Rag and tube construction and computer models?

[BllFs6]

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

[Veeduber]

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

[BllFs6]

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

[Harry O]

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

[[email protected]]

On 07 Apr 2004 16:16:16 GMT, (BllFs6) wrote:

>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
Have not done it, don't know anyone who has, but I suspect you could
save close to 50% of the average tube weight on a chromoly fuselage IF
the actual required tube sizes were available - Particularly the fancy
butted tubing like the custom bike builders use.

[Corky Scott]

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

[Stealth Pilot]

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

[Harry O]

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

[Ernest Christley]

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

[Cy Galley]

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
>
>

[Morgans]

"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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Checked by AVG anti-virus system (http://www.grisoft.com).
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[Richard Lamb]

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

[nauga]

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

[Blueskies]

That's a keeper...

--
Dan D.



..
"Richard Lamb" > wrote in message ...
> 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

[BllFs6]

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

[Veeduber]

Coupla extra points. (Not for Richard, for you other guys.)

>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!

-------------------------------------------------------

Don't stop there. In fact, don't even start there... not if it's a
tail-dragger. Cuz if you got the little wheel in back and the fan up front,
your worse-case isn't going to be your in-flight tail loads but the kink you'll
put in the fuselage when you're having a bad hair day and try rotating too
soon... or leveling out your flare too late. One reason for the kinks is the
fact the moment for the tail wheel is usually more than for the elevator.

So get a handle on that one first, making sure the fuselage has enough strength
for an occasional bad landing. When you get to the flight loads, odds are
they'll be less than your worse-case landing/take-off loads. (All the better
to appreciate a trike gear, with the engine mount doing double-duty for the
landing gear loads.)

-------------------------------------------------------

>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)
>
-----------------------------------------------------

Second Point: Listen to the man. Or build yourself some practice airfoil
sections, make up a wind tunnel and spend a lot of time watching smoke trails.
Because if you keep the NOSE of your airfoil clean back to at about 25% of the
chord, the remaining 75% of the upper camber can look like cottage cheese and
the silly thing will still fly jus' fine.

NACA figured this out in the 1920's which makes it something of a
head-scratcher to see the Famous Designers of today degrading the main working
portion of their wings with protruding rivet heads. Keep that portion of the
wing clean, you'll see a lower stall and higher cruise. (And if you don't,
I'll give you back the money you paid for this :-)

-R.S.Hoover

PS -- I don't mean to say NACA figured out the cottage cheese. I figured that
one out myself when I was designing my All-Dairy composite... the one with the
bricks of butter for the landing gear.

[Stealth Pilot]

On Sat, 10 Apr 2004 01:07:18 -0400, "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?

in all the flying I've done it the tailwind I still have no idea what
the tail download figure is.
I think it peaks about 65lb but I'm not sure.

The tailwind has a hybrid aerofoil that has no published data.
(so all that you posted richard is accurate but quite useless )

the trim with 20degrees of flap is full back, with light fuel in
cruise it is full forward, suggesting to me that it may have tail
upload in cruise when light. dunno. this genuinely interests me since
I wanted to do a reverse engineering exercise to see just where the
strengths and weaknesses lie in the design.

I've been stuck on my first question for 2 years now.
"what is the magnitude of the tail download?"

still dont know.
Stealth Pilot

[Richard Lamb]

Stealth Pilot wrote:
>
> On Sat, 10 Apr 2004 01:07:18 -0400, "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?
>
> in all the flying I've done it the tailwind I still have no idea what
> the tail download figure is.
> I think it peaks about 65lb but I'm not sure.
>
> The tailwind has a hybrid aerofoil that has no published data.
> (so all that you posted richard is accurate but quite useless )
>
> the trim with 20degrees of flap is full back, with light fuel in
> cruise it is full forward, suggesting to me that it may have tail
> upload in cruise when light. dunno. this genuinely interests me since
> I wanted to do a reverse engineering exercise to see just where the
> strengths and weaknesses lie in the design.
>
> I've been stuck on my first question for 2 years now.
> "what is the magnitude of the tail download?"
>
> still dont know.
> Stealth Pilot


I've always felt the same way about that one.
But before we can analyze the master's work...

Not knowing what the airfoil performance curves look like makes it a
guessier answer. We don't know exactly where the center of lift is
located.

But we can work from you trim description and the known numbers and
come up with a guess.

Wittman's airfoil _is_ a little strange.

>From the plans, the CG Range is 15% to 28% of the chord.
Figuring the chord as 48 inches, that means 7.2 inches forward limit,
(aft of leading edge) to 13.44 for the aft limit (also aft of LE).

(That 15% forward limit seems awful far forward to me - but it woiks)

Anyway, the difference between those two points is 6.25 inches.
That is the maximum range of CG locations. We might assume that the
center of pressure is at or forward of the forward limit.

So, working from that...

1425 pounds gross times 6 inches = 8550 inlb moment (worst case).
Tail is about station 144, and the main spar is at station 24.
Sounds like 10 feet (120 inches) to the tail.

So we can divide 8550/120 and get 71 pounds tail load.
(real close to your 65 pound number!)

But different assumptions will give different results.
All depends on how valid our assumptions are at the beginning...

I'm real skeptical that the tail on the Tailwind is producing an
upload at high speed. The airplane handles too well for that to
be the case.

Remember that the leading edge of the stab is probably a little nose
down. Full forward trim _should_ still result in a net down load on
the tail.

Any better, Stealth guy?

Richard

[Harry O]

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]

On Sat, 10 Apr 2004 16:06:27 GMT, Richard Lamb >
wrote:


<hedge trimming>
>
>>From the plans, the CG Range is 15% to 28% of the chord.
>Figuring the chord as 48 inches, that means 7.2 inches forward limit,
>(aft of leading edge) to 13.44 for the aft limit (also aft of LE).
>
>(That 15% forward limit seems awful far forward to me - but it woiks)
>
the actual flight tested values for my aircraft are 10.4" forward
limit and 16.5" aft limit.

1300lb auw (well actually 590kg with an empty weight of 362kg.
empty cg moment arm is 214mm.

<hedge trimming>

>But different assumptions will give different results.
>All depends on how valid our assumptions are at the beginning...
>
>I'm real skeptical that the tail on the Tailwind is producing an
>upload at high speed. The airplane handles too well for that to
>be the case.
>
>Remember that the leading edge of the stab is probably a little nose
>down. Full forward trim _should_ still result in a net down load on
>the tail.
>
I tend to agree although I'm sometimes not sure. I'd love to see a
windtunnel test on the "new" aerofoil to see just what the pitching
moment was.
Stealth Pilot

[Richard Lamb]

Stealth Pilot wrote:
>
> On Sat, 10 Apr 2004 16:06:27 GMT, Richard Lamb >
> wrote:
>
> <hedge trimming>
> >
> >>From the plans, the CG Range is 15% to 28% of the chord.
> >Figuring the chord as 48 inches, that means 7.2 inches forward limit,
> >(aft of leading edge) to 13.44 for the aft limit (also aft of LE).
> >
> >(That 15% forward limit seems awful far forward to me - but it woiks)
> >
> the actual flight tested values for my aircraft are 10.4" forward
> limit and 16.5" aft limit.


That sounds like a lot more reasonable range to me.
20.8 to 34.3 percent chord.


> 1300lb auw (well actually 590kg with an empty weight of 362kg.
> empty cg moment arm is 214mm.

See, THAT's the problem with the metric system...

Is that supposed to be mm, or cm ???

Also, where is your datum for this measurement?

Knowing where the datum is located, we could "load 'er up"
and really see whazzappening.

>
> <hedge trimming>
>
> >But different assumptions will give different results.
> >All depends on how valid our assumptions are at the beginning...
> >
> >I'm real skeptical that the tail on the Tailwind is producing an
> >upload at high speed. The airplane handles too well for that to
> >be the case.
> >
> >Remember that the leading edge of the stab is probably a little nose
> >down. Full forward trim _should_ still result in a net down load on
> >the tail.
> >
> I tend to agree although I'm sometimes not sure. I'd love to see a
> windtunnel test on the "new" aerofoil to see just what the pitching
> moment was.

Man, me too. If we had solid numbers on the airfoil it would be then
be possible to do a little comparative shopping.

This one has the thickest part of the airfoil WAY forward.
Contemporary theory moves the thickest part of the airfoil aft
to increase the laminar area on the front of the wing.
THIS one can't possibly have that much laminar wing flow.

Steve's airfoil is supposed to be a TLAR modification.
Something like a 2309 top curve and M6 (or?) bottom?

Like someone else pointed out in another thread,
TLAR is definitely in the eye of the beer holder...

On the other hand, we _are_ talking about the Wittman Tailwind.
The little airplane that somehow flies better than it should.
I'll bet a steak dinner it's not just the airfoil.

Richard

[Stealth Pilot]

On Sat, 10 Apr 2004 21:08:10 GMT, Richard Lamb >
wrote:

>Stealth Pilot wrote:
>>
>> On Sat, 10 Apr 2004 16:06:27 GMT, Richard Lamb >
>> wrote:
>>
>> <hedge trimming>
>> >
>> >>From the plans, the CG Range is 15% to 28% of the chord.
>> >Figuring the chord as 48 inches, that means 7.2 inches forward limit,
>> >(aft of leading edge) to 13.44 for the aft limit (also aft of LE).
>> >
>> >(That 15% forward limit seems awful far forward to me - but it woiks)
>> >
>> the actual flight tested values for my aircraft are 10.4" forward
>> limit and 16.5" aft limit.
>
>
>That sounds like a lot more reasonable range to me.
>20.8 to 34.3 percent chord.
>
>
>> 1300lb auw (well actually 590kg with an empty weight of 362kg.
>> empty cg moment arm is 214mm.
>
>See, THAT's the problem with the metric system...
>
>Is that supposed to be mm, or cm ???
>
millimeters.

>Also, where is your datum for this measurement?
>

214mm is a tad over 8 3/8" back from the only datum used on the
tailwind :-)
recall that it will always have a pilot added to the empty weight and
you dont actually fly with empty fuel :-)

crumber nunch away. I'll bet you get close to 65lbs as the maximum
download.

(just remembered where my number comes from. future son in law is
doing his commercial studies, worked the numbers with a few
guestimates where there are no aerofoil data. aerofoil data is what's
needed.)
Stealth Pilot

[Richard Lamb]

Stealth Pilot wrote:


> >> 1300lb auw (well actually 590kg with an empty weight of 362kg.
> >> empty cg moment arm is 214mm.
> >
> >See, THAT's the problem with the metric system...
> >
> >Is that supposed to be mm, or cm ???
> >
> millimeters.
>
> >Also, where is your datum for this measurement?
> >
>
> 214mm is a tad over 8 3/8" back from the only datum used on the
> tailwind :-)
> recall that it will always have a pilot added to the empty weight and
> you dont actually fly with empty fuel :-)


Actually, 1425 is the GROSS. If yours is that heavy without pilot
and fuel, then yes, maybe you'll have a 600 pound load on the tail.

Please recall that I was talking about straight and level flight.
Pull G's and the load goes up.

But 75 pounds S&L load still shouldn't make 650 pounds at 4 Gs.

Look. how about a reality check?

Have your bright boy check to see how much the stab spar will bend
at 650 pounds load.

> crumber nunch away. I'll bet you get close to 65lbs as the maximum
> download.
>
> (just remembered where my number comes from. future son in law is
> doing his commercial studies, worked the numbers with a few
> guestimates where there are no aerofoil data. aerofoil data is what's
> needed.)
> Stealth Pilot

[Morgans]

"Veeduber" > wrote
>
> PS -- I don't mean to say NACA figured out the cottage cheese. I figured
that
> one out myself when I was designing my All-Dairy composite... the one with
the
> bricks of butter for the landing gear.

It does make it easy to "grease" a landing, doesn't it? <groan>
--
Jim in NC


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[Stealth Pilot]

On Sun, 11 Apr 2004 14:16:32 GMT, Richard Lamb >
wrote:

>Stealth Pilot wrote:
>
>
>> >> 1300lb auw (well actually 590kg with an empty weight of 362kg.
>> >> empty cg moment arm is 214mm.
>> >
<snip>
>
>Actually, 1425 is the GROSS. If yours is that heavy without pilot
>and fuel, then yes, maybe you'll have a 600 pound load on the tail.
>

you've lost me totally there.

590kg and 362kg are the measured weights of the aircraft, full to the
gunnls and empty. they are not the downloads.
65kg download was the worst case calc at cruise and max auw as I
recall.
at 4g we could say that it is the equivalent of 4 people, 2 a side,
standing on the elevator spar. it might deflect a little but I think
it is feasible.
the tube concerned is 1 1/8" dia by 65 thou wall thickness.

Stealth pilot