chute size calculations... am i reading this right?

[RocketSquirrel]

I'm getting my 4" Loc iv ready for dual deploy, and added some weight in the process. after adding everything up the total weight without motor is 1984g. i have a 50" chute i got from the LOC black friday sale i was planning on using as the main, and the calculator i used "decentratecalculator.net" says it will slow it to around 20ft/s assuming main is around 350ft. that seem a little fast. It says it would recommend a ~60inch parachute. looking for any rule of thumb on this topic, are these calculators online useful or should i follow my own calculations? is 15fps the ideal landing speed or do you think im able to wiggle it with 20fps?

thanks!!

 

[sriegel]

Your numbers are reasonable for descent with those chute sizes. A lot of HPR kits come with chutes (or recommendations) that give 20 fps.

I like 15 fps myself. 20 fps is ok with a stout build, especially if you fly on a sod farm. I would upsize the chute for a dry playa, etc.

 

[teepot]

For that weight I use a Rocketman 48" 2.2 CD chute. I land on a dry lakebed and try to come down at about 12fps. I think 15fps is what is recommended by chute manufacturers. But it also depends on what your landing on. Nice fluffy grass you might get away at 20fps. For me what ever I'm landing on I still would not go over 15fps. For a 70oz rocket I would use an 80" regular chute. But it's up to you what speed you land at. If a fin breaks then you have learned something. If not then you have learned something.

 

[Buckeye]

are these calculators online useful or should i follow my own calculations?

What are your calculations?

Online calculators are merely using terminal velocity relationship:

https://en.wikipedia.org/wiki/Terminal_velocity

 

[David Schwantz]

teepot hit it on the head. Yding area has a lot to do with chute choice. We fly off sod farm so I can get away with a bit smaller chute than a hard lake bed might need. Also, height does not matter when chute deploys, when it deploys it will slow to that decent speed.

 

[Banzai88]

As has been pointed out, construction (fiberglass vs. cardboard/plywood) and landing area (hard pack vs. sod) as well as fin design (flush/sharp corners/overhang) plays into what you're comfortable with.

I usually target a max of 22fps for fiberglass and 15ish fps for cardboard/plywood.....and I land on sod or plowed field.

 

[Handeman]

Be careful of online decent calculators. They use chute diameter, but that is a slippery number to come up with. Some manufacturers use the diameter of the cloth when laid out flat as the chute diameter while other use the diameter of the chute when inflated, others use the circumference of the chute across the top to determine size. When you plug those numbers into a calculator, you're using apples, oranges, and grapes. You need to have the right fruit to get the right answer.

The best is to use the manufactures decent rate calculations. Those will be the most accurate.

BTW, I can't get to decentratecalculator.net. It comes up as server not found.

 

[GalantVR41062]

https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator
I have used this and open rocket to simulate decent rates. Then after flights and recording actual decent rates I adjust my simulations.

 

[Handeman]

https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator
I have used this and open rocket to simulate decent rates. Then after flights and recording actual decent rates I adjust my simulations.

That is great if you are buying your chutes and the manufacture is on the pull down list. I make my own chutes. I know the weight of the rocket with burned out motor, and I make elliptical chutes and need to know what inflated diameter elliptical chute to make to get the desired decent rate. Not so helpful with that, which is where I'm having issues with most of the online calculators.

 

[Buckeye]

That is great if you are buying your chutes and the manufacture is on the pull down list. I make my own chutes. I know the weight of the rocket with burned out motor, and I make elliptical chutes and need to know what inflated diameter elliptical chute to make to get the desired decent rate. Not so helpful with that, which is where I'm having issues with most of the online calculators.

The "A" in the descent rate calculator (terminal velocity) is the same "A" used to define the drag coefficient, CD.

 

[Rocketclar]

An after the fact comment....I look at my altimeter's data on descent to see how the chute(s) did. I can see how the drogue did along with the main. Where it landed makes a difference on damage or not. Amazing how they seem to seek hard pavement.

 

[Handeman]

The "A" in the descent rate calculator (terminal velocity) is the same "A" used to define the drag coefficient, CD.

The "A" is the variable that I'm trying to get to. The terminal velocity (ft/s) the chute comes down at. You need to define the Cd first to get "A". To get the Cd, you have to know the shape of the canopy. Most calculators make an assumption on Cd based on chute diameter, or let you put it in, and then calculate your ft/s based on the assumed Cd and the weight of the load. The problem is, if you don't know the manufacture, how they define the chute size and the Cd of their canopy, it's very difficult to get any accurate ft/s decent rate.

 

[Buckeye]

The problem is, if you don't know the manufacture, how they define the chute size and the Cd of their canopy, it's very difficult to get any accurate ft/s decent rate.

Yep, that's the rub. The manufacturer rarely gives you Cd and A.

The manufacturer will often give you mass and descent speed, though. From that, you can calculate the product ACd, which is what really matters. Make a logical guess for A, then you get Cd. Or, vice-versa.

 

[sriegel]

The "A" is the variable that I'm trying to get to. The terminal velocity (ft/s) the chute comes down at. You need to define the Cd first to get "A". To get the Cd, you have to know the shape of the canopy. Most calculators make an assumption on Cd based on chute diameter, or let you put it in, and then calculate your ft/s based on the assumed Cd and the weight of the load. The problem is, if you don't know the manufacture, how they define the chute size and the Cd of their canopy, it's very difficult to get any accurate ft/s decent rate.

For Fruity's classic elliptical, use the Cd of 1.5 with a circular area based on the stated diameter. Will work the same for your ellipsoid.

 

[RocketSquirrel]

Be careful of online decent calculators. They use chute diameter, but that is a slippery number to come up with. Some manufacturers use the diameter of the cloth when laid out flat as the chute diameter while other use the diameter of the chute when inflated, others use the circumference of the chute across the top to determine size. When you plug those numbers into a calculator, you're using apples, oranges, and grapes. You need to have the right fruit to get the right answer.

The best is to use the manufactures decent rate calculations. Those will be the most accurate.

BTW, I can't get to decentratecalculator.net. It comes up as server not found.

Maybe i copied it wrong. Long gone. Lol thanks though!

 

[RocketSquirrel]

An after the fact comment....I look at my altimeter's data on descent to see how the chute(s) did. I can see how the drogue did along with the main. Where it landed makes a difference on damage or not. Amazing how they seem to seek hard pavement.

Or trees

 

[lr64]

Seems like, if you have your own chute that you made, and a big fan, you should be able to roughly estimate the diameter of the chute in use.