Interpretation

[theshortbus 0]

I´m reading "The parachute and it´s pilot" at the moment, and there is this one bit I´m having trouble to understand. Maybe someone can help me. I really want to understand what I´m reading. I I don´t there is little point reading it
I quote: "The pitch does in fact change somewhat as a result of rear riser input, but much more slowly than in brake application. This is why it is far easier to "stall" the wing with rear risers, even at high air speeds. The angel of attack can be changed quicker, without immediately changing the direction of flight. The slower increase in angel of attack during brake input is due to the fact that the suspended weight must move several feet forward to make a significant impact on the angel of attack"

I am confused. As i read it, it sounds like rear riser input changes the angel both quicker and slower at the same time. It says in the beginning that it changes the pitch slower, and then it says that it increases the angel of attack quicker? Isn´t the "pitch" the angel of attack?? Witch is it? Does the wing stall quicker with slower applications?

Would really help if someone gave me a short explenation.

Thanks a lot!

Party ´til impact!

[diablopilot 2]

I'd hate to put words in Brian's mouth, but.....

When using brakes there is a dramatic reduction in the speed the canopy is flying, this causing the suspended weight (you) to swing foward, affecting the pitch of the system.

With rear risers the speed change is much more gradual, however you are re-trimming the wing without causing the suspended weight to swing forward so radically. This immediately changes the AOA relative to the airflow prior to the riser input. If done suddenly without allowing the system to come back towards equilibrium, this can cause a violent stall.

Of course this is just my interpretation.....

----------------------------------------------
You're not as good as you think you are. Seriously.

[quade 3]

I have an alternative view point about this. Take it for what you will.

Generally speaking and talking about "normal" canopies, when a pilot pulls down on his brake lines only a portion of the trailing edge of the wing is distorted; the outside. While the altered outer sections of the wing have changed their AoA significantly as a result of brakes being deployed, the center section of the wing is, more or less, still its original shape and continues to create lift up to an AoA of about 13 degrees. The upshot of this is that the outer sections of the wing will stall first, but the center section still provides a bit of lift.

When stalling the wing using rear risers, the entire trailing edge has been distorted from "normal" and, more or less, the entire wing stalls at once.

quade -
The World's Most Boring Skydiver

[GLIDEANGLE 1]

Funny... I was struggling with that very same paragraph a few hours ago.... it is still greek to me!

The choices we make have consequences, for us & for others!

[theshortbus 0]

I think it´s because I, for my part, read pitch as AoA. It kind of is in a way also. But given the replies here I think I get it.

So to add a question - is a there any good reason for rear riser input instead of "brakes"? If "brakes" are an option that is? Will it cause reduced air speed without so much infliction of the AoA?

....and, how serious/dangerous is a full "stall"? collapse?

Party ´til impact!

[StreetScooby 5]

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....and, how serious/dangerous is a full "stall"? collapse?

Depends on how close to the ground you are

We are all engines of karma

[DougH 270]

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Depends on how close to the ground you are

Or what you do in reaction to it!

"The restraining order says you're only allowed to touch me in freefall"
=P

[quade 3]

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I think it´s because I, for my part, read pitch as AoA.

Pitch (aka attitude) is the angle between the horizontal plane and the wing.

Angle of Attack (AoA) is the angle between the relative wind and the mean cord line of the wing.

The two are not connected in any way shape or form except sometimes by coincidence during straight and level flight.

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So to add a question - is a there any good reason for rear riser input instead of "brakes"? If "brakes" are an option that is? Will it cause reduced air speed without so much infliction of the AoA?

A judicious use of rear risers can be used to reshape the wing slightly and improve its glide ratio.

quade -
The World's Most Boring Skydiver

[KrisFlyZ 0]

The relative wind is coming at the glide angle. The angle of attack(AoA) of a glider with a glide ratio of L/D is

AoA = ArcTan(D/L) - pitch

pitch in the equation above is measured below the horizon.

The canopy is trimmed nose down so that the AoA is less than the stall angle. Pitch is determined by the difference in line lengths in the front and the back of the canopy and the canopy's chord length. While giving the canopy different riser inputs, we are changing the pitch and consequently, the angle of attack.

While pitch and AoA are not directly related, pitch is the input we can give(while using risers) to that equation above and consequently change AoA.

Why can we stall the wing easily with the risers? Because the pitch can be reduced drastically by yanking down(a 10 degree change in the pitch can be affected by pulling down the risers on a Sabre2 170(chord length 7.2ft) by 15 inches) on the risers and increase the AoA beyond the stall point.

Using brakes does not change pitch but changes L/D by changing wing shape.

Decrease AoA = increase pitch and reduce L/D == pull on front risers

Increase AoA = Decrease Pitch (and possibly increase L/D) == pull on rear risers

Applying brakes = change L/D.


Kris.

[quade 3]

Uh . . . maybe we have a language barrier issue here.

I get my definitions from NASA and the FAA.

quade -
The World's Most Boring Skydiver

[KrisFlyZ 0]

Maybe I am missing something here( when I say pitch I meant pitch of the wing only...not the system) but...

I think the definitions we have used are the same.

Pitch and Angle of Attack

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The two are not connected in any way shape or form except sometimes by coincidence during straight and level flight.

Level flight is just a special case AoA = Angle of incidence and pitch is zero, so is gliding flight, the freestream velocity is coming at the glide angle. If the wing of the canopy was horizontal, AoA is equal to the glide angle.

2.5 to 1(generally aceepted as the glide of a modern square parachute ) is an Anglw of Attack of 22 degrees. If the glide is less than that the angle of attack is more. That is why the wings on all the canopies point downward by atleast 5 degrees.

Kris.

[quade 3]

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2.5 to 1(generally aceepted as the glide of a modern square parachute ) is an Anglw of Attack of 22 degrees.

Lemme just state flat out that there is no way in hell the AoA of a parachute in straight, level flight is 22 degrees.

Somewhere, somehow, you're using a significantly different way of figuring AoA than is accepted by aeronautical engineers or you have an incomplete understanding of the principles involved.

22 degress AoA is well beyond the critial AoA for most rigid wings, which cap out around 15.

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

quade -
The World's Most Boring Skydiver

[pilotdave 0]

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Level flight is just a special case AoA = Angle of incidence and pitch is zero

Well, that's not really true... at least not necessarily. You can be in level flight at any angle of attack. Take a parachute as an example... angle of incidence is always going to be negative (the nose is lower than the tail), but the angle of attack is always going to be positive (otherwise it'd collapse). Pitch is a function of angle of attack and angle of incidence... could be anything.

Dave

[billvon 2,463]

>Level flight is just a special case AoA = Angle of incidence and pitch is
>zero . . .

Pitch is generally not zero in any aircraft except under specific conditions i.e. high speed cruise in a powered aircraft. At low speeds, the pitch is higher than that to maintain enough lift to keep the plane in the air.

>2.5 to 1(generally aceepted as the glide of a modern square parachute )
>is an Anglw of Attack of 22 degrees.

A parachute with an AoA of 22 degrees has stalled.

>That is why the wings on all the canopies point downward by atleast 5 degrees.

They point downwards because canopies rely on canopy trim to set their pitch; they are so stable in pitch that you cannot just use elevators or their equivalent to control pitch of the canopy. And in unpowered flight a canopy must descend to maintain forward speed. The angle of flight is somewhat higher than the trim (pitch) angle; angle of flight (glidepath, or L/D) equals angle of attack plus trim angle.

[KrisFlyZ 0]

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2.5 to 1(generally aceepted as the glide of a modern square parachute ) is an Angle of Attack of 22 degrees. If the glide is less than that the angle of attack is more. That is why the wings on all the canopies point downward by atleast 5 degrees.

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Lemme just state flat out that there is no way in hell the AoA of a parachute in straight, level flight is 22 degrees.

You are only reading parts of what I wrote. I did not say that a parachute flying at 2.5 to 1 has an angle of attack of 22 degrees. The glide angle is 22 degrees. if the parchutes wing had a zero pitch, the angle of attack would be 22 degrees but...

The wing of the parachute points downward to reduce the Angle of Attack. A Sabre2 135 has a chord of 7.23 ft...if the rear lines(brakelines without any slack) are longer than the front lines by 15 inches that is a 10 degree reduction in Angle of Attack.

Parachutes are low aspect ratio wings(typically AR of less than 3) and we cannot use FoilSim for everything. Foilsim does not take into account vortex lift from leading edge seperation. My understanding is that this delays stall.

Check out this link.

The following sections are quoted from the link above. While there is not a number for the angle at which wings of 2< AR < 3 stall, it does illustrate one important point.

Not all wings stall at 15 degrees or thereabouts.

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We define a low aspect-ratio wing as a wing having AR < 5. Squared wings and wings with even lower aspect ratios are still interesting from a theoretical point of view, but of uncertain application.

Theoretical analysis was performed by Betz in the 1920s. Later a number of non-linear lifting line theories (Multhopp, Gersten, Truckenbrodt, and others) provided simple means to predict the vortex lift created by the leading edge separation.

Short wings for low speeds have been studied for a long time. The high angle of attack stall of the low aspect-ratio wings has been known since the early days of aerodynamics. Handley-Page (1911) found that a squared wing (AR=1) stalled at angles above 40 degrees, while a moderately slender wing (AR=6.25) stalled at incidences as low as 10 degrees.

The values of the maximum lift coefficient are largely independent from the aspect-ratios at aspect-ratios above 2 and a Reynolds number 1 million. At aspect-ratios 1 < AR < 2 the most evident effect is the shift of the angle of attack at which maximum lift is reached. This angle increases progressively and easily reaches 30 degrees.

[KrisFlyZ 0]

I am a bit confused...Are you talking about pitch of the canopy + jumper as a whole? Because pulling down on the risers is definately changing the pitch of the wing( just the airfoil part of the canopy...no lines, no jumper).

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angle of flight (glidepath, or L/D) equals angle of attack plus trim angle.

or
angle of attack = angle of flight - trim angle

Which is what I have been saying.

Kris.

[KrisFlyZ 0]

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Level flight is just a special case AoA = Angle of incidence and pitch is zero

Well, that's not really true... at least not necessarily. You can be in level flight at any angle of attack. Take a parachute as an example... angle of incidence is always going to be negative (the nose is lower than the tail), but the angle of attack is always going to be positive (otherwise it'd collapse). Pitch is a function of angle of attack and angle of incidence... could be anything.

Dave

Yes, for level flight at zero pitch, AoA = AoI.


pitch = angle of incidence for the wing of the parachute. Note that I have not included the lines or the jumper. However, if the whole system changed pitch, the pitch of the wing would have a direct relation to the pitch of the system.

If the pitch of the system was 5 degrees below the horizon. The pitch of the wing would be increased 5 degrees and so on...when there is no slack in the lines, the geometry of the system is maintained.

Angle of Attack is then a function of glide angle and pitch of the wing or the pitch of the system.

Kris.

[AFFI 0]

So all this is going to help my canopy piloting how?

[StreetScooby 5]

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So all this is going to help my canopy piloting how?

Don't stall close to the ground.

We are all engines of karma

[quade 3]

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So all this is going to help my canopy piloting how?

I'm no expert in such things, but I think knowing the difference between using brakes and using rear risers might be a bit of a help in canopy piloting.

quade -
The World's Most Boring Skydiver

[AFFI 0]

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I'm no expert in such things, but I think knowing the difference between using brakes and using rear risers might be a bit of a help in canopy piloting.

Yeah, understood – but all the equations are confusing to me.

I keep it simple when I am teaching, and for me cause I am not too bright.

Using a toggle creates drag along the trailing edge of the canopy; rear riser input is more sensitive because you are using more surface area of the canopy right?
-

Mykel AFF-I10
Skydiving Priorities: 1) Open Canopy. 2) Land Safely. 3) Don’t hurt anyone. 4) Repeat…

[mr2mk1g 10]

Actually I don't think that's quite what Brian's getting at.

Using rear risers directly adjusts the angle of the wing to the relative wind. As such, if you go too far with this direct adjustment you can stall the wing out very easily.

Using brakes isn't a direct adjustment to the angle of the wing to the relative wind. It simply adds drag to the canopy which slows the wing itself down while you continue to travel at the same speed as you were before pulling on the brakes.

As the wing moves rearward relative to your body it's angle to the relative wind gradully changes, thus reducing the risk of you inducing a dynamic stall.

The sensitivity of rear risers comes because you are directly controlling the angle of attack of the canopy by pulling on them. With toggles you don't, you merely causing a system to indirectly alter the angle of attack, which takes time.

At least... that's my take on it. I guess maybe someone needs to PM Brian...

[quade 3]

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You are only reading parts of what I wrote. I did not say that a parachute flying at 2.5 to 1 has an angle of attack of 22 degrees.

I dunno man, to me there is simply no other way to interpret THIS sentance, "2.5 to 1(generally aceepted as the glide of a modern square parachute ) is an Angle of Attack of 22 degrees." In it, you've CLEARLY said it is.

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Not all wings stall at 15 degrees or thereabouts.

I never said they did. I said, "22 degress AoA is well beyond the critial AoA for most rigid wings, which cap out around 15." Which is actually true. MOST do.

As I've said before, I think you have an incomplete understanding of the basic definitions and priciples involved. Quoting texts from other people isn't going to change my opinion about that when you continue to write on your own things such as, "The wing of the parachute points downward to reduce the Angle of Attack."

The angle of incidence actually has nothing to do with the AoA.

Angle of incidence is the relationship of the wing to the rest of the "aircraft".

AoA is the angle between the relative wind and the mean chord line of the wing.

The two are not connected in any way, shape or form.

quade -
The World's Most Boring Skydiver

[quade 3]

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Actually I don't think that's quite what Brian's getting at.

Using rear risers directly adjusts the angle of the wing to the relative wind. As such, if you go too far with this direct adjustment you can stall the wing out very easily.

Using brakes isn't a direct adjustment to the angle of the wing to the relative wind. It simply adds drag to the canopy which slows the wing itself down while you continue to travel at the same speed as you were before pulling on the brakes.

As the wing moves rearward relative to your body it's angle to the relative wind gradully changes, thus reducing the risk of you inducing a dynamic stall.

The sensitivity of rear risers comes because you are directly controlling the angle of attack of the canopy by pulling on them. With toggles you don't, you merely causing a system to indirectly alter the angle of attack, which takes time.

At least... that's my take on it. I guess maybe someone needs to PM Brian...

I think you understand what Brian is trying to get at, but there is a minor point I'd like to make here;

Using the brakes does directly change the AoA for those portions of the parachute affected by the brake lines. It increases both the lift and drag for those portions of the wing by reshaping them. This is roughly analogous to deploying flaps on an aircraft wing.

I agree with your statement that using rear risers, more or less, changes the relationship of the entire wing to the relative wind.

quade -
The World's Most Boring Skydiver

[d123 1]

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I think you understand what Brian is trying to get at, but there is a minor point I'd like to make here;

Using the brakes does directly change the AoA for those portions of the parachute affected by the brake lines. It increases both the lift and drag for those portions of the wing by reshaping them. This is roughly analogous to deploying flaps on an aircraft wing.

I agree with your statement that using rear risers, more or less, changes the relationship of the entire wing to the relative wind.

Sorry for puting more wood on the fire but ...

If I understand correctly there is a BIG diff between flaring a canopy with toggles and rear risers. I was thinking that everything just scales down from toggles input to rears input. Like if 0 is no toggle input (just remove the slack from the steering lines) and if 100 is the stall point with toggle input and if the flaring sweet spot is around 40 I've imagined that the input distance gets smaller, when moving from toggles to rears input but the ratios remains the same. My assumption was not true, wasn't it?

In the next season I was planing of doing some rear risers landings. Now I'm thinking that this manoeuvre is for experienced pilots only. Is it safe for someone like me to try a rear-riser landing? If is not recomanding to do so with my limited experience, in your oppinion is it better to cut away when I have a steering line broken?

Does canopy control lessons covers in practice those aspects?

Cheers,
Jean-Arthur Deda.

Lock, Dock and Two Smoking Barrelrolls!