Stalls (& Line Trim?)

[jfields 0]

Here is a newbie-ish canopy control topic I'm currently dealing with...
I'm continuing to learn about my canopy and its flight characteristics. I'm a novice (58 jumps) flying a Sabre 190 at 1.1:1.
I'm trying to explore the speed range of my canopy. From full flight, I very gradually pulled the toggles down. I noticed the loss of windspeed and increasing "mushiness" of the canopy. I continued pulling the toggles down until my arms were extended all the way down at my sides. I was expecting a stall, but it didn't happen.
When I landed, I was talking to an instructor, who suggested I take the toggles and loop them around my hands once then try the stall again. I was wondering if having to do this is an indication of a line trim problem.
Also, since I haven't been successful at it yet, could someone describe what exactly occurs when you do stall the canopy? I know to let up very gently, but I don't know quite what to expect during the stall. Will the canopy fold in the middle, or collapse some other way, or what? If you continue holding the toggles down, will the canopy progress into something looking like a streamer?
To reassure those wondering about it, I've been doing all of my intentional drastic canopy maneuvers (incl. stalls) between about 7000 and 14000 feet. So far, I've done three 3-second delays from altitude. They are a lot of fun and I feel safe getting crazy with my canopy at that altitude.
Thoughts? Suggestions?
Justin
My Homepage

[Jimbo 0]

I was playing around with brake settings on my Safire a while back and I needed to stall the canopy completely to figure a few things out.
Anyhow - When I did it I would look up at the canopy while applying more pressure to the brakes. Right before the stall the canopy looks like it's about to fold in half. The tail is pulled back and towards the center and the front looks more like the 'top' of a triangle (Over all the whole canopy looked something like this: / \ ) - give it a little more input and the whole thing just goes to shit and collapses. You'll know when that happens, and when it does just let the toggles back up and the canopy should reinflate. If the canopy doesn't reinflate, well, you've got another one.
It's something everyone should do - you might need to be right on the edge of a stall to land your canopy in a tight spot someday. It's nice to have this figured out before something like that happens.
-
Jim

[PhreeZone 15]

Another possiblity is that the previous owner of the canopy had extremely long arms and adjusted the brakes to be really long. if this is the case, its a simple matter to shorten the brakes to the right length.
A stall will feel like you have completly stoped moving then the canopy might rock back and forth for a few times. If its rocking then you are on the stall point where the canopy can't get enough drive to go forward nor is in a stall yet. Once you hit the stall, the canopy will move backwards on you then depending on the type canopy it can do various things. My Spectre's end cells would collapse and she would sugre forward after having the lines go slack. I want to touch the sky, I want to fly so high ~ Sonique

[aufreefly 0]

While it is probably not a big deal under a lightly loaded canopy, I have been told by very seasoned canopy pilots that I should not take a wrap on the break lines when exploring my canopy's range. And I found out why on my next jump. After I stalled out my canopy it recovered into line twists(5). Luckily I was at seven thousand feet and had plenty of time to recover. If my hands had been wrapped in my brake lines and I had to cutaway my problem might have been significantly worse.
(Just my personal opinion...feedback is welcome)
michael hunt (yes, that is my real name...and i've heard them all)

[freeflyguy 0]

Justin
The longer brake lines aren't a problem. On our student rigs, the lines are set long on purpose, and the canopies won't stall. That just keeps a student that follows instruction really well from collapsing the canopy on himself. They are taught to flair all the way down. Putting a wrap on your hands would likely get the trim closer to the setting you would have for yourself. I don't know if it is your canopy, but if it was, you would want the brakes to be set so you could just stall it.
Play with it, but when you do get it to stall, you would probably not want to just let it flutter forever, you would probably end up in line twists.
You can also play with stalling on the rear risers too. It reacts different then when stalled on the toggles. But it is good to know how it flys, and the stall point, in case you have occasion to land on the rear risers
j

[freeflir29 0]

I don't think my canopy will stall all the way. It will get very slow and rock a little back but not actually collapse. I like my brakes a little long. So it hits a nice level glide with my arms straight out. I don't think I would want to be able to completely stall it. That might make for a very bad landing someday. Seen people do it. It's not pretty. Ask Chuck about that Stilletto 120 cocyx breaking incident......"and I'm not easily impressed...Ooohh look...a blue car!" -Homer Simpson

[Spectrejumper 0]

The following is taken from the Sabre 2 Flight Characteristics article by PD.
>>Stall Characteristics: As with the Spectre and the original Sabre, you may find that you can pull the toggles all
the way down to full arm extension without stalling the Sabre2, especially if you are jumping a larger one. It?s
important to remember that you do not have to be able to stall your canopy in order to land it correctly.
Technically, a stall means the canopy has dramatically reduced its lift and increased its rate of descent. A stalled
canopy is not really ?flying? any longer. If you want a soft landing, you really want your canopy to keep creating lift
and maintain a low rate of descent until your feet are on the ground. If you adjust your steering toggles so that
your canopy is easier to stall, it may actually become more difficult to land softly.<<
I was unable to stall the demo Sabre 2 I had back in September. I also couldn't stall my Spectre when I had the steering lines at the proper length. I had great landings on both canopies, and I've put the Spectre down in some pretty tight areas off the DZ.
Mike D-23312
"It's such a shame to spend your time away like this...existing." JMH

[Jimbo 0]

Quote

I was unable to stall the demo Sabre 2 I had back in September. I also couldn't stall my Spectre when I had the steering lines at the proper length. I had great landings on both canopies, and I've put the Spectre down in some pretty tight areas off the DZ.

Steering lines at 'proper length'? What exactly is 'proper length'? A lot of jumpers leave the steering lines at the factory default, a lot of jumpers lengthen them, and a lot of jumpers shorten them. Which one is 'proper'?
-
Jim

[Spectrejumper 0]

In my opinion (I'm not a rigger or canopy designer), the factory setting is the proper setting. I figure they designed and built the thing, and their settings will yield the flight characteristics they intended. That being said, if an experienced jumper prefers a different setting they should use it. From personal experience I find the factory setting works great. YMMV
Mike D-23312
"It's such a shame to spend your time away like this...existing." JMH

[quade 3]

A stall is different from a canopy collapse.
Aerodynamically, a stall is a condition wherein the critical angle of attack has been exceeded. Past the critical angle of attack, the wing's ability to create lift falls off precipitously. Functionally, the wing is not creating enough lift to support normal flight. Depending on the aggressiveness of the stall entry, the departure from normal flight can be gentle or violent.
***PLEASE read this next part carefully!***
A stall -can- happen at any airspeed and at any attitude.
***Resume reading in normal careless manner***
It's -my- opinion that this could be the primary factor in some botched hook-turn landings.
My guess is that most ram-air canopies can be stalled without causing a canopy collapse. I know that I can stall my canopy (Spectre 190 @ 1.3:1) in straight and level flight using slow decelleration with few, if any, negative effects. The canopy rocks me in my harness slightly and there is a noticable increase in verticle decent, but the canopy itself appears quite normal.
That said, if you were to continue to pull down the brake lines farther and farther, eventually, you could probably get almost any canopy to collapse (including ones with airlocks?).
Paul
http://futurecam.com/skydive.html

[quade 3]

Quote

Which one is 'proper'?
and
In my opinion (I'm not a rigger or canopy designer), the factory setting is the proper setting.

The proper setting is the one which will provide the best control inputs for the canopy pilot.
For me & my rig . . . that means that at full flight (toggle full up) there is only a -slight- bowing of the brake lines and that within an inch or so of travel the brake lines begin to reshape the trailing edge of the canopy. With this setting, I have a full range of control input.
If the brake lines were longer, I'd be denying myself the full use of the bottom end of the control stroke.
If the brake lines were shorter, then the canopy would already have a deformed trailing edge at full flight.
Factory settings may or may not be correct. Risers don't usually come with canopies so it's nearly impossible for the factory to know exactly how long to make the brake lines for YOUR particular rig. Risers can come in different lengths.
Paul
http://futurecam.com/skydive.html

[Jimbo 0]

Thanks Paul.
The point is that there isn't just one 'proper' setting for steering lines. Each jumper has different requirement, some risers are longer or shorter than others, etc...
SpectreJumper - Just because it came that way from the factory doesn't make it right. It may be right for you but wrong for me, or wrong for you but right me, but the 'factory defaults' don't necessarily translate into 'correct'.
-
Jim

[Spectrejumper 0]

That's where the "in my opinion" portion of my post comes in to play.
Mike D-23312
"It's such a shame to spend your time away like this...existing." JMH

[mnischalke 0]

"That said, if you were to continue to pull down the brake lines farther and farther, eventually, you could probably get almost any canopy to collapse (including ones with airlocks?)."
Prior to collapse from a flat stall, you will begin reverse flight. I was encouraged to put my canopy in reverse flight on jump 14 and it was quite a memorable experience. If you haven't done it, you should to know where it happens. I am not advocating jerking the toggles to you hips and hanging out there until a total collapse. I have held mine until I could see the canopy start to flutter across the bottom skin and then start to drift backwards (from my point of view). Knowing where this occurs up high will can your ass down low.
With air locks, you will probably be able to get actual rearward acceleration out of reverse flight prior to collapse.
Peace
mike

[jfields 0]

Through a number of people's posts and in a roundabout way, my questions were actually answered. As initially stated, my goal in the whole stall discussion wasn't to fly around like that or do it regularly, but to know where and how it happens on my canopy. That way I'll be knowledgeable and experienced (at least a little) without learning at low altitude or when I need to react quickly. Doing it at high altitude a number of times will give me the familiarity and practice.
Thanks for responding, folks! Justin
My Homepage

[alan 1]

Paul, maybe you could share some information on a static stall vs. a dynamic stall. I think this is kinda the direction you are going here. Many jumpers are not aware of the dynamics here. During the recivery arc, when the canopy has a relatively high induced airspeed (creating additional lift), it is also carrying a heavier load due to the additional G forces, correct? What happens when the pilot finds himself "in the corner" and attmepts a quick recovery by radically changing the camber of the airfoil and aoa at the same time, while pulling 1.5 Gs on a canopy that was already .5# over the recommended wing loading? Also, as an aside regarding the "speed=lift" comments in another thread, I understand the point you are trying to make, but please understand the context in which that comment is usually made. In my opinion, it is not made as a defintion or or a limiting comment, but rather as a rule of thumb for canopy flight to instill the idea that it is a good idea to maintain a safe airspeed, especially during the landing phase, and to fly the canopy efficiently. I all too often see pilots pumping the brakes on final and during the flare itself, reducing the efficiency of the wing. I think that can account for many of the unexplained stalls/canopy collapses that we hear about and see, especially if there is some turbulence. Would a CFI instruct his student in a 152 to alternately and rapidly increase and decrease the flaps on final, using a slow approach, in turbulence? How often do you have to remind the student to "keep the nose down"? That, is where I believe the speed equals lift comments are mostly directed. There is nothing wrong with maintaining a safe airspeed, letting it bleed off in ground effect, and landing with a wing that is still flying. Go with it and keep a good perspective.
alan

[freeflir29 0]

"keep the nose down"
Not me.....but I will go to sleep tonight hearing Dutchboy saying....Carb heat........Carb heat......Carb heat...........I make the poor guy sound like a parrot...
Pumping the brakes is bad....but I have found that I needed to do it sometimes. Normally after I screwed up the approach in the first place and had to "dig out" I have found it almost impossible to dig out without popping up. Maybe I just suck....LOL
"and I'm not easily impressed...Ooohh look...a blue car!" -Homer Simpson

[Aviatrr 0]

Quote

I was unable to stall the demo Sabre 2 I had back in September. I also couldn't stall my Spectre when I had the steering lines at the proper length. I had great landings on both canopies, and I've put the Spectre down in some pretty tight areas off the DZ

Are you sure you weren't able to stall either one? Maybe you couldn't collapse it......but could you STALL it? I have never had any problem stalling my Sabre2 or my old Spectre.. I never got either one to collapse, though..
Stalling the canopy and collapsing it are two totally different things..
Mike

[riggerrob 558]

Three points,
First of all, my definition of a stall is when you lose smooth air flow over the top skin.
Secondly, Performance Designs brake line lengths were defined by John LeBlanc who is taller (about 6' 4") and heavier (over 200 pounds) than most skydivers. Most smaller people like their brake lines a bit shorter. Shortening the brake lines can make a canopy so responsive that it turns with toggles at ear level, but that deteriorates landing performance. The disadvantage of shortening brake lines too much is that the canopy's top speed suffers, which means that you carry less momentum into the flare, which means you have less energy to convert into lift, etc.
Personally, I prefer my brake lines close to factory settings there is a slight bow in the steering lines at full flight. This translates to more air speed at the start of the surf, which translates into a longer surf.
Which brings us to my final point. Fashions have changed. Back when I bought my first square in the late 1970s, it seemed that everyone competed in accuracy, even if it was only bets for beer. That was fine on canopies that stall gently like Strato-Stars and Strato-Clouds, but small modern canopies stall violently, so the emphasis has shifted to squeezing every last erg of energy out of canopy just as your feet touch down. Modern blade runners don't want to stall their canopies.

[quade 3]

Quote

Paul, maybe you could share some information on a static stall vs. a dynamic stall.

Gladly.
First a quick definition of some terms about stability.
Static stability is how stable something is when it's just sitting there. For instance a marble on a level table top would be said to have neutral static stability. It'll just sit there. A marble on top of a bowling ball would have negative static stability. Leave it there and it probably "wants" to fall off the sides. A marble at the bottom of a bowl has positive static stability, it doesn't "want" to move up the sides of the bowl.
Dynamic stability refers to what happens after something has been disturbed. Does it continue to be disturbed -- a rolling marble? Does it accelerate in it's departure from being disturbed -- a marble falling off a bowling ball? Does it return to it's position held before being disturbed -- a marble pushed up the sides of a bowl, returning to the bottom.
My understanding of the terms as used in the skydiving community is that a static stall refers to what airplane pilots would call a "slow deceleration", "approach to landing" or a "normal" stall. Basically, at an altitude from which it will be "safe" to recover from the stall (above 2,000 AGL wouldn't be a bad idea), slowly increase the angle of attack and let speed bleed off. Keep increasing the angle of attack, gently, until the stall happens.
Under a parachute, this is fairly simple. Slowly pull the toggles down and wait. Do not to pull down the toggle so quickly that you swing forward. Hold the toggles ALL the way down and wait.
You -should- notice the sound of the airspeed decreasing, perhaps a slight rocking in the saddle and then perhaps a noticeable increase in descent rate.
This is your basic stall and for all intents and purposes the most genteel stall your canopy will ever have.
Recover from the stall by decreasing the angle of attack -- let up the toggles and resume flight.
The dynamic stall is a different beast. This type of stall doesn't really have an airplane counterpart -- in a moment you'll see why.
Again at an altitude from which it will be safe to recover begin the maneuver from full flight -- toggles all the way up. This time instead of slowly pulling the toggles down, pull them down as quickly as you can and hold them there.
A few different things are happening this time around.
Because your airspeed (initially) hasn't changed all that much, but you've increased the angle of attack dramatically, the wing is now creating a lot more lift. A function of creating lift, however, is also the creation of drag. Your canopy, as a result, "wants" to not only go up, but also slow down. Your body, on the other hand, is following Newton's first law of motion and "wants" to continue at it current speed and direction. Unfortunately, this also has the effect of pulling on the rear risers more than the front risers and this continues to increase the angle of attack.
It's a vicious little circle there for a moment or two as increasing the angle of attack slows the canopy more and more as your body swings farther and farther forward. Rapidly, the wing exceeds the critical angle of attack and the wing is stalled.
Your body may have been thrown quite a bit forward of the leading edge of the wing and even slightly upwards of you normal place under the canopy. Your body may feel a much more definite "falling" or even "backward" motion than it did during the static stall.
Now, bear with me because I've also heard the term dynamic stall used in skydiving when referring to what airplane pilots would call an accelerated stall and this is the type of stall I believe Alan was referring to when he said . . .

Quote

During the recivery arc, when the canopy has a relatively high induced airspeed (creating additional lift), it is also carrying a heavier load due to the additional G forces, correct?

You -may- remember in a post a little earlier in this thread I said . . .

Quote

***PLEASE read this next part carefully!***
A stall -can- happen at any airspeed and at any attitude.
***Resume reading in normal careless manner***

Up to this point in this discussion, we've been looking at at stalls in a fairly normal manner. If you didn't know better you -may- have thought that the stall had something to do with the speed of the wing or it's attitude in relationship with the horizon.
Ok, that's normal. Lot's of folks do that. After all, we've demonstrated the stalls from a slow deceleration and with a fairly "normal" relationship to the horizon, earth and sky. And that's about where most discussions on the subject begin and end. So, I can see where just about anyone might make the mistake of thinking that a stall can only happen if you're flying too slow or if the leading edge of the wing is pointed toward the sky.
Unfortunately . . . this is just dead wrong.
To make matters worse, some maneuvers that you may perform -- a turn for example -- creates g-forces. Basically, your body wants to continue in a straight line but is getting pulled in another direction. As the bank angle increases, so do the g forces. In an airplane, maintaining altitude during a turn, the g force increases at a rate equal to the tangent of the bank angle. So, at bank angle of 45 degrees the g force would be 1.414. while at a bank angle of 60 degrees the g force would be 2. These are some "useful" numbers to remember so I have carried them around in my head since my Private Pilot days.
Beyond 60 degrees of bank the G increase is very rapid with 3 Gs coming in around 72 degrees, 4 Gs around 76 degrees, 5 Gs about 78 degrees . . . you get the picture.
You -might- not get exactly 2 Gs under a parachute turning with a 60 degree bank though since you're not maintaining the altitude and the equation becomes quite a bit more complicated taking into account your descent rate, but suffice it to say it -will- be greater than 1!
So what's this got to do with stalls?
As the G load increases, so does the amount of lift required to offset it. With the same angle of attack, the airspeed at which the stall occurred would be increased by the square root of the load factor.
So, let's say you're pulling a sustained 2 Gs (you're really carving a turn or pulling out of a steep swoop), your canopy -will- stall 1.414 times it's "normal" stall speed at any given angle of attack. (hummm, those numbers keep popping up!)
The really insidious part of this is when a person is snapping those toggles down to pull himself out of that too steep and too low swoop, the other part of the dynamic stall, the one where I described the body continuing in a straight line, increasing the angle of attack and aggravating the stall comes into play with really bad results.
This is what I meant when I said in a previous post in this thread . . .

Quote

It's -my- opinion that this could be the primary factor in some botched hook-turn landings.

Anyway, I hope this explained a bit more about stalls in general and stalls under canopy.
Paul
http://futurecam.com/skydive.html

[freeflyguy 0]

Uh, yah. But I think you really meant to talk about the reverse tangent of the square root.
Or maybe just the knudsen connector. But I really don't know about that stuff.

[alan 1]

Thanks Paul, your posts in this forum have been very informative and should be beneficial for those who take the time to read and understand them. That may take a little studying, BUT.........................
Todays high performance canopies require that the pilot have skill, experience, and knowledge in order to fly them safely and efficiently. Unfortunately, many of the pilots I see today get under these canopies and are deficient in at least one of these areas and often times more than one. To some extent, most get away with it by making up for it in one or two of the other areas, ie., they lack knowledge, but compensate with more experience and maybe better skills. That is a real shame, because the really good pilots are strong in all three areas. Enough said.
alan

[freeflyguy 0]

Hmm. Ya maybe my reply was a bit snide there. Sorry, Paul.
You do know what you're talking about when it comes to aerodynamics. Most of what you said is correct and does come straight out of a pilot's manual.
I should say where I disagree. Not just poke.
That said. I disagree.
The original question on this thread was about stall and line trim. That was pretty well answered. One thing not really talked about was whether a person chose to do front riser turns or not, in that persons eqaution of toggle length.
Some mentioned they like only a small bow in their lines, at full flight, with toggles up. That is fine. It will work great and is likely close to factory specks as well. But if you have that setting, and choose to start doing front riser turns, it might be a problem. As you pull you're front riser, at the same time, that hand will tighten your brake lines and deform the rear of your canopy. It may not collapse it, but if not, it certainly is not efficient.
The point where I disagree with Paul's post is this. He mentioned the dynamics of a stall, using toggles. That is very good, and is likely all true, to a point. In his description, he makes it seem a stall can occur, using toggles only, at some speed above very slow. That isn't true, if we are talking about parachutes, and toggles only. On the vast majority of the parachutes we jump. It is practically impossible stall it, with toggles only, until after it has been slowed down to it's stall speed in that configuration. I wish I had a better way to explain it. Anytime I have ever seen anybody in a full dive, then stab the toggles for all he was worth, the parachute simply does it's best to claw back up. It may be that you haven't really fully stalled it. If it is flying at all, even mushy, it is flying. Although likely partially stalled. The bad air starts at the tail, and as the parachute gets slower, the bad air creeps forward on the top skin. When it stalls, it quits flying and falls off. Usually back.
Rear riser stalls are totally different, they can occur at any speed. Although the turbulant air creeping up the top skin is the same. Just likely quicker, and more violent.
A major reason why it wont stall on the toggles, is that when you pull them down, you reshape the parachute so much. It is then 'designed' to fly at a much slower speed. Your dive is not likely over more than 90 degrees, it will swing you down, and quite quick to level, then be increasingly resistant to swinging you back up. You and the parachute catch up to each other, then it just climbs until it runs out of speed, and stalls.
An airplane is totatlly different. You have the elevator. When you pull back on the stick, it forcibly changes the angle attack of the wing. The wing itself isn't changed, it has no more lift, and you can easily exceed the critical angle of attack. It is stalled. The rear risers do the same, forceiblely change the angle of attack.
Where this matters to us, and the only reason I bring it up, is if you are too deep in the corner, pretty much your only choice is stab the toggles. I wouldn't want someone to read something that doesn't completely apply to a parachute and think there is danger of stalling, when you have to stab. That is the last ,and often only, choice that has saved many lives.

[quade 3]

***
And this is where -I- disagree.
What all of this theory should really be telling you is that if you're too low, too fast and still pointed straight at the ground, you're already screwed and no amount of control input is going to change that fact. The -correct- response is to not to allow yourself to be in that position in the first place.
Let me repeat it one more time . . .
A stall can happen at any airspeed and at any attitude.
I -guarantee- this applies to every wing that has ever flown -- including yours.
Anyone that doubts this will eventually have it proven to them. I just hope the demonstration isn't fatal.
Paul
http://futurecam.com/skydive.html

[freeflir29 0]

"A stall can happen at any airspeed and at any attitude"
Ummmm....OK.....I'm lost on this one. I am pretty sure you have to have either too little airspeed or be outside the critical angle of attack to stall a wing. Of course, those two factors are constantly changing as you fly. I see what you are getting at with wing loading during a turn but.....the stall speed increases as more load is put on it.....It's just not as simple as that statement. These two factors have a dynamic relationship.
"Carb Heat On....Carb Heat On.....Carb Heat On..."-Phil Polstra Clay