Flying in turbulence (was: Fatality - Deland, FL)

[RMURRAY 1]

Peter,

not that turbulance was confirmed but I thought that I would point out the relatively recent technical article by John Sherman and Nancy LaRiviere. Here is part..."Understand we are not advocating that you fly at just above a stall. This would be dangerous, as a sudden gust could change the airflow inducing said stall. We do, however, recommend that you fly near deployment brake configuration or depth. This is the flight mode you would want to go to if your canopy did lose lift. Remember, pilots are taught to “reduce to maneuvering speed” when encountering turbulence. One of the considerations for this procedure in powered aircraft is to reduce the structural load, a minor concern for ram-air parachutes. But recognize that at deployment brake depth we have greater lift and a stronger boundary layer, which would be more difficult to separate"

[mark 104]

Quote

"Understand we are not advocating that you fly at just above a stall. This would be dangerous, as a sudden gust could change the airflow inducing said stall. We do, however, recommend that you fly near deployment brake configuration or depth. ... But recognize that at deployment brake depth we have greater lift and a stronger boundary layer, which would be more difficult to separate"

John and Nancy are very smart people, but I do not understand the basis for this assertion.

Lift is a function of forward speed: more speed results in more lift.

As for boundary layer, in what sense is the "boundary layer" (in quotes because I don't know what the term means when applied to skydiving canopies -- I do know what the term means when applied to laminar flow) stronger at slower speeds?

Finally, if a canopy has no deployment brake setting (Icarus tandem main, for example), is toggles all the way up the best way to go? And if a canopy has very deep deployment brake setting, near stall, like VR360 tandem reserve, how does one not fly at just above a stall if one is flying near deployment brake configuration?

Mark

[pchapman 262]

It's starting to look like a new thread on turbulence will be needed elsewhere...

(And I'm not very confident either about the John & Nancy quote.)

Some of the many turbulence threads:
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_view_flat;post=256878;
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_view_flat;post=458267;
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?post=2502098;
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_view_flat;post=759083;
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_view_flat;post=2050501;
http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_view_flat;post=2051698;

[davelepka 4]

This is just a guess, but is it possible the article my be specific to the Firebolt canopy produced by Jump Shack.

[billvon 2,435]

>Lift is a function of forward speed: more speed results in more lift.

Not really. If your exit weight is 180, and you're in stabilized flight, it doesn't matter whether you are doing 10mph on a Manta in brakes or 25mph on a Xaos-27. In both cases the lift is exactly the same - 180 lbs.

Now, if you have more speed, more lift is _available_ temporarily. This is why people swoop; they increase their speed and then use that additional speed to (temporarily) generate more lift. It's also why front riser turns are generally safer than toggle turns, since toggle turns temporarily decrease canopy speed.

>Finally, if a canopy has no deployment brake setting (Icarus tandem
>main, for example), is toggles all the way up the best way to go?

Per John LeBlanc of PD, full flight is generally the safest way to fly in most turbulence. If your canopy has actually collapsed, and needs to be reinflated, then generally deployment position is the safest position to be in, since that's the brake position the canopy is designed to inflate in.

[Travman 6]

If I recall correctly, Scott Miller taught us the same thing in his canopy course - full flight to pass through turbulence, brakes to help reinflate.
Not surprising since he was (is?) one of PD's test pilots.

[MajorDad 0]

Quote

(snip)
Per John LeBlanc of PD, full flight is generally the safest way to fly in most turbulence. If your canopy has actually collapsed, and needs to be reinflated, then generally deployment position is the safest position to be in, since that's the brake position the canopy is designed to inflate in.

Since John L. is making a distinction between modern canopies and the older canopies, would it be fair to say the drill for a Manta in turbulence would be 1/4 to 1/2 brakes while the drill for a Sabre 2 or a Stiletto be full speed?

Major Dad
CSPA D-579

[kallend 1,651]

Quote

>Lift is a function of forward speed: more speed results in more lift.

Not really. If your exit weight is 180, and you're in stabilized flight, it doesn't matter whether you are doing 10mph on a Manta in brakes or 25mph on a Xaos-27. In both cases the lift is exactly the same - 180 lbs.

Now, if you have more speed, more lift is _available_ temporarily. This is why people swoop; they increase their speed and then use that additional speed to (temporarily) generate more lift. It's also why front riser turns are generally safer than toggle turns, since toggle turns temporarily decrease canopy speed.

>Finally, if a canopy has no deployment brake setting (Icarus tandem
>main, for example), is toggles all the way up the best way to go?

Per John LeBlanc of PD, full flight is generally the safest way to fly in most turbulence. If your canopy has actually collapsed, and needs to be reinflated, then generally deployment position is the safest position to be in, since that's the brake position the canopy is designed to inflate in.

Consider this speculation:

For most reliable inflation (as AoA temporarily changes in turbulence) I'd venture the best (average) place for the stagnation point is centered on the ram air intakes. Looking at various canopy designs we see some with huge intakes right at the front, and others with tiny intakes somewhat below the LE, and variations on both. I can't help feeling that the best flight regime for pressurization is going to be different for the different designs.

...

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

[billvon 2,435]

>Since John L. is making a distinction between modern canopies and the older canopies . . .

Honestly, the similarities between a Manta and a Sabre2 are far greater than the differences. I think that the same techniques work for both. Note that on bigger canopies turbulence FEELS worse, but generally they will land you more safely in turbulence than a smaller canopy will.

[phoenixlpr 0]

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would it be fair to say the drill for a Manta in turbulence would be 1/4 to 1/2 brakes

Any explanation? IMHO this does not make any sense. We are not a part of a fixed winged aircraft, it won't break in pieces or get overloaded. We have to keep on flying and for stability we have to keep lines under tension, only way to to is break if lines getting loose. Why to sacrifice speed for nothing?

[RMURRAY 1]

here is the entire article - I am hoping John or Nancy respond (I let them know the subject is being discussed).....

What to do in Turbulence?
1/23/2004
Necessary Action when Encountering Turbulence

The purpose of this article is to illuminate on the facts necessary for jumpers to make an informed decision, so that they may better plan their corrective/preventative action when encountering turbulence while flying their ram-air parachute.

We are aware that others have published information contrary to what will be presented here. It is for that very reason we write this article. We believe this is a major safety issue, and that there should be a hearing and finding by the USPA Safety and Training Committee.

Encountering turbulence under a ram-air canopy can be unnerving at best. It can and has led to canopy collapse and sudden impact with the ground. Turbulence can happen at any altitude, however, it is the most dangerous when it occurs close to the ground because of the limited recovery distance.

To fully understand the problems created by turbulence we must first understand some basic aerodynamics. The principal, which keeps a ram-air flying, is one of pressure differential. It’s the same principal that makes airplanes fly. The pressure on the top of the canopy aft of the thickest point is reduced to less than ambient. This pressure reduction is caused by the flow of the air over the top surface, and the differential of this flow from the flow over the bottom surface. When the air mass is encountered by an airfoil it divides at what is known as the stagnation point. From here some of the air goes over the top of the canopy and some of it goes below the canopy. The air going over the top is accelerated, ‘cause it has to go further, to keep up with the air going under the bottom. The further the air goes the greater the pressure reduction. This pressure gradient is called the “Boundary Layer”. It is progressive in its flow characteristics from the surface of the skin to the “Free Stream Velocity” (the full uninterrupted flow of the air around the airfoil). At the surface (of the canopy) there is no flow, only a reduced pressure. This is why when you stick your fingertip out the door of an aircraft you don’t feel much air flow. As you progressively expose your finger, the airflow becomes greater. It is this boundary layer we are concerned with. If this boundary layer becomes detached from the skin of the canopy, a loss of lift is encountered and what looks like a collapse ensues. This boundary layer can be blown off the top of your canopy as a result of turbulence.

Considering the above, and understanding the axiom “The moment of greatest lift on an airfoil is just prior to an impending stall”, which is taught to every student pilot, tells us that the boundary layer gets stronger as we approach a stall. Why is this? It is because the air flowing over the top must go further and necessarily faster to meet with the air passing on the other side of the wing. Therefore the pressure differential is greater, and the boundary layer is stronger, making it harder to blow off.

Understand we are not advocating that you fly at just above a stall. This would be dangerous, as a sudden gust could change the airflow inducing said stall. We do, however, recommend that you fly near deployment brake configuration or depth. This is the flight mode you would want to go to if your canopy did lose lift. Remember, pilots are taught to “reduce to maneuvering speed” when encountering turbulence. One of the considerations for this procedure in powered aircraft is to reduce the structural load, a minor concern for ram-air parachutes. But recognize that at deployment brake depth we have greater lift and a stronger boundary layer, which would be more difficult to separate.

Think of it in these terms, if you should be so unfortunate to lose your boundary layer due to turbulent conditions, your canopy may fully or partially collapse. When we want to make our canopies open or re-open faster, what do we do? We set the deployment brakes. If you want your canopy to re-inflate quickly you’d best be in some stage of braking (toggles pulled at or near brake setting). This will cause your canopy to recover more quickly- no matter what kind of canopy we’re talking about.

The only time we need to be overly concerned about turbulence is as we get close to the ground. If your canopy bumps and breathes or even partially collapses at 1000’ or 500’ feet - so what! There is plenty of time for it to recover. The time that is takes to recover is about equal to your deployment time.

The notion of “sacheting” or front “risering” out of the turbulence doesn’t make sense aerodynamically, nor will it help you avoid the problem in the first place. Turbulence travels around with the air mass, rising with the thermal activity, descending in the downdrafts. Understanding thermal activity will prevent surprises, and help you to avoid mishaps. Remember for example, not to land directly downwind of tall trees or buildings on a windy day. Land on the upwind side of the runway on a hot (thermally) day, rather than the downwind side. Try to land where the clouds cast great shadows on the ground – you’ll more likely encounter smoother air.

For a wonderful explanation of turbulence as it relates to aviation, read Peter Lester’s Weather Smart article at http://www.met.sjsu.edu/~lester/ws_aug03.html.

Written by:

John Sherman, USPA I/E, Master Rigger, Commercial, Instrument, Multi-engine Pilot, Medallist In 4 Way, 10 Way, Style & Accuracy, US National Championships, Designer of the Racer, AngelFire Reserve, & FireBolt Elliptical Canopy, Tandem, Student, Military and BASE Canopies.

Nancy LaRiviere, USPA I/E, Tandem Examiner, Senior Rigger, Commercial, Multi-Engine Pilot, Double Bronze Medallist 2003 World Championships, Multi-time U.S. Nationals Medallist and U.S. Team Member, Holder of 10 World Records, 5600 jumps, Canopy 102 Coach, President of Jump Shack

[phoenixlpr 0]

Thanks for the article!

I still don't agree with it.

I think Brian Germain covers this really well in Collapses and Turbulence

[mark 104]

Quote

here is the entire article - I am hoping John or Nancy respond (I let them know the subject is being discussed).....

What to do in Turbulence?
1/23/2004
Necessary Action when Encountering Turbulence
...
To fully understand the problems created by turbulence we must first understand some basic aerodynamics... The air going over the top [of the canopy] is accelerated, ‘cause it has to go further, to keep up with the air going under the bottom. The further the air goes the greater the pressure reduction. This pressure gradient is called the “Boundary Layer”. It is progressive in its flow characteristics from the surface of the skin to the “Free Stream Velocity” (the full uninterrupted flow of the air around the airfoil). At the surface (of the canopy) there is no flow, only a reduced pressure. This is why when you stick your fingertip out the door of an aircraft you don’t feel much air flow. As you progressively expose your finger, the airflow becomes greater. It is this boundary layer we are concerned with. If this boundary layer becomes detached from the skin of the canopy, a loss of lift is encountered and what looks like a collapse ensues. This boundary layer can be blown off the top of your canopy as a result of turbulence.

For the aerodynamicists here: is "pressure gradient" a plausible definition of "boundary layer?"

Is "boundary layer" the reason one feels little air flow near the door and progressively more as one exposes a finger or arm, or is it simply a function of surface area and lever arm?

Mark

[pilotdave 0]

I have a degree in aerospace engineering and I feel like I just read an explanation of how atmonauti generates lift.
The whole argument that a wing produces more lift near stall is great, but I don't get it. Yes, the coefficient of lift is higher, but so what? A little disturbance in angle of attack and you'll stall. That's why planes generally fly faster on approach when there's turbulence. Parachutes are so different from planes anyway... I just can't really understand their argument. Seems like they're saying that flow is less likely to separate at a higher AOA because the pressure gradient is bigger. That's nonsense... if turbulence adds a little AOA, flow is more likely to separate. Seems like they're just throwing a lot of aerodynamics terms into a blender and hoping something good comes out.

So I'm not gonna say that they're wrong about what to do in turbulence (I tend to believe full flight makes more sense, but i don't know for sure), but I don't understand their explanation of why quarter brakes or so is any better.

Dave

[3331 117]

John Sherman asked me to post this response.  

"All pilots know that the point of greatest lift is just prior to an impending stall. This means that the lifting component is strongest at that point or AoA. The lifting component or boundary layer is the distance from the surface of the airfoil to the free stream velocity. Certainly the free stream velocity is at ambient pressure and the surface of the airfoil is at a lower pressure. This pressure gradient is a component of lift.

When a rotor or turbulence hits the top of the canopy it can shear the boundary layer off of the surface. This is a stall where the canopy de-pressurizes and the skins wrinkle all lift is lost. I have seen it occur on only half of the canopy causing a violent spin. Certainly it can occur at any brake setting but is less likely to occur with a strong boundary layer which is commensurate with deep brakes. To release brakes at this point would only cause the canopy to go into what I call an accuracy drop. Accuracy jumpers will occasionally dump the brakes which will drop the canopy straight down onto the target. Never done above 5 feet. If it is done at altitude the canopy will effectively be required to execute the inflation portion of the deployment. About 150 feet.

All canopies deploy faster with brakes set. Some tandem canopies don't require brakes but their openings are dis-orderly and slow, if you applied brakes to them they would open faster (to fast).

I ask you, would it not be better to have the canopy in deep brakes executing max lift while riding out the turbulence and be in the deployment or recovery mode if the separation of the boundary layer does occur? This seems safer and faster to recovery to me."

I Jumped with the guys who invented Skydiving.

[billvon 2,435]

>This means that the lifting component is strongest at that point or AoA.

Actually, it's not. If an aircraft is in level flight at an AoA of 2 degrees, vs an AoA of 13 degrees, the lift it generates is exactly the same. I think he means that at any given speed, increasing the AoA increases the lift (at least until the wing stalls.)

>When a rotor or turbulence hits the top of the canopy it can shear
>the boundary layer off of the surface.

The boundary layer isn't a "thing" that can be removed. It's a mathematical construct used to describe the natural tendency of air moving over a surface to be slowed by friction with the surface. Thus the air a micron from the surface isn't moving much at all, whereas the air a foot from the surface is moving along at almost normal speed.

On most wings flying normally, the boundary layer is a bit turbulent. (Layers that are not turbulent are called "laminar" and this is a very difficult flow to achieve; canopies certainly do not meet that criterion.) As the angle of attack increases, lift, turbulence and drag increase. The flow begins separating from the rear of the wing.

At some point, drag increases dramatically while lift levels out and starts decreasing. We call this a "stall." At this point the wing is no longer an efficient lifting body. A wing being flown at a high AoA (as in deep brakes) can enter a stall if a gust causes the airspeed over the wing to decrease suddenly. Since parachutes stall in a pretty soft manner, and recover by themselves, this isn't an issue unless it happens close to the ground.

>would it not be better to have the canopy in deep brakes executing
>max lift while riding out the turbulence . . .

Again, a canopy flying along a stable trajectory in deep brakes, full flight and front risers are all generating exactly the same lift - the amount that equals the exit weight. The difference is that the canopy in deep brakes can be stalled more easily by gusts. (It is also in a better position to _recover_ from such an event, which is why half brakes when the canopy has collapsed is a good idea.)

[mark 104]

Quote

John Sherman asked me to post this response.

"All pilots know that the point of greatest lift is just prior to an impending stall...

I ask you, would it not be better to have the canopy in deep brakes executing max lift while riding out the turbulence and be in the deployment or recovery mode if the separation of the boundary layer does occur? This seems safer and faster to recovery to me."

In other words, in turbulence you should fly as close to the stall point as possible. Is that what Mr. Sherman is saying?

Mark

[RMURRAY 1]

Quote

Quote

John Sherman asked me to post this response.

"All pilots know that the point of greatest lift is just prior to an impending stall...

I ask you, would it not be better to have the canopy in deep brakes executing max lift while riding out the turbulence and be in the deployment or recovery mode if the separation of the boundary layer does occur? This seems safer and faster to recovery to me."

In other words, in turbulence you should fly as close to the stall point as possible. Is that what Mr. Sherman is saying?

Mark

no, he is not. here is what is in the article. "We do, however, recommend that you fly near deployment brake configuration or depth"

Nancy is going to comment on this subject. She feels it needs urgent attention.

What I am hoping is that several industry experts can rethink what they have stated in the past and come up with a simple unified statement we can all go by in the future. I tend to feel immune to turbulance (highly loaded x brace) but tend to go to quarter brakes when it gets real bumpy close to the ground - just because I feel more in control and ready to act.

rm

[billvon 2,435]

>I tend to feel immune to turbulance (highly loaded x brace) . . .

Now THAT is a scary statement.

[pilotdave 0]

Quote

Actually, it's not. If an aircraft is in level flight at an AoA of 2 degrees, vs an AoA of 13 degrees, the lift it generates is exactly the same. I think he means that at any given speed, increasing the AoA increases the lift (at least until the wing stalls.)

Doesn't even need to be level flight, just unaccelerated. But if you're climbing or descending you get a vertical component of drag (and thrust) that complicates things. At high AOA, the coefficient of lift does increase (allowing the wing to generate the same amount of lift at a lower airspeed).

So basically I still don't get their argument for brakes. But they still might be right...

Dave

[RMURRAY 1]

yes, you are right - poor choice of words. anyway, this discussion is not about cross braced...

[phoenixlpr 0]

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"We do, however, recommend that you fly near deployment brake configuration or depth"

Eh, Worst possible choice? Its a really weak argument for flying in deployment breaks because canopies open/reinflate better in that configuration.

[pchapman 262]

Quoting from John Sherman's article:

Quote

The pressure on the top of the canopy aft of the thickest point is reduced to less than ambient.

Technically it is right, but the pressures are even lower closer to the front of the canopy. So by mentioning the thickest point at all, it shows a lack of understanding of aerodynamics.

Quote

The air going over the top is accelerated, ‘cause it has to go further, to keep up with the air going under the bottom. The further the air goes the greater the pressure

Wrong. One of the myths of aerodynamics. It doesn't matter what the air somewhere else is doing. There's no "keeping up" involved. I can't offhand teach the right way to describe it, but the air speeds up to get around the curved airfoil, and the pressure drops as the air speeds up.

Quote

tells us that the boundary layer gets stronger as we approach a stall. Why is this? It

Huh? I'm not sure what a "strong" boundary layer is. Approaching a stall the speed of the air in the boundary layer is particularly slow and about to break away from the airfoil, leading to large areas above the airfoil of disturbed turbulent airflow that isn't zooming across the airfoil in neat, straight, fast lines -- that would produce lift. A strong boundary layer to me would be one with extra energy injected into it, such as from vortex generators. Because there's extra energy and speed in the airflow, it would have less tendency to separate and go turbulent, and thus delay the stall to an even lower speed.

If turbulent air does have an effect on the boundary layer, which makes sense as it will mess up the smooth flow of air, I would not want to already be close to the stall, with an airflow about to break away from the airfoil and lose me a lot of lift.

I think Mr Sherman knows something about aerodynamics but isn't really current on synthesizing it all, and has let a few incorrect concepts creep in.

I did an aerospace engineering degree but am not in the business so am a bit rusty.

In addition, FWIW, I'll also side with billvon and pilotdave's posts.


For Mark about 'pressure gradient' vs ' boundary layer':

One has to be careful with the term because when aerodynamics types talk about 'the' pressure gradient, they usually mean the static pressures front to the rear of the airfoil, the pressures that help speed up or slow down air in its journey over the airfoil. I think you're describing the gradient of the dynamic pressure going outward -- the wind-on-hand type pressure.

So for your quote:

Quote

Is "boundary layer" the reason one feels little air flow near the door and progressively more as one exposes a finger or arm, or is it simply a function of surface area and lever arm?

Obviously it can be both depending on how far you climb out. But yes next to the fuselage, well away from the nose of a larger airplane, there could be, who knows, an inch or two of boundary layer where the air's speed has been slowed quite noticeably by friction over the aircraft, and expanded from a very thin layer up at the nose.

I recall tales of people on the catwalks on top of Zeppelins. Well back from the nose it wasn't very windy to crawl, somewhat windy if crouching, and quite hard to stand up and walk, as one stuck oneself more and more out of the slowest parts of the boundary layer which was a few feet thick. The layer slowly grew over the length of the large aircraft. In other words, if a door were available, doing camera slot from the side of a Zeppelin would be easy.

[Samurai136 0]

http://www.aviation-history.com/theory/lift.htm


The article covers the basic physics that apply:

"To begin to understand lift we must return to high school physics and review Newton’s first and third laws. (We will introduce Newton’s second law a little later.) Newton’s first law states a body at rest will remain at rest, and a body in motion will continue in straight-line motion unless subjected to an external applied force. That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, there is a force acting on it. Newton’s third law states that for every action there is an equal and opposite reaction. As an example, an object sitting on a table exerts a force on the table (its weight) and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction."

And it explains the boundary layer and laminar flow of air around the wing:

"The natural question is "how does the wing divert the air down?" When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface. To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass as is shown in figure 8. This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton’s first law we know that for the fluid to bend there must be a force acting on it. From Newton’s third law we know that the fluid must put an equal and opposite force on the object that caused the fluid to bend.

Why should a fluid follow a curved surface? The answer is viscosity: the resistance to flow which also gives the air a kind of "stickiness." Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. The relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car and why there is dust on the backside of the fans in a wind tunnel.) Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached (note that this occurs in less than an inch). Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer"."

"Buttons aren't toys." - Trillian
Ken

[strop45 0]

My experience of flying a canopy in turbulence is very limited, one very uncomfortable flight through a high (1000-2500') layer of wind shear.

Not directly comparable, but I fly model sailplanes on the slope where turbulence and being close to the ground are normal. The turbulence is mainly associated with high wind speeds, but is also generated by terrain and thermal activity. Flying models suggests to me that stalls and lack of control are the things most likely to cause you to hit the ground at high speed and that generally speed is your friend through turbulence.

Therefore full flight seems like a better option than quarter brakes in turbulence, since it reduces the risk of a stall caused by adverse wind speed/direction across all or part of the canopy. It also gives you more toggle range to deal with the turbulence, i.e. if the gust creates more lift, you don't need to do anything but hang on (and wait for the corresponding sink), if it reduces your lift, you may need to use the toggles to keep the canopy flying.

As usual, comments from more experienced canopy pilots appreciated.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein