How does air spillage help keep cargo chutes apart during descent?

[jostpuur]

How are parachutes like this made work:

http://dolloutfits.com/vs/23.jpg

Why do the parachutes remain on the sides nicely instead of colliding in the middle?

 

[Ranger Mike]

it is all in the D bags..deployment bag each chute is pack into. these bags use a static line attached to the airplane/ helicopter and a A piece of parachute line ( string) is tied to the opening loop at the apex of the canopy(top of the parachute). the weight of the pallet ( object to be dropped) pulls the shroud line out and then the folded chute from the D bag and once fully deployed but not opened , the tie is broken from the D bag. ( the jump master has to haul in the static line and D-bag before the plane can land or he has to cut it away) Anyway, each canopy begins to open simulataneoulsy. Notice the shroud lines running up to the panels prevent entry of the deploying chutes. All three caponies are fully deployed and air starts to fill each chute..no one chute is under the other thus preventing the possibility of one chute tangling into another.
parachute riggers are highly trained people who rig the pallet for the air drop...

 

[jostpuur]

I understood nearly nothing of that explanation.

no one chute is under the other thus preventing the possibility of one chute tangling into another.

I wasn't speaking about possibility of one chute to get tangled with other ones ropes, but about the possibility of one chute colliding with other chutes.

Could it be that the chutes are not symmetrical in weight distribution, but instead have the outer sides heavier than the inner sides?

 

[Ranger Mike]

you asked how they work and some times they do smack into each other. they are identical in construction. it is in the rigging, amigo!

 

[FredGarvin]

Not sure if it really answers the overall question, but I didn't see anything in particular in the Apollo system report on special rigging. The one thing I thought was neat was that the three chutes were ejected 90 degrees from the vertical. I would think that by doing this simultaneously that the parachutes would reach equillibrium in the 120 degrees configuration before they could run into each other perfectly vertical.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730062665_1973062665.pdf

 

[nvn]

jostpuur: I have an answer, but is this question a school assignment?

 

[Phrak]

Let me restate the original question. Why are the chutes forced apart?

They are free to spin on their axis, as they are only supported by cord. Thus, there is nothing in asymmetrical rigging that would help keep them separated while each is free to rotate around its point of suspension. Any I wrong about this?

Why aren't they colliding?

 

[jostpuur]

jostpuur: I have an answer, but is this question a school assignment?

I don't know how to prove this to you with certainty, but I can promise that I've already got my master's degree in mathematics, and I'm not attending lectures or classrooms about stuff like this anymore.

 

[DaveC426913]

I can't be sure but I'll bet there are vents in the chutes that cause spillage. The spillage acts as a propulsive force, pushing the chute the direction opposite the vent.

The only thing I can't be sure is how they would keep the chutes from spinning on their own axis, bringing the vents to the outside, causing the propulsive force to act inward.

 

[nvn]

jostpuur: I think the parachute cords on the outer perimeter of the three-parachute configuration are slightly shorter than the inner parachute cords. This causes more of the trapped air to escape underneath the inner edge of each parachute. This radially inward net air flow creates a horizontal, radially outward propulsive force on each parachute. The parachute therefore moves radially outward until the outward propulsive force is balanced by slightly higher tension on the inner cords than on the outer cords.

I'm currently not sure what guarantees one of the three parachutes will not spin. Perhaps there is a cord linking the three inner cords of each parachute, which would prevent spinning.

 

[Ranger Mike]

cargo chutes do not have gores or open panels in them. a steerable parachute like a square or a MC1-1 steerable troop chute has a gore located at the rear of the canopy. this gore can provide significant forward thrust...SF teams exit at 25000 feet AGL and can land 25 miles from the jump point. A jumper must steer the chute into the wind to stall it out before landing..if he did not do this the forward motion would add to the wind speed and he would get busted up on landing. Ref: cargo chutes- you can not have a 40 ton vehicle wandering around in the air all over the Drop Zone. you want the pak to exit the rear of the airplane and land where the DZ officer wants it to. This is why so there is no steerable gores or forward motion for these cargo chutes..they are all the same length and dimensions and they do not spin..spin is bad for chutes and yes they do smack into each other but the air fills each canopy to the maximum expansion and this pressure is increased the closer to ground level the pak descends...

 

[DaveC426913]

cargo chutes do not have gores or open panels in them.

Well correct me if I'm wrong but even round chutes sometimes have slots in them. This may not be the case for personal chutes, but that's not the same as multiplie chutes for cargo.

This is why so there is no steerable gores or forward motion for these cargo chutes..

Of course there's no forward motion, that's not what the OP is asking about.

and yes they do smack into each other but the air fills each canopy to the maximum expansion and this pressure is increased the closer to ground level the pak descends...

None of this answers the question as to why the chutes are, in fact, staying (for the most part) separated from each other. You can see it in the pics and in videos.

 

[Phrak]

cargo chutes do not have gores or open panels in them. a steerable parachute like a square or a MC1-1 steerable troop chute has a gore located at the rear of the canopy. this gore can provide significant forward thrust...SF teams exit at 25000 feet AGL and can land 25 miles from the jump point. A jumper must steer the chute into the wind to stall it out before landing..if he did not do this the forward motion would add to the wind speed and he would get busted up on landing. Ref: cargo chutes- you can not have a 40 ton vehicle wandering around in the air all over the Drop Zone. you want the pak to exit the rear of the airplane and land where the DZ officer wants it to. This is why so there is no steerable gores or forward motion for these cargo chutes..they are all the same length and dimensions and they do not spin..spin is bad for chutes and yes they do smack into each other but the air fills each canopy to the maximum expansion and this pressure is increased the closer to ground level the pak descends...

Mike, perhaps you can answer this one. I think the answer may lay in the rigging. Do all cargo shoots used in multiples have the extra long rigging? The lines are about 4 chute diameters in length. In personnel chutes they seem to be about twice the chute diameter.

 

[Ranger Mike]

ok I really did not want to get into this but inquiring minds insist. Parachute are application specific. if you want to concentrate a force in a small area you do not want wide dispersment via steerable chutes so no open gores should be used. Typical cargo chute come in 34, 64 and 100 ft. diameters. depending upon the requirement.. they may be single door bundle or rigged in clusters up to 8 chutes for platform delivery system.
Now the wrinkle..the military has developed a steerable cargo chute delivery system that can exit the Ac at 25,000 ft AGL and land 22 kilometers away..it has been done and it does work...and the chute is steerable. This is not the typical cargo delivery system used in airborne resupply operations..back to the round canopies. I am most familiar with the 64 foot model that has 64 gores and no openings. I understand improvements have been made since my jumpmaster days and there may in fact be cargo chutes with open side gores but if this is the case...most certainly these are spaced in such a way as to not to provide forward thrust. Regarding cargo chutes that are bundled..they do bump into each other if excess wind is present other wise they descend in a separated manner..now my turn..why would you think they would all want to share the same space?

 

[Ranger Mike]

i will give you another couple of hints...
hint number 1...think garden hose and a stream of water..what happens when you stick your palm into the stream?
hint number 2.
the air fills each canopy to the maximum expansion and this pressure is increased the closer to ground level the pak descends...
Hint number 3 . they do bump into each other if excess wind is present other wise they descend in a separated manner..

think of the chute as a giant air brake...
what is happening while the pak descends?

 

[MacLaddy]

So the air being released from each chute on the sides is forcing them apart? Sort of a spill-over effect.

Probably didn't even remotely describe that correctly, but hey, I'm new at this.

Sorry for just jumping in here.

 

[mheslep]

I can't be sure but I'll bet there are vents in the chutes that cause spillage. The spillage acts as a propulsive force, pushing the chute the direction opposite the vent.

Yes indeed there are vents, per a NASA documentary I viewed on the Mar's rover landings. Calculating the optimum vent size and shape is a difficult problem, involving iterative wind tunnel tests.

 

[Ranger Mike]

Trapped air inside the canopy is low speed high pressure and spills out around the periphery of the canopy where it meets high speed low pressure airstream moving over the outside canopy. Now you got a boundary layer of flowing air.
If wind speed at altitude us high enuff to overcome this, the chutes will smack. Newer cargo chutes for very high speed deployment have four open gores at 12,3,6 and 9 o'clock to prevent panel blow out due to the very high exit speed
a C130 aircraft used to slow to 120 knots before dropping
this made one big slow target over the DZ
so the new chutes permit higher drop speeds

 

[mheslep]

http://marsrover.nasa.gov/spotlight/20040826.html

After more discussion and design modification, engineers fixed that problem, too, by adjusting the size of the vent that allowed air to flow through the chutes. It turned out that because the chutes were stuffed into a smaller space, they were much denser and didn't float through the air as before. The team made the chutes smaller for better fit, and sewed them from thicker, stronger material.

 

[Phrak]

Perusing the images from a google search on 'cargo parachutes', it appears that the rigging from chutes to common tie point tend to get progressively longer with the number of chutes deployed.

The large arrays of 4 or more don't appear to be in any general order, but appear free to find their own position in the group.

It appears that only aerodynamic forces push the parachutes apart without the aid of any special asymmetrical rigging or vents, and are capable of pushing each parachute about 10 degrees from vertical under static conditions.

The more parachutes deployed in a group, the longer the rigging must be with respect to the parachute diameter to meet the ~10 degree value.

 

[russ_watters]

It appears that only aerodynamic forces push the parachutes apart without the aid of any special asymmetrical rigging or vents, and are capable of pushing each parachute about 10 degrees from vertical under static conditions.

Yeah, I was thinkinig that a single round parachute is inherrently unstable and that instability causes 3 to be inherrently stable.

If a parachute tilts to one side, air spilling out the other side will cause it to slip further in the other direction, until the weight of the payload makes it swing back. With three parachutes, due to the angle each is from the vertical, these aerodynamic forces would tend to pull them apart.

 

[Phrak]

Yeah, I was thinkinig that a single round parachute is inherrently unstable and that instability causes 3 to be inherrently stable.

I hadn't thought of it that way--not thinking of asymmetrical spillage. I believe you've nailed it.

There's even a half skirt visible in the picture on the left chute that would tend to encourage asymmetrical spillage. I wonder if random rotational forces together with interaction with the other chute would tend to force the skirt to the outside.

 

[russ_watters]

Here's an interesting photo from the wiki page on parachutes: http://en.wikipedia.org/wiki/File:Apollo_15_descends_to_splashdown.jpg

One of the three parachutes on apollo 15 failed to inflate!

 

[Phrak]

Here's an interesting photo from the wiki page on parachutes: http://en.wikipedia.org/wiki/File:Apollo_15_descends_to_splashdown.jpg

One of the three parachutes on apollo 15 failed to inflate!

Very cool. Redundancy paid off on that one.

Belatedly, I vaguely recall that all the venting done to a parachute was done to 'stabilize' it. I hadn't understood what that meant up to this time.

 

[Ranger Mike]

round canopies oscillate big time..this is a major draw back..the shape is the biggest drag and slows the load to max for softest landing...and has minimum drift so wil land where dropped better than other shapes,,in fact the military has developed mechanical devices to pull the risers on a cargo chute to slip it in one direction or the other

 

[jostpuur]

I can't be sure but I'll bet there are vents in the chutes that cause spillage. The spillage acts as a propulsive force, pushing the chute the direction opposite the vent.

The only thing I can't be sure is how they would keep the chutes from spinning on their own axis, bringing the vents to the outside, causing the propulsive force to act inward.

I became convinced that the air flow creates some kind of effective repelling force. But I don't think that the vents are in key role for this.

So the air being released from each chute on the sides is forcing them apart? Sort of a spill-over effect.

It appears that only aerodynamic forces push the parachutes apart without the aid of any special asymmetrical rigging or vents, and are capable of pushing each parachute about 10 degrees from vertical under static conditions.

This must be the way it is.

Here's an interesting photo from the wiki page on parachutes: http://en.wikipedia.org/wiki/File:Apollo_15_descends_to_splashdown.jpg

One of the three parachutes on apollo 15 failed to inflate!

Seemingly not so uncommon: Ares 1-X Launch Parachute Failure

Interestingly the Ares test occurred after the previous discussion in this thread.

 

[Ranger Mike]

MALFUNCTION
every paratroopers nightmare
improper packing of the chute is one of the main reasons
that is why the entire parachute rigger platoon is required to make parachute jumps MONTHLY
and they have the least DWI record in the Division
if you knew the chute you are packing may be the chute you jump with you would adhere to the riggers motto
" I will always be sure"

 

[Borek]

why would you think they would all want to share the same space?

If you will draw forces acting on the parachutes it is obvious that they are being all pulled to the center. In the case of one parachute equilibrium position is exactly above the load. It doesn't mean there are no other forces causing separation in the case of three parachutes, but the idea that they would be dragged to the center seems reasonable.

 

[pallidin]

The answer is tension lines.
This causes not only vertical separation but horizontal as well, as those lateral chutes are tilted by tension!

 

[DaveC426913]

The answer is tension lines.
This causes not only vertical separation but horizontal as well, as those lateral chutes are tilted by tension!

And where do these tension lines go from and to that they would draw the chutes outward from the centre?

 

[pallidin]

@Dave

The tilting of the lateral chutes(by virtue of the tension lines) causes them to be off-centered.

The tension lines are placed to provide that a particular lateral chute is aerodynamically 'lop-sided" This causes the chute to move away from the center, very quickly.

 

[pallidin]

Hmmm.. I think I'm way wrong here.
I think I'll just shut-up and listen :)

 

[sophiecentaur]

What worries me about the explanations. so far, is that no one has mentioned the Bernouli effect which could imply that, if there is a lot of air rushing fast through the gap between the 'chutes, there could be a low pressure in there, pulling them together. Perhaps, though, there is, in fact, MORE air, going faster, past the periphery of the set up and that is producing a slightly lower pressure on the outside of the three 'chutes, pulling them apart, against the tensions in the lines which will be pulling them together. Yep - that must be something like the reason.

 

[jostpuur]

Bernoulli principle only applies to noncompressible fluids, so you can forget it when dealing with gases.

 

[sophiecentaur]

OK Then whatever you call it in the explanation of the operation of the aerofoil is what I mean. You still get a drop in pressure when a gas is flowing fast - don't you?

 

[jostpuur]

OK Then whatever you call it in the explanation of the operation of the aerofoil is what I mean.

I would try to understand airplane wings from the point of view that the wings will shoot air downwards when they move forward. So the lift follows as result of Newton's laws.

You still get a drop in pressure when a gas is flowing fast - don't you?

Well yes it is true that there are lot of phenomena where fast flowing gas is in lower pressure. Perhaps I should have not tried to deny that...

Anyway it is a fact that the Bernoulli principle relies on the assumption of noncompressability.

 

[mheslep]

Bernoulli principle only applies to noncompressible fluids, so you can forget it when dealing with gases.

Not true. Bernoulli's law was developed from the study of uncompressibles, but it also applies to gases http://en.wikipedia.org/wiki/Bernoulli's_principle#Compressible_flow_equation", for instance when the flow rate is slow compared to the speed of sound as it is in the topic of this thread.

 

[sophiecentaur]

Jotspur
"I would try to understand airplane wings from the point of view that the wings will shoot air downwards when they move forward. So the lift follows as result of Newton's laws.
"
I don't think the simplistic argument based on Momentum change is sufficient to explain the force generated by an Aerofoil or, as an extension, to the fact that a yacht sail provides windward 'lift'.
I know you can fly an aeroplane upside down by using the momentum change of the air but that is a very inefficient mode of flying and 'planes are designed much better than that when they need to be efficient.

Also, how else is drag to be explained? Your simplified argument would imply that vehicles should be pointed at the front and the back end shape would make no difference at all - ideas which were tried long ago and found not to work.

 

[ScienceGuyOrg]

With water rockets as one of my hobbies, I can sure relate to just one parachute tangling in ways that do not seem possible. I have never attempted using more than one but understand for larger parachutes using a drogue parachute is a good idea.

Bill Kuhl

 

[DaveC426913]

but understand for larger parachutes using a drogue parachute is a good idea.

Are you sure that isn't a drouge parachute?

( Running gag)

 

[Cyrus]

With water rockets as one of my hobbies, I can sure relate to just one parachute tangling in ways that do not seem possible. I have never attempted using more than one but understand for larger parachutes using a drogue parachute is a good idea.

Bill Kuhl

I knew your name looked familiar, you fly at FSA. Your picture is on the January newsletter.

 

[sophiecentaur]

Are you sure that isn't a drouge parachute?

( Running gag)

How are running and drougeing connected? Are they both methods of flight?

 

[DaveC426913]

How are running and drougeing connected? Are they both methods of flight?

https://www.physicsforums.com/showpost.php?p=2361908&postcount=16

https://www.physicsforums.com/showpost.php?p=2469690&postcount=2
https://www.physicsforums.com/showpost.php?p=2469686&postcount=1
etc. etc.

Doing a PF search for 'rouge' will turn up dozens of posts...

And finally:
https://www.physicsforums.com/showpost.php?p=2516732&postcount=112

 

[mugaliens]

Quite a few answers given herein are just plain wrong. Some answers were technically correct but failed to answer the question as to what keeps the chutes apart (it's not "in the rigging," though my hat's off to riggers).

"Air spillage" is the technically correct answer, and some of the loads we spit out the back had eight chutes. I've seen heavier systems for C-17s with up to twelve chutes. It still works.

Symmetrical chutes will separate, so asymmetrical design isn't the answer.

Some chutes have vents, some don't. But aside from JPADS, vents for cargo chutes are symmetrical.

The parachute cords are not of differing lengths.

Cargo chutes for Apollo most certainly had vents.

Cargo chutes for JPADS most certainly have vents (gores) and are computer-steered to precision landing at the PI, even with 60k lb loads.

Personal found chutes often have vents to aid in stability and provide some forward motion. They're normally closed at opening, and the parachutist performs a "four-line modification" which simply releases the panels and allows him/her to steer the chute.

MacLaddy: "So the air being released from each chute on the sides is forcing them apart? Sort of a spill-over effect." Bingo! It's that simple. Kudos for the new guy!

Someone asked about the length of lines for cargo chutes - yes, they're longer, proportionally, to canopy size as compared to personnel chutes. This reduces the deflection angle required for complete separation.

"Tension lines" aren't the answer.

Borek mentioned forces acting on the parachute are all pulling them towards the center. Mostly true. However, with the spillage keeping them apart, they're canted, and will "fly" a bit the same as a towed parachutist (training and entertainment) behind a boat or vehicle rises when towed, even with a plain vanilla round T-10C. However, this only lessens the centering force, making it easier for spillage to help keep them apart.

Sophiecentaur is correct in stating that the air traveling past the periphery is moving more rapidly than the stagnation air spilling out from under the chutes which rises between them. However, the momentum change is precisely that required to counter the weight of an airplane or slow the descent of a parachuted object. Wings are typically designed to work rightside up and do a better job of imparting that change in momentum. Some acrobatic wings are largely symmetrical, however, and work nearly as well upside down as rightside up.

Well, that's all, folks. Hope this clears up some things.