# Proper Fin Size, Help Needed

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#### cdma77

##### Well-Known Member
My rocket measurements are 66" in length and 3" diameter for the body. For a 3 fin rocket is a fin size of 6" in length by 6" in span sufficient? They will be right angle trangles. Or if somebody has a great rule of thumb guide that would allow me to calculate my own.

Thanks,

Jeff

You don't indicate where your CG is. If your rocket is tail heavy, even large fins like these may not make it stable.

Stability is determined by the CG/CP relationship:

https://www.grc.nasa.gov/WWW/K-12/airplane/rktstab.html

...you can always prep your rocket as ready for flight, then give it a swing test. Or, download the demo version of Rocksim from www.apogeerockets.com, and check it out that way.

You can determine the center of gravity mathematically or experimentally.

The math approach is outlined in this thread:

To find the c.g. experimentally, load your rocket with motor, ejection wadding, and all components that will be onboard at the flight condition (parachute folded and stored inside, NC inserted in front of BT, of course). Hold the rocket in a horizontal position and balance it on your finger. OK, if you want to get fancy and accurate, balance it on the edge of something a bit more narrow. Note the position of the c.g.

You can find the approximate center of pressure using various mathematical models. One of the standard ones is the Barrowman approach; these equations are available at several internet sites as well as in the back sections of the Handbook of Model Rocketry (if you don't have one, GET ONE). Other stability calculations are programmed into design/simulation software such as RockSim. You can download a free trial version (an older version, and you don't get all the bells and whistles) of Rocksim at the Apogee website.

You can also determine the center of pressure experimentally, although this approach is somewhat conservative. Draw a profile view of your rocket, including fins in the correct orientation. (If you have a three-fin design, at least two of the fins will project into the profile view with 'shortened' span.) Cut out the paper drawing of your profile view, and balance it on the edge of something. Note the position of the c.p.

You would like the c.p. to be at least one body diameter (or 'caliber') behind the c.g. so if you have a three inch BT the c.p. would be three-plus inches aft. If it is not, you need bigger fins, or a bit of nose weight.

For short, fat rockets (with relatively low pitch/yaw inertia) this stability margin might be less, down to as little as 1/2 caliber. For long, skinny rockets (with relatively high pitch/yaw inertia) this stability margin may need to be several calibers.

Is that the info you are looking for?

Oh yeah, Welcome to TRF!!!

Based on information you gave.
3" dia. 66" long. 6" fins should be more than enough. In fact I would be surprised if you do not find this somewhat over stable.
For best performance I go along with previous post.

Are my right angle triangle fins at 6" and 6" ok ? Most I see are trapazoids.

Thanks,

Jeff

Well, how about this: If your rocket uses the 11" long LOC ogive cone for 3" LOC tube, and your airframe is 55" long (for a net length of 66"), and there's nothing else on the exterior of the rocket except for launch lugs or rail buttons (paint and fillets have little to no effect on the CP) your CP will be located between 1" and 1.5" in front of the your fins (where their leading edge attaches to the airframe.)

If you use the nose cone described above and your airframe is 66" long (net rocket length being then 77") your CP will be located between 2" and 2.5" in front of your fins.

Now. Using this information: Build your rocket the way you want to. Use the fins you're describing and everything. When finished, install a motor and the recovery system and get it ready just like you want to launch it.

When it is prepped, lay it on its side and find the balance point. If the balance point is closer to the nose of the rocket than the location of the CP described above, you have a stable rocket. In fact, because you are using a 3" airframe, you want the balance point to be *at least* 3" in front of the CP.

If you find that the balance point is behind the CP or is too close (less than 3") just add some nose weight until you get the balance point right. There are many methods of adding nose weight, and if needed, we can cross that bridge when we get there.

Hope this helps!

(BTW: I came up with those CP locations using RockSim v.5, the LOC 3" ogive nose cone from the database, 3" LOC tubing for the airframe, and three 6" x 6" right triangle fins that have a 6" root edge, a 6" trailing edge at a right angle to the airframe, and the hypotenuse of the triangle being the leading edge of the fin. I figured once that was inputed, the guts and details on the inside of the rocket really only affect the CG, not the CP.)

Originally posted by cdma77
Or if somebody has a great rule of thumb guide that would allow me to calculate my own.

Tim Van Milligan has a rule of thumb for basic rocket design criteria
in "Model Rocket Design and Construction" [1]

o CG 1 to 2 calibers in front of CP
o 12 caliber length (minimum)
o fin root edge 1.5 calibers
o fin width (root to tip) 1.5 calibers

Being a 'rule of thumb' this will be a bit conservative (some will
say overly).

Also, Tim's drawing in the book shows square fins, you have to read
between the lines to understand the total fin area is what's
important.

So your 6x6 in. right triangles are equiv to just under 4.25 square
fins. Which puts you in the range of 1.5 calibers minus some of
the conservative-ness... IME, that's pretty good. But as others have
anything outside of the simplistic model.

HTH!

[1] https://www.apogeerockets.com/design_book.asp

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