Fins form

Hello everybody,
i'm just doing my graduation work i would have a quick question about the fins forms.

i wrote: Trapezoid or elliptical the flight difference is very tiny. It's just the optic beauty that decide whether take the one or the other.

(sry about my bad english)

Can i write that? Or is it completely false?

I know that by the nose cones it isn't like that.

Oval ones are better under a certain speed. But i don't know what to write about the other nose cones. Can somebody help me? Would appreciate!

Ricci

I think if you use any of the traditional designs for fins and nose cones you will be fine. Pick something that looks good to you.

For professionally designed and fabricated rockets, you seldom see elliptical fins but often see trapezoidal, delta, or clipped delta, so that is the conventional wisdom. But the speeds at which these rockets fly are much greater than that in our hobby world. If you don't care to optimize performance, you can almost get away with any fin shape, as long as they are on straight and have symmetrical aerodynamics, all while keeping the CP adequately behind the CG. Fin design begins to matter the closer you get to the transonic region, and sometimes even earlier.

For nose design, this is a good start:

https://en.wikipedia.org/wiki/Nose_cone_design

Greg

What you are bumping into is the part of fin design that crosses over into aerodynamic lift

Classic aero theory teaches that in three-dimensional aerodynamics the best or most efficient lift is generated by a spanwise lift distribution that has an elliptical shape. If you use the same airfoil section from root to tip (like in a rocket fin) one way that you can tailor the spanwise lift to get an elliptical distribution is to use an elliptical chord distribution.

However, you can also get much of the same aerodynamic effect by using a trapezoidal fin shape with the right combination of tip chord and root chord. There is a small range of values for lambda (planform taper ratio; equal to [tip chord]/[root chord]) where you achieve something like 95% efficiency. And a trapezoidal fin (or wing) is MUCH easier (therefore, also cheaper) to design, build, and maintain.

Thanks at all!!! great information! I putted alot of things in my work!!!

In addition to what powderburner has said, span efficiency factor is pretty close to irrelevant for rockets anyways. Because well-aligned fins operate at very close to zero angle of attack throughout the flight, the induced drag is pretty minimal, so even with a terrible efficiency factor and a far-from-optimum shape, the losses are small.

(Note that this assumes a subsonic rocket)

I have been told that sharp corners on fins cause drag. Thus a trapezoid or clipped delta is superior to a plain delta (broader corners at the tip) and an elliptical is better yet (no corners at all). Dedicated builders of contest rockets therefore often use elliptical fins. If you're not interested in extracting every last decimal place of efficiency then, as powderburner says, straight-edged fins are so much easier to build that they are more common in most other rockets.

adrian said:

I have been told that sharp corners on fins cause drag. Thus a trapezoid or clipped delta is superior to a plain delta (broader corners at the tip) and an elliptical is better yet (no corners at all).

Click to expand...

I know it's been a while since I sat in an aero class, but what I remember for drag parameters are wetted area, frontal area, fin airfoil shape, and surface roughness. "Corners" were not on the list.

You may have heard someone talking about airfoil shapes (as opposed to fin planform shapes)? For subsonic flight, you get the lowest drag with a teardrop shape (rounded leading edge and a trailing edge with thickness that tapers down to zero). In a subsonic airfoil you don't want to have sharp corners on the airfoil (which would appear on a three-dimensional fin surface as a ridge line running along the span) because that tends to trigger flow separation (and drag) in subsonic flight.

For a supersonic airfoil (or even one which spends most-but-not-all of its time in supersonic flight) you DO want a sharp leading edge and a sharp trailing edge. The mid-chord can be gently curved (lens-shaped) or rise to a high point (diamond-cross section).

And all of this is sort of fussing over nothing, because (as cjl posted) these rocket fins spend their flight at very low angles of attack and generally cause little aero drag regardless of what shape they are. (You almost have to try on purpose to create a really bad fin design before it becomes a serious drag problem.)