Stalls (& Line Trim?)

[freeflyguy 0]

Ok. Just figured I would give my thoughts. Take from it what you will. Talk to you later.
james

[quade 3]

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Ummmm....OK.....I'm lost on this one. I am pretty sure you have to have either too little airspeed or be outside the critical angle of attack to stall a wing. Of course, those two factors are constantly changing as you fly. I see what you are getting at with wing loading during a turn but.....the stall speed increases as more load is put on it.....It's just not as simple as that statement. These two factors have a dynamic relationship.

I'll say it again . . .
A stall can happen at any airspeed and at any attitude.
Notice I didn't say; must, shall or will . . . I said, can.
I -think- I know where you've been confused.
Attitude is the wing's roll, pitch and yaw orientation with respect to 3D space.
"Angle of attack" is the angle between the wing's mean chord line and the relative wind.
They are not usually the same thing.
The first part of the phrase "A stall can happen at any airspeed and at any attitude" means is that a stall is not dependant on a particular airspeed.
The wing may be stalled at a higher speed if there is an additional load factor being placed upon it. The wing may also stall at a lower speed if there is less load being placed upon it.
The second part of the phrase "A stall can happen at any airspeed and at any attitude" means that it does not matter if the leading edge of the wing (or nose of the airplane) is pointed up or down. What matters is the angle of attack -- which is a an entirely different thing altogether.
If we were performing a loop in an Aerobat 152, we could stall the airplane at the beginning of the maneuver if we entered it too aggressively. We could stall the airplane at the top of the loop when we're upside down at minimum airspeed and we could stall the airplane at the bottom of the loop as we pull out of the vertical descent and we're rapidly gaining airspeed.
Hmmm, "bottom of the loop as we pull out of the vertical descent and we're rapidly gaining airspeed." Kind of sounds like a hook turn doesn't it?
Paul
http://futurecam.com/skydive.html

[freeflir29 0]

"Kind of sounds like a hook turn doesn't it?"
Yes...but...this is done by the elevator pushing the wings outside of the "critical angle of attack" Or putting such a load on the wing that it stalls. (Over G) Is it possible to push a HP canopy past the critical AOA? I would say it's almost impossible on my PD 190 but lets take a Crossfire 65, Cobalt 65, etc loaded say in the 2.5-2.8 range. Is it possible to have that much control input?
"Carb Heat On....Carb Heat On.....Carb Heat On..."-Phil Polstra Clay

[quade 3]

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I would say it's almost impossible on my PD 190 but lets take a Crossfire 65, Cobalt 65, etc loaded say in the 2.5-2.8 range. Is it possible to have that much control input?

I would say that it's possible with any ram-air canopy, but especially with the canopies that are designed to go faster.
Lemme explain . . .
An airfoil that is designed to go faster, usually does so by having a thinner cross section and less camber that creates -less- lift and therefore less drag.
Think about the difference in relative thickness and camber between a wing on a C-172 and the wing on a F-16 or, if you don't like that example, think about the difference in thickness and camber of a Spectre and a VX.
I'll wager a guess that a Spectre and a VX of similar size and wing loadings will have -vastly- different static stall speeds. I can only -guess- as to which one would have the lower stall speed because there are many more factors involved here because of the flexible nature of the wings, but if they were rigid wings, I'd put my money on the Spectre every time. But now we ARE getting into the area of speculation.
As for having enough control authority, both an airplane and a ram-air canopy have more than enough to cause not only static stalls, but accelerated stalls as well. You're right that in the case of the airplane, it's the elevator that provides the control. In the case of the ram-air canopy, it's not only the brakes, but also the swinging of the body/pilot underneath.
Paul
http://futurecam.com/skydive.html

[cobaltdan 0]

side note:
the spectre uses a stilletto airfoil,
the vx also is based on a stilletto airfoil...
sincerely,
dan

[Jimbo 0]

Dan,
You've said that before and now I have to ask. What exactly does that mean? I'm not trying to be difficult, but genuinely curious.
Thanks,
Jim
Help with cancer research here.

[quade 3]

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the spectre uses a stilletto airfoil,
the vx also is based on a stilletto airfoil...

You wouldn't happen to know the NACA number of that Spectre airfoil would you? (Man would -THAT- be useful in explainations!)
As for the VX, is that -based on- the same airfoil or -is it- the same airfoil. Even a slight difference could mean a lot.
Paul
http://futurecam.com/skydive.html

[cobaltdan 0]

as far as i know the stilletto, spectra, and vx/fx use exactly the same airfoil.
canopy designers in jest reffer to it as the "pd airfoil" as so many "new" parachutes simply copy this airfoil.
ps. i have this airfoil on my 2d cfd program, but i will have to check if i we have a naca # for it.
sincerely,
daniel

[freeflir29 0]

"naca # "
While we are learning stuff....What is that anyway?
"Carb Heat On....Carb Heat On.....Carb Heat On..."-Phil Polstra Clay

[quade 3]

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"naca # "
While we are learning stuff....What is that anyway?

In the 1930s, the National Advisory Committee for Aeronautics, NACA, which later became NASA, researched and developed airfoils.
They had to figure out a way of describing these different airfoils by their shape and originally devised a system of four digits to do that.
In a 4-digit airfoil, the first digit is the value of the maximum camber (in percent of the chord), the second digit is the position of the maximum camber from the leading edge in tenths of the chord, and the last two digits denote the maximum thickness of the airfoil in percent. For the NACA 2415 airfoil, the maximum camber is 2%, the position of the maximum camber is 0.4c, and the thickness is 15%.
Basically, a NACA airfoil is composed of a camber line and a thickness distribution. The thickness distribution is a single equation, while the camber is usually two joined quadratics.
I'll quote this next part because it's way too much for my little brain to memorize.

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the equations for the upper and lower coordinates are:
x(upper) = x - yt*sin(theta) y(upper) = yc yt*cos(theta)
x(lower) = x yt*sin(theta) y(lower) = yc - yt*cos(theta)
where tan(theta) = d(yc)/dx
in these equations, yc is the camber line, yt is the
thickness distribution.
A common approximation (small-angle) is to assume theta
is small, so that sin(theta) is approx. 0 and cos(theta)
is approx. 1. The equations become:
x(upper) = x y(upper) = yc yt
x(lower) = x y(lower) = yc - yt
for 4-digit airfoils, the camber lines and thickness are:
(yc/c) = (f/c)*(1/(x1^2))*(2*x1*(x/c) - (x/c)^2)
for 0<=(x/c)<=x1
-and-
(yc/c) = (f/c)*(1/(1-x1)^2)*((1-2x1) 2x1*(x/c)-(x/c)^2)
for x1<=(x/c)<=1 with x1=(xf/c)
(yt/c) = 5t*(0.29690*x^0.5 - 0.12600X - 0.35160*x^2
0.28430*x^3 - 0.10150*x^4)
where t = thickness/chord
x = position along x-axis
xf = position of maximum camber
f = maximum camber

This -is- pretty basic. ;^)
They also eventually developed 5 digit descriptions as well as 6, 7 and 8 digit descriptions. All of which get more and more (freekin') complex and ---really--- difficult to visualize just using your mind's eye!
If you want to draw an 8 digit NACA airfoil, you better have a computer and a program that understands how to do that.
Do a search on-line of NACA airfoils and you'll come up with some commercially available programs as well as some JAVA applets that will do the simple ones.
Paul
http://futurecam.com/skydive.html

[freeflir29 0]

Hmmm.....Schoolz in session! Thanks......thats a pretty neat system that lets you have some good mathmatical numbers instead of 4,000,000 differen't measurements and everyone measuring in differen't ways. (Kinda like Sq/Ft now) Thanks for the lesson.
"Carb Heat On....Carb Heat On.....Carb Heat On..."-Phil Polstra Clay

[nacmacfeegle 0]

Paul, I think we should teach all that aerodyamic stuff on the first jump courses, instead of wasting time on other trivia!!!!!
Maybe make better canopy pilots out of them.
D

[quade 3]

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Paul, I think we should teach all that aerodyamic stuff on the first jump courses, instead of wasting time on other trivia!!!!!
Maybe make better canopy pilots out of them.

Well, the three priorities of skydiving are still probably the -most- important thing for a first jump newbie, but at -some- point before getting their A license, it would be nice if they knew something about basic aerodynamics.
Clearly, knowledge of aerodynamics would make for a better and safer swooper and should be -required- of anybody teaching the subject of swooping or canopy control.
I'm not saying canopy flight instructors need a degree in the subject, but they should at least have a basic understanding at the level of say . . . a well informed Private Pilot.
Paul
http://futurecam.com/skydive.html

[alan 1]

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as far as i know the stilletto, spectra, and vx/fx use exactly the same airfoil.

I asked John Leblanc and Simon Mundell about this and here are their replies, in part, as I didn't want to include any comments not strictly pertaining to the airfoil comment.
John LeBlanc:
"Yes it does use the same airfoil, and for good reason. When the Stiletto was in the design phase, the skydiving public was getting very concerned about the stability of the Nova. I knew there was a lot of effort put into that canopy, and look what happened. For that reason, we wanted to take a very conservative approach to the Stiletto as far as airfoil. That way we would reduce the possibility of "surprises" after the Stiletto introduction. Therefore, we used the Sabre airfoil for the Stiletto and the Spectre, as it was designed to have stability with a large speed range, emphasizing good low speed handling. Changes in planform (ellipticalness),aspect ratio, and trim are responsible for the performance improvement over the original Sabre.
We've gone beyond that airfoil on the Vengeance, the Velocity, and the Sabre2, by the way."
Simon Mundell:
"The only similarity with the VX and the Stiletto is that they have 9 cells and are made of the same ZP fabric. With the exception of these points they are very different designs."
alan

[Jimbo 0]

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Simon Mundell:
"The only similarity with the VX and the Stiletto is that they have 9 cells and are made of the same ZP fabric. With the exception of these points they are very different designs."

Isn't the VX a 21 cell? Maybe I'm just showing my ignorance here, but I thought that the 21 cell canopies were based on a 7 cell design.
-
Jim
Help with cancer research here.

[weid14 0]

vx = 27 cells
fx & PD velocity = 21 cells

[RemiAndKaren 0]

isnt it
vx= 9 cells with 3 chambers
fx = 7 cells with 3 chambers
?
Remi
Muff 914

[weid14 0]

semantics: 9x3 = 27....etc. if you take the open area as a "cell" than there are either 21 or 27. if you take the way it was developed (7 or 9 cell with angled ribs) then you can look at it the other way. right?

[RemiAndKaren 0]

right... but then lets call sabres and stilettos 18 cells....
Remi
Muff 914

[SkymonkeyONE 3]

That would be more accurate, but I doubt the general skydiving public would go for it.
Chuck
My webpage HERE

[cobaltdan 0]

cells are defined from loaded rib to loaded rib.
loaded rib being one with attached suspension line.
beyond that designs obviously can have differences
ie. an fx is a 9 cell canopy with tri-x construction,
a sabre is a 9 cell canopy, a cobalt is a 9 cell canopy with 1/2 cell construction (18 top skin panels),
etc...
sincerely,
dan

[Geoff 0]

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an fx is a 9 cell canopy with tri-x construction,

I think by your definition, an fx is a 7 cell, and a vx is 9-cell. Was that a typo, or have I misunderstood?
Geoff