Lofting Ribw??

[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

[Dean Pappas]

Hello,
Is this Steve Teerlink?
If so, then we have successfully corrupted you!

Dean P.

[John Miller]

A good lofting program, such as Compufoil, can quickly and easily loft all the ribs, including ribs at specific locations. I don't believe the 3D wing modeling with slicing at the rib locations is any more accurate. What they are doing in essence, is making a digital version of Hunts "Lost Foam" system, without the cradle.

I've seen wings built in the cradle with ribs generated by a good lofting program. The lofted ribs fit was very accurate in the cradle.

A few other advantage of the lofting programs are, They are generated and inserted into the CAD program as DXF files. A few key clicks and less than 1/2 an hours time, can make them ready the laser cutter. There are functions that will set the spars size and location, type or style of leading edge, thickness of the trailing edge, web sizes, and lightening holes, size and location of jig rod holes, and more.

I can then render the entire wing 3D and check out all the fits, if I desire.

I can import another view, the top view as generated by the program, and use it to form the basis for the top view of the wing, again, saving time.

These features save a lot of time for me when lofting, and laying out a wing.

I've used the method Pat and Mark use, and it will indeed generate a good set of rib lofts. I've also used a profile of the airfoil curve at the root, and attached copies at the leading edge, then rotated to meet the trailing edge. ( think of building a digital I-Beamer) to generate a rib loft. The problem in my cases has been the amount of time it took, compared to a lofting program.

Add to the mix, the airfoil library, and the ability to generate, and modify airfoils, then use them to loft. I feel that the price paid is well worth the investement.  


 

[Steve Helmick]

I once borrowed a program called " 'Foiled Again", a program developed and sold by Bernie Crowe, a FF guy from So. Calif.  I didn't know squat about computers, and didn't get the instructions, but I figured it out. Leading me to suspect that it must be pretty user friendly. It comes with a lot of airfoils, and I don't recall if you can load more in or not, but suspect so. I do recall that you can move the highpoint, change the thickness, generate ribs for Warren Truss or "Union Jack" wings, and plot ribs for an elliptical wing. The one thing I didn't find was a way to truncate the TE to add flaps, but maybe it could be done.  I tried to call up Bernie and buy a legit copy, but his wife answered, and it seemed they were in the midst of a    "discussion" that wasn't going well...but that was years ago, and I'm sure he's probably got that sorted out by now, one way or another!   I never got to talk to Bernie...his ears were considerably occupied.  

I have seen Bernie's name recently on SCATen, so it may be possible to get a copy. No idea what the tab would be, but maybe reasonable...   Steve

[Bob Kruger]

Steve;

Do you still need some drawings on this, or have you been able to successfully loft some ribs in AutoCAD?

V/r

Bob

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.

[don Burke]

Profili from Italy.  Files can be imported to any CAD.

[Bob Kruger]

Profili from Italy.  Files can be imported to any CAD.

Don;

Profili is nice, and it is pretty simple to use.

Problem is the lofted rib accuracy is spotty at best.  I've found considerable variation in trailing edge thickness, leading edge placement, etc.   Also, the smoothness of the surface, especially going towards the trailing edge, gets lost somewhere.  After seeing your post this weekend, I downloaded the latest version to see if that had been fixed.  I got the same results as before.  I even tried playing with the scale, e.g. if the root was 10" in length and the tip 8.5" in length, increasing them by a factor of 10 (along with the spar and LE sizes) to see if that would have any impact.  My rationale was that once back in CAD, I could scale it back to 10% of original size.  I had the same results as before - surfaces with ripples, non centered LEs, etc.

Its OK if you are hand cutting ribs from templates and size them slighty oversize so that you can sand them back.

But , I would not use if for generating/lofting ribs that would be laser cut.

Bob

[don Burke]

Didn't notice those characteristics when I last used it.  But then I'm just "tracing and cutting" not laser cutting.

[Bob Kruger]

Didn't notice those characteristics when I last used it.  But then I'm just "tracing and cutting" not laser cutting.

Dick;

Follows is an example that I am seeing.  I may be setting up something wrong.

V/r

Bob