Why are Vatssas chutes so much smaller than expected?

[paparoof]

Hey Geof -

In post #34 in this thread you mention making the Vatsaas chutes.

Every time I've tried making one of those the chute comes out WAAAAAY smaller than expected. I'm quite sure I'm doing/interpreting something incorrectly, but I've seen mention on TRF of this same problem before, so thought it might be helpful to myself and others if this issue could be clarified.

Can you tell me what I'm doing wrong?

 

[jderimig]

Nothing. The 'diameter' is the flat canopy diameter not the inflated diameter in their spreadsheet.

 

[kandsrockets]

Hey Geof -

In post #34 in this thread you mention making the Vatsaas chutes.

Every time I've tried making one of those the chute comes out WAAAAAY smaller than expected. I'm quite sure I'm doing/interpreting something incorrectly, but I've seen mention on TRF of this same problem before, so thought it might be helpful to myself and others if this issue could be clarified.

Can you tell me what I'm doing wrong?

I make and sell the chute they use and I found that if you scale there size by 1.5 times they work out. I used one in my L2 bird and also in my Blue Moon and they work great if you scale them by what I said. I will be doing a 48" for my drogue and a 12' for my main in my L3 bird.

 

[geof]

I believe jdermig is correct, or close to it. Personally, I think that the "diameter" is taken to be the length of all the straight edges, laid flat out, possibly even including the gaps in between. When I wrote in that other thread, I almost said that I had found the Vatsaas chutes to be very fast/small for their stated size. I make mine markedly bigger than suggested by the calculator, but I can't offer a rule of thumb for the multiplier.

The neat thing about the Vatsaas chutes is their style. The Nakka chute is as effective or perhaps even more effective than would be expected for its stated size (I don't include the spill hole). In one sense, the Nakka chute is easier because all the pieces fit together as expected, whereas with the Vatsaas chute it takes a bit of figurin' to fit the parts together. However, the Nakka chute requires a lot more sewing!

I like them both. But be prepared for fast descent with Vatsaas.

G

 

[paparoof]

I make and sell the chute they use and I found that if you scale there size by 1.5 times they work out. I used one in my L2 bird and also in my Blue Moon and they work great if you scale them by what I said. I will be doing a 48" for my drogue and a 12' for my main in my L3 bird.

Well alrighty then! I'll try that this weekend. Thanks!

 

[paparoof]

Personally, I think that the "diameter" is taken to be the length of all the straight edges, laid flat out

The neat thing about the Vatsaas chutes is their style.

Thanks Geof -

I agree with the style comment - that's why I want these to work out, I understand hemi is s'posed to be more efficient, but I really think cupped chutes just look so cool.

Sounds like I just need to adjust my expectations and maybe not start with the weight calculation. I'll just make a bunch of 'em and after they're done figure out which one goes with which rocket.

And for the un-initiated, here's a link to the Vatsaas chutes calculator I'm referring to:
LINK

By the way, if you go to their site, spend some significant time reading. Their site includes some of the funniest stuff I've ever read on the web. Specifically, this page.

 

[geof]

BTW, the Nakka chutes are also cupped, but not X-form.

Here's a photo of one.

 

[jderimig]

Also the default Cd value in their spreadsheet is 1.5 which is very optimistic IMHO. If you change that to 0.75 to 1 you may get a size that you feel more comfortable with.

 

[denverdoc]

Also the default Cd value in their spreadsheet is 1.5 which is very optimistic IMHO. If you change that to 0.75 to 1 you may get a size that you feel more comfortable with.

Bingo! I have been disappointed with the descent rates of the Vatsaas chutes--I fly in thin air, but I don't think that is the whole story. I heartily concur with jd..'s rec.