Downwind Stall

[f94sbu 0]

>As a flight instructor, I can tell you that when a new pilot turns an airplane, it dives.
I don't doubt that, but when I talk about turn, I mean a _turn_, not a turn and then dive. What I was trying to say is that when you are doing a frontriser turn, you are also diving even if you call it a turn, but when you talk about a plane turning downwind, you mean turning and compensating for the loss of airspeed to maintain altitude. That's why you cannot really compare the effect of a turning airplane and a turning (and diving) parachute
regards,
Stefan

[ramon 0]

When I hear sail plane I think a glider type thing.
I am specifically talking about freefall deployed devices, I could desgin a semi-rigid wing that would go further than 418 and with a real aeronautical engineer probably somehting that would go miles on thermal currents.
.
(neuvres is like a paraglider but I am unsure if it is on the market)
So what is a competition sail plane? Is it not a rigid wing glider and what does it have to do with a pendulum weighted inflatable wing deployed in freefall and intended as a life saving device?
ramon

[quade 3]

Stefan --

Quote

That's why you cannot really compare the effect of a turning airplane and a turning (and diving) parachute.

A wing is a wing and they both interact with aerodynamic forces in the same way.
As far as wind shear and turbulence goes (you had given an example of the wind near a building), those are other topics entirely and are not what is being discussed here.
quade
http://futurecam.com

[kallend 1,633]

<<< BTW, it's
tough to explain to a pilot all about ground reference when he insists he trims differently up/downwind at
10000'.>>>
There's no restriction that prevents morons from getting pilot's licenses.

[SkyMissy 0]

Quote

and your flare will be different due tue different airflow over the wing.

No, the airflow is the same, that is the point. The groundspeed will be different.
13,500' to the ground. We're dressed like clowns. The door's open. Hit it!

[amir1967 0]

I think that at list half of the people including myself when talking about "downwind stall" mean turbelent air and shear of some sort.
So maybe it's a different topic but it seem to that it still exist ,and again I might be wrong again.
amir

AM67

[quade 3]

Then you need to go back to the top of this thread and reread it from the beginning. We have been talking simply about the wind in general and not turbulance or wind shear.
If you'd like to talk about turb or wind shear, then yes we can, but again that's a totally different topic. Both turbulance and wind shear can take down, not only canopies, but very large passenger jets as well. However, it's the near instantaneous change in the direction of the wind and not simply turning into it that causes the problems.
quade
http://futurecam.com

[kallend 1,633]

<<Subject: Re: Downwind Stall
In reply to:
and your flare will be different due tue different airflow over the wing.
No, the airflow is the same, that is the point. The groundspeed will be different.>>>
Your post says it was a reply to mine (and it comes up that way when viewing the thread), but I didn't write that!

[f94sbu 0]

>A wing is a wing and they both interact with aerodynamic >forces in the same way.
Of course, but only if they are manuvered the same way. I said that it was wrong to compare a diving parachute with a plane turning with constant altitude.
>As far as wind shear and turbulence goes (you had given an >example of the wind near a building), those are other topics >entirely and are not what is being discussed here.
Maybe not. BUT the point I (and others) are trying to make is that you can hurt yourself badly if you are not aware of the wind when you are doing hook turns. Personally it has never been anything worse than I couldn't walk away by myself, but still bad enough that I had trouble sitting for 2 weeks! Even though the initial question was about a proof that I and others are 'wrong' I don't want other people to hurt themselves when obviosly people have made this misstake before. What is said about turning etc. in a constant air mass is of course right. What I am trying to explain is what happens close to the ground where the air mass is not constant in most cases.
Btw, how did you include the stuff from my post the way you did instead of having to resort to the old usenet way that I am using ;-)
(2nd reply)
>Then you need to go back to the top of this thread and reread it from the beginning. We have been talking simply about the wind in general and not turbulance or wind shear.
What is wrong changing the subject just a little. Some of us are trying to tell people how to not get hurt ;)
Let me try to explain a little better what I mean by this example:
You are traveling downwind at 100 m above the ground where the wind speed is 15 m/s with a relative forward speed of 10 m/s. Your relative ground speed is then 25m/s. Then you make an aggressive upwind (180) turn and you start to dive. Lets say that at 25 m above the ground you have forward speed of 15 m/s relative to the ground (in the other direction). This speed has been created by changing your potential energy into inertial energy (chaning your speed from 25 m/s into -15m/s). The physics behind this does not take the wind into account, only your initial position and speed (which was of course created by the wind)
The problem however is now if the speed of the wind is lets say 5m/s at this altitude. This could be due to turbulence from the ground or windsheer or something else)
This means that the relative speed in the wind is 20 m/s which creates a certain amount of lift.
Now, lets assume that we are doing the same thing without the wind.
Then the numbers would be 10 m/s ground speed at 100m, at 25 m it would be (10-40) = 30m/s in the other direction.
Since there was no wind, this would be 30 m/s relative to the air. 30 m/s creates more lifting power than 20 m/s which means that the recovery arc of the parachute (or plane if you tried the same stunt with a plane ;) will be smaller.
Of course you can change the numbers into anything because I didn't make more thorough calculations other than assuming that
the speed is different at 100 and 25 m and thus proving the different lifting force experienced with or without the wind.
A funny side effect of this is of course then that if you actually turn downwind, your lifting force would actually be greater.
Hoverer I wouldn't recomend anyone doing a downwind hook if the air speed at 100m is about 15 m/s :-)
regards,
Stefan

[hookit 0]

Quote

Your post says it was a reply to mine (and it comes up that way when viewing the thread), but I didn't write that!

Yup. It confused me too at first. He replied to the wrong post. The quote was from JDHILL's post.

[hookit 0]

Quote

Btw, how did you include the stuff from my post the way you did instead of having to resort to the old usenet way that I am using ;-)

You preclude the quoted material with: [ quote ] (minus the spaces) and conclude the quoted material with [ /quote ] (again minus the spaces).
If you hit the FAQ button near the top of the page you'll see many neat little markup tricks that are available in the forums.

[quade 3]

Stefan --

Quote

Of course you can change the numbers into anything because I didn't make more thorough calculations other than assuming that
the speed is different at 100 and 25 m and thus proving the different lifting force experienced with or without the wind.

If I understand you correctly and I'll admit that we seem to have a little language barrier here, then no. That's not the way it works.
quade
http://futurecam.com

[RichM 0]

There is one relevent point to turbulence - I guess that the amount of turbulence near the ground will increase as the wind increases. This would lead to different overall flight characteristics for a canopy as it lands in turbulent air, resulting in truly different lift characteristics on windy days, although with no difference up/downwind.
Also, is it possible that on windy days there are reasonably different wind speeds at 0ft and 20ft agl, so the mass and inertia of the skydiver at 0ft might cause the canopy at 20ft to fly through more air and so more turbulence when flown into wind than when flown downwind, resulting in truly different landings.
All the above is pure guess work. I'm just trying to think why so many people have beliefs that are not supported by the simple fluid dynamic model. Any thoughts?
Rich

[f94sbu 0]

Quote

If I understand you correctly and I'll admit that we seem to have a little language barrier here, then no. That's not the way it works.

What is it that doesn't work like this? You fail to tell us why you disagree. That the wind speed is different at different levels? Or that the physics behind this is wrong? I didn't earn a degree in engineering physics for nothing...
Let me know and I'll redo the maths for you.
regards,
Stefan

[crazy 0]

Quote

All the above is pure guess work. I'm just trying to think why so many people have beliefs that are not supported by the simple fluid dynamic model. Any thoughts?

Maybe for the same reasons why some people think that you cant see the full moon during daytime. If your grandma, or your instructor, hammers a stupid idea into you, it takes a lot to change your mind. Faith against thought is unfair because it's easier to believe than to understand.
The striking news is that i checked and noticed that in australia it's just the opposite: the aircraft climbs better downwind and the canopies tend to turn upwind.
BB
Come

[hookit 0]

Quote

What is it that doesn't work like this?

Stefan, I'm a bit confused as to the point you're trying to make so I'll start off by saying that I agree with your math. It's the model upon which it's based with which I disagree. Your model assumes that on the windy day the wind is blowing 66% slower (relative to the ground) near the end of the hook than it was at the beginning of the hook. If that were the case then this would not be a normal high performance landing...it would be a bumpy unpredictable ride because the canopy isn't flying through a stable air mass. I believe you're talking more about turbulence than you are normal winds.
In your original post you state the following:

Quote

BUT the point I (and others) are trying to make is that you can hurt yourself badly if you are not aware of the wind when you are doing hook turns.

If you replaced the word wind with turbulence then I agree wholeheartedly and feel that your model proves this point well.
However, if you're proposing that it's normal to experience a 66% difference in wind speed (relative to the ground) during a normal hook turn then I would have to disagree. I purposefully set up such that I'm not downwind of obstacles when making my hook (or carve) because I want fly through the cleanest most stable air mass possible. I'm quite sure that even on the windiest of days I've jumped I haven't experienced such a drastic change in wind speed during last 100 meters of descent (thankfully!).
Blues,
Trey

[kallend 1,633]

A few years ago Scientific American had an article on "popular physics". They asked people what they thought happened with simple actions (such as throwing a ball in the air while walking, dropping a ball while walking, swinging a weight on a string around your head and then releasing the string, etc). It was amazing how many people (a large majority) came up with incorrect descriptions of what happens, even some people that have degrees in engineering physics and the like.
As Richard Feynman said, just because you can remember some formulae and solve some problems to pass an exam doesn't mean you have any idea what it all means.
There was also a famous TV segment some years back where they interviewed graduates from (I think) MIT and asked what causes the seasons. Almost all of them gave an incorrect explanation.

[f94sbu 0]

Quote

Your model assumes that on the windy day the wind is blowing 66% slower (relative to the ground)

Ah, now we are getting somewhere ;) I wasn't assuming that this is the case everytime, everywhere. I was merely assuming that this was the case when you will experience the effect that I was talking about.

Quote

I believe you're talking more about turbulence than you are normal winds.

By definition turbulence is curl in the windmass. Curl is created when the sheer gets larger than a certain point. Which means that sheer can occur without curl up to a certain point. Just look at a floating river. When the river is wide, you will see the sheer very easy by looking at the water with different speed in the middle and close to the edges. When the same river gets narrow (and transporting the same water but with a different speed) you will clearly see when the sheer forces gets too large and curl is created.

Quote

windiest of days I've jumped I haven't experienced such a drastic change in wind speed during last 100 meters of descent (thankfully!).

You should have joined us today then ;) Most of us got quite a few bumpy rides just becase of the wind. But of course this was caused by the fact that it wasn't a stable air mass, due to both the nature of the wind and obstacles on the way. (Quite far away, but still enough to create turbulence.)
However, I have to disagree that the air mass is always stable. If you study fluid dynamics of a pipe, you'll be able to see that the speed of the fluid is different in the middle of the pipe compared to the speed at the edges. Air flowing past a surface (ie the ground) is subject to the same phenomena. One important factor here is the friction between the air and the ground so I guess that in some places, you hardly notice any sheer (or turbulence which happens when the sheer gets too large). In other places this effect will be very obvious and then you need to take care.
The bottom point I was trying to make is that if you don't believe what I say, go ahead and hook like you always do, but be prepared to hurt yourself! The wind will not be equal everywhere and even if you feel the wind is free of turbulence, it can still contain sheer.
Be safe, have fun!
regards,
Stefan

[hookit 0]

Stefan,
I definitely see the point you're driving at now.

Quote

If you study fluid dynamics of a pipe, you'll be able to see that the speed of the fluid is different in the middle of the pipe compared to the speed at the edges. Air flowing past a surface (ie the ground) is subject to the same phenomena.

I was wondering about this very thing as I responded to your previous post. I agree that air is subject to the same phenomena however, since air is significantly less viscous than water, the phenomena should be greatly reduced. It could certainly be a factor though.

Quote

The wind will not be equal everywhere and even if you feel the wind is free of turbulence, it can still contain sheer.

Good point...and food for thought. Thanks for the posts!
Blue Skies and Long Safe Swoops,
Trey

[Tonto 1]

No. This is the problem with a little intelligence. It goes a long way. Once you leave the ground, there is only airspeed. Most people cannot grasp this concept.