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kelel01

Physics/Canopy Question

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Funny conversation with lots of digressions and non-pertinent info.

angle of attack, distance, airspeed, none of that matters to the question. The only thing that matters is descent rate.

The half braked canopy will land second since its descent rate is less.

The distance question is much more interesting.



This is correct.



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

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The two canopies will have about the same glide ratio, but the unbraked canopy will land first.


I agree that the canopy at full flight will land first due to his angle of attack, but it is impossible for the same cxanopies to have the same glide ratio.



No it's not. All they need is the same Cl/Cd, and that can be achieved at two different AOAs.
...

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

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Does there exist a canopy that lands faster in half brakes than in full flight?


Purpose of brakes?
The mind is like a parachute - it only works once it's open.
From the edge you just see more.
... Not every Swooper hooks & not every Hooker swoops ...

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The canopy in half brake will have a profile that creates more lift, decreasing it's descent rate and increasing drag. Due to the increased drag, it's foward rate will decrease, resulting in covering less ground in 0 wind conditions.
The AoA will not necessarily change permanently, but it will change during the time that the suspended weight is moving as the canopy slows down.
The mind is like a parachute - it only works once it's open.
From the edge you just see more.
... Not every Swooper hooks & not every Hooker swoops ...

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The AoA will not necessarily change permanently, but it will change during the time that the suspended weight is moving as the canopy slows down.



And how long is that gonna take? Yes, initially the braked canopy will create more lift, but once it has reached an equilibrium, it will create the same amount of lift as the unbraked canopy...by flying at a lower airspeed, etc. What happens for the few seconds after the brakes are applied has little bearing on the question at hand. Course neither does the fact that both canopies generate the same amount of lift. But, the braked canopy doesn't have a lower descent rate because it generates more lift. That's the point I'm trying to make.

Dave

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I meant faster in the context of the question, not with faster airspeed.

-- Toggle Whippin' Yahoo
Skydiving is easy. All you have to do is relax while plummetting at 120 mph from 10,000' with nothing but some nylon and webbing to save you.

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Reading this thread is like listening to political candidates answering a straight question. Eveyone talks about something else.

Or like a construction site where each worker is given the plans to different structures.

Or a working meeting at an engineering company.

...
Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

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Sorry I mistyped "B"is at full flight and therefore has a greater angle of attack and will land first.
there are no other factors that will change this as long as all the constants are the same.
Would anyone care to dispute this?
"Dropzone.com, where uneducated people measuring penises, has become an art form"

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2 same sized canopies trimmed the same with jumpers being the same height and weight with the same conditions,
It is impossible to have the same glide ratio with one in 1/2 brakes and the other at full flight.
As far as your Cl/Cd I don't know what you are referring to.
"Dropzone.com, where uneducated people measuring penises, has become an art form"

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Cl/Cd = Lift/Drag ratio. Glide ratio is basically the same thing as lift to drag ratio. If pulling half brakes changes lift by the same factor it changes drag, your L/D ratio won't change. In reality I doubt that would ever happen, but it's possible.

And maybe I'm just missing something here, but how do you know a canopy at half brakes flies at a lower angle of attack than a canopy in full flight? I'd assume otherwise, but maybe I'd assume wrong. What am I missing?

Dave

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I suppose you mean sooner/earlier.
Fast is an attribute of speed, sooner/later one of time. [/knittpicker] :P
The mind is like a parachute - it only works once it's open.
From the edge you just see more.
... Not every Swooper hooks & not every Hooker swoops ...

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I suppose you mean sooner/earlier.
Fast is an attribute of speed, sooner/later one of time. [/knittpicker] :P



You mean nit, not knit. (And if one looks nit up in the dictionary, nit picker comes from the animal nit, not from the luminous nit.)

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Let's suppose, as someone said, that the full flight trim is faster than that for best glide ratio. As you start pulling down the brakes, the glide ratio improves. If you keep pulling down the brakes, the best glide ratio will be reached. Pull them down some more, and the glide ratio decreases again. Pull it just to the right point, and the glide ratio will be the same as in full flight, albeit at a slower speed and slower descent rate.

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Let's suppose, as someone said, that the full flight trim is faster than that for best glide ratio. As you start pulling down the brakes, the glide ratio improves. If you keep pulling down the brakes, the best glide ratio will be reached. Pull them down some more, and the glide ratio decreases again. Pull it just to the right point, and the glide ratio will be the same as in full flight, albeit at a slower speed and slower descent rate.



Exactly. And most modern canopies are trimmed for full flight faster than best glide speed.
...

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

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Both canopies are producing the same amount of lift...


Point taken well taken. I guess I'm focusing of my flare speeches where you have lots of forward speed when you hit the flare. Once the speed reduces, you go back to the same lift.

Lets try another angle (pun intended)
Lift is constant but is the direction of the lift is not.
A plane in a nose dive is producing the same amount of lift as a plane in level flight. The problem is that lift is produced 90 degrees to glide angle. The lift is not being used to keep the plane up.
However, if the glide angle is changed to be closer to the horizontal, the lift becomes closer to the verticle.

I think that mean that the wing has more usable lift.

Am I getting close?


I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

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Am I getting close?



No. Not really.

The amount of lift created by a wing has no direct relationship to its attitude relative to the horizon. An airplane performing a loop, for instance, will usually have to generate enough lift to support 3.5 or so times the weight of the airplane when the loop is started (going up) and when the loop is finishing (going down).

The ONLY angle that matters is the Angle of Attack (AoA) which is the angle between the wing and the relative wind. The horizon doesn't really enter into it except it does make a handy reference when flying straight and level.

I think you might have quite a few misconceptions about aerodynamics because we usually don't teach that very well to most skydivers.

A very basic course in aerodynamics suitable for private pilots would clear up most of your misconceptions. A very good on-line resource is http://www.av8n.com/how/.

(Ah poop, the site is down right now, but it IS good when it's working.)
quade -
The World's Most Boring Skydiver

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Brakes does not affect the angle of attack, risers do.


Brakes do affect the angle of attack one way or another. If you fly at full brakes you will get a lower glide ratio yet you havn't changed the pitch of the wing (at least not matching the decrease of the glide ratio). You've got an higher angle of attack.

Pilotdave - as far as I have understood lift and drag (as described at http://www.av8n.com/how/htm/4forces.html) the lift at steady state certainly are different since the drag has a different vertical component (the sum of the vertical components of the forces must be zero).

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Pilotdave - as far as I have understood lift and drag (as described at http://www.av8n.com/how/htm/4forces.html) the lift at steady state certainly are different since the drag has a different vertical component (the sum of the vertical components of the forces must be zero).




Yeah, way up above I mentioned I wasn't taking drag into account so saying the two canopies have equal lift isn't exactly true. But for small descent angles and low speeds (like parachutes), I don't think the vertical component of drag will make a huge difference in the amount of lift that each canopy needs to generate.

And teason...

Forget that drag stuff for a min. When I say the lifts are gonna be equal, I'm only talking vertical component of lift, not total lift. If the braked canopy created more lift vertically than the exit weight of the jumper, the canopy would accelerate upward. Course that's exactly what it does right when you pull the brakes. But when you leave them down, the canopy will eventually reach a steady state. At that point, the lift equals the exit weight. If the vertical component of lift doesn't exactly equal weight, the canopy will accelerate up or down.

For a plane in a steep dive, drag becomes very important. But a plane at a very shallow descent angle, which is not accelerating vertically, is producing the same amount of vertical lift as a plane flying straight and level.

Dave

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Brakes does not affect the angle of attack, risers do. But since the question was simple who will land first, then B.

Actually, brakes do change the angle of attack. Consider that application of brakes changes your lift to drag ratio, or glide angle, as it's also called. Since your canopy's angle to the horizon hasn't changed, but the steepness or shallowness of it's path thru the air has, you've changed the angle of attack. Think of an accuracy canopy coming almost straight down to the target. Obviously a high angle of attack, using only the brakes.

All airfoils stall because you exceed their critical angle of attack, the point at which they stall. By your reasoning, you could never stall a canopy using only the toggles.

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Brakes does not affect the angle of attack, risers do. But since the question was simple who will land first, then B.

Actually, brakes do change the angle of attack. Consider that application of brakes changes your lift to drag ratio, or glide angle, as it's also called. Since your canopy's angle to the horizon hasn't changed, but the steepness or shallowness of it's path thru the air has, you've changed the angle of attack. Think of an accuracy canopy coming almost straight down to the target. Obviously a high angle of attack, using only the brakes.

All airfoils stall because you exceed their critical angle of attack, the point at which they stall. By your reasoning, you could never stall a canopy using only the toggles.



Depends how AOA is defined, really. If you define it as the angle between the airfoil datum and the freestream, then changing the datum by changing the camber (brakes) ipso facto changes the AOA.
...

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

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Depends how AOA is defined, really.



Is that in question?

http://www.av8n.com/how/htm/aoa.html#sec-def-aoa

A general approximation is the angle between a line created from the leading to trailing edge and the relative wind.

Deploying wing flaps or using brakes on a canopy changes the line between the leading and trailing edges -quite- a bit.
quade -
The World's Most Boring Skydiver

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Depends how AOA is defined, really.



Is that in question?

http://www.av8n.com/how/htm/aoa.html#sec-def-aoa

A general approximation is the angle between a line created from the leading to trailing edge and the relative wind.

Deploying wing flaps or using brakes on a canopy changes the line between the leading and trailing edges -quite- a bit.



Yes it is in question. A quote from your source (my emphasis added).:P

"The exact alignment of the indicator stick relative to the airplane is not critical. The most elegant scheme is to orient the stick in the zero-lift direction so that zero angle of attack corresponds to zero coefficient of lift. That choice will be used throughout this book; see section 2.14 for a discussion of other possibilities."


However, pretty much any definition changes if the camber line is altered (as by pulling down the TE with brakes) so the AOA does change with brakes, contrary to what was claimed by someone above.
...

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

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Ohhhhhhhhhh . . . . (shuffling of feet) . . . . ok.

Although the zero lift line does make the most logical sense, does it not? I mean a zero angle of attack making zero lift . . . makes sense to me.
quade -
The World's Most Boring Skydiver

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