# Up scaled wrap around fin guides

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#### MKP

##### Well-Known Member
I'm up scaling a Estes Crossbow SST, from BT-20 to BT-60 and was wondering about how to upscale the fin guide since it has five fins in an unusual arrangement. Just take the measurements and times them up, or is there an easier way with software?

Edit: I thought I'd add that I've scanned the wrap in, so its on a computer.

View attachment finguide.pdf

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You are scaling the BT20 circumference up to the BT60 circumference. So:

(BT20 diam)x(pi)x(scale factor) = (BT60 diam)x(pi)

(BT60 diam)x(pi) = scale factor
----------------
(BT20 diam)x(pi)

which is the same as

(BT60 diam) = scale factor
-----------
(BT20 diam)

and putting in the actual numbers:

1.637 = 2.2242 = scale factor
------
0.736

So you would have to lay your original fin marking guide on the Xerox machine and enlarge it 222 percent (222.42, to be precise).

Did any of that make sense?

The Crossbow aka "SWAT" has 7 fins. Take a piece of paper and wrap it around the tube,make a mark on both pieces where they overlap. Measure the distance between both points and devide by 4. Make marks on the paper that distance apart. A cheap marking guide. As you can see,I am a BIG fan of the SWAT (Crossbow). Other then size, the only difference in the 2 I see is the tubes on the wings.

I'm up scaling a Estes Crossbow SST, from BT-20 to BT-60 and was wondering about how to upscale the fin guide since it has five fins in an unusual arrangement. Just take the measurements and times them up, or is there an easier way with software?

Edit: I thought I'd add that I've scanned the wrap in, so its on a computer.

Just buy a SWAT,its made from a BT60. I have an unopened one. They come up on ebay regularly. I can copy the guide 4 u if u want.

Thanks, looks like I have two avenues then.

I know it has, 7, but the fin guide has 6.(so I was off by 1)

Just buy a SWAT,its made from a BT60. I have an unopened one. They come up on ebay regularly. I can copy the guide 4 u if u want.

That's all right, I've got it figured out now. Besides, I don't Ebay anymore.:y:

Thanks, looks like I have two avenues then.

I know it has, 7, but the fin guide has 6.(so I was off by 1)

Nope, You weren't wrong. The Crossbow has only one lower fin, the
S.W.A.T. has two>

Well, I just built a Crossbow and it does indeed have two bottom fins, or sub-fins, as they are called in the instructions. And when I looked at the S.W.A.T instructions it only had one "sub-fin". Unless we're confusing what a "lower fin" is.

Actually, when I count balsa slabs on this thing, there's 8.

I think I see where the confusion's come from. Everyone assumed the S.W.A.T. and Crossbow are identical, but in fact the crossbow has one more fin. (and minus the tubes) The fin guide is NOT a simple 90 degree separation like it is on the S.W.A.T.

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Nope, You weren't wrong. The Crossbow has only one lower fin, the
S.W.A.T. has two>

SWAT has ONE rear fin angled forward, one tail fin(rudder) 2 rear wings( aelirons?) 2 wings, and one canaard fin,7 total. I havent built a Crossbow,but it looks identical to me minus the wing tubes. The stock pic I have of the Crossbow shows all 7 fins,minus wing tubes.

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Actually, when I count balsa slabs on this thing, there's 8.

QUOTE]

I give up,how do you count 8? The pic I see from EBAY listing shows 7 fins.

2 "wings", 2 "ailerons" or "elevators", or whatever, 2 "sub-fins" or "lower fins"

That's 6

Then there's the rudder and the dorsel fin ahead of that. That's 8. If you look at the box picture, there's a sub-fin hiding.

I counted the dorsal,but I called it the canaard,my mistake, one of the "sub fins" IS the tail...7. I am man enough to admit I am wrong,if it really does have 8,I will buy you one.

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Still won't believe me even if the models right in front of me, eh?
Hopefully this scan helps, there are TWO foward angled fins on the bottom.
See the parts labeled "I"

Still won't believe me even if the models right in front of me, eh?
Hopefully this scan helps, there are TWO foward angled fins on the bottom.
See the parts labeled "I"

You sir,are correct. But I swear those 2 fins on the instruction sheet look nothing like the picture in my opinion. I stand by my deal,let me know where to send your kit.

Don't worry about it. Actually, these were some of the worst instructions I've seen from Estes. Pretty confusing, and not enough information. So some deviance from the box art and the instruction art doesn't surprise me.

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