CAD Tool For Design Tiny Aircraft

On Tue, 25 Sep 2007 21:17:19 -0700, Le Chaud Lapin wrote in :

RCCad? It's 3-D.

I don't know whether it supports scripts.

A design program from a different standpoint:

It is CAD. I don't know whether it does 3-D.

Marty

My recommendation is to get an overview introduction through a school's CAD training class. CAD is only one small part of any design & engineering project.

Cheap: As is commonly said, you get what you pay for. SolidWorks at $4000 US may be considered expensive. A more important consideration in the end, is when you have to supply 3D Solids files to people you collaborate with and manufacturing companies for CNC work. You will need to send them files in the format they need to do their work.

Learning to model in a 3D CAD program will NOT give you the elements of mechanical engineering, design, & aircraft engineering specifics. I would expect you to spend even more time learning engineering issues, than the CAD side of the project.

Bo

To produce the aircraft specifications use a program called AirplanePDQ

Quite cheap as well.

This program will also produce a 3 view of the aircraft in 2d and also export the 3 view as a DXF I think.

You can then import this as construction geometry into a 3d CAD, we used Solidworks.

Can you get the student version of Solidworks? this would be a cheap way to go.

I know this works as we have done it for a new 12000lb utility single prop aircraft design similar to the Dehavilland Otter.

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The word you are looking for is parametric. SolidWorks fits this bill. There are several ways to make components in SW work together. At the most basic level geometry in one part can be tied to that in another. To this can be added equations that relate different dimensions. To this can be added design tables which are nothing more than spreadsheets built into a part or assembly of parts that drives part dimensions. And to this can be added control from an external program like Excel (the most common) to Access to a custom written API program.

As to cheap, well the question there is whether this is a hobby interest or a business interest. If it is a hobby, no doubt the cost of SW at 3,995 plus yearly maintenance might be a bit high, but for a business it isn't much at all. In addition SW requires a fairly high end PC to do the kind of thing you are talking about.

As with anything as complex as airplane design (you didn't say it had to fly, but I am guessing it will) to do the things mentioned in the first paragraph will require some training, some practice and probably more questions on this forum.

TOP

If your model plane doesn't require very organic shapes, then Alibre might be a good choice and is cheaper than SolidWorks. If you need more organic shapes and smooth transitions, SolidWorks would be a better choice, but it can take a lot of work to get it right and the models will be less robust. (You'll change a parameter and some feature far away may break.) If you need really nice shapes, and don't have the time or patience to mess around, you might need to go to the expensive guys, like CATIA and UGS. Jumping into them from a non-CAD background would be really scary.

Jerry Steiger

Autocad is NOT the program you want for something like an aircraft design. "3D aware" is meaningless; even a program is 3D or it isn't. Autocad is 3D nowadays, but its interface dates back to days when it wasn't, and it shows. And parametrics (where one change can automatically send changes rippling through the design) is highly overrated (I'm sure I'll hear from the guys on CCS about this), though it can also be very useful.

A fully 3D program is, IMHO, a must for any kind of design, anything else is silly. A parametric modeler, however (like SWX and many others) can be very cumbersome to use... and I've used a lot of them over the years. Yes, if the design constraints are set up correctly from the start, minor changes can be ridiculously easy... but if not, or if you don't have a clear idea of where you're going from the start, you can find yourself boxed into a corner and have to start from scratch.

Personally, I prefer a pure geometry based modeler. Simple dimensional changes affecting many components may take longer, but it's far easier to make large sweeping changes if necessary, or switch to an alternate design approach. Most of my work nowadays is large machine design (though my degree is in aero engineering), for which I use KeyCreator (formerly Cadkey). Same price range as SWX, though, which I don't define as "cheap".

-Dana

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I read all the responses and looked around the 'Net, and it seems that SolidWorks, if not what I'm looking for, is create by people who had the mindset I was looking for.

But now I am confused. I thought parametric modeling was good.

I program computers from time to time, and being able to change the structure of a component and have everything that depends upon it change accordingly is simply invaluable, so I cannot see why this would be bad. That's precisely the behavior I want.

For example, in my miniature aircraft I envision, there is only one fuel tank, and it's cylindrical, but its radius and length are a function of several other parameters.

I am guessing that, like in programming, there is an art to structuring the interdependencies so as to minimize likelihood of running into dead-end that you mention.

Finally, I was really surprised to learn that parametric modeling was not fundamental in all CAD programs. I cannot imagine what it would be like to try to optimize a design without it. What do people do without parametric modeling? Tweak every single component manually during optimization phase?

[I am going to give Alibre a look also.]

-Le Chaud Lapin-

Creating a 3D parametric model can be likened to programming. Some people make spaghetti code and some make nice tight robust models. This is called capturing design intent.

It can also be likened to a database capturing spatial information.

You can iterate in a non-parametric world too. It takes good revision control.

TOP

Don't let idiots like jon banquer confuse you. Solidworks, ProE, Inventor, UG, CATIA, Alibre, SolidEdge and some others would all work fine for what you want to do. It mostly depends on your budget, your suppliers/customers, your pool of potential designers/drafters, level of support required, and maybe corporate culture.

Le Chaud Lapin wrote in news: snipped-for-privacy@g4g2000hsf.googlegroups.com:

Parametric modeling IS good. But you must use the right tool for the right job. Some people's jobs are not well suited for parametric modeling, or they never learned to use it well, so they think it's bad in general. As TOP said, a well constructed parametric model captures design intent and allows easy generation of new or altered designs. People who structure models poorly or need to deal with radical changes to design intent can be better served by non-parametric modelers. For optimizing, parametrics are not just good, they should be a requirement. And if you need to make radical changes in a hurry with no regard to capturing design intent, that can be done fairly easily in a parametric modeler by making models that would otherwise be thought of as horribly structured.

But you'll be wanting well structured models again when it is time to make fine changes. This may require that the design be completely remodeled, but that can be mitigated by structuring your models in a way that can support radical alterations. Look into techiniques like skeletal modeling or so-called horizontal modeling.

Be warned, there is a steep learning curve, even for tools with a UI as streamlined as SW. A tool that is easy to use, may also be easy to use incorrectly. This goes double for complex shapes such as an airplane will have. If you do choose SW, we can recommend some good resources and examples for study.

I have built and flown some radio controlled model airplanes, and I think SW could handle modeling something of that complexity fairly easily, if the models are structured well. If not, it will be a painful experience. Having said that, even with a well-structured model, there will be frustration with rebuild errors and the like. Hopefully, R/C airplanes are similar enough in scope to your project that my opinions will be useful in your decision.

innews: snipped-for-privacy@g4g2000hsf.googlegroups.com:

Ooooh...rebuild. I like rebuild. I guess it's the same concept of compilation in programming. I was wondering in my OP if there were an equivalent with parametric models. After all, with all the interdependencies..something has to be synthesized from the expression of interdependencies, just like in programming.

This is my first design of anything using more than a ruler and scraps of paper, let alone an airplane. I know it will take years, so I'm going to just take my time and learn what I need to know as I go.

-Le Chaud Lapin-

I think I get it now. Hit me while walking my dog.

Example:

1. One dimension of fuel tank depends on required fuel capacity.
2. Fuel capacity depends on mass of certain parts.
3. Mass of parts depend on geometry and density of material of those parts and load requirements, etc..
4. Load requirements depend on configuration of other structures.

And it would seem that there is a right way and a wrong way, and again, finding the right way is more art than science. "Reaching" too deep into model to extract parameters to be used elsewhere might be a bad idea. Deliberate indirection and hierarchy would be important. There would also be opportunity for circular references. Also, there should be some kind of "on/off, part is there, part is not there" programmability.

I was wondering about this. It seems like parametric is a superset of non-parametric in some ways.

-Le Chaud Lapin-

The concept of rebuild isn't quite like compilation... well maybe. The analogy breaks down rapidly, but I think you have the concept. Just in case the analogy is leading to a misunderstanding: parameters are the rules by which the CAD program creates and manipulates the individual elements that make up the models. After a change to one of the parameters, the rebuild process has to be repeated so that the model is correct to the parameters. Features (as in "feature based parametric modeler") are very closely analogous to subroutines, if the rebuild is an execution of code to build a model. The parameters would then be constants within those subroutines. For example: a hole feature generates a hole in the model based on diameter and depth parameters, when the model is rebuilt.

If you want to do automated optimization, you are going to need to program some macros for SW, maybe add-ins. There is an excel spreadsheet from about 8 years ago that uses Excel's optimazation tool, but it's limited to a single variable, IIRC. I saw in you OP that you were interested in programming some relationships, but these should be the first things that require programming on you part.

Also, I see in your conversation with TOP that you might be interested in some parameters being driven by masses and volumes. You will need to do some coding for that too, at least in SW.

Other CAD programs may not need macros for this particular task. Some of the add-on analysis tools (FEA, CFD) have such features, but that will get very expensive very quickly.

Anyhow, parametric CAD seems to be the tool you need, and you will grasp the mechaniics of using it very quickly. A good coder should easily, almost intuitively grasp what it is to organize the models well. I think you will be ok, but there will be bumps along the road (such as managing strange dependency behavior). SW will serve for your task, but something like Alibre may be sufficient and much cheaper. Demo versions of most CAD packages can be had, and standard advice is not to pay any cash until you have taken several for a test drive.

This isn't quite aircraft, but a while back I wrote a VB macro that, in conjunction with a very crude model of a concrete mixer, was capable of predicting the CG of aforesaid truck with a fraction of an inch of what the customer was measuring. For such studies you make a rude and crude model that will update quickly but that captures the intent of the study you are doing.

I will many times not even use a 3D model if I can do it in Excel. Nobody likes this because you can't see the pretty pictures, but you can run through lots of scenarios very quickly that way. And with the solver in Excel being what it is there is little that can't be done to get close to the right answer.

TOP

PS Also search the NG for configurator. There are some very good ones out there for SW.

I think I will try Alibre. I was sure SW was what I wanted, but Alibre looks good from website too.

I've been to the SW website about 5 times today hoping that, by magic, the price would drop to $100 for full version. Strangely, this has not happened, so I need get an eval version soon. :D

-Le Chaud Lapin-

This is going to be very tricky. There HAVE to be circular references in your optimization. When you change the weight of the fuel tank, you have to reevaluate the size and weight of all of your other components to account for the new load. But now you have changed the weight of the rest of the components, so the fuel tank needs to change again. If you are lucky, the solution converges and you end up with a design that works. If you start from the wrong spot, it might never converge.

The good news is that you seem to have the type of mind set that would allow you to work through this type of problem. The bad news is that it is an extremely complex problem that requires a lot of deep knowledge in many areas of design.

Jerry Steiger

On Tue, 2 Oct 2007 13:14:34 -0700, "Jerry Steiger" wrote in :

Speaking only from practical experience with RC models (~14 years total), the envelope for small aircraft is extremely forgiving for ordinary flight regimes. Most RC aircraft can double or triple their fuel load without noticeably affecting flight performance.

Look at the college competitions for evidence. A few years back, a weight-lifting competition was won at around 19 pounds of payload for an aircraft powered by a plain-bearing 0.40 ci engine. Engines like that are usually used to fly ~5 pound trainers with a wingspan between 40" to 48" or so.

Drat. The American Institute of Aeronautics and Astronautics website is down right now ... Here's a report on the 2006 competition:

If you're trying to fly an RC aircraft across the Atlantic with a gross weight of 5 kg (11 pounds), then you DO need to consider a multitude of tradeoffs such as you describe:

An engineer friend of mine likes to say, "One observation is worth ten thousand expert opinions." Regardless of what the design software predicts, the product needs to be tested in flight to see whether the theories work. (I'm assuming that the initial post in this thread was about a flying model.)

Something funny happens as you go down in scale. It has something to do with Reynolds Numbers and the volumetrics of small aircraft (volume decreases far more quickly than area). This means that the power-to-weight ratio favors the model aircraft and that, as a general rule, the ratio of the strength of materials to G-forces increases. A 1/4 scale model (using 1/4 of the linear dimensions) has 1/16 the area of the prototype and just

1/64th of the volume.

One way to get in the ballpark when designing a new model is to select dimensions from aircraft that are already known as good flying designs. The airfoils that work well for full scale do not work well on small aircraft (as a general rule--Clark Y airfoils probably scale OK; fighters and bombers from WW II on generally do not scale well).

So a lot depends on the kind of aircraft the other poster wants to build, whether it is supposed to fly, and what kind of performance is to be optimized.

Marty

Oh, I forgot: Keycreator:

Jon Banquer! Small world, eh? I wondered what you were doing on rec.aviation until I realized this is crossposted to comp.cad.solidworks. Been quite awhile since we got into pissing matches on the Cadkey webforum... I gave history based parametric modelers a good try, I really did (UG, SWX, and Inventor) but I got tired of having the inability to model something the way I wanted based on the constraints of an earlier design version... and want back to a pure geometry modeler (KeyCreator). It certainly has some warts, too many IMHO, and I think Greg Marr still gets annoyed when I bitch too much, but warts and all it's still the best tool I've found for the kind of work *I* do... YMMV.

-Dana

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