Does wind speed and gusts affect descent rate?

[chuckakers 369]

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******Thanks! Any chance anyone with a physics background can explain this in physics terms? My friend is not convinced lol

Maybe an illustration would help??

I am a physics professor, and this topic was discussed AT LENGTH many years ago in the canopy control forum on DZ.COM, to include the effects of gusts and wind shears.

You could do a search.
Dude, this is the wrongest place to pull out your diploma.

Might want to study up before you call out Professor Kallend. Intellectually he'll eat your lunch and take your soda money.

Chuck Akers
D-10855
Houston, TX

[JohnMitchell 14]

chuckakers

Might want to study up before you call out Professor Kallend. Intellectually he'll eat your lunch and take your soda money.

He's my go-to guy for physics answers.

Okay all you doubters, imagine a goldfish swimming aimlessly in his fish tank, which is in a train traveling smoothly down a straight track at 30 mph. Now pretend that fish is you flying your canopy in a giant airmass moving smoothly across the face of the planet at 30 mph. Can you feel the constant airspeed and stabilized rate of descent no matter which way you face???

Oh Hell, if you can't, I don't care. Too many pilots still can't wrap their head around that principle either. Just make sure you give me 45 degrees before you follow my group out the door.

[monkycndo 0]

John, you were close in your description.

An even better question would be, Your fish in a tank is on a cart rolling the opposite direction as the train that is going 30 mph. The train is experiencing a 5mph crosswind. How long would it take the fish to get to Chicago?

50 donations so far. Give it a try.

You know you want to spank it
Jump an Infinity

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ahh those magnificent sounds

What goes around, comes later.

[mjosparky 3]

monkycndo

John, you were close in your description.

An even better question would be, Your fish in a tank is on a cart rolling the opposite direction as the train that is going 30 mph. The train is experiencing a 5mph crosswind. How long would it take the fish to get to Chicago?

3 days.
Sparky

My idea of a fair fight is clubbing baby seals

[JohnMitchell 14]

mjosparky

***John, you were close in your description.

An even better question would be, Your fish in a tank is on a cart rolling the opposite direction as the train that is going 30 mph. The train is experiencing a 5mph crosswind. How long would it take the fish to get to Chicago?

3 days.
Sparky

I'm giving you a B-. You didn't show your work.

[kallend 1,620]

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******Thanks! Any chance anyone with a physics background can explain this in physics terms? My friend is not convinced lol

Maybe an illustration would help??

I am a physics professor, and this topic was discussed AT LENGTH many years ago in the canopy control forum on DZ.COM, to include the effects of gusts and wind shears.

You could do a search.
Dude, this is the wrongest place to pull out your diploma.

Really? He SPECIFICALLY asked for someone "with a physics background". You should work on your reading skills.

...

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

[billvon 2,380]

>Dude, this is the wrongest place to pull out your diploma.

I'm thinkin a physics degree qualifies as having a physics background (which is what the poster asked for.)

[kallend 1,620]

Quagmirian

Reading this thread, I am feeling much more confident about my knowledge of how to design a parachute.

As said previously, the only way a canopy's descent rate can be affected is by sudden gusts, turbulence, wind shear or updrafts/downdrafts. Flying upwind or downwind doesn't mean shit to a canopy or any other untethered flying machine, because it's flying through the airmass.

Given that low level windshears usually have directionality associated with them (wind speed is lower near the ground), upwind or downwind WILL make a difference when you descend through a windshear near the ground.

...

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

[hangdiver 1]

Something I discovered flying hang gliders...if the gust velocity flying downwind is greater than the difference between your airspeed and stall speed the gust will stall the wing...not a real good feeling especially when your ground speed is 60 plus.


hangdiver

"Mans got to know his limitations"
Harry Callahan

[cbjetboy 0]

k_marr08

Thanks! Any chance anyone with a physics background can explain this in physics terms? My friend is not convinced lol

Maybe an illustration would help??

The wind speed only matters if the object is anchored to the ground or another object. As long as your wing is free to move in the air mass the wind speed is irrelevant.

Chad B Hall
Woo hoo!
My goal is to make every jump a fun and safe one. Blue skies!
Some of my videos...

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k_marr08

Maybe an illustration would help??

According to Canopy Glide Simulator

with canopy 220ft WL 0.8, flying up-wind, with no brake input:

- stronger the wind - slower vertical speed.

Vertical speed varies roughly 10% between 'no wind' and '13 mph wind'.

[matt002 0]

Its funny to see the many recycled analogies given to answer this one every few months.
Most funny as its presented as absolute fact by those that post it.
The truth is of course that the water in the fish bowel, or air in the plane/train is often moving or swirling or interacting with the other objects that share the space, and this will have an effect on the paper plane/fish/ball moving inside of it.

In most skydiving situations the basic analogy holds good and is a vast improvement in understanding over the basic ground school explanations. Occasionally tho, you will see a canopy partially collapse, or find yourself dropped or lifted a few metres when flaring.
Best advice is to jump in low, steady winds and then the basic explanation will see you right.
If you speed fly, para glide or skydive in ridiculous winds, you will require a better understanding tho.

[pchapman 261]

matt002

In most skydiving situations the basic analogy holds good and is a vast improvement in understanding over the basic ground school explanations. Occasionally tho, you will see a canopy partially collapse, or find yourself dropped or lifted a few metres when flaring.

People need to understand the basics before moving to the more complex stuff, building knowledge upon knowledge, and many have a very hard time visualizing the 'moving block of air' thing in the first place.

Otherwise all one can teach is "um, any sort of shit can happen out there, good luck".

Which may be true if you get hit by a lee side rotor or whatever you personally might be getting into, but doesn't help anyone understand how to analyze what might happen.

The answer to the original question, "Does wind speed and gusts affect descent rate?" is still:
1. No (Wind at any given speed can't, when only talking about wind speed)
2. Yes (Gusts can, whether horizontal or vertical).

[billvon 2,380]

>.if the gust velocity flying downwind is greater than the difference between your
>airspeed and stall speed the gust will stall the wing...not a real good feeling
>especially when your ground speed is 60 plus.

Agreed - and this can happen flying either downwind or upwind. Thus keeping some speed margin on gusty days is a good idea.

[hangdiver 1]

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>.if the gust velocity flying downwind is greater than the difference between your
>airspeed and stall speed the gust will stall the wing...not a real good feeling
>especially when your ground speed is 60 plus.

Agreed - and this can happen flying either downwind or upwind. Thus keeping some speed margin on gusty days is a good idea.

I agree although there area couple of differences...when flying upwind the wing will most likely stall when the gust stops thereby reducing your airspeed past the stall point with the wing angle initially higher relative to the horizon than the stall downwind...Of course this all depends on the length of the gust...flying downwind the wing will stall when the gust starts and reaches that magic #...the difference between airspeed and stall speed...with the wing angle lower to the horizon compared to the upwind stall also with an obvious higher ground speed.

I've experienced both...the upwind gust feels like being lifted by the nose...the downwind gust feels like the tail is being lifted.

As far as flying low speed wings in those conditions it isn't very smart in the first place.


hangdiver

"Mans got to know his limitations"
Harry Callahan

[kallend 1,620]

format

***Maybe an illustration would help??

According to Canopy Glide Simulator

with canopy 220ft WL 0.8, flying up-wind, with no brake input:

- stronger the wind - slower vertical speed.

Vertical speed varies roughly 10% between 'no wind' and '13 mph wind'.

There is, of course, no guarantee that the author of the simulator used accurate physics in the simulation.

...

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

[format 0]

That would be nice of him.
Aside from "Created unique real time aerodynamic parachute simulations", he's listed skidiving as one of hobbies but I can't find out if he is member on dz.com

[JeffCa 0]

format

***Maybe an illustration would help??

According to Canopy Glide Simulator

with canopy 220ft WL 0.8, flying up-wind, with no brake input:

- stronger the wind - slower vertical speed.

Vertical speed varies roughly 10% between 'no wind' and '13 mph wind'.

My guess is that this concept is correct, even if there are errors in the size of the variation. The assumption that many people seem to be using is that the airspeed of the canopy is always exactly the same, regardless of windspeed relative to the ground. My thinking is that this cannot possibly be correct. Would it not have to assume the idealised case that the aircraft (parachute and pilot) system is essentially massless, and that it has no form that disrupts the wind around it? Like it just blows completely freely at the exact speed of the wind, without providing any kind of resistance or disruption to the flow? Assuming a canopy that moves forward at 20 knots, will a 20 knot headwind actually give the canopy exactly zero groundspeed? I don't think this would be the case. There must be some reduction in the power of the wind from the real physical properties, the mass and form, of the aircraft (which blows faster in the same wind, a feather or bowling ball?), so the headwind would result in somewhat increased airspeed for the canopy and a corresponding increase in lift.

Does this make sense?

"So many fatalities and injuries are caused by decisions jumpers make before even getting into the aircraft. Skydiving can be safe AND fun at the same time...Honest." - Bill Booth

[Nathan_123 0]

In general wind speed does not affect canopy flight (unless you are really close to the ground such as landing).

Think of it this way: if your in a swimming pool that has a glass bottom will you interact any differently with the water if the pool is on the ground or suspended 100 ft up in the air? How your body interacts with the water would stay the same if you do the same movement.

The fluid dynamic stays the same as air is a fluid and its affects are the same, just harder to see. Wind affects ground speed but its a constant as far as the air flowing into your parachute at any given time as long as your in full flight.

The only that would change the pressure inside the parachute (the fluid pouring into it) would be if you hit turbulence as that's like hitting an air bubble in the pool.

Strong winds can have an effect of you are flying sideways to the wind and its strong enough to push on one side of your parachute because that can affect the shape of the wing. The change in shape is what would affect the flight, not the wind.

If you are landing then the air can be turbulent causing constant little changes in inflation pressure, causing potentially less lift.

The other affect can be if you are flying near a hill or mountain. As the air cools near the mountain is tends to 'slip' downhill because the air further away from the hill is warmer. Since cool air drops (or warm air rises) this affect can push you down more. This affect also affects BASE wingsuiters.

Atmospheric pressure will cause a lot larger changes in canopy rate of descent when compared to wind.

[unkulunkulu 0]

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Assuming a canopy that moves forward at 20 knots, will a 20 knot headwind actually give the canopy exactly zero groundspeed?

Sure, why not. You see, speeds are kinematics, they're the result. If you know that windspeed is 20 knots and airspeed is 20 knots in the opposite direction, then you defined the kinematics, there's no way they don't sum up to zero :)

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I don't think this would be the case. There must be some reduction in the power of the wind from the real physical properties, the mass and form, of the aircraft (which blows faster in the same wind, a feather or bowling ball?), so the headwind would result in somewhat increased airspeed for the canopy and a corresponding increase in lift.

Yes, but even if you fly downwind all the same effects are happening in the airmass. The only thing that changes is the direction of the windspeed vector that will be added to your airspeed to get your groundspeed.

[JeffCa 0]

unkulunkulu

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Assuming a canopy that moves forward at 20 knots, will a 20 knot headwind actually give the canopy exactly zero groundspeed?

Sure, why not. You see, speeds are kinematics, they're the result. If you know that windspeed is 20 knots and airspeed is 20 knots in the opposite direction, then you defined the kinematics, there's no way they don't sum up to zero :)

You're still in the idealised system. A parachute forward speed of 20 knots, combined with a windspeed of 20 knots BEFORE they interact. Then they do interact, and there are other effects because this is not an idealised imaginary scenario with no masses, etc. When I jump in the air, I don't start to blow away at the velocity of the wind, because I have mass/inertia. If I was massless, I would move at the windspeed. It takes time for wind to accelerate the object with mass, and there must be numerous other real-world effects caused by the materials, lines, disruption to airflow, interaction between suspended mass (jumper) and wing, etc.

I want to suggest that we're simplifying this too much. This is not a high-school level problem in which we can draw a few vectors and make a conclusion. In the absence of our ability to create a controlled test, we must account for all of the variables, then make a computer model to try to simulate it. Computational fluid dynamics is an entire field on its own. That is apparently what one guy has attempted, and it shows the windspeed affecting descent speed slightly. What variables were included in his model? I don't know. Did he miss anything? Almost definitely. But what?

Do you think I'm wrong and this truly is a simple problem with a simple solution? Am I over-thinking it?

Edited to add that I do see both sides of the debate (the simple and the complex), I just can't decide which one is correct in the real-world situation.

"So many fatalities and injuries are caused by decisions jumpers make before even getting into the aircraft. Skydiving can be safe AND fun at the same time...Honest." - Bill Booth

[wolfriverjoe 1,340]

You are right in that it takes a bit of time for the wind to overcome your inertia. That is easily demonstrated by the "forward throw" you get when you leave the plane. The plane gives you forward velocity that you bleed off as you fall.
But by the time you open (presuming a typical freefall) you are going more or less straight down (relative to the airmass), aren't you?

By the time you open, you are "one with the airmass." You really become "one with the airmass" as far as the ground reference is concerned when the plane leaves the ground.

Even if you just do a H&P, by the time you are fully open, the canopy is only "seeing" the air it is moving through. What the air is doing (presuming a steady wind) has zero effect.

Go back to throwing a toy glider in a 747. Does it matter if you throw it toward the front or back? Or side to side?

Or take a balloon in the jump plane (only inflate it part way on the ground to allow expansion). Will it float differently if you bounce it toward the front or back? Will is sit still if you don't bounce it?

Edit to add: This is all presuming a steady wind. Steady in both speed and direction. No shifts in direction, no changes in speed, no turbulence or burbles. Which really never happens.

Also:

Nathan 123

...Strong winds can have an effect of you are flying sideways to the wind and its strong enough to push on one side of your parachute because that can affect the shape of the wing. The change in shape is what would affect the flight, not the wind...

No. Just like forward speed through the air isn't affected, this won't happen either. If the wind is steady, the entire canopy is going sideways with the wind, no matter how strong it may be. Without a change in velocity (gust), the canopy won't care how fast or what direction the wind is blowing.

Does a flag ever flutter if it's hanging off of a balloon? (in the real world, variations in speed and direction will cause minor movement)

"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

[unkulunkulu 0]

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Do you think I'm wrong and this truly is a simple problem with a simple solution? Am I over-thinking it?

The relativity principle is very strong and buried deeply in the laws of physics, you can rely on it very confidently. It's way more essential than aerodynamics, lift etc (if you can speak like that). So _my_ opinion is yes, this is simpler in a way, because we only need to know one principle, we don't have no know anything about aerodynamics, because it's dynamics, but the question is about kinematics, i.e. known velocities.

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A parachute forward speed of 20 knots, combined with a windspeed of 20 knots BEFORE they interact.

What exactly do you mean that parachute forward speed is 20 knots BEFORE interaction? It is measured when air and the parachute are interacting. So everything is accounted for when we say that parachute's airspeed is 20 knots. Then, we add wind. But wind -- is just another frame of reference. And even the simplest Gallilean invariance tells us that the ground speed will be 0. It's that simple.

You see, no one has discovered any changes to the laws of physics when moving to another (inertial) frame of reference.

Disclaimer: I'm talking Newtonian physics here, relativistic effects and near-light speeds issue is omitted. In near-light speed scenarios the change in lift might become possible :)

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Hmm.. yes
but
you have an opinion on original question?