This "Enter the pattern at 1000 feet" concept we teach

[AggieDave 6]

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If people are using 1000 AGL downwind and abeam the target, then that is, in my opinion, too high.

I teach students about the changes in the patterns when you have winds vs. no winds. It starts on the first tandem when I teach them a landing pattern. I continues with the second when I purposefully screw up the pattern to teach them about plans and reality and how to do things safely. It continues in the transistion course when I teach them that with our canopies, every 300ft of altitude equals roughly 520ft of distance across the ground (in perfect, no thermal, no wind, etc conditions).

I don't think the 1000' is too high, I think the training isn't good enough!

--"When I die, may I be surrounded by scattered chrome and burning gasoline."

[lawrocket 3]

I learned it, and it worked pretty well. For me, the only thing that took some gettign used to is the "Gee, I seem pretty far from the target right now" factor which would make me cut short my downwind leg and land long.

Other techniques then allowed me to make the needed adjustments to that, and my last three landings were dead in the X.

My wife is hotter than your wife.

[cvfd1399 0]

When the wind is 10-14 mph I usually go on final at 1000. I make a few "saches" (sp.) to tweek it, but overall my canopy does not get any forward speed. I usually get out first so once I open I am flying up wind jump run to the LZ. It is a 370sf loaded at .83. I am usually last to land, so I am not interfering with the pattern, and if so I either merge, or land on the edge of the LZ.

[Shark 0]

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Other techniques then allowed me to make the needed adjustments to that, and my last three landings were dead in the X.

Except that one by the motocross track...
Oh, wait! That was with me!

Actually that was a very good approach and landing that you did. The MX'ers enjoyed our demo.

[FallinWoman 1]

One of the things that Gary and I have talked about is the number lock that some studetns seem to get. Those of us who know how to land just do it....whether it is entering the pattern at 1000ft or lower. I think that students entering this high puts them well above the other jumpers, and that is not safe.

I guess my question then is how to teach some sort of benchmark numbers iwhtout having students become fixated on those numbers. The numbers do change based on conditions...how does one teach that?

~Anne

I'm a Doll!!!!

[masher 1]

I was taught to be at 1000' at 45deg from my target. From that point, I start my downwind run.

--
Arching is overrated - Marlies

[PhillyKev 0]

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Please correct me if I'm wrong, but doesn't the Spectre have a flatter glide ratio than the Sabre2? I dunno, just thought I heard that somewhere.

Marketing may say that it does, my experience says that it doesn't.

-
Jim

I just read that in the other thread. I call BS. I flew both at the same wingloading a lot. There's no way.

[JohnMitchell 14]

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don't want to prove you wrong, but AFAIK the glide ratio is determined for/from certain conditions :
altitude
pressure
wingloading
temperature
humidity
etc etc

if anyone with better knowledge could give precision...

I've always heard glide ratio is the L/D ratio, lift to drag ratio. All those things you list affect true airspeed, but the lift to drag ratio should remain fairly constant within certain parameters. If you get to the extremes of the curves with wingloadings, you will get some variation. We need Kallend in here with his math skills to tell us the right answer.

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[piisfish 136]

sorry, might be a difference in understanding, I thought glida ratio was the equivalence of the french FINESSE, which is the ratio of units of horizontal distance travelled per unit of vertical distance lost...
for example when you go down 1 meter and you travel 4 meters horizontally, you have a finesse of 4

scissors beat paper, paper beat rock, rock beat wingsuit - KarlM

[larsrulz 0]

I'm not sure if I'm good enough as Kallend, but I am getting my master's in aerospace engineering not too far from him.

Issue #1: Glide ratio refers to the horizontal distance traveled for any given unit of vertical decent. So a parachute with a glide ratio of 4.5:1 moves 450 feet forward (in a no wind condition) for every 100 feet it drops.

Issue #2: As for lift to drag ratio, this is (related to) the efficiency of a rigid wing. A parachute does not work the same way in which a aircraft wing works. I'm still personally uncertain about what force causes a (square) parachute to "fly," but (I believe) the majority of the force causing flight is air deflection as opposed to pressure differential. A non-airlocked canopy (airlocked don't help that much in this instance anyway...but) does not have enough rigidity to resist the force of lift if it was due to a pressure differential.

Issue #3: Back to your original question, those listed parameters do not directly effect the glide ratio. The glide ratio is effected by the trim and airspeed of the canopy. I assume everyone has noticed that their canopy levels off after releasing it from a front riser dive (hence how one makes pretty swoops) while a canopy will make a steep dive after releasing it from full breaks (hence why you don't do full breaks at 50'). Now those parameters listed by piisfish effect the speed at which a canopy will fly, which effects the glide ratio of the canopy flight itself.

So yes, piisfish is right that those parameters affect glide ratio, but they do not effect it directly; rather the airspeed is effected by those parameters and hence alters the glide ratio.

I got a strong urge to fly, but I got no where to fly to. -PF

[billvon 2,476]

>Issue #1: Glide ratio refers to the horizontal distance traveled for any
>given unit of vertical decent. So a parachute with a glide ratio of 4.5:1
>moves 450 feet forward (in a no wind condition) for every 100 feet it drops.

Agreed there . . .

>Issue #2: As for lift to drag ratio, this is (related to) the efficiency of a
> rigid wing.

Also agreed; indeed, sailplanes rate their efficiency in L/D, which in turn corresponds directly to best glide ratio. A sailplane with a max L/D of 40 can cover 40 miles if released from 5280 feet.

> but (I believe) the majority of the force causing flight is air deflection as
>opposed to pressure differential.

Those two are the same thing. Any normally flying wing deflects air downwards; you can use newtonian physics to prove the aircraft is exerting a downwards force on the air around it that is exactly equal to its weight. Another way to explain this is that there is a pressure differential between the bottom and top of the wing that keeps the plane in the air. This is because the wing is forcing air downwards, and thus generates more pressure below the device that's forcing air downwards (the wing) than above it.

Bernoulli is often a source of much debate because his theory (i.e. "the air flowing over the top of the wing is going faster so there's less pressure" etc) seems to be in contradiction to a more newtonian view of lift. But they are really all parts of the same effect.

>Now those parameters listed by piisfish effect the speed at which a
>canopy will fly, which effects the glide ratio of the canopy flight itself.

It's odd, but loading an aircraft more heavily will not generally affect its L/D. However, we're not aircraft, and if a bigger person jumps the same canopy he's also increasing the drag of the system because his body is just plain bigger and more draggy.

[larsrulz 0]

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> but (I believe) the majority of the force causing flight is air deflection as
>opposed to pressure differential.

Those two are the same thing. Any normally flying wing deflects air downwards; you can use newtonian physics to prove the aircraft is exerting a downwards force on the air around it that is exactly equal to its weight. Another way to explain this is that there is a pressure differential between the bottom and top of the wing that keeps the plane in the air. This is because the wing is forcing air downwards, and thus generates more pressure below the device that's forcing air downwards (the wing) than above it.

Not exactly, an aircraft wing works because the path over the wing is longer than that under the wing. The means that a little block'o'air going over the wing must go quicker than its buddy who goes under. The block'o'air going quicker has a lower pressure which causes that block'o'air to want to displace the higher pressure buddy above the wing. This causes the fluid (air) around the wing to lift the wing up, which is basically how lift works. What I was talking about was the fact that (I believe) parachutes do not work like rigid wings work. If a pressure differential produced lift in the case of a parachute, this would cause the canopy to collapse from the bottom up, which obviously doesn't happen. What (I believe) occurs is that a parachute is deflecting the airflow, so newton's laws are what is keeping the parachute aloft, as opposed to those of bernoulli.

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Bernoulli is often a source of much debate because his theory (i.e. "the air flowing over the top of the wing is going faster so there's less pressure" etc) seems to be in contradiction to a more newtonian view of lift. But they are really all parts of the same effect.

I'm a little confused by the above comment. But fluid mechanicians agree that bernoulli's theories of dynamic pressure are correct. They don't contradict any of newton's laws...care to explain what you mean here?

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It's odd, but loading an aircraft more heavily will not generally affect its L/D. However, we're not aircraft, and if a bigger person jumps the same canopy he's also increasing the drag of the system because his body is just plain bigger and more draggy.

See, that's a completely different issue all together. Loading a glider more heavily DOES change its L/D, albeit indirectly like a parachute. The only thing that affects an objects L/D or glide ratio is the airspeed at which that object is moving. An aircraft has a variable thrust which can keep that airplane at whatever speed it needs to be flown at no matter its payload. A parachute is under the constant "thrust" of gravity. When there is more force (forward and downward) due to a more heavily loaded canopy, the thrust to keep the canopy moving forward does not change. This means the trim condition is such that the speed for trimmed flight is higher. I've kind of lost track at where I was going with this, but rereading your reply...I wholeheartedly approve of the use of "more draggy."

I got a strong urge to fly, but I got no where to fly to. -PF

[quade 3]

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Not exactly, an aircraft wing works because the path over the wing is longer than that under the wing.

(quade bangs head agaisnt wall)

Stop, please . . .

While that is one way to model what is happening, it is not the entire explanation.

For a slightly more complete explanation that applies to rigid, rotary and even flexible wings . . . go here.

quade -
The World's Most Boring Skydiver

[billvon 2,476]

>Not exactly, an aircraft wing works because the path over the wing is longer
>than that under the wing.

That's what I teach my FJ students, but it's not _quite_ true. If it were true, airplanes with uncurved wings, or symmetrically curved wings, could not fly. Yet 59 cent balsa airplanes with perfectly flat wings still fly, and even a normally cambered wing can fly upside down. So "the air goes faster over the curved surface and exerts less pressure" is an incomplete answer - although easy to understand, and useful to explain to first jump students why a parachute must have airspeed to fly.

> If a pressure differential produced lift in the case of a parachute, this
>would cause the canopy to collapse from the bottom up, which obviously
>doesn't happen.

Correct. The lines prevent the canopy from collapsing from "the bottom up." (Note that if the AOA changes so much that the top is loaded, the wing collapses instantly - there is no rigidity in the other direction.)

>What (I believe) occurs is that a parachute is deflecting the airflow, so
>newton's laws are what is keeping the parachute aloft, as opposed to
>those of bernoulli.

I agree that Newtonian physics apply here - but I think you will find in the final analysis that Bernoulli and Newton are explaining exactly the same thing in different ways. Circulation, the Kutta-Joukowski theorem, the venturi effect, Newtonian mechanics - all provide the same answer, just give you different tools to get there.

>But fluid mechanicians agree that bernoulli's theories of dynamic
> pressure are correct. They don't contradict any of newton's laws...care
>to explain what you mean here?

You gave a good explanation right there. Neither contradicts the other, because both are explaining the same thing in different ways.

> Loading a glider more heavily DOES change its L/D, albeit indirectly
>like a parachute.

From a student manual for glider pilots:

---------------------
Effects of Water Ballast

The purpose of adding water ballast is to increase glide performance at higher speeds. A sailplane has a best glide ratio at a specific speed - which is weight dependent. Fly at any other speed and your glide ratio will be less. A glider and pilot may have a best glide ratio of 40:1 (for instance) at 50 knots. Add 400 pounds of water and that 40:1 (pronounced "forty to one") is now achieved at around 60 kts.

For the techies among us, the performance speeds (stall, maneuvering, best glide, etc.) vary with the square root of the weight change ratio. If a 750 pound plane and pilot gets its best glide ratio of 40:1 at 50kts, adding 400 pounds water changes the weight to 1150, and so the new best glide speed is sqrt(1150/750) times 50 = 61.9kts. The best glide ratio of 40:1 stays the same!
-----------------------

It's counterintuitive that adding weight does not change the L/D, but it's true. People are sometimes confused because a heavier aircraft DOES require more power to stay aloft, but that's due to basic laws of power and motion (the same force against a medium that's moving faster requires more horsepower to maintain that force.)

[PhillyKev 0]

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Not exactly, an aircraft wing works because the path over the wing is longer than that under the wing. The means that a little block'o'air going over the wing must go quicker than its buddy who goes under. The block'o'air going quicker has a lower pressure which causes that block'o'air to want to displace the higher pressure buddy above the wing. This causes the fluid (air) around the wing to lift the wing up, which is basically how lift works.

For that to be 100% correct, then the velocity of the fluid moving over the wing should be able to be accurately predicted using Bernoulli's formula. However, in reality, the velocity of fluid over a lifting wing is much higher than what Bernoulli's formula would spedify.

The explanation for that is Newton's law on action/reaction forces which deals with deflection of air, in effect push from the underside, rather than being sucked upward by lower pressure as Bernoulli suggests. The implementation of Newton's law has a great deal to do with angle of attack. In fact, changing angle of attack alone changes lift which in effect, negates the Bernoulli formula as being the sole contributor to lift.

It's actually a little of both.

[larsrulz 0]

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That's what I teach my FJ students, but it's not _quite_ true. If it were true, airplanes with uncurved wings, or symmetrically curved wings, could not fly. Yet 59 cent balsa airplanes with perfectly flat wings still fly, and even a normally cambered wing can fly upside down. So "the air goes faster over the curved surface and exerts less pressure" is an incomplete answer - although easy to understand, and useful to explain to first jump students why a parachute must have airspeed to fly.

Yes, it's true that wings needn't be curved, but Bernoulli's theory still explains why a balsa wood airplane flies. The weight is in place in such a way that the aircraft trims itself for best glide. If you take a flat piece of balsa and place it horizontally in a wind tunnel, absolutely no lift will be produced. Once you make an AOA w.r.t. the relative wind in the tunnel, then the pressure on top of the "wing" decreases and lift is produced. I was merely using a standard GA wing because it best explains why a wing with an AOA equal to zero (steady and level flight) can continue to stay aloft in such a configuration. Look at an aircraft tail (which has no camber), without deflection of the rudder there is no yawing occuring. As soon as you deflect the rudder, the airflow "inside" the yaw is slower and hence of higher pressure, which pushes the tail out.

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I agree that Newtonian physics apply here - but I think you will find in the final analysis that Bernoulli and Newton are explaining exactly the same thing in different ways. Circulation, the Kutta-Joukowski theorem, the venturi effect, Newtonian mechanics - all provide the same answer, just give you different tools to get there.

Kutta-Joukowski and Bernoulli fundamentally explain lift in the same way, one approaches it from a circulation standpoint, one from a dynamic pressure standpoint. What we are talking about here (with Bernoulli's theory) is basically venturi effect, yes. Now simple Newtonian mechanics themselves do not explain lift. They are the basis for continuum mechanics and hence can be used to apply conservation of energy and prove Bernoulli's theory, but they alone do not explain lift, i.e. you can't sit there and just say "F=ma explains why an airplane flies."

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You gave a good explanation right there. Neither contradicts the other, because both are explaining the same thing in different ways.

In that both theories are derived from continuum mechanics, i.e. conservation of energy. They don't explain lift the same way

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It's counterintuitive that adding weight does not change the L/D, but it's true. People are sometimes confused because a heavier aircraft DOES require more power to stay aloft, but that's due to basic laws of power and motion (the same force against a medium that's moving faster requires more horsepower to maintain that force.)

If it sounded like I was saying it directly alters the L/D, that is not what I meant. Altering the weight alters the speed a glider flies out, as there is no other way to alter the speed of a glider (as gravity is constant). This means that changing the payload changes the speed which changes the glide slope. You idealy want to ballast a glider for best glide.

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(quade bangs head agaisnt wall)

Stop, please . . .

While that is one way to model what is happening, it is not the entire explanation.

Bernoulli's theory in my above response explains how lift is produced. No, it isn't the entire explanation of how an aircraft flies. But go ask an aerospace academic the equation for lift and they'll give you L = q*S*Cl, where q is dynamic pressure, S is planform area, and Cl is coefficient of lift. Even a barn door, inverted wing, or hanging-body-under-parachute will have a coefficient of lift, so this equation will give the lift force due to any object through any fluid.

I got a strong urge to fly, but I got no where to fly to. -PF

[quade 3]

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L = q*S*Cl

Um, you mean like this?

quade -
The World's Most Boring Skydiver

[riggerrob 563]

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I'm not sure if I'm good enough as Kallend, but I am getting my master's in aerospace engineering not too far from him.

Issue #1: Glide ratio refers to the horizontal distance traveled for any given unit of vertical decent. So a parachute with a glide ratio of 4.5:1 moves 450 feet forward (in a no wind condition) for every 100 feet it drops.

Issue #2: As for lift to drag ratio, this is (related to) the efficiency of a rigid wing. A parachute does not work the same way in which a aircraft wing works. I'm still personally uncertain about what force causes a (square) parachute to "fly," but (I believe) the majority of the force causing flight is air deflection as opposed to pressure differential. A non-airlocked canopy (airlocked don't help that much in this instance anyway...but) does not have enough rigidity to resist the force of lift if it was due to a pressure differential.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

You have had your head in a textbook too long!

When you get out in the real world, you will find that you have to apply huge "fudge factors" to all your aerodynamic formulas.
The study of air flow is at best a crude science, in part because parachutes are really crude air foils. They get away with huge inaccuracies because of their low wing loadings.

"Glide ratio" and "L/D" mean the same in practical terms.

Bernoulli has a pretty theorum, however, airflow over the top skin of most fabric wings (i.e. parachutes) is so crude that you cannot maintain laminar flow more than 10 or 15% (of the chord) aft of the leading edge.

However, I tend to side with Newton.
Since ram-air parachutes have such rough top skins, I believe that most of their lift derives from the bottom skin deflecting air downwards.
To illustrate my point, I suggest that you watch the film "Pushing Tin" or stand in the bowl when your buddies are landing.
Or read what your textbooks have to say about wing tip design affecting lift to drag ratios.

Many companies (i.e. Atair, Barrish, Handbury Irvin, Pioneer, etc.) have experimented with single skin skydiving canopies (V-rib, Sailwing, Paradactyl, Delta II, Hornet, etc.). They all reported that single-skin canopies flew well, except in turbulence.
Which brings us to my theory the top skins on ram-air canopies have little aerodynamic function and are primarily structural members, trapping air to stiffen the canopy in turbulence.

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[larsrulz 0]

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You have had your head in a textbook too long!

Come on, I hardly ever purchase the textbooks for class, let alone actually read them.

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When you get out in the real world, you will find that you have to apply huge "fudge factors" to all your aerodynamic formulas.
The study of air flow is at best a crude science, in part because parachutes are really crude air foils. They get away with huge inaccuracies because of their low wing loadings.

My two paragraphs you are responding to don't really mention all that much about any sort of aerodynamic formulas. The majority of aerodynamic formulas are empirical anyway. As for airflow in the case of parachutes, all theories go out the window in my opinion, and it's due to many other things other than low WL.

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"Glide ratio" and "L/D" mean the same in practical terms.

Yes and no, but mostly yes.

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However, I tend to side with Newton.
Since ram-air parachutes have such rough top skins, I believe that most of their lift derives from the bottom skin deflecting air downwards.

Read my original reply: "A parachute does not work the same way in which a aircraft wing works. I'm still personally uncertain about what force causes a (square) parachute to "fly," but (I believe) the majority of the force causing flight is air deflection as opposed to pressure differential." So I do agree with you.

Quade, in response to your clicky, I don't believe lift equations should be spat out at FJ students...hell I recently wouldn't even go into this with an instructor during a [A and B license] canopy class he was teaching. None of this is necessary for a student nor PSTer to understand. My comments are no longer in response to Gary's original question about the teaching for pattern issues, they were merely disputing and explaining the second half of the discussion.

I got a strong urge to fly, but I got no where to fly to. -PF

[quade 3]

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Quade, in response to your clicky, I don't believe lift equations should be spat out at FJ students.

Which is why I don't use them for most things.

That said, I, being a former airplane flight instructor, do have all of that in the back of my head at all times.

Most people's heads would explode if you dragged out the formulas for lift.

For most of what people need to be able to fly their canopies (or airplanes for that matter), the typical models will suffice. That said, if someone says they know how aerodynamics works by trotting out only Bernoulli . . . I think it's important to re-educate them.

quade -
The World's Most Boring Skydiver

[billvon 2,476]

>Now simple Newtonian mechanics themselves do not explain lift.

In the final analysis, an airplane will exert a force on the air (which is viscous, and thus moves) that is exactly equal to its weight in full flight - thus satisfying the most basic of Newton's tenets. Additionally, if you performed a finite element analysis on every chunk of air near the plane, you would note that the forces on each one of them (which results in the acceleration of the air downwards, usually) all added up to a force equal to the weight of the plane.

Now, that in and of itself isn't a very useful observation; it doesn't tell you much about how efficient the plane is, which is why Newton isn't all that useful when it comes to airplanes. However, if you deflect a bunch of air downwards with a piece of plywood, and the force used to accelerate that mass of air (from F=MA) is equal to the weight of the plywood - the plywood will fly. May not be stable, but it will generate enough lift to keep itself (at least momentarily) in the air.

In the final analysis, I think that everything from flying squirrels to wingsuits to canopies to cessnas to helicopters flies pretty much the same way, by deflecting a mass of air downwards. There are half a dozen ways to analyze that, but the varying methods of analysis don't change the underlying forces at work.

>This means that changing the payload changes the speed . . .

Agreed there . . .

>which changes the glide slope.

I don't think that's right. The glide _slope_ remains the same, even though the descent rate and forward speed have increased. From a mile up you can cover exactly the same distance in a glider whether you have no ballast or a ton of ballast (in zero winds of course.) Now, if there's a tailwind, you'll go a lot farther with no ballast, because you will be in the air longer. If there's a headwind, you will go a lot farther _with_ ballast, because your speed is greater and your groundspeed is less affected (on a percentage basis.) Hence there are some uses for ballast even if it doesn't change your no-wind glide angle.

>I don't believe lift equations should be spat out at FJ students..

Equations? Agreed. I think the minimum they have to know is that their parachute has air flowing around it, and it is the motion of that air that causes lift, and lift will give them a good landing. The important notion there is that if the airflow goes away so does the lift.

[larsrulz 0]

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The glide _slope_ remains the same, even though the descent rate and forward speed have increased. From a mile up you can cover exactly the same distance in a glider whether you have no ballast or a ton of ballast (in zero winds of course.)

I can pretty much agree to disagree with you on the other stuff. But I still have a problem here. Having flown a glider, they act (in flight) very much like a canopy acts, which I believe everyone here can relate to. I mentioned something in my original response about how you can see the different glide slopes of a canopy. When you let out of a front riser dive, a canopy planes out and momentarily picks up a very high glide slope. If you go into full breaks and quickly release that, a canopy will dive at the ground gaining a very steep glide slope. This is because at any given wing loading, the trimmed glide slope requires a certain speed for the canopy to fly at. This speed is necessary for the proper amount of lift to be produced. Too quickly and there is more lift than is requested, too slow and there is too little lift. This here simply explains why a canopy dives and planes out. The exact same thing holds for gliders.

I got a strong urge to fly, but I got no where to fly to. -PF

[billvon 2,476]

> Having flown a glider, they act (in flight) very much like a canopy acts,
> which I believe everyone here can relate to.

?? I've flown gliders too, and I see them as very different. You can trim them for different speeds, and they are far less dynamically stable than a canopy. You can hold a glider in a dive, or trim it for a very high (or very slow) hands-off glide.

> This is because at any given wing loading, the trimmed glide slope
>requires a certain speed for the canopy to fly at.

Not if you fly a paraglider. You can change the trim speed of a paraglider, and it will happily cruise at either the higher speed or the lower (trim) speed. You can do the same with a parachute; we don't have the same systems paragliders have (called a "speed bar") so it's a lot harder.

One of my side projects, if I ever have time, will be to take a Nitron and add a speed system to it. That way the canopy will have two speeds it will cruise at, and the glide slope will remain about the same.

>This here simply explains why a canopy dives and planes out.
>The exact same thing holds for gliders.

You're talking about pitch stability, which has more to do with design than with the total amount of lift the wing (or tail) produces. A glider does not pitch up until the amount of lift equals the amount of weight; it pitches up until the negative lift on its tail declines to the point where the nose begins to pitch down again, which is how most non-canard aircraft accomplish pitch stability.

[quade 3]

See http://142.26.194.131/aerodynamics1/Performance/Page3.html

quade -
The World's Most Boring Skydiver

[pilotdave 0]

Do we need to get into a discussion of phugoid and short period modes here?

Dave