High speed stall Vs. slow speed stall

[grega 0]

We know that the stall occurs, when the canopy's angle of attack is too big, right?
To achieve this you have to slow down the canopy while the body (most of the mass) swings forward and aoa increases.

At slow speed stall you slowly pull down the toggles to the point where the aoa becomes too big and the canopy stalls.

The question is, where is this point at high speed stalls? Do you have to pull the toggles to the same point to induce a high speed stall? Or is this point much higher at higher speeds?

Some technical explanation is welcome.

p.s. If i'm wrong at any point please correct me.

"George just lucky i guess!"

[Hooknswoop 19]

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The question is, where is this point at high speed stalls? Do you have to pull the toggles to the same point to induce a high speed stall? Or is this point much higher at higher speeds?

The angle of attack a canopy will stall at is much lower on rear risers than on the toggles, hence the higher stall speed. A canopy will stall at a much lower airspeed using the toggles because the shape of the airfoil is being changed to one that can fly at lower airspeeds. This is similar to lower the flaps on an airplane to lower the stall speed.

So an airfoil stalls at the same angle of attack, regardless of airspeed, unless you change the shape of the airfoil.

Derek

[grega 0]

So if i get this right:

Slow speed stall is induced with toggles?
High speed stall is induced with rear risers?

Is it always the same speed and aoa when stalling with toggles, no matter what was the speed and aoa was, before i induced the stall?

"George just lucky i guess!"

[Hooknswoop 19]

A high speed or acclerated stall can be done with either toggles or rear risers. With rear risers, it will stall at a much higher airspeed than with toggles.

With toggles, as you pull down, you are increasing the angle of attack the canopy will stall at. By pulling hard and quick enough, the canopy will stall at a higher airspeed than it normally would.

With toggles, you are changing the shape of the airfoil. The more you pull down trying to stall the canopy, the more you are increasing the angle of attack the canopy will stall at. You have to pull them down fast enough, creating enough "G" forces (which raises the airspeed the canopy will stall at) to out pace the stall speed reducing effect the toggles have.

Derek

[grega 0]

I thought so.

It's all clear to me now. Thanks

"George just lucky i guess!"

[freeflyguy 0]

For a number of reasons, I am getting a bit tired of seeing people post about a high speed stall with toggles. It just doesn't happen. Yes, Hooknswoop, at a wing loading of 3.0 it is a concern, but for 95% or the readers of these forums, it is not a concern.

If somebody, during a panic situation that is about to smash them into the ground, has it in their mind that they may stall the canopy on toggles, they may not flair enough to save themselves. Your best panic model is the simplest. You are too late for a flat turn, too late to plan your landing better, too late for anything else, just stab. So what, if it stalls?It will dump you on your back, who cares, a broken back is better than massive head injuries.

Regardless. Go Try stabbing yourself out of a turn at altitude. Just practice it. Then make up your own mind.

----------------------------
bzzzz

[quade 3]

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For a number of reasons, I am getting a bit tired of seeing people post about a high speed stall with toggles. It just doesn't happen.

Yes it does. Absolutely.

See http://www.dropzone.com/cgi-bin/safety/detail_page.cgi?ID=20

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If somebody, during a panic situation that is about to smash them into the ground, has it in their mind that they may stall the canopy on toggles, they may not flair enough to save themselves. Your best panic model is the simplest. You are too late for a flat turn, too late to plan your landing better, too late for anything else, just stab. So what, if it stalls?It will dump you on your back, who cares, a broken back is better than massive head injuries.

Well, I guess that's one approach. Another would be to not let yourself get into the position in the first place.

One gets a person broken or dead and the other is a non-event.

You choose.

quade -
The World's Most Boring Skydiver

[Hooknswoop 19]

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For a number of reasons, I am getting a bit tired of seeing people post about a high speed stall with toggles. It just doesn't happen. Yes, Hooknswoop, at a wing loading of 3.0 it is a concern, but for 95% or the readers of these forums, it is not a concern.

I agree that it is harder to stall a canopy above the normal airspeed it stalls at with the toggles, didn't mean to imply otherwise. I wasn't thinking from the perspective of my VX, but all canopies.

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If somebody, during a panic situation that is about to smash them into the ground, has it in their mind that they may stall the canopy on toggles, they may not flair enough to save themselves.

I honestly didn't mean to imply that it easy to put a canopy into an acclerated stall with the toggles, only that it can be done. It is actually difficult to put a canopy into a high speed stall with the toggles. It requires a very fast and max input to increase the "G" load while pendulming back under the canopy. If the canopy is at full flight or slower, it would be very difficult to get the canopy to stall at an airspeed above it's normal stall speed.

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You are too late for a flat turn, too late to plan your landing better, too late for anything else, just stab. So what, if it stalls?It will dump you on your back, who cares, a broken back is better than massive head injuries.

I do disagree with this. A stalled canopy will hand harder than a non stalled canopy. In a panic situation, fly the canopy and keep it over your head.

Derek

[Spizzzarko 0]

You know what?????
There's a simple way to alleviate all of this. Just fly smooth. I see a lot of people who are only flying their canopy at the point in time where they are. They never plan for what can and will happen next. You need to be one or two steps ahead of your canopy at all times. You should never be in the situation where you are to low to turn. If you are truly flying your canopy, then you can see the ever changing traffic situation. But if you are worried about stalling your canopy, then go up high and stall your canopy. Practice practice practice. Once you know how to stall that canopy then don't do it near the ground. Then you don't need to worry about it.

[yourmomma 0]

i was under the impression that a "high-speed" stall was caused by excesssive loading of the wing. i.e when one is in the midst of recovering from a hard turn the stall is actually the inability of the wing to recover to normal flight due to the excessive loads or (increase of apparent weight to wing ratio.)

[Hooknswoop 19]

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i was under the impression that a "high-speed" stall was caused by excesssive loading of the wing. i.e when one is in the midst of recovering from a hard turn the stall is actually the inability of the wing to recover to normal flight due to the excessive loads or (increase of apparent weight to wing ratio.)

"G" forces increase the airspeed a canopy (wing) will stall at. To pull "G's", you have to increase the angle of attack. Once the critical angle of attack is exceeded the wing stalls, regardless of the airspeed. A high speed or accelerated stall is exceeding the critical angle of attack with more airspeed than the canopy normally stalls at.

Pulling to much toggles or rear risers in a dive produces a high speed stall. Pulling too much rear risers or too much toggles too quickly can produce a high speed stall from full flight.

Derek

[pchapman 261]

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Pulling to much toggles or rear risers in a dive produces a high speed stall. Pulling too much rear risers or too much toggles too quickly can produce a high speed stall from full flight.

If we agree with this, and I do, then we have answered WHETHER a canopy will stall at a speed higher than the 'regular' stall speed (achieved through slow deceleration to a stall).

The next question is also vital to the canopy pilot: WHEN will the canopy high speed stall.

In particular, what interests me is whether the toggle position (or rear riser position) will be different for a high speed stall compared to a regular stall.

In aircraft, stick position is typically fairly well correlated with angle of attack: E.g., haul the stick back 6" and the plane will keep flying, haul it back 8" and it'll stall / spin / snap whether in slow flight or higher speed flight.

So if you need a certain amount of brake to stall your canopy in normal flight, won't you need about the same amount when in a diving turn, trying to dig out of the corner?

If that's true, then high speed stalls can theoretically be avoided by simply knowing where the stall point on the brakes (or rear risers) normally is. In practice there will of course be the tendency to pull too much brake when trying to avoid the ground.

Peter Chapman

[Hooknswoop 19]

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If we agree with this, and I do, then we have answered WHETHER a canopy will stall at a speed higher than the 'regular' stall speed (achieved through slow deceleration to a stall).

The next question is also vital to the canopy pilot: WHEN will the canopy high speed stall.

In particular, what interests me is whether the toggle position (or rear riser position) will be different for a high speed stall compared to a regular stall.

In aircraft, stick position is typically fairly well correlated with angle of attack: E.g., haul the stick back 6" and the plane will keep flying, haul it back 8" and it'll stall / spin / snap whether in slow flight or higher speed flight.

So if you need a certain amount of brake to stall your canopy in normal flight, won't you need about the same amount when in a diving turn, trying to dig out of the corner?

If that's true, then high speed stalls can theoretically be avoided by simply knowing where the stall point on the brakes (or rear risers) normally is. In practice there will of course be the tendency to pull too much brake when trying to avoid the ground.

Excellent question. I don't know. I would guess there is too many variables, i.e. line shrinkage (for Spectra), airspeed, "G" forces, DA, WL, etc. to determine an exact toggle position for any condition of where the canopy will stall in any area of it's flight envelope. I would like to hear other people's opinions though.

Derek

[quade 3]

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In aircraft, stick position is typically fairly well correlated with angle of attack: E.g., haul the stick back 6" and the plane will keep flying, haul it back 8" and it'll stall / spin / snap whether in slow flight or higher speed flight.

This is simply not true.

In most conventionally controlled aircraft such as a Cessna 172, how much stick deflection will raise or lower the angle of attack is related to the airspeed the aircraft is moving at the time.

The linkage between the stick and the control surface is fixed and the differences in airspeed will result in different amounts of force roughly the square of the increase in airspeed. For instance, if at 40 knots a 1 inch yoke deflection caused 10 pounds of force at the tail, then at 80 knots it would create 100 pounds of force.

One of the key goals in teaching airplane students stalls and landings is to have them recognise the "mushiness" of the controls at slow airspeeds.

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If that's true, then high speed stalls can theoretically be avoided by simply knowing where the stall point on the brakes (or rear risers) normally is. In practice there will of course be the tendency to pull too much brake when trying to avoid the ground.

No.

quade -
The World's Most Boring Skydiver

[gus 1]

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In particular, what interests me is whether the toggle position (or rear riser position) will be different for a high speed stall compared to a regular stall.

I think they must be. If you stab the toggles/rear risers to pull out of a dive the weight that your canopy is supporting increases (as the g forces increase) and that alone will change the point at which the canopy stalls.

Just my £0.02.

Gus

OutpatientsOnline.com

[pchapman 261]

Quade wrote:

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This is simply not true.

That I'm not so sure about. The assertations remain to be supported by further evidence. Your reply doesn't really add much to support your case; while I don't have any good evidence except anecdotal.

The math:
Slight mistake in the math? I believe you meant that a doubling of the speed would cause a roughly four times (not 10 times) increase in the force applied at the tail. OK, fair enough. But aren't counteracting forces also going to increase with the square of the speed? (Like the negative pitching moment of a wing)

Mushiness:
Yes, the control feel will be different at different speeds. And that in practice does affect how much or how abruptly pilots handle the controls. A pilot may inadvertently pull the stick back too much if it only takes 4 lbs at low speed, rather than 40 lbs at high speed. But doesn’t affect my argument one way or another about stick position.

IN ANY CASE:

I'd still like to see jumpers' reports on whether they can stall (or reach the edge of a stall with) their canopy in higher-g flight, and whether the toggle position varies greatly from that during a normal stall in a straight glide. That's in line with what was asked at the very beginning of the whole thread

Not sure how to safely collect the data though. Stalls while in diving spirals don't sound like too much fun, even though I'm more prone to trashing canopies than most people.

[quade 3]

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That I'm not so sure about.

I am.

There's no mistake in my math. Doubling the speed, squares the forces. 10^2 = 100.

Stick deflection is not, in and of itself, indicative of a stall.

Toggle application is not, in and of itself, indicative of a stall.

A stall can happen at any airspeed or attitude.

Angle of Attack is the -only- thing that matters. Exceed the Critcal AoA and you -will- stall.

Further, there is a stall mode that we as parachutists have that doesn't really exist in conventional fixed wing aircraft -- the dynamic stall. With the dynamic stall, the speed of application and G-Force will be quite variable so any hope of a stall avoidance method based on toggle position would be impossible in practical application.

For more information on the dynamic stall, you can read my article HERE.

quade -
The World's Most Boring Skydiver

[yarik 0]

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There's no mistake in my math. Doubling the speed, squares the forces. 10^2 = 100.

Let me put my 5 kopeyek here :).
Quade, you're 100% right about stick/toggle deflection. One can tell AoA on an airplane only if the wing is in stable level flight at constant certain airspeed. Otherwise - you can't determine AoA by stick deflection.

Now as to the Math, I'm sorry, but doubling speed will not square the force - it will increase it 4 times (or squaring the increase rate of the AS). Aerodynamical force is directly proportional to square of speed so doubling the speed will increase the force 4 times, tripling the speed will increase the force 9 times.

[quade 3]

Damn it, you're right and I was wrong.

Double the airspeed and you get four times the lift.

BTW, I owe you a beer the next time we meet for you pointing out the mistake (and making me go back to the text book to check it out again).

quade -
The World's Most Boring Skydiver

[quade 3]

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The math:
Slight mistake in the math? I believe you meant that a doubling of the speed would cause a roughly four times (not 10 times) increase in the force applied at the tail.

Damn it, you're right and I was wrong.

(Has it really been that long for me? Since I used to -teach- aerodynamics!)

BTW, I owe you a beer the next time we meet for you pointing out the mistake (and making me go back to the text book to check it out again).

quade -
The World's Most Boring Skydiver

[quade 3]

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There's no mistake in my math. Doubling the speed, squares the forces. 10^2 = 100.

Oh yes there was!

quade -
The World's Most Boring Skydiver

[yarik 0]

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BTW, I owe you a beer the next time we meet for you pointing out the mistake (and making me go back to the text book to check it out again

I'll buy you a beer too, for being a good opponent in such discussions, and for making me get back to books as well! So let's have a beer together!

[kallend 1,625]

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Angle of Attack is the -only- thing that matters. Exceed the Critcal AoA and you -will- stall.

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Well, since we're well into the nitpicking stage by now, there is also that little thing called Reynolds Number, which can change the value of the critical AoA.

...

The only sure way to survive a canopy collision is not to have one.

[quade 3]

Quiet you . . .

(Dang physics professors.)

quade -
The World's Most Boring Skydiver

[grega 0]

Well i tried it today. high speed stall and slow speed stall.

I didn't do the high speed stall with toggles, because i didn't try hard enough to bury those toggles (damn that was hard). But i stalled it with rear risers at both high and slow speed.

At high speed (after a 180 turn), i definitely didn't have too pull rear risers as far, as i had to at slow speed to stall the canopy. The canopy barely got over my head (if looking straight up i was below rear of the canopy) and it already stalled, by pulling rear risers really hard.

So much about whether the toggles or rear risers has to be pulled exactly the same to stall the canopy at high or slow speeds. For high speed stalls you need to pull less than at slow speed stall, but you have to pull abruptly to stall it at high speed. And at high speed you really have to use all the strength to stall it, so it's not really easy to stall it at high speed.

If you think it's only me or my feeling please correct me.

"George just lucky i guess!"