Needed: Open Rocket Fin Design Tutorial

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K'Tesh

.....OpenRocket's ..... "Chuck Norris"
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Does anyone have a tutorial for OR's fin designs?

I'm working on a design, it's not available on rocket reviews as a .rkt, and it has trapezoidal fins.

I know the width of the fins (and thus the tip chord), as well as the lengths of the leading and trailing edges, but not the root chord, height, nor sweep. I've always found the freeform fins to be a PITA, unless I can manage to change an image of the fin pattern into a black and white image and import it from there.

Tip edge/Width is 1 1/4inch
Leading Edge is 3 3/8 inch
Trailing edge is 2 25/32 inch

By math, I figured the root edge to be 1.38", but I'm not 100% certain that the tip edge is at a 90 degree angle to the leading and trailing edges.

BTW, the rocket is the Estes Marauder #1922 kit.

Some help here would be appreciated.

Thanks!
Jim
 
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check the lpr/marauder thread, I've posted a fin 'unit' that should work(hopefully). had to go with freeform fins. started with paper and pencil and drew the fin then made measurements, sweep angle is approximately 26.5 degrees.
Rex
 
Tip edge/Width is 1 1/4inch
Leading Edge is 3 3/8 inch
Trailing edge is 2 25/32 inch

By math, I figured the root edge to be 1.38", but I'm not 100% certain that the tip edge is at a 90 degree angle to the leading and trailing edges.

Please see my next post for the answer.
 
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Here's a link from Ye Olde Rocket Shoppe's plans. It includes fin templates:
https://plans.rocketshoppe.com/estes/est1922/est1922.htm

Thanks, I've downloaded it. Problem is, there's no measurements on the fin template, and no way of ensuring that the scaling was accurate.

I've now got a couple of sets of numbers (and a pair of simulations w/the different sets). I think I'll dig around in my fin box (where unused fins go to be lost), and see if I can find my originals.

What I really need to know now is how do people compute the X,Y numbers for freeform fin patterns in OR.

I can easily understand 0,0 and the last value, but computing the intermediate points is the problem for me. I wish that there was a way of just entering in the number of sides, their lengths, and perhaps an angle or two to make sure that everything is properly laid out.
 
Actually, there is an easy way to determine if the fin diagram is correct in scale. The instructions say the bottom edge of the lug standoffs are 7/8s in length. Simply print out and measure the fin templates. If the lug standoffs are correct, then the fins are correct. If not, then use a program such as Photoshop or GIMP to rescale the diagram. Then convert the fins to something OR can read and import.

Or, if your math skills are strong, find out the non-right angle of the fins, the tip chord length, leading edge length and by doing that trig thing, build your fin in OR.

FC
 
It would be awesome to convert rocket or OR fin patterns to cad compatible files.
 
time-out-ref.jpg



Thanks everybody for all the info on the Marauder... Now lets go back to the original request.

I'm looking for the method to figure out the points (x,y coordinates) so I can enter them into OpenRocket, and recreate fins with straight edged polygon shapes based on limited info (eg, lengths only). I don't own photoshop or any image processing software other than what came on my computer (MSPaint). I have limited access to a printer (at school, or a bike ride to the library (2 miles round trip)).
 
draw a rough sketch including the dimensions you do know and scan it and post it here - I'll take a look and see if it's possible to work out the rest for OR.
 
draw a rough sketch including the dimensions you do know and scan it and post it here - I'll take a look and see if it's possible to work out the rest for OR.

Thanks for the offer Zebedee,

I'll go back to the Marauder for a moment... I know that the fin is 1.25" at the tip, is 1.25" wide, with a leading edge that is 3 3/8" long, trailing edge is 2 25/32" long, and the root is for this exercise 1.38" long. I know that the tip has a 90 degree angle with both the leading and trailing edges.

What I need to learn is how to do the process of computing the x,y values in OR so that the fins match the specs given. I want to know this so I'll be able to do it again for some other rocket (such as the Yellowjacket (upscale), Der Big Red Max, or Nova Payloader (upscale)) when I'm working up my own sims in OR.

I attempted to do one of a DBRM (two stage) , with measurements taken from my own fins (so I can get fin tabs on the templates I'm planning on), and the printout was close, but clearly not correct. Printouts of the fins found in the .rkt files from Rocketreviews were just as bad (if not worse).
 
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Hi K'Tesh,

First I drew a rough sketch with the fin root horizontal along the bottom and filled in the measurements we know
see the scanned and attached PDF for my working and diagram. I reproduced the working below in case my handwriting
is not clear on the scan:

Root Chord = 1.38
Tip Chord = 1.25
Leading edge = 3 3/8
Trailing edge = 2 25/32
Angles between leading edge and tip and trailing edge = 90 deg

Firstly we can simply state the co-ordinates of the leading edge/root corner and trailing edge/root corner:
(0,0) and (1.38,0) these will be your first and 4th coordinates.

The next thing to do is find the angle that the leading and trailing edges make with the root. They must make the same angle
since you say they are both perpendicular to the tip chord.

I drew a line (labelled 'q' in my sketch) to complete the rectangle made by the leading and trailing edges and the tip chord.
We know that the length of q must be the same as the tip since it's a rectangle with all 90 deg angles.

From trigonometry sin(x) = length of opposite side / length of hypotenuse
thus sin(alpha) = 1.25/1.38
and alpha = inverse sin(1.25/1.38
= 64.93 deg - I assumed this was supposed to be 65 - maybe the root is not exactly 1.38 inches?

Next I dropped a dashed line from the corner of the leading edge and tip chord down to the root line perpendicular to the root.
Again using trigonometry we can say that:

Sin(alpha) = y1/(3+3/8)
and y1 = sin(alpha)*(3+3/8)
y1 = 3.0587

we can also say that:

cos(alpha) = x1/(3+3/8)
x1 = 1.4263

This gives our 2nd coordinate for OR => (1.4263,3.0587)

Do the same for the corner of the trailing edge and tip chord:

sin(alpha) = y2/(2+25/32)
y2 = 2.5206

Cos(alpha) = (x2 - 1.38)/(2+25/32) *look at the diagram to see why this is so, the triangle we are using starts at the trailing edge and root junction
x2 = 2.5554

So your 3rd coordinate is => (2.5554,2.5206)

and we're done.

Zeb

View attachment Trigonometry.pdf
 
General equations for any trapezoidal fin, knowing only the 4 side lengths:

Trapezoid.svg


Side AD is the leading edge, side CD the tip chord, side BC the trailing edge, and side AB the root chord.
We will assume that point A is at X=0, Y=0, which makes point B at X=AB, Y=0

The general theory I'm using here is that you can remove a parallelogram from a convex trapezoid and the remaining area forms a triangle. Two of those triangle's angles will match two of the original trapezoid's angles. I won't go into more detail unless you want it.

Angle ∠DAB = CB / (CB + AD + AB - CD) × 180°
Angle ∠ABC = AD / (CB + AD + AB - CD) × 180°

D is at:
Dx = AD × cos(∠DAB)
Dy = AD × sin(∠DAB)

C is at:
Cx = Dx + CD - BC × cos(∠ABC)
Cy = BC × sin(∠ABC)

Math check, if Dy is not equal to Cy, something in your work, or my math, is wrong.
 
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