Lofting in Autocad

[proparc]

For all you computer savvy guys out there. I need to "loft a set of ribs in Autocad 2000. I have the tip rib and center rib section drawn out, but I need to loft all the center ribs. Can it be done without rescaling and if so, how? Thanks in advance.

[Mark Scarborough]

Milton,
To draw the ribs, you need actual length of each rib you desire. You can get this by drawing a plan view of the wing layout and then drawing a line at each point you want a rib. Not knowing how you drew your rib up I will  just give my perspective. In order to do this simply you need the rib plot to be a polyline.It should only show the actual exterior skin shape, no spars or cutouts. I start with the root rib. The polyline needs to exclude the leading edge radius and the trailing edge height. you then just scale the upper half of the rib using the upper rear corner of the rib profile as the base, and scale it from that point to each of the lengths that you derived in your plan view drawing. By using the upper corner of the trailing edge you maintain the trailing edge thickness as a constant. If you scale  from the center line of the trailing edge the thickness of the trailing egde will get smaller towards the tip. If this doesnt make sense, email me your cad file and I can plot them out in a few minutes for you. its very hard to explain but very easy to do. After you have lofted each of your ribs, then offset the "skin" profile towards the inside 1/16 or whatever your skin and capstrip thickness is, draw your spars and TE ,LE then mirror  to create the bottom half of the rib.Ta DA its done.

[Paul Smith]

If you have sheet of AutoCad ribs (and other parts), will the lazer cutter instinctively jump from one part to another, or does the cad jockey need to give them a "no cut" path to follow.

As I see it, if I were 'Cading a set of ribs, I would need to scale the airfoil for each rib, and use some hand work to get the LE and spar notches "true size". 

I would draw a plan view with the spars included and begin with a root rib.  Then I'd copy the root rib to each rib station and "stretch" it down to the required chord.  Then hand-draw the spar notches in full size.

[Scott Jenkins]

Paul,

"will the lazer cutter instinctively jump" Computers are inherently dumb no A.I. yet.
No the laser will not jump from one rib to the other, drawing up a set of ribs is only half the battle then you need to program the laser and define movement in X,Y,Z. Z can be used for the jumping to the next part. Or if you want to just stay in the burn so to speak the you can program it to move from one rib to the next. A few typical lines of code looks like this:

G00 Z1.
X26.1007 Y46.3722
G01 Z-.25 F200.
G02 X26.0712 Y46.3392 I-.0312 J-.0018 F600.
G01 X24.6984 Y46.2622
G02 X24.6654 Y46.2916 I-.0018 J.0312
X24.6949 Y46.3246 I.0312 J.0018
G01 X26.0677 Y46.4016
G02 X26.1007 Y46.3722 I.0018 J-.0312
G00 Z1.

This routine will cut you a slot 1.625 long, .25 wide, with .125 radius on each end using a 3/16 cutter.

And to think 35 years ago I wrote millions of lines of code by hand then let the key punch operator put them on to tape. Nowadays I can crank out 100,000 or 200,000 lines of code in a hour or less using my trusty computer. Man I do not miss doing it by hand.

Scott

[proparc]

If you have sheet of AutoCad ribs (and other parts), will the lazer cutter instinctively jump from one part to another, or does the cad jockey need to give them a "no cut" path to follow.

As I see it, if I were 'Cading a set of ribs, I would need to scale the airfoil for each rib, and use some hand work to get the LE and spar notches "true size". 

I would draw a plan view with the spars included and begin with a root rib.  Then I'd copy the root rib to each rib station and "stretch" it down to the required chord.  Then hand-draw the spar notches in full size.

This is the technique I have been using in the past. But, this method cannot loft from one shape to another in even incremental steps.  All that you will do is rescale the initial shape down to a perfect smaller size and then put back in all the proper size spars etc.

The need to loft is important in my case because I cut my own foam wings,(nice skill to have) and I need to get the friggin ACAD to loft.  I know that Solid Edge and Solidworks can do this and I can import the output back into ACAD, but as you can see, at this point, we are starting to take a serious ass whipping.

Larry Cunningham was so kind as to hep me to using his StuntRib program and DFX'ing the output back into ACAD. The only drawback here is that it does not appear to allow you to import your own "no need to practice world beater airfoils" and loft them and then export back to ACAD.  Its looks like you are going to get a NACA something or other whether you like it or not.

[Mark Scarborough]

Milton,
That is the advantage of using my method. I ahve Pat Johnstons airfoil from his bearcat, (a very good airfoil incidently) and I use it in all my designs. by scaling the skin of the wing, it is very very easy and quick from that point to offset(thats a command that moves the selected line a preset distance in the indicated direction) to form the rib profile, then insert at the correct distance from the trailing edge your spars.
I can talk you through it on the phone if that would help. NO import export, or being forced to use a NACA airfoil. Trust me its easier than it sounds in writing.

[proparc]

Mark, I am going to bust a move on your suggestion.  I think your on the case about this one. Thanks a million.

[Mark Scarborough]

I hope it works for you. I certainly dont want to take away from anyone elses methods. One thing I have learned with Cad, for every need there is at least a dozen ways to accomplish the same thing. Having never used any of the other methods, I cant speak for them I just know that the way I do it,(Pat J helped me refine what I was doine) works for me. Best of luck, if youhave any issues feel free to give me a holler..

[John Miller]

Mark, your method will give you a set of lofts, but it won't allow for changes in the high points location, or percentage of thickness. These changes are important for a stunt wing, and are used on most designs. Varying the tip shape from the root will improve the tendency to delay the stall at the tip, causing the root to stall first, which improves the turn.

I'm not aware of an easy method of doing this in AutoCad. I suppose you could construct the wing on 3D, with your chosen root and tip rib, skin it, slice it at the locations you want ribs to be at, and then rotate the view to look straight down on the ribs.

The best, and easiest way I know of is to use a good lofting program, and import the results into AutoCad. Larry's Stunt rib program has been mentioned. I've not used it, but have heard good things about it.

For my work, I invested, some years ago, into a program called Compufoil Pro. At the time, it cost about $125.00 USD. Eric Saunders is the developer, and a nice guy. I've since upgraded several times, and it's always been at minimal prices because I own a registered copy of the program. The most recent version is called Compufoil 3D, and is a very powerful, and accurate, lofting program.

I asked Eric about entering my custom, world beater airfoil, and he supplied the information on how to do it, but I wasn't able to get it to work, so he had me send him the plot. He sent me back a file that now lets me use my airfoil.

That's my experience with Compufoil, and this is not meant to be an adsvertisement for the program, just a heads up on how to do what you want easily and accuratly.

Check the net, there are many good lofting programs out there, some are freeware, but I'll stay with what I have.

John Miller

[Mark Scarborough]

If I understand correctly, you are wanting to use a different airfoil at the root and the tip? If that were the case then yes perhaps you are correct. My method will not allow that. More power to you, I personally cant justify another program expense versus any perceived gains by altering the airfoil from root to tip. by my method, the tip rib actually is a slightly thicker crossection than the root which helps to mitigate the tip stall issues, IMHO the flap configuration actually has as much if not more to do with the actual tip stall condition than the airfoil shape. With regards to chord of the flap, and how far out it goes on the wing. I base most of my wing design on existing successful aiframes, IE most commonly Pat Johnstons designs which are fairly well proven to be successful.HOWEVER, I will not say that I am absolutly right on this, just my opinion based upon readings of different designs and opinions. Also, Its not as though I have 50 years of aero training and design practicum to call from, hence the observation of proven design as a basis for my work. As always there are many ways to skin a cat......
As an additional thought, reducing tip stall tendancies would improve the quality of the turn perhaps, but only when you are walking the actual tightrope of a stall. If you have sufficient lift to make the turn you are attempting, then the tip does not stall and therefore the only factor is the generation of vortices and the associated drag. Again my opinion and observation.

[John Miller]

Mark,

As in all things, there is compromise. I certainly was not trying to undercut or belittle what you were saying. I have gained any knowlege of stunt aerodynamics,  I may have, pretty much the same way you have.

Pat Johnston is a friend of mine, and we often get together at contests to discuss our ideas and theories about stunt design. We don't always agree, but that doesn't diminish any respect either.

Pat is also a very prolific Cad man, with a good output of work over the span of time we've known each other. I tend to take more time with my drawings, because I can't spend as much time doing them, at this point in my life.  Still, I do consider myself on about an even par with Pat when it comes to our designs, and training in CAD.

But back to the subject.

So, yes the percentage of thickness would increase using your AutoCad method of lofting. I see that now, missed it before. I'm still not sure that the shapes would approach optimum, but, what's really optimum, is a moving target most of the time.

I've used AutoCad since Release 7, and have found, that for me, there's enough value in an additional lofting program, to make it worth it to me to spend the money for what I consider the best out there, and I'm a tightwad. Such a program is not necessarily for everyone, I know, but one doesn't have to spend a lot of money to get a decent rib lofting program. Some are, in fact, freeware.

Using the lofting program has made it easier, faster, and more accurate for me to get a full loft of all the ribs, including variations of the airfoil. It also lofts in the spars, to my dimensions and chosen location, as well as style and shapes of the lightening holes in the ribs themselves. I also can include whatever size, and location of jig holes needed.

The basic operation is, choose the root and tip airfoils. ( The tip is usually 2 to 3% thicker, and the high point is moved about 3 to 5% forward, using the same airfoil as the root as a starting point.) Once the foils are chosen, I use the loft function, which through a series of entrys, sets the span of the panel, starting rib possition, spacing, spar sizes, and locations, etc. and then presents me with a complete lofted set of ribs that can be imported into AutoCad, or virtually any Cad program as a DXF file. This usually takes about 30 minutes or less for me to do, unless I'm trying something fancy.

Once into the Cad program, I dress up the lofted ribs, convert the lines to polylines to make for better laser cutting. and I'm done with the ribs.

Now, having said all the above, The original question was answered by yourself, and offered a decent way to use AutoCad to get a set of lofted ribs, though there may be some questions as to minute aerodynamics and personal preference, you would still get a set of usable ribs in the end.

My answer was not answering what you had already answered, and well too, but to try and show the value of using a compatible lofting program to gain more speed and accuracy, as well as broader capabilities than is easily accomplished in AutoCad alone. Apparently I didn't convey this very well the first go around. I hope it comes through better this time.

John Miller

[proparc]

Interestingly enough. I just downloaded a demo of Compufoil 3d,(the latest incarnation) last night. It cost $38.00 for the full version and it can loft and import and export dfx. To smoothly transition from one shape to another can also be done in Turbocad Pro which can also import export. Pro Engineer can do this invoking their "slice" command module.

I talked to Pat Johnston,(Mr. Autocad) and Autocad won't do it, (trust me ,I tried). Their Autodesk Inventor 3D program will.  I have some background and training using these high end 3D programs but for everyday 2D drafting they are a resource hog and a pain in the ass.  It is for this very reason you will see engineering firms and departments using "the old one two combination"-Autocad and Solidworks\Pro E.

In my case, there is no getting around it. The stunt work that I am involved in is taking me pretty deep and I am going to have to get up close and personal again with one of these high end 3D programs.  It's time for me to build another computer.

[don Burke]

"PROFILI" is a 10 ($euro) program from Italy.  Very good and will draw all the ribs with structure included.  The files generated can be xfrd to AUTOCAD.

I believe a google for PROFILI will take you to his web site.

don Burke

[Bob Kruger]

For all you computer savvy guys out there. I need to "loft a set of ribs in Autocad 2000. I have the tip rib and center rib section drawn out, but I need to loft all the center ribs. Can it be done without rescaling and if so, how? Thanks in advance.

As they would say back where're I'm from, "you can't get there from here...." (at least not using Autocad).

There are a number of programs that will allow you to do this once you have your tip and root airfoils / rib profiles drawn.  Turbocad has a lofting function, as does Rhino 3D.  The basic technique is to place your rib profiles along the Z axis as they would be for an actual wing, then loft a skin on the wing.  Then, you "cut" the ribs at various points through the lofted surface, delete the lofted skin, and you have ribs.  You then move them back to "0" on the Z axis, and arrange for your friendly laser cutter.  All of this can be done on a .dxf or a .dwg file, so interoperability with Autocad and other programs that use the Autocad format is not complicated.

The problem with Turbocad ribs are that the cad "points" are too numerous and too close together for effective laser work.  Having a rib with 1000 or 2000 (or more) cad "points" really slows down some laser cutters.  Ribs generated by the newer version of Profili are also very detailed, but this "detail" and point density again is excess of what some laser cutting machines need, and again it slows them down considerably. 

V/r

Bob