Mythbusters - "Turn your canopy into a sail!"

[tdog 0]

I have heard some very experienced “experts” (at the DZ and DZ.com, names removed to protect the guilty ) give some advice that just does not make sense. This has been argued in old threads as hijacks – but since I still hear it often I am pulling out my “Mythbusters” thinking cap and asking a fresh question for today’s audience.

The advice they give goes something like this:

“Dear student/friend. Say you have a long spot. You need everything you got to go as far as you can. What you do depends on the wind. If the wind is to your back, pull down your brakes and make your body as big as possible. BY PULLING DOWN YOUR BRAKES YOU WILL CHANGE THE SHAPE OF YOUR WING INTO A SAIL AND THE WIND WILL PUSH YOU AND MAKING YOUR BODY BIG WILL ACT LIKE A SAIL TOO.”

There is no argument that flying with a major tail wind can be a major advantage on a long spot… BUT HERE IS MY ARGUMENT…. There is no way a canopy floating/flying/in equilibrium with a moving flow of air, can turn into a sail!!!

Imagine a hot air balloon for a second… When it is on the ground, the breeze wants to push it over, and you see the balloon either dragging the basket, or laying down on its side in the breeze. This is a sail trying to overcome the resistance of the basket on the ground.

The second the balloon lifts off and is no longer attached to the ground, it is floating in the moving breeze… For the first few seconds, there IS a sail effect, as the breeze has to push the balloon to equilibrium with its environment… Once the balloon has the inertia energy it gained from the “sail action” from the breeze – its ground speed will be exactly the speed of the wind it is floating in – but it’s airspeed will be zero.

I have proof from my balloon jumps… We were trucking along groundspeed wise, but there was no wind across our face in the basket. We were going exactly the same speed as the molecules around us.

Our skydiving wings are no different than a balloon floating in space – our wings will find the equilibrium with the wind direction and fly with it’s designed forward speed thru that air, regardless of ground speed. Even with deep brakes, most of our skydiving wings will still have some forward speed, and having a massive amount of fabric pulled down will not turn into a “sail” to push us along, as the relative wind is still coming from the front, not back!

Sails only work when there is resistance against the fabric, like a sail on a boat using the wind to overcome the friction of the hull in the water. Or like a parasail pulled behind a boat… Or a kite flown in the park. Or a hot air balloon tethered to the ground. All these examples the fabric is tied to something that is putting equal and opposite force against the “push” of the wind – a sail. Our canopies are not tied to the ground! Our canopies find equilibrium when flying thru the wind, not resistance against the wind pulling an object with ground induced drag.

Here is the simple test… Turn with the wind to your back on a windy day. Pull deep brakes to make a “sail”…. As long as you are not stalled, look at the “sail” your wing made on its tail. Which way is the relative wind coming from? Likely it still is coming from the front, there is nothing that is acting like a sail above your head. (And, imagine if it did act like a sail, the wind would push the back of the canopy forward, changing the whole angle of attack into a downward forward dive. That would not “get you home” safely!)

So anyone else agree – “Myth Busted” – that pulling your brakes to make a “sail” of your canopy is not technically possible or the right explanation of why slow flight with a tail wind might work to your advantage? Or, put me in my place and show me where I am wrong…

Ok, at the sake of making this post even longer… Here is what I learned when I first learned how to fly my first fabric wing, which was a paraglider. I believe the same argument applies to our canopy, and I want to know what you guys think.

For the sake of discussion, the headwind is not so strong that you are flying backwards…

My instructors taught there were two control settings. “Min Sink” and “Best Glide”. Min Sink turns your energy into the most amount of lift so you stay up longer but sacrifices distance. Best glide turns your energy into the farthest distance you can go from that altitude but sacrifices the time you stay up.

If you are flying into the wind, or the wind is less than about 5 MPH in any direction, fly at “best glide”. This control surface setting will provide the most distance for your given altitude. You are going to be fighting the wind, so an efficient descent that consumes your energy quickly giving you the most forward distance is desirable.

If you have a tail wind of 5MPH or more, consider using “Min-Sink-Speed”…. Min sink reduces the distance you will travel on your own, but turns that energy into lift that keeps you up longer. However, since the tail wind is GREATER than the airspeed you LOST by going to min-sink – the net effect is you will go farther using the winds energy added to your own.

The opposite is true too. If you fly at “best glide” with the wind, you will come down faster and not use the wind’s energy to give you the most distance by staying up in the wind as long as possible. If you fly at “min-sink” into the wind, you are losing twice. First, because you are staying up longer, thus fighting the headwind longer, and second, because you have already sacrificed distance for time. So clearly, flying at the wrong glideslope will negatively effect your spot just as much as flying at the correct one will help you out.

Anyone want to comment??? Am I right or wrong??? I am all ears for the sake of learning.

[riggerrob 643]

Sounds like they are using the wrong analogy for the right reason.
Applying brakes helps you fly down wind because it exposes the canopy to winds aloft for an extra minute or two.

[Hooknswoop 19]

Quote

There is no argument that flying with a major tail wind can be a major advantage on a long spot… BUT HERE IS MY ARGUMENT…. There is no way a canopy floating/flying/in equilibrium with a moving flow of air, can turn into a sail!!!

You are right. Relative to the air they are flying through, they are still moving fowards and by making your body bigger, you are decreasing your glide. By getting in brakes, you will be in the air longer, taking advantage of the tail wind longer, but you should still get small for the best glide. Whoever giving the advice doesn't understand relative wind.

As for the sail concept,, that doesn't hold water either since the canopy would have to be moving backwards through the air for the relative wind to hit the back of the canopy and propel it foward.

This is a very basic concept and if they do not uderstand it, they should not be giving advice to others.

Derek

[Airman1270 0]

Quote

I have heard some very experienced “experts” (at the DZ and DZ.com, names removed to protect the guilty ) give some advice that just does not make sense. This has been argued in old threads as hijacks – but since I still hear it often I am pulling out my “Mythbusters” thinking cap and asking a fresh question for today’s audience.

The advice they give goes something like this:

“Dear student/friend. Say you have a long spot. You need everything you got to go as far as you can. What you do depends on the wind. If the wind is to your back, pull down your brakes and make your body as big as possible. BY PULLING DOWN YOUR BRAKES YOU WILL CHANGE THE SHAPE OF YOUR WING INTO A SAIL AND THE WIND WILL PUSH YOU AND MAKING YOUR BODY BIG WILL ACT LIKE A SAIL TOO.”

There is no argument that flying with a major tail wind can be a major advantage on a long spot… BUT HERE IS MY ARGUMENT…. There is no way a canopy floating/flying/in equilibrium with a moving flow of air, can turn into a sail!!!

Imagine a hot air balloon for a second… When it is on the ground, the breeze wants to push it over, and you see the balloon either dragging the basket, or laying down on its side in the breeze. This is a sail trying to overcome the resistance of the basket on the ground.

The second the balloon lifts off and is no longer attached to the ground, it is floating in the moving breeze… For the first few seconds, there IS a sail effect, as the breeze has to push the balloon to equilibrium with its environment… Once the balloon has the inertia energy it gained from the “sail action” from the breeze – its ground speed will be exactly the speed of the wind it is floating in – but it’s airspeed will be zero.

I have proof from my balloon jumps… We were trucking along groundspeed wise, but there was no wind across our face in the basket. We were going exactly the same speed as the molecules around us.

Our skydiving wings are no different than a balloon floating in space – our wings will find the equilibrium with the wind direction and fly with it’s designed forward speed thru that air, regardless of ground speed. Even with deep brakes, most of our skydiving wings will still have some forward speed, and having a massive amount of fabric pulled down will not turn into a “sail” to push us along, as the relative wind is still coming from the front, not back!

Sails only work when there is resistance against the fabric, like a sail on a boat using the wind to overcome the friction of the hull in the water. Or like a parasail pulled behind a boat… Or a kite flown in the park. Or a hot air balloon tethered to the ground. All these examples the fabric is tied to something that is putting equal and opposite force against the “push” of the wind – a sail. Our canopies are not tied to the ground! Our canopies find equilibrium when flying thru the wind, not resistance against the wind pulling an object with ground induced drag.

Here is the simple test… Turn with the wind to your back on a windy day. Pull deep brakes to make a “sail”…. As long as you are not stalled, look at the “sail” your wing made on its tail. Which way is the relative wind coming from? Likely it still is coming from the front, there is nothing that is acting like a sail above your head. (And, imagine if it did act like a sail, the wind would push the back of the canopy forward, changing the whole angle of attack into a downward forward dive. That would not “get you home” safely!)

So anyone else agree – “Myth Busted” – that pulling your brakes to make a “sail” of your canopy is not technically possible or the right explanation of why slow flight with a tail wind might work to your advantage? Or, put me in my place and show me where I am wrong…

Ok, at the sake of making this post even longer… Here is what I learned when I first learned how to fly my first fabric wing, which was a paraglider. I believe the same argument applies to our canopy, and I want to know what you guys think.

For the sake of discussion, the headwind is not so strong that you are flying backwards…

My instructors taught there were two control settings. “Min Sink” and “Best Glide”. Min Sink turns your energy into the most amount of lift so you stay up longer but sacrifices distance. Best glide turns your energy into the farthest distance you can go from that altitude but sacrifices the time you stay up.

If you are flying into the wind, or the wind is less than about 5 MPH in any direction, fly at “best glide”. This control surface setting will provide the most distance for your given altitude. You are going to be fighting the wind, so an efficient descent that consumes your energy quickly giving you the most forward distance is desirable.

If you have a tail wind of 5MPH or more, consider using “Min-Sink-Speed”…. Min sink reduces the distance you will travel on your own, but turns that energy into lift that keeps you up longer. However, since the tail wind is GREATER than the airspeed you LOST by going to min-sink – the net effect is you will go farther using the winds energy added to your own.

The opposite is true too. If you fly at “best glide” with the wind, you will come down faster and not use the wind’s energy to give you the most distance by staying up in the wind as long as possible. If you fly at “min-sink” into the wind, you are losing twice. First, because you are staying up longer, thus fighting the headwind longer, and second, because you have already sacrificed distance for time. So clearly, flying at the wrong glideslope will negatively effect your spot just as much as flying at the correct one will help you out.

Anyone want to comment??? Am I right or wrong??? I am all ears for the sake of learning.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Wow. Wouldn't it be easier to concentrate on getting a good spot?

Cheers,
Jon

[kelpdiver 2]

it's the wrong phrase to describe it, but I don't know that it warrants a 1000 word response.

BTW, the relative wind can be pretty minimal on a sailboat doing a downwind leg.

[ryoder 1,590]

You are right. Among pilots this is commonly known as the "Myth of the Downwind Turn". (The point being that if the a/c indeed "knew" which way the wind was blowing, you would need to take that into account before making a downwind turn, else the a/c might stall.)

I have encountered a shockingly large number of jumpers who believe in the myth, many with instructional ratings. I no longer bother trying to argue with them. People who don't understand physics too often resent efforts to educate them.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[kallend 2,147]

Quote

You are right. Among pilots this is commonly known as the "Myth of the Downwind Turn". (The point being that if the a/c indeed "knew" which way the wind was blowing, you would need to take that into account before making a downwind turn, else the a/c might stall.)

I have encountered a shockingly large number of jumpers who believe in the myth, many with instructional ratings. I no longer bother trying to argue with them. People who don't understand physics too often resent efforts to educate them.

Turns on a windy day can be quite different from turns in still air if there's a wind shear, which there usually is close to the ground on a windy day.

Further, since we can't see the air we generally make our turns by ground reference, which significantly affects the geometry of the turn via our control inputs when there's a significant wind.

So, like many myths, there is some element of truth to it.

...

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

[Extrakt 0]

Firstly I agree with most of what you said, but the only thing that
you don't take into account is that a sail is a wing and uses lift to
move the boat through the water. So therefore a canopy could be considered a sail, but at the same time it would be more if the
operator was connected with the earth in some way.
Also a balloon has no aerodynamic properties so it could never be considered as a sail/wing, its just getting blown over by the wind
when its in connection with the ground. I used to think a sail was pushed by the wind also until I went sailing last year, and my friend kindly pointed out that the sail is an airfoil.

The only thing the wind affects is your ground speed/track, from what i've seen/read. The min sink/max glide stuff you talked about
is spot on as far as I can tell.

Edit:Typo.

"If murder and suicide are illegal, then why is it ok to kill yourself and others with cigarettes?????"

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