Big Canopies in Turbulence

[pchapman 262]

Brian Germain just posted an article in the Safety section, that seems not to have been put in the Forums. I think it is worth getting into the forums too, to be found in searches or discussed.

http://www.dropzone.com/cgi-bin/safety/detail_page.cgi?ID=718

Quote

Big Canopies in Turbulence
Posted Wed Jun 03 2009

By Brian Germain

I have spent much of my life studying parachute stability. It has become an obsession of sorts, spurred by a fairly sizable stint in a wheel chair- funny how that works. I have designed and built many, many canopies with the goal of creating collapse-proof canopy. I have failed. It is impossible.

This is the case because, despite the best efforts of the designer to increase internal pressurization and dynamic stability, the canopy can still be flown badly and become unstable. This will always be so. The job then, falls on the educators, and on the pilots themselves to learn and rehearse the essential survival skills that increase the chances that the correct action will be taken in the spur of the moment.

I stated in my original article on turbulence, entitled “Collapses and Turbulence”, that the key is to maintain lots and lots of airspeed and line tension. I still hold that this is generally the truth. However, upon re-examining the situation, I have realized that my perspective on the situation is based on my frame of reference. I fly sub-100 square foot cross-braced speed machine that falls out of the sky like a homesick bowling ball. I do not really represent the whole. The average-size parachute is 150-170 square feet in the civilian world, and much larger for students and military jumpers. In further exploring the issue from the perspective of lighter wing loading and larger parachutes, I have discovered that this is not necessarily best way to fly a larger canopy in chaotic air. Here is why this is so:

If the parachute has a great deal of drag, i.e. a light wing-loading, thick airfoil or is a large parachute in general, the rules change. Such canopies are less capable of maintaining high speeds unless flown very aggressively. Due to the high drag variable at the canopy end of the drag equation (“Rag Drag”, as I call it) the excess airspeed makes the canopy itself want to retreat behind the jumper far enough to reduce the airspeed far below the unadulterated full flight speed. This momentarily increases the likelihood of a collapse. The parachute levels off in mid air, slows down, and for a brief moment, becomes vulnerable to collapse.

Therefore, when flying a canopy with a short, powerful recover arc, aiming to increase the speed beyond full flight becomes a double-edged sword. If the timing is wrong, such as when leveling out high (prematurely), the situation can become very dangerous. The truth is, leveling off well above the ground is dangerous for any wing-loading, and can happen with any parachute due to an incomplete plan or an imperfect execution.

Parachutes flown below one G, at speeds less than full flight speed tend to be more susceptible to collapse. So, if the pilot is quick with their "Surge-Prevention Input", (what paraglider pilots call "flying actively", the risk of collapse is significantly reduced as the negative pitch oscillations will be minimized, thereby diminishing the likelihood that the wing will reach a low enough angle of attack to actually achieve negative lift and dive toward the jumper (i.e. collapse and scare the daylights out of you).

Given the fact that the only preventative or corrective response to a collapse is to stab the brakes as quickly as possible, the sooner the pilot responds to the forward surge, the less the input necessary to avoid or correct a collapse. Therefore, a canopy with a great deal of slack in the brake lines will require more motion on the part of the pilot to create any appreciable effect. This means that a canopy that is in full glide, with the toggles all the way up in the keepers and three inches of excess brake line trailing behind will take longer to see an increase in the angle of attack due to the control input than one with no slack in the brakes at all.

So then the question is posed: “Do we shorten the brake lines on larger canopies to help the pilot prevent collapses?” The answer to this is no, we cannot. This will result in serious bucking during front riser input. It will also mean that following a few hundred jumps, the canopy will be in significant brakes when they think they are in full flight, due to their “lazy arms” pulling the tail down when they should be flying arms up. This will result in lower average airspeeds that will reduce the parachute’s flare power, as well as it’s penetration into the wind. This will also result in more oscillation and distortion in turbulence.

The answer comes to us from our sisters and brothers in the paragliding world. They teach their students to hold a touch of tension on the brakes when flying through turbulence. The goal here is not to put on the brakes and deform the tail, but to simply take up the slack on the brake lines, in preparation for a 12-24 inch strike on the toggles to prevent a collapse. Some teach their students to hold about 5 lbs of pressure on the brakes, while others teach that we should hold no more than two inches below the “Feel Point”. Either way, taking the slack out of the brakes is like standing ready in the door, even when you can't see the count.

So, on larger canopies, it appears that a light touch on the brakes may help prevent collapses. However, it is not because the canopy is more stable in this configuration, but simply that the pilot is more prepared to prevent the wing from surging forward in the pitch window. Once the wing has passed through that parcel of turbulent air, however, the job remains to regain the full flight airspeed, while maintaining positive G's. Letting the wing surge back into full flight too quickly can send the wing out of the frying pan and into the fire. Get it back to speed gently, but get back there as quickly as possible. These are opposing goals, so the actions of the pilot once again become pivotal, calling upon trained skills and acute attention to sensation.

Ultimately, the best way to handle turbulence is to deny it battle. Despite what your ego is telling you, you already have enough jumps. I know you want more, but sometimes the best way to go is to sit on the ground and watch the inexperienced jumpers get experienced.

Live to fly another day.

Brian Germain Big Air Sportz ">www.bigairsportz.com<

[pchapman 262]

Now that I posted it I'll comment on it:

Looks good.

I've been advocating the "slight brake" idea for some time, to get the feel of what the canopy is doing, NOT to actually brake the canopy. That's likely one reason why people sometimes were more comfortable adding a little brake in turbulence, even if they couldn't articulate why it might be beneficial (rather than causing problems by slowing the canopy).

[Edited to add:] Mind you the paragliding folks do tend to use a little more brake than that, actually applying a couple inches of brake as Brian's article alludes to. Although the canopies are quite different, they do this despite having a very low wing loading. It has pluses and minuses - the canopy is already slower and closer to the stall, but by letting brake up one can faster recover from any swinging back of the canopy induced by a gust. It also dampens the movement of canopy in general, perhaps more of a factor with highly aerodynamic paraglider wings despite their larger size and entrained air mass.
While that's all something interesting to discuss, I still go along with the recommendation to not brake while flying a parachute in turbulence. (Other than to take up brake line slack.) (This rule can be relaxed somewhat for big old F-111 canopies where the tradition had been to use a little brake.)

One could emphasize the idea of keeping the canopy in a "window" over one's head a little more, although Brian has written about that before. Both the window and the tension on the brakes to feel the canopy are part of the paragliding "active flying" style.

One need not over react to every little bounce one gets in turbulence, but quick and precise action on the toggles can reduce the chance of the canopy getting too far away from normal flight.

I've always liked Brian's idea of putting on a little extra G on approach in turbulence, as it is one way in skydiving to "keep the speed up on approach" as one does in airplanes in turbulence. It's just that with parachutes there are fewer ways to add speed safely in turbulence. Straight front risering isn't all that good for canopy stability, so a gentle carving turn works.

I figure a harness turn would be the best way to do the turn. (As opposed to too much front riser or toggle input that could remove some of the margin of canopy stability that's useful to have in turbulence? However those inputs tend to be more abrupt only at the beginning of a turning maneuver.)

It's a good point that in practice it might be harder to keep such a turn going long enough and from high enough to be practical with many medium sized canopies. Still, it might be useful to just do a little harness turn acceleration for the final 90 degree turn to final. Anyone practice the adding-G idea with canopies on the light side of medium wing loading?

[DSE 3]

FWIW, the article is the home page feature.

[d123 1]

http://www.u-turn.de/site/gs/assets/afs/uturn_afs.avi
http://www.u-turn.de/site/rahmen.php?sprache=en&kat=gs&next=06_00.php&claim=innovationen&renderpage

For general knowledge is good to know but finding methods to fly a big parachute (not paraglider) in turbulance is like finding a way to make rape a more enjoyable experience for the victim. We, big canopies fliers, can say I'm taking the copilot place this jump (and still have some fun when the plane dives) and go latter in the tunnel for 5 min.

Lock, Dock and Two Smoking Barrelrolls!

[DocPop 1]

Quote

http://www.u-turn.de/site/gs/assets/afs/uturn_afs.avi
http://www.u-turn.de/site/rahmen.php?sprache=en&kat=gs&next=06_00.php&claim=innovationen&renderpage

For general knowledge is good to know but finding methods to fly a big parachute (not paraglider) in turbulance is like finding a way to make rape a more enjoyable experience for the victim. We, big canopies fliers, can say I'm taking the copilot place this jump (and still have some fun when the plane dives) and go latter in the tunnel for 5 min.

I am sure Brian would agree that it is better not to fly in turbulence, but the topic still needs to be discussed to allow pilots of canopies of all sizes to have a plan for IF they encounter less than ideal conditions.

Just a couple of weeks ago I was on a load where the conditions changed dramatically (15oF temp drop, 180 degree wind change and severe wind shear/turbulence all the way to the ground) between boarding and being under canopy.

Ideally, I would not have been on that load, but as I was, I would have preferred to have had this discussion before that jump!

"The ground does not care who you are. It will always be tougher than the human behind the controls."

~ CanuckInUSA

[hackish 8]

Would be nice to have a better description of the toggle input and suggestions on dealing with the canopy beginning to deform because of turbulence. In my experience the canopy bucks pretty fast and I can't picture what his suggestion is - if we're supposed to stab the brakes every time you feel a jolt or what.

Brian, can you add anything to this?

-Michael

[mdrejhon 8]

I'm interested in what Germain says about this in my perspective:

One of the things I do with a 170, which is classified as one of the larger canopies in this article, is that I do double-fronts to speed my landings, which are often preceded by a brief application and sudden release of half brakes, to make it easier to grab the front risers down on larger canopies.

Basically, I pull my brakes down halfway for a few seconds, then quickly go to full glide by releasing fairly quickly. In this event, my canopy surges forward slightly, and at that point, a perfect opportunity to grab and hold down both front risers down to keep it in accelerated approach, without being bothered by intense front riser pressure. It's a technique for making it easier to hold down front risers of canopies with lots of front riser pressure. I'd be interested in how this article applies to my case; as I often do jump at dropzones of well known turbulence, such as Perris. I believe the moment there is relatively low front riser pressure, is when lines are slightly slack which means increased risk of turbulence for 1 second. As I let go of my brakes, I'm briefly vulnerable, until I quickly replace the line tension with my pulling down the front risers, then I'm at lower risk (after the brief moment of higher collapse risk) punching through any turbulence more efficiently with the extra speed.

[pchapman 262]

Quote

Would be nice to have a better description of the toggle input and suggestions on dealing with the canopy beginning to deform because of turbulence. In my experience the canopy bucks pretty fast

I'm not Brian but my take on canopies in turbulence in general is this:

-- Don't react to every little bump. Turbulence is typically bouncing the canopy around pretty fast. Reacting too much may get one get out of sync with the movement of the canopy or just be slowing the canopy in brakes if stabbing a little (more) brake every time there's a jolt.

-- If the canopy dives a whole lot, then stop it from pitching very far forward with brake.

-- If the canopy pitches back a whole lot, make sure there's zero brake so the canopy gets the chance to pitch forward,

-- If the canopy turns off to the side a whole lot, oppose that with opposite brake, to get back straight or to the original heading for landing or whatever is appropriate. One can be forceful but unless there's a compelling reason (eg forced towards an obstacle when about to land), then don't get so deep in brake that the canopy slows too much compared to normal flight.

-- If part of the canopy folds under, causing a sudden turn and dive, oppose it with opposite toggle to fly straight, and pump the collapsed side brake once or twice through a fairly long toggle stroke to help it reinflate.

-- If the whole canopy stalls
(whether due to a downwards leading edge 'frontal' collapse or a traditional too high angle of attack and/or too pitched up and slow situation)
then use moderate to heavy brakes to avoid too much pitch forward and to help reinflation.
(But one would wait until the canopy actually pitches forward or drops - - one wouldn't want to get on the brakes too fast and turn a minor stall into a more major one by adding a lot more drag when it is about to stall.)
The exact amount of brakes to use will depend on what's happening -- one wants to use brakes to stop an abrupt dive and promote opening, but also not overdo their use if the canopy is slow and mushing along and still in turbulence. I could see that the brake point might be somewhere between the pure stall recovery point slightly above the stall point, and the brake set point (which varies a lot but is suitable for normal canopy inflation).

How often really extreme turbulence events occur is unclear to me. Often one sees stuff happening to canopies that really don't take much input because there's nothing bad enough yet to deal with. The canopy breathes and bucks, and maybe a pressure wave goes through an end cell and it snaps almost shut and then open again in a fraction of a second. But unless the canopy really dove or pitched up, there's nothing the jumper could really have done, even if it is uncomfortable to be in the situation.

Much of that sort of canopy behaviour is in itself fine until one is actually about to land, where the reduced efficiency or changed flightpath or low speed or increased descent rate can cause problems with getting a decent flare to a good landing.

Associated with turbulence there's sometimes wind shear, which is technically separate but can also cause landing problems.

I think all this applies well enough to canopies both big & small / old & modern / low & high wing loading.

There's a lack of good turbulence related video out there, that would help people prepare for dealing with turbulence.

I'd also like to hear what Brian or others think.

[billvon 2,463]

>punching through any turbulence more efficiently with the extra speed.

There are several things wrong with that statement IMO.

Turbulence is wind that changes in a small distance. The severity of the turbulence will INCREASE as you increase your speed, because you are seeing more change per unit of time. It makes about as much sense as saying "my car will slice through those speedbumps more efficiently if I am doing 60mph."

The "air mattress" effect is often overestimated i.e. "if I go fast I will have a more pressurized wing." Take an air mattress outside and try to hold it upright in 10kt winds - and then realize that that air mattress has about 10X the pressure in it than any canopy ever will.

Pilots of powered aircraft reduce their speed in turbulence to prevent structural damage due to gust-induced lift loads. We have essentially infinite strength in the +G direction, but zero strength in the -G direction. I don't care how much pressure you have or how rigid your wing is - if you get loading on the top of the canopy it is going to collapse no matter what.

Canopy collapse can be caused by many things, including moving the stagnation point above the nose, suddenly changing the relative wind left or right etc. By intentionally distorting the canopy you are bringing it closer to collapse even before turbulence.

Canopies are designed to be dynamically stable when flown hands-off. They will weathervane into the wind and recover airspeed if they are slowed, and they do this most reliably when flown with minimal inputs. They also reinflate if they collapse - and they do this best at deployment brake settings (half or so.)

As a final angle, consider that strong enough turbulence will collapse your canopy no matter what you do or how stable the canopy is. If that happens, do you want to start your fall at normal canopy speeds or at front riser speeds?

[d123 1]

If you guys are still interesting in "flying in turbulence" what I can do is to inform you that in paragliding world there are exercises (that you do on the ground) that help you build reactions for dealing with turbulence. Speaking with a real paragliding instructor to refine/adapt those exercises for skydiving should be the right way of doing it.

I also think that similar exercises don't exists in skydiving for a good reason and that's the fact that we don't really need them (think low aspect ratio, higher speeds and small amount time under wing compared with paragliders)

Here's an example of those exercises and I hope that we don't open the pandora box on flying in turbulence because people got hurt from ground handling in gusty winds.

http://www.youtube.com/watch?v=DPo082yXc4E&feature=channel_page

Regards,
Jean-Arthur Deda.

Edit to add: I'm talking about the part where he flies with his eyes close (@ 1:50) and not the part where he climbs stuff!

Lock, Dock and Two Smoking Barrelrolls!

[pchapman 262]

Quote

Turbulence is wind that changes in a small distance. The severity of the turbulence will INCREASE as you increase your speed, because you are seeing more change per unit of time.

I'm concerned with the angle of attack change. A downgust of a certain speed creates less of an angle of attack change for a canopy moving quickly than a canopy moving slowly. That is one reason to prefer to be going fast rather than slow.

(Or, similarly, a turbulence induced horizontal gust of a certain speed will cause a smaller percentage change in the speed of a faster canopy.)

(But one can still argue about different angles of attack to begin with, whether one is comparing the same canopy fast vs slow or a big canopy vs a small canopy. If you speed up a given canopy you might get a smaller angle of attack change, but be flying at a smaller angle of attack to begin with, so I'm not sure what the trade off becomes. Things get messier.)

Sure when going faster one will hit the turbulence quicker but does that matter on the timescales and frequencies we deal with? A fast canopy feels some more sudden hits by turbulence; a slow canopy wallows more, but neither is of the type of G loading that is going to be a problem to the pilot (as opposed to the canopy).

Would I rather be fast or slow if I somehow have a collapse? Well, yes I'd rather be slow if about to hit the ground. But on a given canopy, I'd want to have some decent speed going, as too slow would leave too little energy to allow for pulling out of a dive, flaring etc. But you were likely just arguing that you don't want too fast, eg, not in a front riser dive.

Quote

The "air mattress" effect is often overestimated

Agreed. A soft canopy with big nose openings might breathe more and lose some efficiency in turbulence, but it is angle of attack that is what would cause a canopy to collapse, not the canopy being "too soft".

Referring to mdrejhon's post, a sudden change towards a lower angle of attack or lower line tension, as with letting a canopy pitch forward and then hauling down on front risers, yes that's more dangerous as it gets more turbulent. So I figure any transition like that should be done slower and more smoothly if there's turbulence.

[billvon 2,463]

>A downgust of a certain speed creates less of an angle of attack change
>for a canopy moving quickly than a canopy moving slowly.

I think that's backwards. If you have a wind change where the winds change from zero (relative to the ground) to 10 knots "down", and that happens over the space of 20 feet, your canopy will react a lot more violently if you pass through it in half a second than if you pass through it in a second. If you pass through it in half a second, you see a shear of 20 knots per second. If you pass through it in a second, you see a shear of 10 knots per second.

In both cases, when the canopy exits the shear region, the new wind condition holds. But in the slower case, the canopy has more time to adjust to the new wind condition (by increasing descent rate) reducing odds of a top-loaded condition.

>A fast canopy feels some more sudden hits by turbulence; a slow canopy
> wallows more, but neither is of the type of G loading that is going to be a
>problem to the pilot (as opposed to the canopy).

Definitely, and I hope I did not give the impression that the pilot's G-loading is the issue. It would take thunderstorm-level winds to create enough G forces to give you issues with harness strength, loss of consciousness etc. However, our canopies cannot tolerate any negative G-loading at all; that's where the concern comes in. If our lines could be made rigid after opening, a lot of these issues would go away.

> But you were likely just arguing that you don't want too fast, eg, not
>in a front riser dive.

Yes. I think the best turbulence-penetration speed will generally be the canopy's trim speed. That gives you margin against stall, it minimizes excess speed and canopy deformation, and reduces the speed at which you penetrate shear zones.

[pchapman 262]

Quote

>A downgust of a certain speed creates less of an angle of attack change
>for a canopy moving quickly than a canopy moving slowly.

I think that's backwards. If you have a wind change where the winds change from zero (relative to the ground) to 10 knots "down", and that happens over the space of 20 feet, your canopy will react a lot more violently if you pass through it in half a second than if you pass through it in a second. If you pass through it in half a second, you see a shear of 20 knots per second. If you pass through it in a second, you see a shear of 10 knots per second.

In both cases, when the canopy exits the shear region, the new wind condition holds. But in the slower case, the canopy has more time to adjust to the new wind condition (by increasing descent rate) reducing odds of a top-loaded condition.

I was thinking of an instantaneous "sharp edged" gust, which of course is an approximation. The question here is whether that's a good enough approximation.

You're right that the dynamics are a factor too. There's inertia to deal with, much more so for a skydiver than a dandelion seed.

If gusts hit instantly, then the angle of attack factor says that faster = less angle of attack change.
If vertical winds change very very slowly, then at any speed the canopy has time to adjust back to equilibrium flight.

So a question is, where are we typically in between?

Look at one 'in between' situation:
Let's use that example where the gust ramps from 0 vertically to 10 knots down over 20 ft. Let's pretend the canopy doesn't have time to react much in half a second. The fast canopy gets to the full gust strength in half a second, and therefore has its angle of attack lowered by "x" degrees. The slow canopy in that same half-second only reaches the point where it is at half gust strength. It is flying half as fast, getting half the gust strength, so the angle of attack change is the same, x. Neither canopy has any advantage in this case.

(Naturally I'm making trigonometric simplifications, like using the small angle approximation and pretending the flight is horizontal. But that should be OK.)

Then at the end of the second half-second the slow canopy arrives at the point of full gust strength. If the slow canopy (with the jumper's mass too of course) essentially had "no time to react" due to inertia, angle of attack would go down by a total of 2x -- that's the scenario I had given as an example, with the fast canopy doing much better.

But allowing for some reaction from the canopy, it would pitch nose down due to natural stability to reduce the angle of attack. How fast can it compensate?
If it can pitch down by less than x it will end up with a pitch down of more than x. (Worse than the fast canopy at x degrees down.)
If it can pitch down by x then it will end up at x degrees down. (Same as the fast canopy at x degrees down.)
If it can pitch down by more than x then it will end up with a pitch down of less than x. (Better than the fast canopy at x degrees down.)


So we have a situation where the end result depends on the relative strength of the two factors:
1) the pure angle of attack change caused by a gust that depends on the canopy's speed (as I mentioned), which favours a faster canopy, and
2) the time taken for the canopy & jumper system to adjust to changing angles of attack (depending on inertia & canopy pitch stability) (as you mentioned), which favours a canopy that goes through the gusts slower.


Both factors need to be mentioned.
How does that sound?

So which factor is more important in practice?
I still kind of think that a lot of gusts we hit are in effect 'sudden', before the canopy has time to react much, so I'll still favour factor #1 and say in general that more speed is better in preventing a collapse (even if more dangerous when one happens). But I'm open to other ideas.



Here's another example of how the tradeoffs can be impossible to know unless one actually knows the particular flight characteristics of a given canopy, and what the gust shapes actually look like that it has to fly through:

Say that there's a maximum vertical just strength, and it is such that if the gusts are hit suddenly (reaching full strength over very little distance), then a particular slower canopy could just get to the point of having the nose fold down, losing lift & dropping the jumper until full reinflation occurs. Then if one had a bit faster canopy, it would never collapse in those conditions. And the slower canopy could collapse if the gusts are hit suddenly, but would not if they ramped up to full strength only over a larger distance, which gave the canopy (at that slow speed) time to adjust to the changing air direction. In this example, the faster canopy is by far superior. But that depends entirely on how the gust profiles match up with the speeds and angles of attack the canopies are flying at. If the numbers change, the conclusions can change about which canopy situation is better.

[mdrejhon 8]

Wow -- an interesting discussion triggered by my post.
It's very interesting reading, but I see reason to both sides:

billyvon/pchapman:
Although flying fast definitely has elevated risk, please see page 39-40 of Brian Germain's "Parachute and its Pilot". It's based on this, as well as having read at least one or two other Germain articles, where I came up with the phrase "punch through the turbulence". That said, I should be clear I realize there are OTHER risk metrics that do go up, and this won't work when turbulence gets bad enough...

I think some risk metrics (as billyvon says) definitely go up, while other risk metrics (as page 39 to 40) definitely go down, when I fly a little bit faster through medium turbulence. To a certain extent, anyway.

That said, I definitely DO NOT fly fast on purpose to go through turbulence, and I've tended to automatically punch brakes (like I did when I flew through a dust devil at 1700 feet sometime last year). But, when I'm flying through manageable turbulence -- the kind that slightly buffets you around -- I do feel some better stability (Against instantaneous moments of turbulence) when I'm flying front-riser accelerated, but I do feel slightly vulnerable during transition from brakes to risers, so I don't pull down brakes much during transition. I've aborted front-riser accelerated plans when conditions were adverse.

If Brian Germain wants to write an addendum, have him come to this forum and reply to this thread, to explain how some risk metrics go up and other risk metrics go down, when I fly fast front-riser-accelerated, on a lightly-loaded (170sqft @ 1.15) through moderate turbulence. Also, under my 170, I fly vertically similarly front-riser-accelerated than I do straight-in under some 150sqft and 140sqft full-glide demo-canopy jumps I've done. In many cases, slower, too. It's not 90's or 180's I'm doing, so my speed isn't like mammothly increased, and I don't have other complex dynamics to concentrate on, that arises during swoop turns...

[Edit: Sent PM to Brian Germain. I'd welcome his opinion]

[aphid 0]

I've come to this conversation a bit late, but could you please be more specific regarding the two following statements as I'm not sure I understand correctly:

#1: "Canopies are designed to be dynamically stable when flown hands-off. They will weathervane into the wind and recover airspeed if they are slowed"

I thought from my experience that unadjusted flight allows/causes the canopy to turn and run with the wind, and,

#2: " I think the best turbulence-penetration speed will generally be the canopy's trim speed."

Do you mean, a) hands all the way up (designed full-flight), or, b) a modest input of the rear risers to encourage clean flight (pilot-trimmed flight)?

I look forward to your reply.

Thanks

John

[JohanW 0]

Quote

#1: "Canopies are designed to be dynamically stable when flown hands-off. They will weathervane into the wind and recover airspeed if they are slowed"

I thought from my experience that unadjusted flight allows/causes the canopy to turn and run with the wind, and,

We've been through this before. The canopy doesn't know what the ground is doing, so weathervaning both the canopy and its pilot towards upwind or downwind relative to the ground is not applicable.

But a canopy definitely has a tendency to weathervane into the relative wind (making its pilot follow, too). Which means that (as long as your hanging in the harness symmetrically), you won't be seeing sustained yaw. You won't be slipping the canopy. You (normally) won't be flying into linetwists, which would happen if canopies were unstable yaw-wise.

(My English, and my aerodynamics, seem insufficient to express the aerodynamic reality clearly and concisely. I don't know if it's static or dynamic stability that makes a canopy weathervane into the wind. And weathervaning 'out of the wind' sounds strange. Perhaps BillVon can explain what he and I mean in better words.)

Johan.
I am. I think.

[BrianSGermain 1]

Quote

Would be nice to have a better description of the toggle input and suggestions on dealing with the canopy beginning to deform because of turbulence. In my experience the canopy bucks pretty fast and I can't picture what his suggestion is - if we're supposed to stab the brakes every time you feel a jolt or what.

Brian, can you add anything to this?

-Michael

Hi Michael,
Jabbing the brakes is not often necessary, only in extreme surges. However, contant smooth corrections to the pitch to keep your parachute over your head will minimize the amount if jabbing you have to to in the first place. The truth is, parachutes (current designs) are quite stable. The center lift is quite far forward, so they tend to recover quite nicely. Let her fly fast, and she will most often sail on through the bumps.

I find that it is the low altitude surges that are the problem. When the wing dives just before re flare, the flare may not work very well. As soon as you feel the wing to in front of you, an you feel the G's start to get light, apply the brakes quickly to bring the canopy back to it's original flight path. That way your canopy is ready to level off immediately.

Brian Germain

Instructional Videos:www.AdventureWisdom.com
Keynote Speaking:www.TranscendingFEAR.com
Canopies and Courses:www.BIGAIRSPORTZ.com

[billvon 2,463]

>If it can pitch down by less than x it will end up with a pitch down
>of more than x.

I agree with your trigonometrics, but would also add a second variable - acceleration downwards. In our example, the faster canopy has increased its downward speed by 16 feet per second, best case (i.e. if the canopy has lost all or almost all of its lift.) The slower canopy has increased its downward speed by 32 feet per second. That means the slower canopy has a 16 foot per second "margin" that the faster canopy does not have, given the down-gust we are discussing.

[billvon 2,463]

>I thought from my experience that unadjusted flight allows/causes
>the canopy to turn and run with the wind . . .

Not really. It definitely seems that way, because the one time we really want to be headed into the wind is on final - and that's the time that any turn at all, from any source, will make you turn downwind. So it seems like your canopy always wants to turn and run with the wind.

But that's the same no matter which direction you are flying. If you're flying straight downwind, little gusts/asymmetries will try to turn you _upwind._ But since we rarely fly final downwind, we don't see that as often.

>Do you mean, a) hands all the way up (designed full-flight) . . .

Yes, hands all the way up. If your canopy is trimmed per most manufacturer's directions that will result in no tail deflection and some bow in the brake lines.

[BrianSGermain 1]

Quote

Wow -- an interesting discussion triggered by my post.
It's very interesting reading, but I see reason to both sides:

billyvon/pchapman:
Although flying fast definitely has elevated risk, please see page 39-40 of Brian Germain's "Parachute and its Pilot". It's based on this, as well as having read at least one or two other Germain articles, where I came up with the phrase "punch through the turbulence". That said, I should be clear I realize there are OTHER risk metrics that do go up, and this won't work when turbulence gets bad enough...

I think some risk metrics (as billyvon says) definitely go up, while other risk metrics (as page 39 to 40) definitely go down, when I fly a little bit faster through medium turbulence. To a certain extent, anyway.

That said, I definitely DO NOT fly fast on purpose to go through turbulence, and I've tended to automatically punch brakes (like I did when I flew through a dust devil at 1700 feet sometime last year). But, when I'm flying through manageable turbulence -- the kind that slightly buffets you around -- I do feel some better stability (Against instantaneous moments of turbulence) when I'm flying front-riser accelerated, but I do feel slightly vulnerable during transition from brakes to risers, so I don't pull down brakes much during transition. I've aborted front-riser accelerated plans when conditions were adverse.

If Brian Germain wants to write an addendum, have him come to this forum and reply to this thread, to explain how some risk metrics go up and other risk metrics go down, when I fly fast front-riser-accelerated, on a lightly-loaded (170sqft @ 1.15) through moderate turbulence. Also, under my 170, I fly vertically similarly front-riser-accelerated than I do straight-in under some 150sqft and 140sqft full-glide demo-canopy jumps I've done. In many cases, slower, too. It's not 90's or 180's I'm doing, so my speed isn't like mammothly increased, and I don't have other complex dynamics to concentrate on, that arises during swoop turns...

[Edit: Sent PM to Brian Germain. I'd welcome his opinion]

Holy Pontification of the Navel, Batman!

If your lines get slack, increase the angle of attack. It is that simple.

Everyone already knows that part, right? OK, so then comes the question of whether excess speed is a good thing when airfoil distortion is necessary, i.e., front riser input. But to answer that question, we must ask whether or not there is tail input during the maneuver. If there is, it may very well reduce the stability. If not, it may still open the door for a collapse on some older canopies, but most modern canopies are so front-loaded that the parachute barely notices the input; the load was already there. Therefore, any airfoil distortion is outweighed by the benefits from the extra airspeed. As I mentioned in previous articles, the extra speed increases the canopy's drag, and if the speed can be maintained, the drag will maintain the angle of attack, and this benefit the stability. Due to the powerful Rag Drag, the pRachute is less likely to experience a negative pitch change in a short period of time. The speed prevents it. If the speed cannot be maintaned, then you should stick with full flight and be ready to apply brakes quickly to keep it over your head the old fashioned way.

Instructional Videos:www.AdventureWisdom.com
Keynote Speaking:www.TranscendingFEAR.com
Canopies and Courses:www.BIGAIRSPORTZ.com

[aphid 0]

Thanks Bill.

John