Parachutes that Drift Less

[jadebox]

I made the mistake of disputing a claim on Facebook that a certain design of parachute drifts less than other designs. While my statement that as long as the descent rate is the same, the drift distance for any two parachutes (on average) will be the same is true, I jumped into to comment when it really wasn't necessary.

Then I compounded the issue by bungling my explanation for why the style of 'chute can't affect the drift. Although I apologized for sidetracking the post, some there labelled me a troll.

Since Facebook isn't a very good platform for that kind of discussion, I'm posting here in hopes of making things clearer.

 

[jadebox]

The claim that Parachute Design X will drift less than Parachute Design Y.

This isn't true because the formula for the distance a rocket will drift under 'chute is pretty simple. You just multiply the time the 'chute is in the air by the wind speed. The shape or design of the parachute doesn't matter.

Some parachutes, however, generate lift. So, they will essentially glide. These 'chutes can turn into the wind or circle or whatever. But,if you were to drop them from some altitude in a fixed wind a number of times, the average distance they drift will still be the time the 'chute was in the air multiplied by the wind speed.

 

[jadebox]

When a parachute is deployed at altitude, it is quickly accelerated so that it is moving along with the air, much like a leaf dropped onto a river will flow with the current. So, in the air, a parachute doesn't experience what we refer to, from our reference point on the ground, as wind.

Standing on the ground, we feel wind blowing past us. But, floating in the air, you won't feel the wind. For example, assume that you feel a 20mph wind blowing east. Then you climb into a hot air balloon's gondola and rise into the air. Soon, the balloon will, relative to the ground, be moving east at 20 mph. You won't feel a 20 mph anymore because you are moving with the air at the same speed as the air.

 

[jadebox]

So, some have said that certain parachute designs present a small cross-section to the wind - for example by being flatter. It is assumed this makes them drift slower because there is less wind force accelerating them.

The concept of "wind" blowing on the parachute doesn't apply because the parachute, being carried along in the air, doesn't experience the wind as we do on the ground.

Also, the parachute doesn't feel a wind from the side. It only feels air moving towards it from one direction - the direction in which it is falling.

 

[jadebox]

The idea of a parachute moving slower than the air it is in presents some challenges. The parachute would have to supply energy to counter the force of the air trying to accelerate it and it would have to know in which direction to apply the force.

If you were sitting in a boat on a river without oars or a motor, you would have no way of propelling yourself against the current. You would just float with the current. Your rudder would have no effect because there is no water flowing past it. And, assuming you couldn't feel the wind or see around you, you'd have no way of knowing which way you are moving (or, even knowing that you are moving). In a similar manner, the parachute has no way of resisting the air flow and no way of turning into it,

 

[jadebox]

There was a suggestion that a rotating parachute might drift less because some of the wind's energy is lost in making the 'chute spin. Again, this argument fails because there is no "wind" from the parachute's perspective. The flow of air through the parachute causes it to spin, not wind hitting it from the side.

 

[jadebox]

Borrowing an analogy I posted earlier:

Another way to explain why all (non-guided) parachutes with the same descent rate will drift (on average) the same distance would be to think of two different size and weight rockets falling from the same height under parachutes of differing sizes on a day with no wind. The sizes of the 'chutes have been chosen so that the rockets fall at exactly the same rate. You'd obviously expect them to reach the ground at the same time and they will.

Now, instead of standing still under them, you stand on a moving treadmill. You'll feel a "wind" blowing as you move. From your vantage point on the moving treadmill, the rockets will seem to be drifting instead of falling straight down. But, they'll still reach the ground at the same time. This situation is equivalent to the rockets falling in a wind. If it helps, you can imagine the treadmill to be really large, as large as the earth, and circular. The earth (and you) moving while the air stays still is equivalent to the air moving while you and the earth remain still.

 

[jadebox]

References:
Parachute Recovery Systems Design Manual - https://apps.dtic.mil/docs/citations/ADA247666
Recovery Systems Design Guide - https://apps.dtic.mil/dtic/tr/fulltext/u2/a070251.pdf


Some Previous Threads:
https://www.rocketryforum.com/threads/recovery-drift.28072/
https://www.rocketryforum.com/threads/x-form-vs-classic.56575/ (Even MicroMeister agreed with me. I will miss him.)

 

[Steve Shannon]

I think you and I had this same discussion on Rocketry Planet many years ago.
For plain old dumb parachutes, you’re right; parachutes drift with the wind. Maybe not perfectly if the wind speed changes because the center of pressure of the system is way up at the canopy and the center of mass is the object suspended beneath the parachute, causing air to spill asymmetrically. If wind speed is steady (as you stipulated) eventually the system stabilizes and travels with the wind.
But sport parachutes, the kind that people use, are designed to have a forward airspeed. As long as there’s some intelligent system to control the direction they can result in a much lower ground speed than the wind speed would indicate.

 

[jadebox]

I think you and I had this same discussion on Rocketry Planet many years ago.
For plain old dumb parachutes, you’re right; parachutes drift with the wind. Maybe not perfectly if the wind speed changes because the center of pressure of the system is way up at the canopy and the center of mass is the object suspended beneath the parachute, causing air to spill asymmetrically. If wind speed is steady (as you stipulated) eventually the system stabilizes and travels with the wind.
But sport parachutes, the kind that people use, are designed to have a forward airspeed. As long as there’s some intelligent system to control the direction they can result in a much lower ground speed than the wind speed would indicate.

Yes. I'm referring to parachutes without active control. And, I'm assuming a constant wind speed for simplicity. If changes in wind velocity cause air to spill asymmetrically, the descent rate of the parachute will change. With a changing wind, the amount of drift will still be (for all practical purposes) equal to the descent time multiplied by the air velocity (but now calculated with vectors).

In the Facebook comments, some equations and text from a research paper were posted to refute my conclusions. But, those citations were from a system using active control to vary the descent rate as the parachute passed through areas of changing wind velocity. (Sort of how a hot air balloon is steered.)

 

[jadebox]

Also, the parachute doesn't feel a wind from the side. It only feels air moving towards it from one direction - the direction in which it is falling.

Clarification - I meant "- the direction in which it is moving though the air." The 'chute is falling downwards under the force of gravity. But it is also moving horizontally along with the air around it. So, it is following an angled trajectory towards the ground. The air flow felt by the parachute will be along that vector.

 

[jadebox]

Another counterpoint that was offered is that a parachute doesn't instantly accelerate to match the speed of the air.

This is true, but not really relevant. First of all, at ejection the rocket may be moving any direction in relation to the air and at some random speed. So, the exact time it takes the parachute to accelerate to the speed of the air around it depends on some essentially random variables that will change with every flight.

But, assuming the recovery system deploys as intended, like a helium balloon released into the wind the amount of time it takes to match the speed of the air is very small. Drift distance can be reduced, of course, by delaying the opening of the parachute. But, that increases the descent rate (at least until the 'chute fully opens). So, the drift distance is still a simple function of the time of descent and wind speed.

 

[jadebox]

There was also some take about mass (which may be related to the citations from the paper I mentioned earlier). I think the suggestion is that inertia would keep the parachute from drifting as far. But, the comparison is between two parachute designs providing the same descent rate for the same mass. So, mass can't be an issue because it's the same for the two things being compared.

 

[Handeman]

Everything you've described is correct. I've had this discussion myself. It's amazing how intelligent people can't get the idea of the chute dropping through a moving air mass and the only air flow the chute sees is vertical because of gravity. I think a lot of the issue is that people are creatures of the ground and naturally see things in the air from a ground perspective. Some just can't seem to change that perspective and see it from a point of view of being in the air and moving with an air mass. They can't seem to understand that wind is moving air as observed from the ground. If you move your point of observation off the ground, you no longer have wind.

 

[Arsenal78]

I’ve personally flown rotating chutes and standard chutes in the same conditions. The rotating chute doesn’t drift as far as a regular chute. Also a rotating chute stabilizes a rocket so it doesn’t swing as much, if at all, whereas a standard chute can (and has in my experience) made a rocket fall diagonally and snap a fin. You two confuse yourselves. Yes WIND is a term us people invented to describe moving air, however moving air is moving air, regardless of what you call it. I’ve seen a rotating chute spin a little faster when the wind picked up. Please provide HARD physical evidence of YOU personally flying a rotating chute.... I’ll wait... (BTW I know who you had the discussion with and until you’re as accomplished as he is in the subject, please be cautious as to not let your ego get the best of you like another vendor on here and lose future customers”.

 

[solid_fuel]

Everything up to Steve's first response could have been one post and you might appear less maniacal

 

[jadebox]

Everything up to Steve's first response could have been one post and you might appear less maniacal

I separated the points to make it easier to reply to each one individually.

 

[georgegassaway]

I

agree

with

your

first

eight

posts.

posts.

 

[jadebox]

Yes WIND is a term us people invented to describe moving air, however moving air is moving air, regardless of what you call it.

The airflow past the parachute is created by the parachute falling. It isn't the same as the wind that you feel on the ground. See Handeman's post above. He explains it better.

There is no ego involved other than some frustration at not being able to describe the physics as well as I wish I could.

 

[swatkat]

I've got a 730 bridge nearby... now if we just had two chutes that are certed to fall at the same rate with a given weight...

 

[georgegassaway]

But splitting into 8 was not needed.

I have used similar ways to describe how once a model is in the air, it does not "feel" wind (ignoring gust effects). From the model perspective, it is in "calm" air (again ignoring gusts), while the ground is drifting below it at whatever the wind speed is.

To the claim that a rotating chute, or any "special" chute will NOT drift the same, (IF they both have the same rate of descent), then I propose it is for the ones claiming the difference to prove it as they are the ones making claims against known physics. Fly a big rocket that drops two weighted payloads, one with a normal chute and one with a "low drift" chute (I've seen an R&D project , which won, test two different G Streamers with equal weighted payloads, by carrying both up in an R/C electric sailplane and dropping both for easy testing. That one was testing for the best streamer configuration for duration, not drift, but the test method would work just as easily. Also the streamer that stayed up longer than the other, drifted farther).

The chutes MUST be built/weighted to have the exact same descent rate for the test to be valid, so ideally they would land at the same moment. And it would take some calibrated test drops to determine that, forget computer sims which are useless for precise prediction of descent rate (only very crude ballpark, if the chutes deploy nominally and nothing swings or wobbles).

They would probably land close together. If not, random changes in airflow (including up or down air) may account for that, and the "normal" chute may have a random "glide" to it to a small extent. But that would tend to randomize out such that the normal chute might drift LESS than the claimed low-drift chute (since any glide vector could as likely point INTO the wind as pointing away from the wind direction. Or, crosswind = little significant difference from pad).

Do it at least ten times, with valid drops each time (both chutes deploying at the same altitude, both landing pretty close in time or else finding out why one is landing later than the other). Heck, enter it as an R&D project.

Also note that on an ideal day to fly rockets, sunny with very little wind, there's likely be thermals around and the wind will also change direction a lot, and wind velocity varying rapidly, due to those thermals. So such testing would be better on an overcast day with little to no thermal activity. And oh yeah, taking notes such as wind velocity each minute for 10 minutes before launch, wind during flight, and wind each minute up to 5 minutes after landing. And any notable changes in direction, at what times those direction changes occurred. and of course how far away the chutes landed from each other, any cross-wind vector, and distance from pad (have to know that to determine any % difference in drift, if one lands 20 feet downwind from the other and the closer one landed 1000 feet way..... that's a 2% "noise level" result.

For a two chute drop test, everything else being equal (and same rate of decent from identical deployment altitude and time):

I think the odds of normal chute drifting farther on one test: 50%

I think the odds of "low-drift" chute drifting farther on one test: 50%

Which is why it'd take a good number of flights to get statistically valuable data.

 

[swatkat]

730 foot I should say.

 

[ThirstyBarbarian]

Here is my rotating parachute. You can see that the chute protector is blowing in the wind like a flag. So obviously the parachute is not perfectly matched to the wind speed. If it were, then the chute protector would have no air moving past it to blow it around.

 

[ThirstyBarbarian]

Yes. I'm referring to parachutes without active control. And, I'm assuming a constant wind speed for simplicity. If changes in wind velocity cause air to spill asymmetrically, the descent rate of the parachute will change. With a changing wind, the amount of drift will still be (for all practical purposes) equal to the descent time multiplied by the air velocity (but now calculated with vectors).

I think this is the root of the disagreement. You are assuming a constant wind speed for simplicity, and you are saying things like "for all practical purposes". Those are ways of ignoring the differences in how parachutes will perform in changing wind and saying that small differences are equal to no difference at all.

If the wind speed is perfectly steady, then I agree, the chute will quickly begin to drift along in the wind at the same speed as the wind. But in the real world, wind is not steady and unchanging. There are gusts and eddies and changing currents, and the chute may drop through different air currents at different altitudes. How the chute reacts to those changes will make a big difference in how far it drifts. If a chute responds rapidly to changing wind speed and direction, then it will follow a different path from one that does not respond quickly to those same changes.

For example, if one parachute has a large cross section and another has a small cross section, then they will react differently to a change in wind speed or direction. You said earlier that this would make no difference because:

The concept of "wind" blowing on the parachute doesn't apply because the parachute, being carried along in the air, doesn't experience the wind as we do on the ground.

Also, the parachute doesn't feel a wind from the side. It only feels air moving towards it from one direction - the direction in which it is falling.​

That's only true in your idealized situation of a constant, steady wind that never changes speed or direction. If two falling parachutes are hit by a sudden gust of wind, they do feel that wind from the side. They do not instantaneously accelerate to match the new wind speed and direction. They accelerate in proportion to the force applied to the chute by the wind, and that force will be determined by the cross section presented to the wind. The two chutes will drift a different distance and direction if they accelerate at different rates. It won't take long before they have both matched the wind, but it will take some time, and that time will be different for the two chutes. Now you might say that for all practical purposes, it's not a big difference, but that does not mean there is no difference at all.

 

[jadebox]

The parachute is descending through the air, so, of course the protector will flap around. And, if the parachute generates lift, it may be gliding which also causes air flow around it.

One thing I don't think is understood is that air always flows around an object from one direction. When a parachute is descending, the air only flows past it from the direction that is is heading. There isn't also a second airflow from the side or horizontally parallel to the ground. If you stand still, you might feel a breeze hitting you from the left. But, if you start walking quickly, it will feel like the breeze is coming more from in front of you. You don't feel two separate breezes. You experience the vector product of the wind and the airflow you create when moving forward. So, as the parachute falls through the air, there is no wind pushing it on the side, there is just the flow of air around it from the direction that is falling.

When the wind changes, a parachute isn't hit on the side by a gust of wind. The parachute is falling, so the air is passing it from the same angle as it is descending. When the wind changes, the angle that the air flows around it will change. So, for an instant the cross section of the parchute exposed to the air stream will change. This would change the direction the the parachute is descending and, for a bit, the descent rare. If it reduces the drift, it will be because the rate of descent increases for a bit.

In any case, there is no way for a parachute to "know" which way or how fast the wind is blowing. So, there is no way for one design of an unguided parachute to drift less at the same rate of descent. If you need more evidence, as a pilot how they determine the wind speed and direction when in flight and consider how an unguided parachute could attempt that task.

 

[Maxwelljets]

I think part of the issue here is that horizontal motion under parachutes is due to two separate components, and most people refer to them together as "drift." To make things clearer, it's better to think of drift as the component of the horizontal travel caused by the parachute moving at zero horizontal airspeed, and glide as the component of horizontal motion caused by a nonzero horizontal airspeed. For the most part, the original poster is correct- in most cases, parachute design has very little effect on the drift component of horizontal motion. What it can make a difference in is the glide component. On the one hand, you have parachute designs with large skirts (e.g. Sky Angle, Rocketman, etc). These skirts provide a fairly large resistance to gliding, but cannot provide any resistance to drifting with the wind since they will naturally tend toward zero horizontal airspeed. On the other hand, you have things like parafoils that are specifically designed to glide at perhaps a 5:1 glide slope. If you drop a parafoil on a perfectly calm day, it's going to glide a significant horizontal distance. Flat parasheets, ellipsoid canopies, and pull-down apex (iris) chutes will all have some intrinsic amount of gliding

The point being made by the other individual in the facebook thread was that while "zero horizontal airspeed" is a good approximation most of the time for determining the average landing location, it does take a nonzero time to reach zero horizontal airspeed. If the descent time is short, or the terminal velocity is high, this time taken to approach zero airspeed may be significant enough to reduce the horizontal distance it will travel. Additionally, the claim of a rotating parachute often having a lower total horizontal distance seems like it should have merit, since it should glide less than a comparable non-rotating parachute, though this would only come into play for longer descent times.

The direction of gliding will have nothing to do with the direction of the wind as measured from the ground.

 

[jadebox]

The direction of gliding will have nothing to do with the direction of the wind as measured from the ground.

The point that I was making is that two unguided parachutes with the same descent rate will, on average, drift the same distance. An increased glide ratio will decrease the rate of descent. But, the average distance that a gliding parachute will travel is still equal to the time it takes to descend multiplied by the wind speed, just like for a non-gliding 'chute (or any falling object at terminal velocity). So, it isn't reasonable to say that a parachute that generates lift drifts less (or more). At the same descent rate, sometimes it will drift less, but other times it will drift more.

 

[ThirstyBarbarian]

The parachute is descending through the air, so, of course the protector will flap around. And, if the parachute generates lift, it may be gliding which also causes air flow around it.

If you look at the video, it's pretty clear that the protector is flapping in a wind, not being blown by the descending through the air. That chute was intentionally built to be oversized, so the descent rate is only about 12 fps at most. The protector is not reacting to a 12 fps descent -- it's flapping in air that is moving horizontally, not vertically.

And I don't think the chute is gliding. I was there watching this flight, and the chute hung in the air steady as a rock. And the rocket came down very close to the launch pads, so I don't think it glided. But if it did glide, then doesn't that mean that not all chute designs drift the same way?

One thing I don't think is understood is that air always flows around an object from one direction. When a parachute is descending, the air only flows past it from the direction that is is heading. There isn't also a second airflow from the side or horizontally parallel to the ground. If you stand still, you might feel a breeze hitting you from the left. But, if you start walking quickly, it will feel like the breeze is coming more from in front of you. You don't feel two separate breezes. You experience the vector product of the wind and the airflow you create when moving forward. So, as the parachute falls through the air, there is no wind pushing it on the side, there is just the flow of air around it from the direction that is falling.

When the wind changes, a parachute isn't hit on the side by a gust of wind. The parachute is falling, so the air is passing it from the same angle as it is descending. When the wind changes, the angle that the air flows around it will change. So, for an instant the cross section of the parchute exposed to the air stream will change. This would change the direction the the parachute is descending and, for a bit, the descent rare. If it reduces the drift, it will be because the rate of descent increases for a bit.

What you are saying here does not make any sense to me at all. Maybe I'm not getting it, or maybe you need to explain it better, or maybe it's wrong. OF COURSE, a parachute can be hit on the side by a gust of wind! And of course wind exerts a force on the chute when it hits it from the side. We are talking about chutes drifting in the wind, right? So I think we both agree that a parachute is affected by wind, and a chute does drift with the wind. In order for that to happen, the chute must respond to forces exerted by the wind.

Say, for example, a chute is falling in zero wind, it is going to fall straight down, agreed? Now say that as it is falling, there comes a gust of wind moving horizontally at 10 miles per hour relative to the ground. Can you tell me what you think is going to happen when that gust of wind comes through?

Here's what I think is going to happen. The parachute is going to experience a 10 mph wind blowing on it from the side. It's already descending through the air, so, like you said, the net flow of air around the parachute is not 2 aisrtreams, it's the sum of the two, but now there is a 10mph component that is coming from the side that wasn't there before. That 10 mph wind from the side is going to exert a force from the side and begin to accelerate the parachute to the side. As it accelerates to the side, the airflow coming from the side will start to diminish as the parachute starts to match the speed of the horizontal wind. Eventually the chute will catch up and match the speed of the wind, and it will no longer be accelerating and will no longer be experiencing a force of wind from the side. It will be in equilibrium, descending and also moving horizontally with the wind. Does that sound right?

So back to the different chute designs. I'm saying not all chute designs are going to react to the same 10 mph gust the same way. Some are going to present a different cross-sectional area to the gust than others. That means that the same 10 mph gust will exert a smaller force on the chute with the smaller cross section than it will exert on the one with the bigger cross section. And that means it will cause a smaller acceleration to the side. If the side gust of wind is maintained long enough, then both chutes will eventually come to equilibrium and will drift at 10 mph with the wind, but the one with the larger cross-section will match the wind speed faster, and the one with the smaller cross section will take longer to get up to speed. The smaller one will experience a smaller force but for a longer time, until it comes to equilibrium. By the time the smaller chute is up to speed, the larger one will have drifted further with the wind than the smaller one.

And that's just taking into consideration the cross-sectional area and not other design factors. Some kinds of chutes flutter and collapse a bit on the side when they are hit by a gust of wind. Others don't. Some may tilt up on the side when hit by a cross breeze and will really pick up speed quickly. Other won't. All of that and many more factors will determine how a chute drifts on a wind.

In any case, there is no way for a parachute to "know" which way or how fast the wind is blowing. So, there is no way for one design of an unguided parachute to drift less at the same rate of descent. If you need more evidence, as a pilot how they determine the wind speed and direction when in flight and consider how an unguided parachute could attempt that task.

The parachute does not have to "know" anything to be affected by changing wind currents. Two different non-sentient objects can react differently to outside forces without having to know anything.

 

[ThirstyBarbarian]

The point that I was making is that two unguided parachutes with the same descent rate will, on average, drift the same distance.

What do you mean by "on average" in this sentence? Do you mean something like "more or less" or "for all practical purposes"?

I think the main point of the earlier Facebook discussion, and the point i'm trying to make here, is that there are differences in how different chute designs react to side breezes. Some will drift more than others. If your point is that it's not much of a difference, then maybe that's a matter of opinion, and we can disagree on that. My point is that there IS a difference. That's a fact.

 

[jadebox]

And I don't think the chute is gliding.

If you felt a wind on the ground, yet the parachute appeared to be coming straight down, then it was most definitely gliding into the wind.

I think you're confused about frames of reference. To avoid worrying about the air flow caused by the parachute falling, lets replace it with a hot air balloon that is not rising or falling. When the balloon is tethered to the ground, you, standing in the open gondola, can feel the wind blowing past you because you are anchored to the ground and not moving. Once the rope is cut, the balloon, begins moving at the speed of the wind (relative to the ground). Inside the gondola, you can't feel the wind because you and the air are moving the same speed. If you jump out of the gondola, you will now feel air rushing toward you, but it will be from the direction you are traveling. If you deploy your bat wings and begin gliding, you will now feel air rushing toward you from the new direction that you are traveling. From the time the rope was cut, you have had no way to determine which way or how hard the wind is blowing on the ground (unless you cheat and look at the ground). A parachute has no way of looking at the ground, so it has no way of know which way the wind is blowing.