Canting Tip Cord or Outboard Edge of Fins

[opcon]

I've been looking at the countless possibilities for fin design and I'm wondering why I have not seen many with a canted tip cord or outboard edge? For example:

Is this considered a poor design and if so would anyone be able to tell me why?

I did some light interwebs searching and did not find an answer.

 

[OverTheTop]

By tapering the outside edge to a point you are reducing the area away from the airframe. This is where you get the biggest effect for fin airflow during flight, and the largest torque moments when the fin is not at zero AoA (angle of attack) and generating lift. The fin near the root is generally in laminar flow (boundary layer) and so largely ineffective. So from a mechanical POV you have got built the infrastructure (the part of the fin inboard is reasonably sized to support a decent area) but then you have put a triangle of fin only, giving you around half the available area to generate lift when you want it.

For a little bit better lift (stabilising performance) leave that edge square. That makes it more difficult for the air to flow off one side of the fin and onto the other, giving you just a little more lift .

You can do it whatever way you want in typical HPR, but when you are looking for performance with minimum mass you try to get as much area out wide as possible. Wide and thin are the best for lift, just like an aircraft wing, but the onset of flutter is at a lower airspeeds.

 

[opcon]

Thanks mate, I appreciate the detailed explanation.

 

[GlenP]

You might see that similar shape once in a while. The change in chord length along the span is called taper. The change in the mid-chord line is sweep. You can search for aerodynamic effects of taper and sweep on wing design, but fins don’t need to be as highly efficient lifting surfaces so much as wings. Of course there is an apogee peak of flight newsletter article on the subject, but might not exactly answer your particular question.

https://apogeerockets.com/education/downloads/Newsletter442.pdf

One example
https://estesrockets.com/product/007244-indicator/

 

[jqavins]

Another reason is structural. Tip chords of fins get bumped on landing and in handling, as much or more than any other part of the rocket. Pointy bits are more prone to damage than blunt bits. Angling* the tip edge the way you did makes a pointier corner right where it's most likely to be dinged. So be prepared to do some repairs with wood filler and touch-up paint.

On the other hand, it looks good. Sometimes good looks are the goal, so go with it it you want. The fins may need to be a little bigger, and they may be a little more prone to dings, but so what?

* "Canting" is not the word I'd use, as that usually refers orienting fin planes or motor centerlines such that they are not parallel to the airframe's main axis. Reading the thread title I was expecting to find a discussion of curving fins such that the root edges are parallel to the axis while the tip edges are not. Which would be interesting.

 

[mbeels]

The Estes Optima has a somewhat similar fin shape (which personally I like). As others have said already, no reason not to use it if you like the look. If you're going to for altitude records, there are better options when performance is your main goal.