# Fat boy fin dimensions

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#### BlueNinja

##### Well-Known Member
I want something to fly G75s in now that I have my 180 case, so I decided a FB upscale to 4" would be cool. Can someone give me the fin dimensions? I looked at JimZs page and couldn't find anything.

Thanks

Blue

Root edge = 77 mm
Trailing edge = 65 mm
Tip edge = 52 mm

Oh, heck, it's the computer age. Here's a full size template, in PDF format. There are no TTW tabs.

Hope this helps...

SpaceGarbageMan already gave youthe exact information you were looking for but in the future another alternative is to get a RockSim file off of EMRR & view the fin dimensions from there.

But you still gotta do the math
HTH,

I measured fins from two or three kits and came up with this:

8.85, 1.30 leading edge exposed root
12.65, 3.75 trailing edge tip
12.0, 1.30 trailing edge exposed root
11.50, 1.30 tab
11.50, 0.37 tab MMT edge
9.30, 0.37 tab MMT edge
9.30, 1.30 tab
8.85, 1.30 starting point

That calcs out to a LE length of 3.10 inches, a LE sweep-back angle of 37.8 degrees, an exposed root chord of 3.15 inches, a tip chord of 1.90 inches, and a TE length of 2.53 inches.

I posted an EXCEL spreadsheet that does the calcs for you.

To upscale your FB fins to a four inch BT, the scale factor would be (4/2.6) = 1.53846154. Remember, precision is important.

To use the fin spreadsheet, use the following inputs in column B:

S = planform area = (average chord) x (span) = ((3.15 + 1.90)/2) x 2.45 = 6.1863 sqin per fin

AR = aspect ratio = (span squared)/(planform area) = (2.45 x 2.45)/6.186 = 0.97029

Greek symbol: lambda = taper ratio = (tip chord)/(root chord) = 1.90/3.15 = 0.60317

Sweep angle at leading edge = 37.8 degrees

LEERC = leading edge of exposed root chord = 8.85

Use the up-scale ratio (calc'd above) in column F:

Scale Factor = 1.538

The spreadsheet will spit out your new fin geometry for the exposed portion of the fin. You will have to enlarge the TTW tab yourself to fit your new CR locations and MMT tube size.

I must have done something wrong in the spreadsheet..... These fins for some reason don't look much like the FB fins... could it be the sweep angle? It almost looks like a plain trapezoid, with teh rear point of the fin being at approximately (14, 4). I'll double check my numbers.

Yeah, I guess it does work out fine. I seemed to remember the FB fins swept more. Now, who to get a nose cone from...

I have the rsim demo so i can't save. maybe this christams I'll get to spending some money on the real thing.... I managed to do a weird little sim last night, fins looked pretty accurate but I simmed it on a G75 and then an H128 and teh 2 flights were within 100ft of each other? Must have been my error.

Can someone rocksim this for me?

Not too much selection of the 4" rounded nosecones, plus big nosecones are expensive. Scotglas has the 4" mosquito nosecone, see https://www.scotglas.com/components.htm. If you're patient and vigilant, from time-to-time you will see 4" rounded nosecones on ROL auctions.

You could also make one yourself by fiberglassing over a styrofoam mandrel. Have you done any fiberglassing yet?

--Lance.

Nope, none yet.

No...I couldn't even begin to guess where you could get a 4" Fat Boy nose cone.

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