software for viewing fin designs

[powderburner]

Submitted for your approval . .
(in my best Rod Serling voice)

I use a spreadsheet trick to do my own 'visualization' of potential model rocket designs, and thought some of you might be able to use the same info.

The attached file is a small EXCEL spreadsheet that will draw a fin shape for simple, trapezoidal planforms, based on only a few key geometry inputs. It uses a simple little EXCEL trick to turn the numerical data into straight-line-plots. The graphical image can help your eyeball keep track of the overall fin design while you play with numerical input data, and the final fin geometry data can be taken directly from the spreadsheet to feed other analysis such as RockSim.

Before going to the spreadsheet, a few definitions are in order to be absolutely certain we are all talking about the same things.
Planform Area (or simply, Area): the size of one side of the fin, as viewed from a point on a line perpendicular to the face of the fin, measured in square inches
Exposed Root Chord: side of fin that is attached to body tube, oriented longitudinally, measured from leading edge to trailing edge in inches
Station LEERC: body station of the leading edge of the exposed root chord, measured in inches
Tip Chord: most outboard edge of fin, assumed to be oriented longitudinally (and not clipped or canted), measured from leading edge to trailing edge in inches
Span: the distance from the exposed root to the fin tip, measured perpendicular to the rocket’s longitudinal axis in inches(this is actually 'half-span,' but I'm not going into that now)
Leading Edge Sweep-Back Angle (or simply, Sweep): angle from a reference line (that is perpendicular to the rocket body longitudinal axis) back to the fin leading edge, measured in degrees; a positive value means that the fin is swept aft
From these definitions we can automatically calculate the following:
Aspect Ratio (or AR): a non-dimensional measure of the “slenderness” of the fin shape, equal to (Span2)/Area
Taper Ratio (or the Greek letter lambda): a non-dimensional quantification of the taper from exposed root to tip, equal to (Tip Chord)/(Exposed Root Chord)

The fin size and shape (planform and location, if you want to talk like an engineer) can be defined with a subset of the above variables: Area, AR, lambda (the Greek symbol would not copy into this post), Sweep, and Station LEERC. The purpose of using this particular set of descriptors is to make fin design and sizing as painless as possible, because the other variables can now be extracted from this information by our spreadsheet. (Alternatively, we could design the spreadsheet to use Chords, Span, Sweep, and Station LEERC as input data and automatically spit out the other characteristics.)

Quick side note: Numbering of the body stations (and consequent stations of the fin LEERC and LETC) usually begins at a point in front of the nose of the vehicle, in order to keep all numerical values positive.

So, here’s the EXCEL part. The sample fin is from an Astron Alpha. The cells that are shaded are the places where input data must be provided, and all other cells are calculated automatically. If you have not used EXCEL, here is a user tip: save the original file for reference, save the file again under another name after you have twiddled up a new fin, and the original data and fin design will still be available. This tip also works for versions of the file where you reprogram the spreadsheet: save new versions under new names and the original will be preserved.

(see first page of EXCEL spreadsheet, labeled ‘Basic Fin’)

If your fin planform does not deviate too far from a simple trapezoid, you can still use this spreadsheet, although you may have to use as much “art” as science. A canted or clipped fin tip can be approximately modeled using an average span and equivalent theoretical tip, and then calculating the new tip and planform characteristics (but you will have to work out for yourself how to restore the canted tip when you finish). Similarly, if the exposed root chord is mounted on a boat-tail fairing, and the Z-axis location of the root varies, an equivalent root can be modeled by using an average spanwise location for the root, calculating a theoretical value for the chord, and calculating new fin planform characteristics. Or, if you plan to do a lot of work with almost-trapezoidal fin shapes, go ahead and program a new spreadsheet.

Anyone can do this same little trick and get EXCEL to draw some simple line diagrams. In my example, you can see the programmed calculations in the spreadsheet by clicking on each cell. The X-Z coordinates for each corner of the fin shape are copied over to the right to create a list of end points for the lines we want to see (and the first X-Z pair is repeated, so the full perimeter of the trapezoid will be drawn). A gap (empty cells) in the list causes the EXCEL graphics to terminate the first drawn shape and begin drawing another. Once you have prepared a list of coordinates, use the following sequence:

1) Click the ‘INSERT’ button in the header
2) Click the ‘CHART’ button within the drop-down menu
3) Click the ‘XY SCATTER’ option in the window of chart types
4) Click the image of the straight-line plot (it’s on the lower-RH corner for my version of Windoze)
5) Click ‘NEXT’ at the bottom of the window
6) Fill in the window for ‘DATA RANGE’ by clicking and dragging across your list of X-Z coordinates (you can move the chart-construction window out of the way, to see where to click and highlight your data, by clicking in the header and moving the construction window to the side temporarily). You may also have to play with re-selecting your data series to be in ‘ROWS’ or ‘COLUMNS’ to get it right.
7) For a quick-and-dirty chart, click ‘NEXT’ and ‘NEXT’ and ‘FINISH’ and you will have your first chart.

You will probably need to click on the chart (to highlight the whole thing) and stretch it vertically or horizontally (to where the chart axes are more-or-less equally scaled) before you can see the true shape of your new fins. You can also click on the axes themselves to change the plotted range of numbers, the number of decimals, font size, etc. You can click on the chart field to get access to controls on borders, shading, plotted line widths, and whether you want the symbols to show or disappear. Aw, heck, go play with it yourself.

If you want to get super fancy, you can include the geometry of the nose cone and body tube and draw them also. The following is an example of the geometry for the Estes Fat Boy, complete with elliptical nose cone which is actually represented by a whole bunch of itty bitty straight segments.

(see second page of spreadsheet, labeled ‘Fat Boy,’ by clicking on the tab at the bottom; if this EXCEL file does not make it into the post, just email me and I'll send it to you, it's only about 25-30K)

 

[dluders]

The Excel spreadsheet's visualization of rocket fins is very good. One can easily take and existing design, and then morph it into some unique shape. The information would be useful to give to a custom G10 fin cutting operation (such as Public Missiles Limited's service), so that they can cut your fins out J-U-S-T right. Kudos to you!