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Jay Kempf

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Most CAD software will run on almost any modern computer/laptop. But if you get advanced (like almost all aircraft design) the horsepower of the machine becomes your limitation. Waiting for the machine to do anything means you are waiting. So as you advance you build better machines. For a single homebuilt the horsepower of the machine is almost always not your limitation.


When it gets out of control later in your experience it gets like this: my current 19" rack workstation was about $1250 from ebay open box, had never been run. I added quite a bit to it and currently I run 28 physical cores (56 HT) in 2 processors and I have 192GB (out of 1TB max) of high speed RAM, graphics card has 3200 more cores and 12 more GB of dedicated video RAM. My day to day ops run from large fast SSD's with one tied to a large cloud account through dedicated fiber. Many, too many, TB to count anymore. Everything is geared to not watching the little hour glass or circle go around EVER.
 

KeithO

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Jay, your application sounds more like analysis (FEM/CFD) than what is needed for a cad workstation ? Although it will clearly run the cad application just fine I am sure....

When it gets out of control later in your experience it gets like this: my current 19" rack workstation was about $1250 from ebay open box, had never been run. I added quite a bit to it and currently I run 28 physical cores (56 HT) in 2 processors and I have 192GB (out of 1TB max) of high speed RAM, graphics card has 3200 more cores and 12 more GB of dedicated video RAM. My day to day ops run from large fast SSD's with one tied to a large cloud account through dedicated fiber. Many, too many, TB to count anymore. Everything is geared to not watching the little hour glass or circle go around EVER.
 

Jay Kempf

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Yup, built it for three bottlenecks. First is just large elaborate advanced surface models and assemblies. Second is rendering and animation of the former. Third is analysis. The other concern for remote workers now that are in engineering is the internet bandwidth you can get ahold of. Just before the Covid clamp down we got fiber. Great timing.

If you go to Dell and try to buy the workstation I use it would be in the 8-12k range. I found a surplus, never run workstation and then did some delete adds. Total for just the rack mounted device is about $2500. I have 3D mice, big monitors, fancy ergo keyboards, trackball snobware, yadda... The machine came with 5TB of spinning front panel drives. I turned off 4 but added back a lot of SSD capacity.

Just a data point for those wondering where the other end of the spectrum is. Years ago and engineer working for one of the majors would have killed for this machine. Now anyone can have one. Sure makes a lot of difficult tasks seem easy.
 

stanislavz

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Another try to use graphical solving of trusses have shown me one interesting possibility.

Example 1:

1652953079268.png

Here i have entered all dimensions of truss + 100 (mm) of force - each length of element in top and bottom figures are forces acting on them. C = compression, T - tension. Upper case and lower for different elements.

Units will be messy here..


And the funny part. We could go from other side, i can see that i do have biggest load on my C3 member, which is equal to reaction force of whole truss so we cant change it by mowing internal elements, but we can move them to tailor it length for our tube load.

Now "force" is 276 at our C3. 18x1 tube will hold this load till 1220 mm length. On C2 - 229. So it could be as long as 1330mm. We just enter them. But now load (143) and lenght (900) of C1 is fixed. So 12x1 tube will not hold this load, but 14x1 - will.

And we have saved by this change ~200 grams :)

1652952159180.png

But in first step we could go for 16x1 in place of 18x1. Then our first length of C3 is 1000 C2 is 1100 and C1 is 1320 at 143 of force. Which is ok for 16x1 but not for 14x1. But in sum this one is even lighter than first option with 18x1 + 14x1 by 160 grams per two longerons..

1652953222772.png
 

stanislavz

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What happens when you use an up force instead of a down force?

Sometimes if tail comes down harder than you planned at landing forces can be greater than aerodynamic loads?
It is just an example. Yes, you will have to solve all it for all your force with different directions. Unless in my gut feeling is right, and for up force you will be have same drawings, but different sign. Or mirrored at horizontal axis.

And yes, you could add some analytical road, by solving equation of C3 forcu equal its square root of length with same for C2.

Pros of this one - you are not guessing, drawing calculating and finding correct size of tube. But doing all of this from opposite side.
 

stanislavz

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1652958300058.png

This is an example with 15x tube - it is not ok, C1 will be too long to hold its load. Here we would be forced to add new element to divide it to two.
 

PiperCruisin

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The optimization stuff is cool, but one has to be careful with it.
What is it optimized for? Minimizing weight using Von Mises stress I'm guessing. While not impossible to use optimization tools, I usually don't find them practical because I'm considering lots of other things such as:
Stiffness/displacement, wear, damage tolerance, manufacturability, ease of fabrication, constrained by minimum gage, natural frequency, fatigue, multiple load cases, serviceability, cost, incoming inspection... to name a few. That gets tough to program into a computer.
 

bhooper360

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The optimization stuff is cool, but one has to be careful with it.

You should see some of the structures they're making with additive manufacturing; they're so optimized, and the walls are so thin, that the parts are not even particularly capable of supporting their own weight. If you pick one up the wrong way or sneeze on it, you'll punch a hole through a structural member.

Of course, it's incredibly strong in terms of what you mentioned, maximizing the load carrying capability according to specific, theoretical load cases.

That gets tough to program into a computer.

The main thing is to calibrate the intuition. To recognize the patterns, like completing the square to factor an expression, eventually I find myself more able to identify the tradeoffs that other people make in their own airplanes, in the end mostly what I'm doing is reading about what other people have done, digesting it and then regurgitating it. Lol.
 
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cluttonfred

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The optimization stuff is cool, but one has to be careful with it.
What is it optimized for? Minimizing weight using Von Mises stress I'm guessing. While not impossible to use optimization tools, I usually don't find them practical because I'm considering lots of other things such as:
Stiffness/displacement, wear, damage tolerance, manufacturability, ease of fabrication, constrained by minimum gage, natural frequency, fatigue, multiple load cases, serviceability, cost, incoming inspection... to name a few. That gets tough to program into a computer.

100% agree. There is also the issue of buildability, especially by an amateur without a significant investment in CNC tooling.
 

cluttonfred

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How would you differentiate your term buildability from ease of fabrication?

Regardless of the word choice, you can use fabrication techniques for car that would not be economical in light aircraft, ones in factory light aircraft that would not be economical for any but the most popular kits, and ones in kits that would not be practical for building from plans. The part comparison you posted is a perfect example:
1653048580837.png
I can certainly imagine the day that 3D printing becomes so commonplace and affordable that buying the printer and making your own kit of parts becomes an attractive alternative in money and time to buying a kit or building from plans using traditional techniques, but we're not there yet.
 

stanislavz

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How would you differentiate your term buildability from ease of fabrication?

Easy..

This young boy have a series of movies about making a protypes.



The easiest part to make in order of bigger complexity -
1. just piece of wire.
2. bended wire.
3. Cut sheet metal.
4. Cut and bend metal sheet.
 
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