Rag and tube construction and computer models?

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

Steve did more than just "eyeball" engineering. He had some contacts at the
University of Wisconsin that he sent his drawings and parts down to have
them analyzed.
"Harry O" wrote in message
...
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

Steve said that. However, he also said that it was done many years after
the plans were first offered for sale. I believe it was about the time he
changed it from the W-8 to the W-10. There were a few tube sizes that were
increased in size then, particularly at the top, front of the cabin to carry
the spar loads. Of course, it was because of the heavier Lycoming engines
being used rather than from failures.

"Cy Galley" wrote in message
news:1uJdc.117$xn4.5040@attbi_s51...
Steve did more than just "eyeball" engineering. He had some contacts at
the
University of Wisconsin that he sent his drawings and parts down to have
them analyzed.

"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


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.651 / Virus Database: 417 - Release Date: 4/5/2004

Morgans wrote:

"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


It's all in the numbers, Jim.

Your teachers always told that math would come in handy someday.

Well, take tail loads?

We start with the wing airfoil performance curves.

One curve plots section lift at any given angle of attack.

One curve plots section drag "

Back in the slide rule days the third curve represents "center of
pressure" location (again, per angle of attack but expressed in percent
chord. i.e.: where the summed center of the pressure field is in
relation to the section chord.

Makes an easy model to visualize what's happening.

Now days, the third curve is the Coefficient of Moment, or the
rotational
force the airfoil generates at that angle of attack.
No as touchy feely, but I gotta admit that the coefficient method is
easier to calculate with.

Next, there is the CG question.

For a pitch stable airplane, the center of lift will be behind the
center of gravity. If you visualize this, the nose falls.

A down load on the tail lifts the nose.
How much down load?
Just enough to bring the nose back level.

Knowing where the CG and CL are physically located we also know the
distance between them (the Arm).

For straight and level flight, we know the lift (equal to weight).

So we can do a little arithmetic and find the pitch moment for our
hypothetical airframe.

(We'll skip the airfoil CM for now, ok?
The CG/CP moment is by far the greater issue of the two.
But in reality ALL moments get included.)

So, take that pitching moment and divide it by the Tail Arm (distance
from CG to elevator?) to find out what the load on the tail will be
(pounds).

It's really fairly simple arithmetic so far.
The biggest surprise is how small the actual control loads are.

Some 10 feet back to the elevator makes a very long Arm.

Ten pounds back here can have more impact on CG location than 100
pounds in the back seat. In fact, it better, or the back seat might
be the way wrong place!

Bounds checking shows that many airfoils have a higher CM at higher
AOA.

I think that implies that at low speeds, tail loads are actually higher?
Why?
More force is needed on the tail to hold the nose up that high.
:^)

At higher speeds the CM is generally lower because the AOA is lower.

Ok, too many blank looks again...

Visualize it this way?

At high AOA the center of pressure is generally forward some.
The air is attached to the front third of the wing (or less?),
so the lift force is transferred to the wing in that area only.
(drag too)

As the AOA comes down (and speed is higher to make same lift) the
center of pressure is "blown" aft. (?)

It just makes an easy mental image to help remember how it all\
fits together.

So a steep CM curve (old style) or a larger range of CM values indicate
an airfoil with an active center of pressure. (also ?)


As for the other specific things you mentioned?

Lift distribution is more of a plan form thing but there are other
considerations as well.

One aspect is the planform shape.
Rectangle (Hershey Bar), Elliptical? Delta

Another aspect is wing twist.
Twisting the tips down makes less lift at the tips.

Take a rectangular wing and wash the tips down a bit and you get can a
nice elliptical lift distribution from a Hershey bar.

Do the same thing to an elliptical plan form and it might not even fly.
(washed the lift right off the back of the wing!)

I'm still working on gust loads...
They are described as so many feet per second of gust
but I have trouble wrapping that up neatly.

My best guess for a reasonable approximation is this...

Do a vector diagram with the airplane's forward speed (in fps)
on the X axis, and the gust vector pointing doen (vertical at xxx fps)
and note the angle of the resultant.

Go back to the airfoil performance data and recalculate how much lift
will be generated at that speed if the AOA suddenly increased by that
angle.

Divide that lift number by the flying weight to get the G load that
would be imposed.

There is a lot more to it, of course.
More than I know for sure.
But it's a start.


Richard

Richard Lamb wrote...

There is a lot more to it, of course.
More than I know for sure.
But it's a start.

SHH! If you tell *all* our secrets
*everybody'll* be doin' it! g

Nice post.

Dave 'mystique is 50%, the rest is algebra' Hyde