Stable descents

[BsSmith]

All of my rockets under 6 pounds have the same problem, they all make a perfect boost, and then they deploy the parachutes and start spinning on the way down. By the time most of them hit the ground, they're parallel to the ground. Putting the parachutes on the ends of the shock cord helps, but they still spin on the way down. It seems like the main problem with them is the nose cone spinning up the shock cord and parachute, because I get them all back with twisted shock cords. How do you make a lightweight rocket fall stable?

 

[privateer]

Haven't tried it yet, but isn't a spill hole supposed to cut down on the spinning?

As a BAR, I seem not to have alot of swinging issues but I see it alot at the range. I've gone using long shock cords - maybe that has something to do with it?

 

[dedleytedley]

I've had the same issues with the rocket swinging in wide circles under the chute(s). I use swivels on all my chutes and when using pairs of chutes I attach an extra swivel to the pair. I suspect the problem comes from using a shock cord that is too long. Any thoughts?

 

[BsSmith]

Haven't tried it yet, but isn't a spill hole supposed to cut down on the spinning?

As a BAR, I seem not to have alot of swinging issues but I see it alot at the range. I've gone using long shock cords - maybe that has something to do with it?

All of my new rockets have Spherachutes with spill holes. They don't spin as much, but they still have some problems.

Part of the problem may be the length of my shock cords too, the longest one I have in a single deploy rocket is 15 feet long, with a Sherachute, and it still spins. It's the rocket in the second picture. My Titan has 15 feet of shock cord too, and that one doesn't spin at all, but it also has dual chutes.

 

[dedleytedley]

How about attaching a streamer to the shockcord near the BT? It seems to me that should work to reduce the swinging or at least slow it. I'm going to try it on my Rubicon next time I fly. It's notorious for swinging and losing fins. Ted

 

[Sailorbill]

I have a 3.9" Fat Boy that spins when I use a parachute with out a spill hole. I have added a small parachute on the recovery harness close to the body tube and this helps some.

 

[qquake2k]

I thought the main idea behind a spill hole was to reduce the swaying caused by air spilling out of the sides of the parachute.

 

[Micromeister]

I thought the main idea behind a spill hole was to reduce the swaying caused by air spilling out of the sides of the parachute.

You are exactly correct QQake2K!
The Oscillation is caused by the air filling the chute having to go somewhere. it takes the path of least resistance or the highest edge of the chute, this then raise the corrosponding edge and so on around the chute. a center spill hole helps to relieve the pressure stablizing the chute and load under it. To small a hole with not stop the oscillation, to large will increase the decent rate, it's a balancing act

 

[troj]

Spinning like you describe is typically caused by uneven shroud lines, something that a spill hole won't resolve.

As others have stated, a spill hole resolves side-to-side oscillation, but it doesn't resolve spinning.

-Kevin

 

[Micromeister]

Spinning like you describe is typically caused by uneven shroud lines, something that a spill hole won't resolve.

As others have stated, a spill hole resolves side-to-side oscillation, but it doesn't resolve spinning.

-Kevin

Kevin:
I must disagree strongly: A Spill hole can and does resolve spin oscillation as well....Quite well I might add. This Option has been used for a very long time as a remedy for BADLY tied uneven shroud lines. If you've done a mass build session or two with younger kids I'm sure you'll have seen this. We generally remedy with a pair of scissors on the field, cutting a 3/4" to 1" hole in their 12-18" parasheets. wham-bam most excess oscillation is completely removed from both Estes or Quest plastic parasheets. Some of the worst parasheet oscillations I've ever seen have be all but completely eliminated by the addition of a close to proper size spill hole.

OBTW: How many shroud lines do you believe are exactly the same length? I'd be willing to bet not to darn many. Even on my homemade nylon hemi's which are measured and sawn in there is still an amount of Variation in the lines length and in the stretch of the fabric gore. I'd almost be willing to say NO two shroud lines are exactly the same length. there will therfore alway be a high side "edge" from which air can or must spill. The more non-porus the canopy fabric the more oscillation well be started.

 

[troj]

John, a spill hole may reduce or remove spin, but that's not the purpose they're typically put on a parachute.

As far as all lines being exactly the same length, it depends on what's considered "exact". Kinda like in framing a house -- 1/4" is "good enough". On 550 cord on a 28 foot C-9, no, the lines likely aren't "exact", but they're darned close, and a C-9 doesn't spin.

The only time I've had problems with parachutes spinning is when the lines are a significant amount out of whack. "significant" is relative -- 1/4" on a 12" parachute is a *lot*. 1/4" on a 10 foot parachute isn't much.

-Kevin

 

[luke strawwalker]

I wonder about the location of where the parachute is attached to the shock cord and it's influence on spinning. I've seen quite a few rockets eject the chute, fill it, start coming down, natural side to side swinging from the ejection/deployment starts an oscillation that turns into a rotation, picking up speed and becoming VERY fast on the descent-- usually it seems that the spinning assemblage finds a "balance point" with the rocket body on the end of the shock cord turning in wide circles at the bottom, with the nosecone turning small circles opposite the chute, with the chute spinning in the opposite direction just above it. You can tell by the rotation that the thing eventually finds a "harmonic oscillation" where everything is swinging just right to create a feedback loop-- the nosecone will swing "tip straight out" to the side, and the body can swing nearly horizontal, with the chute spinning "on it's side", virtually gliding down in a 'death spiral', picking up speed and feeding that rotational energy into the swinging rocket/nosecone to keep them going or even accelerate the spin. Perhaps tieing a "loop" into the shock cord and attaching the chute at this "midpoint" between the cone and rocket body would minimize this "harmonic" effect. I don't know, but it'd be an interesting research project! Where that shock cord loop should be, and how long each remaining component is from the chute, would certainly influence how the thing flew under a chute and any "harmonics" set up by the swinging nosecone and body below the chute.

Spill holes are a good solution, as they definitely help. Another possible solution is multiple chutes (clustered chutes) They seem to stabilize one another and minimize swinging/spinning. Clustered chutes also will inflate and come down at an angle to each other and the direction of travel, which helps minimize the spinning/swinging since they're already at an 'angle of attack' and have a 'stable' raised area to spill the air from on the high side between the two chutes...

Later! OL JR

 

[BsSmith]

I'm starting to thing that the swing is just an effect of having a short shock cord. All of my dual deploy rockets fall perfectly flat with little to no swing, but all of those have at least 30' of shock cord. Once that length is shortened to 15', which I use in smaller single deploy rockets, they start swinging. I even have two rockets that swing under Spherachutes, which have a spill hole and shroud lines closer in length than I can get myself.

I agree that there is some sort of harmonic oscillation going on. It seems like the intial swing from deployment starts an oscillation that gets larger and larger after every swing, with the air spilling out the side of the chute on the 'maximums' fueling it. If you think about it, a shorter shock cord with a small mass on the bottom is easier to get going than a long shock cord with a large mass. The longer cord needs more of an initial swing to get it going.

 

[Rocketman248]

Use a high quality swivel. That way the spinning of the parachute doesn't travel down to the rocket.