Forums: Skydiving Disciplines: Swooping and Canopy Control:
How a rear riser stall differs from a braked stall?

 


p3h  (B License)

Dec 25, 2011, 9:45 PM
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How a rear riser stall differs from a braked stall? Can't Post

How a rear riser stall differs from a braked stall? Rear riser stall seems to happen because of too big angle of attack which separates the airflow from the upper side of the wing so it cannot be diverted down at the end [reference „Understanding Flight” by Anderson & Eberhardt]. And it looks that way, as at first an airfoil still is pressurized, but want to „break” chord wise.

So how the braked stall (by pulling toggles) works and how both differs, because from experience I can tell that they look differently (braked stall looks more like a butterfly where an airfoil breaks span wise and end cells want to touch themselves), so it seems for me that they work differently as well?

It seems that when I pull toggles I increase drag in the tail section of a canopy, which in result pulls nose portion up increasing AoA. Also pulled toggles create drag via pulled fabric in the tail section which slows canopy's forward speed.

So does the braked stall happen because:

a) Airflow over the top skin separates from the wing and is not creating downwash at the end

b) The AoA is so big that the nose inlets are too high angled to let the air in (the bottom skin blocks it) to keep cells pressurized

c) Drag created by braked fabric in the tail and increased AoA (if increased AoA increase also drag which I don't know but it seems so) reduce canopy's airspeed so much, so there is less and less air coming into the cells to keep them pressurized. But why the center cell/cells seem to be better pressurized than the outer cells that are breaking to the inside?

d) All of them together in some proportions

e) Something else like the fact that brake lines are not placed in the center of the tail.

For me it seems like the reason is that the pressure in the cells drops by either option B), option C) or both of them, and then when pressure drops, brake lines have enough power to pull end cells inward as they are connected only at the outer cells end. But at that time, canopy still has forward speed, just not enough pressure. So this stall which we see in a shape of a „butterfly” is just the begining of a stall as there is still air coming into cells, but once the airflow will be blocked to the inside of cells by increased AoA (by the bottom skin) or by zero airspeed created by increased drag/AoA, then a canopy will totally collapse. But which one is it? Would a headwind let a canopy to be pressurized longer, as it could inject itself into the cells even when a canopy brakes to zero airspeed, like when you kite a canopy on the ground in high wind?

This is totally messed up for me at this point – and maybe the explanation is really simple? Your turn.


gaz511  (D 107859)

Dec 26, 2011, 5:26 AM
Post #2 of 7 (1141 views)
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Re: [p3h] How a rear riser stall differs from a braked stall? [In reply to] Can't Post

The riser stall occurs when you exceed the critical AoA, this can occur at high speed or low speed, it's simply when you've pulled the tail down so much that you no longer have a wing, its more or an airbrake instead. Just like putting you hand out the window of a speeding car and change the angle of your hand, you can use the angle to create lift but at a certain point you'll change the angle so much that the wind no longer forces your hand up and instead it looses lift get gets blown backwards.
The toggle stall is pretty much a combination of what you've said, basically you've slowed down your "ram air" wing so much that it's no longer creating lift, but also there is little or no ram air, so the wing will loose shape and eventually collapse, the wing tips bend back and touch because the brakes (bend down fabric) cause drag and pull the trailing edge tips back. Even at speed if you pull your toggles the brakes are applying a force to the trailing edge tips, but they don't bend backwards because that force is opposed by the pressure in the wing from the ram air/forward speed. The only thing you got wrong was the bit about head wind. A head wind would only make a difference if you were stationary on the ground. In flight, essentially there is no wind, i say this because you are travelling through it like a boat in a river, you canopy will fly at the same speeds it always does regardless of headwind/downwind etc, it's only your ground speed that changes. Lots of people struggle with this concept, but use the river idea, you are in the river getting pused by the flow and the river bank is the ground/intended landing area, no mateer how fast you boat travels it's always being pushed a certain direction by the flow, so if you face into the flow travelling at 20kts and the flow is 20kts, then the result is you aren't moving forward along the river bank, but your still doing 20kts!


Pulse  (D 16387)

Dec 26, 2011, 7:53 AM
Post #3 of 7 (1107 views)
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Re: [p3h] How a rear riser stall differs from a braked stall? [In reply to] Can't Post

The canopy stalls because of too much AoA either way. The canopy become s horseshoe shape in the brake stall because the AoA reaches (or close to) 90-degrees. At that point the air is deflected forward and the two sides wrap towards one another.

Stalls can occur at high or low speeds.


chuckakers  (D 10855)

Dec 26, 2011, 8:01 AM
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Re: [p3h] How a rear riser stall differs from a braked stall? [In reply to] Can't Post

All stalls occur when the angle of attack becomes to high for the wing to produce lift, regardless of the cause.


davelepka  (D 21448)

Dec 26, 2011, 8:08 AM
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Re: [p3h] How a rear riser stall differs from a braked stall? [In reply to] Can't Post

Quote:
This is totally messed up for me at this point – and maybe the explanation is really simple? Your turn.

It's pretty simple. A stall occurs at a given AOA, as already stated. What happens to the canopy after that is a function of shape. A stalled canopy with the toggles pulled all the way down is one shape, and a stalled canopy with the rear risers pulled down is another shape. Neither of them are 'flying', they're both just trailing in the wind above you.

Go jump off a building holding a bedsheet. Hold it by one corner on the first jump, two corners on the second jump, and three corners on the thrid jump. Each time it will assume a different shape, none of them are airfloils or are 'flying', but they're all different. Same basic idea.

Don't really jump off a building.


wolfriverjoe  (A 50013)

Dec 26, 2011, 9:28 AM
Post #6 of 7 (1076 views)
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Re: [p3h] How a rear riser stall differs from a braked stall? [In reply to] Can't Post

In reply to:
This is totally messed up for me at this point – and maybe the explanation is really simple? Your turn.

The stall is the same. The shape is different because of where the pull is coming from. The lack of pressure in the cells is far more from the lack of airspeed than the AoA. In a braked stall, the center cells aren't any more pressurized than the end cells, the end cells are getting the "pull" from the steering lines and there isn't enough pressure for them to keep their shape.

The steering lines are attached to the outer part of the canopy. The rear risers are attached to all of the C & D lines.

Try this: Lay the caonpy on the ground, bottom skin up. Put the container (and the lines) in front of the nose. Get everything nice and even, then pull on the rear risers. The canopy will (more or less) smoothly pull over.
Next, do the same thing, but just pull on the steering lines. The pull will come from the back corners, not the entire trailing edge. The canopy should end up looking a lot more like the "butterfly."

And a headwind (or a tailwind) will have zero effect on the stall. Your airspeed is the speed through the air. Unless you are touching the ground, how the air is moving is totally irrelevant.


RichLees

Dec 26, 2011, 10:37 AM
Post #7 of 7 (1061 views)
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Re: [wolfriverjoe] How a rear riser stall differs from a braked stall? [In reply to] Can't Post

depending on the canopy, the stalls can be pretty different.

some BASE canopies can "fly" backwards on the toggles. I've heard it called sheeting. most (?) skydiving canopies dive off to one side or straight back if you give them enough stick.

some lightly elliptical (eg Silhouette) can go backwards surprising well on the rears. it can't be said to be flying because the pressure has gone and so its sheeting, but its controllable and the Dbag and PC can end up in front of the wing. get the rears down to your knees and the 9-cell span is retained, but the chord collapses to a couple of feet and you come straight down at 60fps which is pretty neat for vertical panning when camera behind CREW.



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