] Can't Post

>With larger canopies, 'L' increases, which increases angle 'a'.

Nope. You have neglected Ls, length of the suspension line. That increases in large canopies as well, keeping angle 'a' relatively constant.


dgw  (C License)

Sep 21, 2007, 1:27 PM
Post #86 of 99 (1311 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

Bollox! Wink You are certainly right. There is an error in the resolution of the forces.

My alternate suggestion, based on the same diagram (with zero credibilty):

The bigger angle 'a', the further you need to pull down the riser to achieve a set amount of deflection. This would require more/different muscle power, making it seem harder to pull, accepting other factors.

I leave the original post as a testimony to my stupidity Blush


dgw  (C License)

Sep 21, 2007, 1:35 PM
Post #87 of 99 (1308 views)
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Re: [dgw] 180 degree front riser requirement for "A" [In reply to] Can't Post

Well, that completely scuppers all ill-fated aspects of my theory. Frown

I'm pleased to have been through the exercise though.

Thanks for the input.


denete  (B 33880)

Sep 21, 2007, 4:54 PM
Post #88 of 99 (1288 views)
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Re: [UDSkyJunkie] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
when you let up on the brakes, your glide flattens and speed slows. Result is less air pressure in your canopy. Less pressure = easier to deform (think of a nearly flat vs. very tight air mattress. Centrifugal forces and such come into play, but not that much, 'cause you aren't spinning super fast like a toggle spiral. Your airspeed increases, and the pressure increases, so it's harder to deform.

Wow, I didn't even get down to reading your post before I posted. I actually came up with the same theory as an AE?? (That's actually very cool...for an arm-chair aerodynamics fan like me.)

- David


denete  (B 33880)

Sep 21, 2007, 5:04 PM
Post #89 of 99 (1286 views)
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
> Result is less air pressure in your canopy. Less pressure = easier
> to deform (think of a nearly flat vs. very tight air mattress.

This has almost zero effect. The "air mattress" effect simply can't resist you much, and is negligible compared to the stabilizing force of the lines opposing the lift the canopy is generating. Consider a tight air mattress and then consider that the pressure in a canopy is many times less.

Okay, try this. Put that same partially inflated air mattress in the swiming pool with suspension lines attached like a canopy. Put yourself on the bottom of the deep end with a harness on (and a ton of weight to hold you down).Wink (The air inside the mattress will move opposite of what it should do in the sky, but the idea is similar.) Pull down on one front riser and watch the pressure try to equalize on the surface of the water (which would be where the air meets the bottom of the canopy and the open nose while in the sky). Now fill the mattress fully with air and try the same thing. Having all of the lines under tension doesn't allow it to pop up like a beach-raft, but it makes it incredibly hard to pull under. Sure, this is an extreme example (a pool full of water pressure versus air in a mattress), but it works.

By the way, how many injuries / fatalities were there during the days of pioneering the square canopy before cells were cross-vented? Would a radical turn collapse the canopy?

- David


(This post was edited by denete on Sep 21, 2007, 5:10 PM)


d123  (B 6134)

Sep 21, 2007, 6:45 PM
Post #90 of 99 (1279 views)
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Re: [denete] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
In reply to:
> Result is less air pressure in your canopy. Less pressure = easier
> to deform (think of a nearly flat vs. very tight air mattress.

This has almost zero effect. The "air mattress" effect simply can't resist you much, and is negligible compared to the stabilizing force of the lines opposing the lift the canopy is generating. Consider a tight air mattress and then consider that the pressure in a canopy is many times less.

Okay, try this. Put that same partially inflated air mattress in the swiming pool with suspension lines attached like a canopy. Put yourself on the bottom of the deep end with a harness on (and a ton of weight to hold you down).Wink (The air inside the mattress will move opposite of what it should do in the sky, but the idea is similar.) Pull down on one front riser and watch the pressure try to equalize on the surface of the water (which would be where the air meets the bottom of the canopy and the open nose while in the sky). Now fill the mattress fully with air and try the same thing. Having all of the lines under tension doesn't allow it to pop up like a beach-raft, but it makes it incredibly hard to pull under. Sure, this is an extreme example (a pool full of water pressure versus air in a mattress), but it works.

By the way, how many injuries / fatalities were there during the days of pioneering the square canopy before cells were cross-vented? Would a radical turn collapse the canopy?

- David

Hi David,

An air mattress is as hard to deform when you pull from the 1st half (front risers) as it is when you pull from the 2nd half(back risers).

If the front riser tension/pressure/resistance to "pull" (when you do a double front risers) comes from the internal pressure of the wing (the "air mattress" effect) this means that it will be as hard to deform the wing (think mattress) with the back risers as it is with the front risers and this is not true. Is a LOT more easier to deform the wing with the back risers.

We are having different tension/pressure/resistance to "pull" between back and front risers because of different distribution of the lift as billvon suggested it. The center of lift is a lot more closer to AB lines than it is to CD lines and that's the truthTongue.
As Kallend suggested and hackerguySmile (OP) noticed as you begin to pull on both front risers you decrease the wing AoA and the center of lift moves away from the nose changing the pressure/tension distribution between front risers and back risers. That's exactly what the OP noticed as you begin to pull on both front risers it's hard but after a few cm is easier.

Take a look again over the pictures kallend attached. If you have any question I'll be more than happy to PM you with a more visual and vulgarized explanation.

Front risers turns is a different thingy and the "equation" gets a lot more complex and shit is way too much for me to even speculate and shit.Smile

Anyway I want to thank billvon, kallend the OP and all the others who put up with this thread. It really cleared few things for me. A nice friday and happy landings to everybody.


hackish  (No License)

Sep 22, 2007, 3:36 PM
Post #91 of 99 (1242 views)
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Re: [d123] 180 degree front riser requirement for "A" [In reply to] Can't Post

Okay on Friday before I got to chopping (or is that hacking) my main off I did have some time to play with the sabre making observations.

Indeed the first few inches were more difficult and I was easily able to commence a spiral. As soon as I felt myself swinging out in an arch I was unable to maintain the riser's position and this is consistent with what the experienced diver's comments had stated.

I consulted with my instructor and he felt that commencing the spiral more than satisfied their requirements for a 360 degree front riser spiral. It was too windy today so I spent time filling out the hack-away report plus hunting down the PC and free bag in the thick bushes. Tomorrow I will try holding it with 2 hands to see if I can maintain a spiral at all.

-Michael


kallend  (D 23151)

Sep 22, 2007, 7:43 PM
Post #92 of 99 (1224 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I consulted with my instructor and he felt that commencing the spiral more than satisfied their requirements for a 360 degree front riser spiral. It was too windy today so I spent time filling out the hack-away report plus hunting down the PC and free bag in the thick bushes. Tomorrow I will try holding it with 2 hands to see if I can maintain a spiral at all.

-Michael

2 hands on one front riser?

What do you plan to do with the toggles?


Premier billvon  (D 16479)
Moderator
Sep 22, 2007, 8:16 PM
Post #93 of 99 (1219 views)
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Re: [denete] 180 degree front riser requirement for "A" [In reply to] Can't Post

>Now fill the mattress fully with air and try the same thing.

I think you are confusing buoyancy with rigidity. The air-mattress effect (in air) has a very small effect overall on the rigidity of the canopy. In water, it's very buoyant, and that dominates all other forces acting on the mattress.


denete  (B 33880)

Sep 22, 2007, 10:00 PM
Post #94 of 99 (1209 views)
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Re: [billvon] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
>Now fill the mattress fully with air and try the same thing.

I think you are confusing buoyancy with rigidity. The air-mattress effect (in air) has a very small effect overall on the rigidity of the canopy. In water, it's very buoyant, and that dominates all other forces acting on the mattress.

Good point. I was going for the lower surface (of the mattress/canopy) effects when looking at torsional stiffness.

- David


hackish  (No License)

Sep 23, 2007, 5:03 AM
Post #95 of 99 (1200 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
2 hands on one front riser?

What do you plan to do with the toggles?

Darn 3rd hand... I'll ask if it's going to be safe to just leave the toggles on their stops. I did this last time - never really thought about it. Also bear in mind I'm doing this at about 3000'.

-Michael


d123  (B 6134)

Sep 23, 2007, 6:13 AM
Post #96 of 99 (1194 views)
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Re: [kallend] 180 degree front riser requirement for "A" [In reply to] Can't Post

I've tryed that but you just can't put 2 hands on 1 front risers. When you try to put your left hand on the right front riser the erected left risers and MLW are in the way. At least on Navigators. Going with the left hand behind MLW might be possible but IMHO that's asking for trouble.


(This post was edited by d123 on Sep 23, 2007, 6:14 AM)


brettski74  (C 3197)

Oct 2, 2007, 12:13 AM
Post #97 of 99 (1112 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
Just trying to work out the physics of what happens and if it's possible on the gear a student would fly...

<snip>

There's more going on then you might think. As a student, I did 360 front riser turns on a Navigator 220. Use two hands. It's hard work, but you should be able to do it.

In regards to the load on that riser, don't think that a smaller canopy will necessarily have lighter front riser pressure. I currently fly a 210 square foot PD 9-cell and can hold a 360 front riser spiral with one hand. I've also flown a Sabre-2 170 - a full 40 square feet smaller, but there's no way I could do that on that canopy. The front riser pressure was significantly heavier. Different fabric. Different glide ratio. Different canopy design (eg. aspect, nose shape, etc). There could be more factors here than just size, but I think that's kinda been covered already.

Also "G forces" implies inertia or similar effects. I think you'll find that it's drag that increases the forces on risers or toggles as the turn continues and speed increases. I'm not convinced that any centripetal forces induced by a fast turn would act counter to your inputs.

In terms of why a canopy turns right when you pull on anything on the right side, well, I haven't read Brian Germain's book, but this is how I resolved it for my own purposes. I make no guarantees that this is 100% accurate. If you pull on the toggles or the brakes, you induce a turn because the dominant effect is that of slowing down that side of the canopy. The lift on the slower side of the wing decreases, and this induces the banking you get in such a turn. Front risers cause the canopy to dive, which increases your speed, however, you still turn the same direction, so the change in speed is obviously not the dominant effect because that would induce a turn in the opposite direction. What also happens is the dive - ie. when you pull the right front riser, you cause the right side of the canopy to fly lower than the left, banking the canopy and thereby inducing a turn in much the same way as the ailerons on a plane induce a turn, by creating a horizontal component to the lift induced by the wing.

I hope that helps.


(This post was edited by brettski74 on Oct 2, 2007, 2:00 AM)


hackish  (No License)

Oct 2, 2007, 8:20 AM
Post #98 of 99 (1083 views)
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Re: [brettski74] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
If you pull on the toggles or the brakes, you induce a turn because the dominant effect is that of slowing down that side of the canopy. The lift on the slower side of the wing decreases, and this induces the banking you get in such a turn.

I believe the dominant effect is the change of AoA on the side of the canopy which explains the dive associated with a toggle turn.

I did a 360 spiral on the front and my instructor indicated he would sign it off. I wasn't able to hold the riser down the entire spiral but enough that it was completed by the time the riser was released.

-Michael


brettski74  (C 3197)

Oct 2, 2007, 1:18 PM
Post #99 of 99 (1052 views)
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Re: [hackish] 180 degree front riser requirement for "A" [In reply to] Can't Post

In reply to:
I believe the dominant effect is the change of AoA on the side of the canopy which explains the dive associated with a toggle turn.

The change of angle of attack is the change you're applying to the canopy. The effect I'm talking about is the effect on your flight by changing the shape and/or orientation of the wing in such a way.

The increased rate of descent in a toggle or rear riser turn happens as a parasitic effect of the banking induced by the turn - ie you have less lift directed downward and less drag opposing vertical movement due to the changed orientation of the canopy. The change in AoA that you're applying when you pull rear risers or toggles is actually the opposite of that required to induce a dive.

My point was that when you pull down the rear of the canopy, a turn starts because you've increased the drag on that side of the canopy causing it to slow down and creating the turn. This speed difference then induces banking in the canopy. When you pull the front, you induce the banking first, which, in turn, causes the turn.



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