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

[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.