Downwind Stall

[Martini 0]

Does anyone know of info regarding canopy/airplane flight in wind that I can print out to try to convince skydivers and pilots that there is no such thing as a downwind stall, airplanes don't have a faster climb rate (once aloft) when pointed "upwind", canopies don't dive faster when swooped downwind and in fact that in a stable moving airmass there IS NO WIND for airplanes, canopies, balloons, birds or anything else up there because you are drifting with the moving air all the time no matter which direction you fly? I could use a source or two with better explanations than I can give.

[SkydiveMonkey 0]

Just explain it's a matter of airspeed vs ground speed. It's all relative.
Let's go play chicken with a planet !!

[flyingferret 0]

This sounds pretty interesting, and I think I can kinda understand what he is saying, but can someone elaborate on this a little bit?
malachi

[SkydiveMonkey 0]

What he's saying is that (for example) a canopy will move at the same speed through the air at all times (with no inputs), but the ground speed will be different depending on which way the canopy faces.
Let's go play chicken with a planet !!

[jdhill 0]

I guess I'm kinda confussed... why do we teach students to land into the wind?
a wing will stall (no longer provide lift) at a given airspeed with a given loading... that airspeed is the same upwind or downwind... say your canopy has 10mph fwd speed at full flight and stalls at 0mph airspeed and you are landing into a 10 mph head wind... the canopy will come straight down and when you flare it will go backwards up to 10mph(ground speed) before stalling... fly the same canopy down wind at 10 mph... you would have a ground speen of 20mph (10mph of airspeed 10mph of airmass movement - wind), you begin to flair you slow down to and hit the ground at 10mph gound speed because your airspeed went to 0mph while your ground speed was still 10mph (the speed of the "stable air mass").

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canopies don't dive faster when swooped downwind

Canopies may not dive more in a downwind swoop... but their ground track will change during the dive because they are pushed by the airmass, and your flare will be different due tue different airflow over the wing.
http://www.aerialfusion.com

[f94sbu 0]

I would say that theoretically, there is no difference between a down wind swoop and an up wind swoop. But my back tells me otherwise ;) When I am doing 180 turns in windy conditions I tend to do them higher and I guess it is because of the different air speeds at different altitudes.
When the parachute is recovering, it depends on some air to fly on. If it decends quickly it looses cell preasure since the relative speed to the air is decreasing simply because you are moving down.
I have once hit the ground pretty hard when my parachute didn't recover as it normally does and this was in very windy (>10 m/s) condition. The second time it happened to me I immediately felt it and had time to flare higher than I usually do and then all was fine.
Stefan

[quade 4]

Sometimes my jaw drops.
Just this last weekend I was in the packing area at Perris and a couple of us were watching the pro-swoops. They were going for distance and the course was downwind. Massive swoops in the +350 foot range with the winner, I think, at 418 feet. A lot of them were taking butt slides and a couple even went head over heals at the end.
Here's where it gets interesting.
One guy in our group -- a guy with over 2,000 jumps, massive CRW experience, Pro-rating and that regularly gives advice to people about landings -- was telling us that the reason that so many people were having difficulty stopping landing was that going downwind there was less lift!
Ok, now, clearly this guy has some experience but obviously no real clue when it comes to aerodynamics, yet, like I said, he has over 2,000 jumps and gives landing advice to anyone that comes up to him - including the ever popular "ya need more speed to get a good landing" and this isn't for swoop landings mind you, he means tip-toe stand ups.
Nothing I could do would convice him otherwise -- and I'm a U.S. government certified explainer of such things! This guy just would not listen.
Finally, I just shook my head and walked away.
What could I say? He actually has his Pro-rating and I've yet to get mine. (sigh)
quade
http://futurecam.com

[jdhill 0]

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was telling us that the reason that so many people were having difficulty stopping landing was that going downwind there was less lift

did he actualy mean less lift or that the canopy runs out of air speed to stay up while it still has a fair amount of ground speed, which may give the impression of having less lift?
http://www.aerialfusion.com

[quade 4]

Josh --
He said and meant lift.
Believe me, I gave him every benefit of the doubt when it came to this issue. I made absolutely certain I was understanding his position on the subject because it's was just so dang clearly wrong and I couldn't really believe I was hearing this come out of his mouth.
quade
http://futurecam.com

[jdhill 0]

well you gave him a chance... I was looking at the results from the Pro meet earlier and was thinking that they ad to be going downwind to be getting distances like those... how fast were the winds?
When I was out there for the intermediate event the winds would come up in the afternoon to about 10-15... a few brave souls made practice runs on the distance course and were getting way out there, but comming real close to getting hurt (in fact one did sprain an anckle that prevented participation the following day).
http://www.aerialfusion.com

[amir1967 0]

I'm sure that you are right with you'r physics and arody. but win as air mass is not constant close to the ground and every time that it increases abit the canopy and the pilot need to catch up and the canopy will dive a bit while gaining that wind mass speed ,so if you swoop 6" above the ground I think the only way is crash landing.
or did I got that wrong too
amir
AM67

[quade 4]

The winds were maybe 15ish, and for the distance runs I think I only saw one guy not get dirty. Of course, I wasn't really watching all that closely so there may have been more.
Looked pretty freekin' scary to me.
quade
http://futurecam.com

[quade 4]

It's not all that complex. The real issue is at what airpeed you touchdown and what is the wind speed.
Let's say that in no-wind conditions you'd normally put your feet on the ground while still moving at 10 mph. Facing a 10 mph headwind you'd be essentially stopped with no forward movment, but for a 10 mph downwind landing you'd be traveling at 20 mph. Since most people can't run quite this fast, you'd either slide or stumble.
That's really all there is to it.
quade
http://futurecam.com

[kallend 2,027]

There IS a difference, and it's a difference of PERCEPTION.
You don't fly by reference to the airmass because, unless you come from LA, you can't see it. You fly by reference to what you can see, and at low altitudes the most prominent visible object is the ground. You adjust your control inputs to make your ground track coincide with your mental picture of what it should look like. Hence although your canopy can't tell the difference between upwind and downwind, your control inputs WILL be different with the result that the canopy will appear to fly differently.
And then, of course, there are wind shears that are obviously direction sensitive.

[hookit 0]

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There IS a difference, and it's a difference of PERCEPTION.

Well said.
I've heard many people (some of them very experienced) claim that a canopy dives harder on windy days. When I ask why they believe this they usually say it's because, as you dive your canopy on a windy day (when landing into the wind), the high winds are hitting the topskin and are preventing it from coming out of the dive as quickly as it would on a no-wind day.
In reality what I believe to be the cause of the PERCEPTION of the canopy diving harder on a high wind day is that the canopy doesn't generate forward GROUND speed (again when landing into the wind) until further along in the recovery arc than normal therefore from the pilot's perspective the canopy didn't actually come 'around the corner' until it was at a lower altitude than normal ergo...'it dove harder'. However, with respect to the path of the canopy THROUGH THE WIND it's the same as any other hook.
Comments and corrections are more than welcome.
-Trey

[eames 0]

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how fast were the winds?

Actually Josh, the winds were very light... maybe 5ish. I came in right after Shanon set the record. I must say though, the conditions were perfect: the morning had been foggy and it was really beginning to heat up, I swear we were catching thermals off the ground. But Shanon just hit the gate right in the power band and really milked it. I couldn't believe it when I saw him come out of the course from up above. Hard act to follow.
Jason

[quade 4]

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Actually Josh, the winds were very light... maybe 5ish.

Ok, it seemed a lot faster from where I was standing, but I'll admit that was maybe 1/4 mile away.
quade
http://futurecam.com

[Martini 0]

Thanks for all the input guys, it's good that most of us understand what's up here but for those stubborn ones out there a source outside this forum (or a nice professerly bit by Quade) would be helpful. BTW, it's tough to explain to a pilot all about ground reference when he insists he trims differently up/downwind at 10000'.

[quade 4]

This particular myth is very closely related to a similar one in the airplane world known as the "Downwind Turn Myth".
Try these;
http://www.ipilot.com/forum/message.asp?PID=7840
http://www.flyingmag.com/Columnist/PrintArticle.asp?ArticleID=115
http://store.yahoo.com/avshop/pp.html
quade
http://futurecam.com

[Martini 0]

Thanks Quade, the iPilot site had just what I was looking for.

[ramon 0]

hmm.. getting your feet just below the entry gate at maximum centripital force pulling the risers out of your hands at the same time must be a beautiful thing.
You know there is a thermodynamic and aerodynamic limit to swoop length. 418 is really freaking long.
I wonder how long it will be before someone in a gravity powered, freefall deployed, life saving device manages to beat that one without a catchnet or giant airbag.
Right on Shanon...like catching the perfect wave..really
ramon

[billvon 2,998]

>You know there is a thermodynamic and aerodynamic limit to swoop length. 418
> is really freaking long.
A competition sailplane can "swoop" over a mile (L/D of 50 to 1) then pull up and land somewhere else. We haven't even gotten close to the limits of aerodynamics yet.
-bill von

[billvon 2,998]

>Ok, it seemed a lot faster from where I was standing, but I'll admit that was
> maybe 1/4 mile away.
Not suprising - later in the day, I landed in no wind and had a 10kt wind 50 feet away blowing the flags around.
-bill von

[f94sbu 0]

The iPilot thing is not really applicable since a parachute dives when you turn it, a plane does not (unless you make it
dive)
And when the parachute dives, it experiences different windspeeds at different altitudes. That said, I think that we can agree that there is no such thing as a downwind stall, however I think you'd all agree that if the wind suddenly dies, everything will stall. Anyone tried landing towards a building knows what I mean ;)
regards,
Stefan

[markbaur 0]

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The iPilot thing is not really applicable since a parachute dives when you turn it, a plane does not (unless you make it)

As a flight instructor, I can tell you that when a new pilot turns an airplane, it dives. The energy needed to turn the airplane has to come from somewhere, and it's only with practice that flight students learn to convert some of their airspeed to additional lift to maintain altitude, instead of just letting the airplane fly at its trimmed airspeed.
Mark