1. ## Fin Surface Nomenclature

I am sorta new at the rocket thing and am confused by the fin surface nomenclatue. Leading edge, Trailing edge, semi-span, root length equal or greater than half the diameter of the tube it is mounted to. Tip Chord length, Root Chord, Sweep length, and sweep angle??? A drawing would help.

2. I'm sure someone with a better memory than I can answer your question directly. I would suggest you get a copy of the Handbook of Model Rocketry by G. Harry Stine it'll have the information you want and more. Then you can memorize it or keep it around so you don't have to.

Mine's on loan to my son's science teacher. He working through the Barrowman equations.

John

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1. Root Chord
2. Sweep Angle
3. Sweep Length
4. Tip Chord
5. Body Tube Diameter
6. Semi Span
TE Trailing Edge
Last edited by RandyT0001; 30th March 2010 at 04:18 AM. Reason: Fixed pic

4. Originally Posted by nukemmcssret
I am sorta new at the rocket thing and am confused by the fin surface nomenclatue. Leading edge, Trailing edge, semi-span, root length equal or greater than half the diameter of the tube it is mounted to. Tip Chord length, Root Chord, Sweep length, and sweep angle??? A drawing would help.
Leading edge = the front edge of the fin. The edge facing into the wind.

Trailing edge = the rear edge of the fin. The edge facing towards the rear of the rocket.

Semi-span = the distance the fins extend out from the body tube, measured perpendicular to the tube.

Root length = length of the edge of the fin that is attached to the body (the "root" of the fin). Also known as "root chord".

Tip chord = length of the tip of the fin, parallel to the body tube. Similar to the root chord, but measured at the tip of the fin rather than the root.

Sweep angle = Angle between the leading edge of the fin and a line perpendicular to the tube. This describes how far backwards the fin is swept

Sweep length = another way of defining the sweep. This is the distance between the point where the leading edge meets the root of the fin, and where the leading edge meets the tip of the fin, measured parallel to the body tube.

A diagram would clarify this immensely, but I don't have one handy at the moment.

5. Originally Posted by RandyT0001

1. Root Chord
2. Sweep Angle
3. Sweep Length
4. Tip Chord
5. Body Tube Diameter
6. Semi Span
TE Trailing Edge
Your sweep angle is wrong. Other than that, everything looks good. The sweep angle should be measured from a line perpendicular to the body tube to the leading edge of the fin.

6. nukem,

cjl is correct. The full name of the variable, at least as used in the aerospace industry, is sweepback angle. And yes, it is measured from a line perpendicular to the rocket's longitudinal axis, back to the line of the fin leading edge

Edit: Does this help?
PM me or post qstns here if you need help.

Another thought about fins: when cutting them out from balsa or basswood the grain of the wood should be aligned with the fin leading edge (at least, that's the most simple way to describe it). Grain direction could also be aligned with the 50% chord line of the fin, or anywhere between the 50% chord and the leading edge. Grain direction should specifically NOT be aligned with the root chord.

For fins cut from plywood, or G10 fiberglass panels, or graphite composite, the structural properties are much more uniform in all directions and the fin template can be laid out on these materials in just about any orientation.

For competition model rockets, or for all-out performance, you really should airfoil the fins (round LE, tapered down to a thin edge from the 50% or 60% chord line down to the TE) and you should taper the thickness from the root to the tip. For sport rockets, it still helps to airfoil the fins (or at least to round the LE) but it is also OK to leave the fins square.

The only time you "want" a sharp (beveled) fin LE is when your rocket is going to spend a substantial portion of its flight at transonic and supersonic speeds. By substantial, I mean like HALF the flight or more. If your rocket will be subsonic for most of the ascent, then just barely get trans- or supersonic for a few seconds, then immediately go subsonic again when the motor cuts out and the rocket coasts, you will be better off with subsonic fin airfoil shapes. (And probably only 1 in a 100 of the guys who think their rocket goes supersonic actually achieve that even for a couple seconds; it's a lot more complicated than just believing what a simulation tells you.) Even if your rocket is truly rated to get up to supersonic speeds, the fin edge bevels that you commonly see in photographs are HORRIBLE (high drag). A good fin leading edge bevel needs to reach back to the 20% or 25% chord line, needs to be SHARP on the LE, and needs to be smooth and clean. Same thing goes for bevels on the TE of supersonic rocket fins. For a nice supersonic fin design, study the Nike-Smoke fins (lots of documentation on them out there on the net).

The fin nomenclature used for a certain rocket simulation software uses some terms in common, and they have also come up with some of their own. (Why they did this, instead of using standard industry terms, I don't know, but it's what we have.) The alternate software terms are not better, they are not really worse, they are just different and a little confusing to some people. If you have questions about rocsim fin variables, this is an excellent place to ask qstns and get help, a whole bunch of these TRF guys are experts at turning the crank on rocsim.
Last edited by powderburner; 30th March 2010 at 04:42 AM.

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Originally Posted by powderburner
For fins cut from plywood, or G10 fiberglass panels, or graphite composite, the structural properties are much more uniform in all directions and the fin template can be laid out on these materials in just about any orientation.
PB, I agree with most of that statement, but you should still line the grain of the outer plies of the plywood up with the leading edge (or mid-chord line) of your fins. All of the plywood I have worked with has an odd number of plies, so it still has a directional preference.

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