Design > Engineering board

Lofting Ribw??

(1/3) > >>

MrSteve09:
Hi,..

I have designed and drawn plans, in AutoCAD, for R/C planes in the past.  However, now that I've "converted" to C/L Stunt, I'm curious as how to draw ribs in AutoCAD.

With R/C planes when the wing tapers the ribs get thinner (linerally) as you progress to the tip.  I've noticed with C/L wings the taper isn't as linearly, and the ribs don't get much thinner.  In fact the trailing edge of the rib stays constant, (1/4" or so to mate with the flap).

Is there a trick to lofting C/L ribs??  I'm using an older version of AutoCAD R14.

Sincerely,
Steve T.

Mark Scarborough:
Steve,
The method I use is pretty simple, but Its harder to explain in words.
Basically what it boils down to is to create a poly line for the rib contour , I start with the root rib . This contour does not include the trailing edge, I lay out how many ribs I want in planform. Then I copy and paste the upper contour onto each rib station. Using the top of the half inch trailing edge line as the basepoint. I have already established the wing tapers at this point. So then, what you do is scale each rib using the base point  and reference setting each upper rib contour to the correct length. The basepoint again is the top of the 1/2 inch trailing edge. Then I simply mirror the upper contour to get the lower contour and you end up with a complete rib with constant trailing edge height.This method does NOT encompess the flaps, they are simply drawn in manually as needed.

The thing I like about this is that it allows a marginally thicker tip airfoil percentage, I like the idea of a thicker tip. If this isnt clear let me know and I will whip up a CAD sketch with notes to illustrate it better for you. I can then post a JPG of it on line if people want to see it.

MrSteve09:
Mark,...

Thanks for the response!!  I think I understand what you're talking about.  If you are using the top of the trailing edge as a base point (or insertion point) and then scaling your rib polyline to the specific chord length, at each rib station, how does that affect the leading edge of the rib profile??   If you've got the time, a picture would clarify things.

Thanks I certainly appreciate it!!

Steve T.

phil c:
You can lay out a trapezoid front view to establish the wing thickness at each station.  Combine it with the plan view and get the chord length for each station.  Use Mark's method to fix the TE height.  Add the leading edge form at each station.  Then the spar, if one is used.  If you use a spar, I find it easier to use separate curves or polylines for the section ahead of the spar and behind the spar.  It is easier than using a polyline with a spar in it for the top curve and then readjusting the spar size at each station.

No matter what airfoil you use, when you start stretching it six different ways it will change a bit.  No more than it did though carving a stack of ribs, which still works pretty well.

Larry Cunningham has a program he wrote called stuntrib which does much of this more or less automatically.  Or you can buy Compu-Foil.  I've used that and it does a pretty good job of lofting ribs.

Larry Cunningham:
My ancient StuntRib program defines two thickness taper methods: linear (straight line) or elliptical (through an ellipse). The three SN articles are posted on the PAMPA web site which describe the methods used by StuntRib in detail. You can download Stuntrib and documentation from the PAMPA web site.

I've subsequently created an Excel spreadsheet which calculates and emits DXF contours for individual ribs - a "modified" version of the NACA00nn airfoil, which can slide the airfoil high point fore or aft of its 30% of chord. The spreadsheet, unlike StuntRib, processes only a single airfoil at a time; you have to do your own thickness interpolation for each rib. However, it's easy to make the spreadsheet calculate thicknesses at rib stations for your wing panel. Email me if you want a copy of the XLS spreadsheet.

The "big boys" of 3D CAD have elegant graphical methods of lofting the entire wing
and cutting section slices for the ribs.

L.

"The greatest homage to truth is to use it." -Ralph Waldo Emerson

Navigation

[0] Message Index

[#] Next page