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lazyfrog

any risk of dynamic stall ?

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in case of high wingloading or radical move (hookturn), is there any risk of a dynamic stall ??



A wing can stall at any speed and at any attitude (something which is taught in primary flight training). So yes, the risk of a stall always exists when performing radical moves.


Try not to worry about the things you have no control over

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A stall is the result of exceeding the maximum angle of attack. This is not a function of airspeed at all. Turn inputs that involve a reduction of the angle of attack will not result in a stall. That comes from tail input.

For instance, if you apply a single toggle with a great deal of energy, a high-speed stall is possible on that side of the wing. Front riser input will not cause an increase in the angle of attack.

If you are referring to any deformation as a "stall", any aggressive maneuver can cause a problem. Changes of direction should always be fluid and slow enough to provide the system adequate time to remain coordinated with respect to the relative wind.

The smaller the canopy, the greater the possibility the pilot will have the strength to destabilize the parachute.

Caress the air into cooperation rather than ordering it around like beast of burden. It can bite you...
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I try to do this up high sometimes, just to see. But ya like what everyone says. I bet a hard rear riser pull would be the quikest way to instagate.... It would be really bad if you are down low and in such a manuever and let say ( ok you have gotten away with something like this many, many times ) there are some rotors or small chop to make this even worse for you..... aawwwwoouch.

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Correct me if I am wrong, but I think when people refer to high speed stalls in swooping, they mean that someone applied too much tail input after a dive (usually begun with non-tail input). In other words, if you are diving straight down, and jam the toggles, you are quickly increasing the angle of attack, and could induce a stall. Of course, you usually jam the toggles down because you are too close to the ground, so stalling out can make the situation even worse.

A wing will stall at a slightly higher angle of attack when the freestream velocity of the air is higher, but I don't know if there will be a noticable difference in the speeds we see in skydiving.

- Dan G

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I think that some distinction needs to be made with regards to the nature of brakes and rear risers with as it relates to high-speed stalls. The following is part of my upcoming book "The parachute and its Pilot".

The risk of high-speed stall using steering toggles certainly exists, but is less likely than a rear riser stall. This is because toggles increase the angle of attack in a different way than rear risers.

When you apply the toggles, the induced drag causes the wing to retreat behind you, which increases the pitch angle and thereby the angle of angle of attack. Since it takes time to actually move the suspended load on the pitch axis, the angle of the relative wind changes slowly enough to prevent the critical angle of attack from being exceeded. Furthermore, the physical strength required to change the angle of attack with the steering toggles is prohibitive of quick changes to the angle of attack. This does not mean that the pitch angle does not change quickly, merely that the relative wind angle has ample time to stay in the window of angle of attack that works for flight on that particular wing.

Rear risers are a different story entirely. Application of the rear risers requires less effort and a shorter critical distance in order to exceed the maximum angle of attack. This allow the wing to be stalled with less physical effort, and at a higher airspeed. This is true for several reasons. The most significant reason is that the pilot is actually changing the angle of attack of the wing itself, without the necessity of a pitch change. In other words, by altering the trim of the parachute by shortening the C and D lines, the wing is actually tipped on its own in a very short period of time. Further, the stall speed of a wing with such a flat trim is higher than the originally designed trim specs of the parachute.

In short, you need to watch yourself when performing rear riser landings. If you need to dig yourself out of the corner, do it with your toggles. You have already killed your surf distance by diving the canopy too low and for too long. A steep entry never produces a very long swoop. Your priorities have changed from glory-seeking to self-preservation mode.

Knowing when to abort is perhaps the most important skill of all;
it is the skill that allows us to get old.
Instructional Videos:www.AdventureWisdom.com
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I'm trying to relate this to my limited fixed wing knowledge. Doesn't toggle input relate to the change of the wing chord as in flap input thereby changing the critical speed and angle needed for a stall also?

Brian, I have learned more about canopy flight and dynamic theory from your posts than anything else. Thanks for your time in educating us!

Bill

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You are correct that a reduction in the chord of the airfoil will result in a higher critical angle of attack in order to precipitate a stall. In common terms, shortening the distance from front to back on the wing will generally cause it to stall at a higher speed.

Due to the nature of ram-air canopies, however, angle of attack cannot be increased without either applying the brakes or the rear risers. The fact remains, despite fixed wing theory to the contrary, that parachutes will stall sooner and at a higher airspeed in rear risers. I know it is counter-intuitive, but it is empirical fact.

Due to the change of the wing shape from flat lower camber to curved, the slow-flight characteristics improve. Further, by applying the toggles, the internal pressure is increased and shifted forward, especially on airlocked canopies. The center of gravity shifts forward as well, while the center of lift shifts back due to the altered airfoil shape. This in effect alters the equation, and the net result is a reduced “stall speed”.

There are dynamics of rear riser application that are very relevant when considering the different stall characteristics between toggles and rear risers. When rear risers are applied, the trim of the wing is flattened. This is a significant change to the angle of incidence; that is, the angle of the mean chord line compared to the fuselage, which is of course imaginary on ram-air canopies. By reducing the difference between the B and C, the airfoil also becomes distorted and therefore causes a disruption in the efficiency of the wing as a whole. Furthermore, the suspended weight is carried further aft, skewing the center of gravity as well. All this results in a higher stall speed.

The difference between tail input methods is obviously quite complicated. What we know as scientists is the data, which points to the conclusion that toggles are a better way to finish the flare. This assumption is fundamentally based on the present state of the art, which will inevitably change. The so-called truths that are asserted here are therefore contextual, based on a particular timeframe. Future airfoils may in fact perform quite differently.

For instance, the Sensei prototypes that we are now testing have a very low stall speed on the rear risers. Specific alterations have been made to the airfoil with the goal of improving rear riser flight. Perhaps one day we will release the toggles after opening, and never touch them again. Maybe we won't need toggles at all...
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To that end, I know quite a few people who do exactly what you just said: unstow brakes after slider stowage/removal, then fly to a complete stop on rears. The upper strata of canopy pilots reguarly perform this "art" on very-small canopies, yet the great majority of skydivers nowadays have never once even tried to turn their canopy with rear risers, much less practice landings at altitude in case they ever break a control line and need to actually land on rears under their canopy.

Practice using all of your control inputs on a regular basis up at altitude. One of the first things a pilot does when flying a new type of airplane is practice stalls, both power off and full power. Most people I see nowadays who downsize or change to a more HP planform simply try to live through the landings, barely touching the toggles or risers. Senseless and dangerous.

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You are correct that a reduction in the chord of the airfoil will result in a higher critical angle of attack in order to precipitate a stall



I don't think that is the case. The speed at which the stall occurs will change but the angle of attack remains virtually the same.

A reduction is chord given the same surface usually equals less drag.
Memento Audere Semper

903

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Actually, I diagree with my statement as well. Sometimes I need to proof-read more.

The point I was making, so very late at night, was that toggle-braking provides a slower "Minimum Sink" mode than rear risers, which represents "Maximum Glide."

Hope that does ya.
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Instructional Videos:www.AdventureWisdom.com
Keynote Speaking:www.TranscendingFEAR.com
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The way I see it Brian is with toggles is almost like putting the flaps on while with rear risers is like pulling back the stick and increasing the angle of attack via the stabilizer. Of course a canopy does not have neither!

I agree that a dynamic high speed stall is more likely to occur with abrupt rear risers input than toggles mainly due to the fact that rear risers are much more effective in increasing the angle of attack and that toggles also increase drag more than the risers.

Bottom line is that aerodynamics of rigid wings is understood better than the one of a pile of soft fabric!
Memento Audere Semper

903

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I wish it were that simple. I agree that there are some similarities to airplane flight, but using this kind of comparison is a metaphor at best . The fact is, both inputs effect the angle of attack, they just do it in different ways.
As long as the pilot understands the distinctions, the appropriate inputs can be utilized when the circumstances call for a specific functionality.
Instructional Videos:www.AdventureWisdom.com
Keynote Speaking:www.TranscendingFEAR.com
Canopies and Courses:www.BIGAIRSPORTZ.com

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Just an FYI here...if you have the "Out of the Blue" dvd from Team Extreme you can see a rear riser stall @ the 18minute 45second mark. It is amazing at how abrupt and quick the stall happens. I'm not sure which pilot it is but it's interesting to watch it frame by frame.

Scott
I read somewhere to learn is to remember and I've learned we all forgot

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