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180 degree front riser requirement for "A"

 

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hackish  (No License)

Sep 20, 2007, 4:04 PM
Post #76 of 99 (1714 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? 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 (1698 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 (1698 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
Post #79 of 99 (1694 views)
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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 (1685 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 (1688 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 (1677 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 (1629 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)
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kallend  (D 23151)

Sep 21, 2007, 12:50 PM
Post #84 of 99 (1619 views)
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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 (1611 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 (1610 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 (1607 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 (1587 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 (1585 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 (1578 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 (1541 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 (1523 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 (1518 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 (1508 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 (1499 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 (1493 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 (1411 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 (1382 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 (1351 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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