Rag and tube construction and computer models?

Hi all...

Many moons ago I worked at place doing finite element modeling....and the work
I did was with solid objects...dont worry..I get back to the rag and tube
question eventually...

The way that works (for a solid object, say like a crankshaft, a bracket,
etc,,,) is you build your big object outa lots of little objects...lets call
them bricks.....Now, with lots of nasty math, physics, and engineering you can
develop equations that say if you put this much force or displacement (or
whatever) on this part/side of one brick, then such and such will happen here
and there and there on the "brick".....now the corners/sides of that brick are
mathmatically tied to the next brick and so on and so on.....and what you end
up with in the end is a GIGANTIC mass of equations (often thousands if not tens
of thousands of them) that the computer works hard to find the solution to...

Now, its not quite as bad as it sounds (as long as you werent the poor soul who
had to write the program in the first place) because what you generally did was
use another program to make a geometric model of the object of interest (kinda
like a fancy autocad)....and that spit out another nasty file that got feed
into the first program I described above...

The nice thing about this was you could input ALL kinds of material properties
describing each brick like strength, rigidity, fracture toughness (and
MANYmore) and as importantly it allowed an object to "constructed" with
different materials....and then you told the program where to put forces, or
displacements or whatever....and off it went to crunch numbers....

Once all the computing was done, you used a third program to visualize
stresses,deformations etc etc...and with that you could "see" where you had
more material than you needed, or where the stresses were too high, or where
something was likely to buckle etc etc....so it allowed you to optimize a part
in ways standard textbook engineering equations never could...

The other cool part is you could even do things like create a "crack" here and
see if was likely to propagate....or "break" a part there and see how the load
was redistributed among the remaining parts.....allowing you to check out lots
of "what if" scenarios you'd never have the time or money to do otherwise)

Now, I never used this capability of the program...but it also had the
capability to construct objects out of plates, shells, infinitely thin rods
(wires?), and hollow tubes....

It occured to me the other day that virtually all the rag and tube designs
being built today were designed before this computer capability existed (or at
the time only at the real high tech computer power houses of the day)...

So, 2 questions...does anyone know of any small plane "rag and tube" designs
where any significant computer modeling was used?

And secondly....any guesstimates on how much weight percentage wise you could
shave of the typical tube structure by using such modeling and still maintain
the same structural margins....?

take care

Blll

Dear Bill (and the Group),

This isn't an answer to your questions. But maybe it is, in a way.

When designing an optimized tube-frame structure such as a rocket mount or
off-shore drilling platform you are allowed to let the task drive the
properties of the material in that you can spec whatever alloy, diameter and
wall-thicness that might be required. The assumption here is that the budget
is large enough to allow you to let contracts to have the materials made to
your specs. (Don't laugh. It got us to the moon & back.)

But when applying the new software (ie, circa 1960's) to more mundane tasks,
such as the engine mount for an R-2800... or the fuselage of a Formula One
airplane, you were forced to use the materials that were commonly available.
Then you ran into an interesting problem with tooling costs and fabrication
skills, interesting in that in most cases, implementing your new, computer
optimized structure will cost millions of dollars and several years, since it
dictates the need for new jigs & fixtures, different welding & inspection
procedures and retraining your work-force.

Bottom line is that with an existing structure any benefit of structural
optimization usual fails the Practical Factors test.

Starting from scratch? Then that's a different story and there are some nice
examples of steel-tube airframes, including square & rectangular tubing (!)
that have taken full advantage of computer-aided design, MIG welding (it's
faster) and so on.

When applied to home-building I suggest you turn the equation around. Use
CAD&D to come up with a welded tube structure that uses the LEAST number of
different diameters and wall-thicknesses as well as the least amount overall,
combined in a structure optimized for unskilled weldors working without
elaborate jigs & fixtures. This speaks directly to the Practical Factors of
one-off, home-built construction, the most critical of which is cost.

The answer to your questions can be found in any number of airplanes flying
today. Unfortunately, they start at about $100k and go up. Alas, such
airplanes and the attention devoted to them virtually guarantee the demise of
grass roots aviation in America.

-R.S.Hoover

Mr Hoover

Another good/interesting point by you as always and I "grok" what your
saying....

Let me take another stab at this....

Take some "typical" rag and tube design that your "cost challenged"
homebuilders are building these days with minimal tools and skills....

Most likely (IMHO) it is of a design that was not computer
optimized....somebody long ago probably just eyeball/rule of thumb/comparision
with previous successful designs engineered it till it seemed light enough,
simple enough, and when given the static loads it was likely to encounter it
didnt break.....at which point the designer said "praise the lord" and moved on
to other tasks....

Now, take THAT design, and do the modeling (of course the modeler needs to KNOW
what they are doing)....

Look at the computer results....the model might show some areas that have ALOT
of stress.....which at the very least tells the builder "make sure THOSE welds
are damn good".......

or the model might show some tubes are under very low tension compared to its
strength .....so you realize you can spec out those tubes one or two standard
sizes down in diameter/thickness.....without any penalties

Or the model might show an area prone to buckling which when fixed with an
extra brace adds only a little weight to the overall structure but makes the
entire structure significanty stronger (ie high rewards to cost ratio there)...

Or by playing with the model you might find out that you can leave out this
tube here, that tube there, and those over yonder and you've lost little or
nothing in the strength of the design.....

I understand what your saying about the big projects....you optimize the design
in a biggggg way and then special material or sizes are no big deal.....

I'm asking/proposing the opposite....take a standard design....and see if you
can tweak it and still use standard materials and parts....and with a little
luck you might end up with something that is a bit lighter or stronger or if
you are really lucky has a lower part count....

The good thing about that kinda project is the only thing its gonna cost you is
your computer time (assuming your using free software)....

Just some wonderings on my part....

take care

Blll

I remember using one of the first finite element programs outside of the
aerospace industry. Back then, you did not have a nifty program to create
all the "bricks" something was made up of. You had to define each node in
three dimensions -- and then define the end conditions at each node in 9
ways (3 for each dimension). Took a lot of work. Then it was sent by
telephone to St. Louis (to McDonnell-Douglas I believe) for processing. The
results were sent back by telephone in one or two days. No graphs. No
colors. Just numbers. The last finite element program I used was on a PC
about 10 years ago. Much easier. As you may have guessed by now, I am a
registered structural engineer.

Anyway, to get back to your question, it depends. I have run some tube and
fabric designs through finite element analysis. If you were to check the
Tailwind design, you will not find ANY reductions in tube size or thickness.
You will undoubtedly find some suggested tube increases. I checked the
design on one of the later programs and also built a Tailwind airframe. I
believe that he probably used every tube size and wall thickness there is
available in that design. There are little itty-bitty tubes branching all
over the place. I know Steve's design has a long history of troublefree
use, so I would be suspicious of the finite element model. I did not have
the inclination or time to refine the model any more.

If you were to check just about any of the EAA airplanes (such as the EAA
Biplane or the EAA Acro Sport), you will find many reductions in size or
thickness. I believe that the Poberezny designed airplanes were made
"hell-for-stout" for beginners and also to minimize the number of different
tube sizes needed. BTW, I looked carefully at the designs for these, but
did not do an analysis of them.

I doubt that optimizing the EAA airframes (metal tubes only) would cut more
than 10 pounds from them. If you were to race airplanes (like Wittman did),
any improvement would be worth the work. If you don't, is it really that
important? Each designer decides that himself.

"BllFs6" wrote in message
...
Mr Hoover

Another good/interesting point by you as always and I "grok" what your
saying....

snip

Just some wonderings on my part....

take care

Blll

On Wed, 7 Apr 2004 18:29:03 -0500, "Harry O" wrote:



Anyway, to get back to your question, it depends. I have run some tube and
fabric designs through finite element analysis. If you were to check the
Tailwind design, you will not find ANY reductions in tube size or thickness.
You will undoubtedly find some suggested tube increases. I checked the
design on one of the later programs and also built a Tailwind airframe. I
believe that he probably used every tube size and wall thickness there is
available in that design. There are little itty-bitty tubes branching all

It is interesting to look at the airframe of the nesmith cougar and
the w8 tailwind together. as you say the wittman uses the one tube for
each longeron. the nesmith steps down in diameter at every cluster.
the tailwind looks to be about half the fiddle factor of the nesmith.

in australia there was an eyeball designed high wing tube and fabric
that was in the run up to production when it hit airworthiness snags.
the CASA engineer determined (it I recall the secondhand info
correctly) that in areas of the fuselage it did not have sufficient
margins of strength. stress checking was then done (dont know what
method was used) to correctly match the tube sizes to the loads.
the second iteration of the design then went into production.

design as I recall was a knock off clone of an avid flyer or a kitfox
but I cant recall the design's name.

so yes there is an instance where a design was optimised by structural
evaluation after initial design.
TLAR only gets it correct is the eye is exceptionally practised.
(tlar - that looks about right)
Stealth Pilot
Australia

I have never seen the plans for the Nesmith Cougar, but I pulled out my old
set of plans for the Wittman Tailwind to check tube sizes. BTW, in talking
with Mr. Wittman, I quickly learned that you don't even mention the Cougar.
He was very sensitive about someone who wasted a lot of his time asking
questions, then stole his design, and then ruined it with bad modifications.

Anyway, there were 22 different sizes and/or wall thickness of tubing listed
in the Tailwind plans. That is a lot more than I remember seeing in the
plans for the others I mentioned. The did step down the further back they
got. I doubt that anyone ever did a stress analysis for the Tailwind (at
least before it was built) and it was done by "eyeball". However, I have a
lot more faith in Mr. Wittmans eyeball than the numbers from some structural
engineers I know.

Another off-topic comment about the Tailwind. I talked to Steve Wittman
several times. One time was about the engine. I bought a Lycoming
0-290-D2. He looked down on that. He used an "85hp" Continental at the
time. Much lighter and delivered as much power (?). I asked about the
pitch of the propeller and the speeds he was getting. They did not match.
I talked to him again. I found out that he was running the little engine at
about 3,200rpm. Way, way over the manufacturers "redline". The propeller
pitch and speeds he was getting matched at the higher rpm. He did say that
he only got about 400 hours from the engine between rebuilds, though. Since
he did them himself, he did not think that was much of a problem. No doubt
he balanced and blueprinted the engines, too.


"Stealth Pilot" wrote in message
...
On Wed, 7 Apr 2004 18:29:03 -0500, "Harry O" wrote:

Anyway, to get back to your question, it depends. I have run some tube
and
fabric designs through finite element analysis. If you were to check the
Tailwind design, you will not find ANY reductions in tube size or
thickness.
You will undoubtedly find some suggested tube increases. I checked the
design on one of the later programs and also built a Tailwind airframe.
I
believe that he probably used every tube size and wall thickness there is
available in that design. There are little itty-bitty tubes branching
all

It is interesting to look at the airframe of the nesmith cougar and
the w8 tailwind together. as you say the wittman uses the one tube for
each longeron. the nesmith steps down in diameter at every cluster.
the tailwind looks to be about half the fiddle factor of the nesmith.

Stealth Pilot
Australia

"Stealth Pilot" wrote in message
..
TLAR only gets it correct is the eye is exceptionally practised.
(tlar - that looks about right)
Stealth Pilot
Australia

I do love TLAR, but where does one find figures needed for things like
downforce required by the tail, gust factor loadings, lft distributions for
varios airfoios and configurations, ect?
--
Jim in NC


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Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
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On 07 Apr 2004 22:31:51 GMT, (BllFs6) wrote:

Mr Hoover

Another good/interesting point by you as always and I "grok" what your
saying....

Let me take another stab at this....

Take some "typical" rag and tube design that your "cost challenged"
homebuilders are building these days with minimal tools and skills....

Most likely (IMHO) it is of a design that was not computer
optimized....somebody long ago probably just eyeball/rule of thumb/comparision
with previous successful designs engineered it till it seemed light enough,
simple enough, and when given the static loads it was likely to encounter it
didnt break.....at which point the designer said "praise the lord" and moved on
to other tasks....

Now, take THAT design, and do the modeling (of course the modeler needs to KNOW
what they are doing)....

Look at the computer results....the model might show some areas that have ALOT
of stress.....which at the very least tells the builder "make sure THOSE welds
are damn good".......

or the model might show some tubes are under very low tension compared to its
strength .....so you realize you can spec out those tubes one or two standard
sizes down in diameter/thickness.....without any penalties

Or the model might show an area prone to buckling which when fixed with an
extra brace adds only a little weight to the overall structure but makes the
entire structure significanty stronger (ie high rewards to cost ratio there)...

Or by playing with the model you might find out that you can leave out this
tube here, that tube there, and those over yonder and you've lost little or
nothing in the strength of the design.....

I understand what your saying about the big projects....you optimize the design
in a biggggg way and then special material or sizes are no big deal.....

I'm asking/proposing the opposite....take a standard design....and see if you
can tweak it and still use standard materials and parts....and with a little
luck you might end up with something that is a bit lighter or stronger or if
you are really lucky has a lower part count....

The good thing about that kinda project is the only thing its gonna cost you is
your computer time (assuming your using free software)....

Just some wonderings on my part....

take care

Blll

Bill, my understanding of the most certified tube and fabric airplanes
is that they WERE structurally engineered using the old methods of
analysis. I would guess that you might save a pound or two off the
J-3's fuselage but then again, maybe not.

I'm building a Christavia Mk 4. The designer, Ron Mason, deliberately
overdesigned it. It's way heavier than it needs to be but he has his
reasons for doing it that way. I'm not happy with the extra weight
but I'm not a structural engineer and I'm not about to second guess
him. He designed the airplane with missionary use in mind and
pictured it pounding along in jungle thermals over gross. Then he
added a fudge factor for idiot/unskilled builders and you end up with
a really really stout airplane, but heavy.

Like I said, he had his reasons.

Corky Scott

Veeduber wrote:
Dear Bill (and the Group),

Bottom line is that with an existing structure any benefit of structural
optimization usual fails the Practical Factors test.

-R.S.Hoover

John Dyke told me his own self that a local college analyzed his Delta
design. They told him that he could have saved weight in the spar by
stepping down one size at each station.

If he had of done that, I think I would have walked to Iowa to beat him
to death with an intricate, fully optimized, warped-like-hell spar.

I've also drawn the Delta up in Pro/Desktop, a 3D CAD package. My
lesson was that some things are easy on the computer, and some things
are easy in real-life, and the two don't always correlate.

There's a VRML section on my webpage now if you want to see a 3D model
of the Delta.

--
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
alleviated by information and experience."
Veeduber

Many thanks for the comments and interesting stories guys....

As usual, the answers range ALL over the spectrum so I am not sure much was
resolved...

But interesting things were told, good points were made, and somebody somewhere
probably learned something....

So, all in all I think it was worth it....

take care

Blll