Thread: [Jsbsim-devel] Aitfoils at high alpha

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Very interesting link. I wish I could find the Sandia report referred 
to here. Anyone from Sandia paying attention? :-)

  http://www.aerospaceweb.org/question/airfoils/q0150b.shtml

Jon

Can you reach this from outside?

http://infoserve.sandia.gov/sand_doc/1980/802114.pdf

On Wednesday 21 July 2004 11:00 am, Jon S Berndt wrote:
> Very interesting link. I wish I could find the Sandia report referred
> to here. Anyone from Sandia paying attention? :-)
>
>   http://www.aerospaceweb.org/question/airfoils/q0150b.shtml
>
> Jon
>
>
> -------------------------------------------------------
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> _______________________________________________
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> http://www.JSBSim.org
> _______________________________________________

On Wed, 21 Jul 2004 13:43:32 -0600
  "Peter C. Buechler" <pc...@sa...> wrote:
>Can you reach this from outside?
>
>http://infoserve.sandia.gov/sand_doc/1980/802114.pdf

Hi, Peter:

It appears that I cannot. Is the report releasable?

Jon

----- Original Message ----- 
From: "Jon S Berndt" <js...@ha...>
To: <jsb...@li...>
Sent: Wednesday, July 21, 2004 3:08 PM
Subject: Re: [Jsbsim-devel] Aitfoils at high alpha

> On Wed, 21 Jul 2004 13:43:32 -0600
>   "Peter C. Buechler" <pc...@sa...> wrote:
> >Can you reach this from outside?
> >
> >http://infoserve.sandia.gov/sand_doc/1980/802114.pdf
> 
> Hi, Peter:
> It appears that I cannot. Is the report releasable?
> Jon

    The pdf is  DL'ing to me right now. 

    Ron

On Wed, 21 Jul 2004 17:45:10 -0500
  "Ron Freimuth" <ron...@at...> wrote:
>     The pdf is  DL'ing to me right now. 
>
>     Ron

Yep. I eventually got it.

Jon

----- Original Message ----- 
From: "Jon S Berndt" <js...@ha...>
To: <jsb...@li...>
Sent: Wednesday, July 21, 2004 3:08 PM
Subject: Re: [Jsbsim-devel] Aitfoils at high alpha

> On Wed, 21 Jul 2004 13:43:32 -0600
>   "Peter C. Buechler" <pc...@sa...> wrote:
> >Can you reach this from outside?
> >
> >http://infoserve.sandia.gov/sand_doc/1980/802114.pdf

    I see the shtml answer on High Alpha was from Jeff Scott.  Who did a lot
of the FlightGear models that were posted on the WEB. Of course
aerospace.org is Jeff's current site.

    I have some images of curves in "Horner" that give Cd and CL for some
airfoils up to 90 deg alpha.  They are about what Jeff mentioned.  After the
initial drop in CL past stall, it starts to peak, reaching a max at, I
think, 45 deg.  While Cd is also about what one would expect. It is not
simply proportional to CL^2 in these regions.

    In fact, now that the images in Jeff's reply have appeared, I see they
are similar to the Horner curves.  Though, I think Horner's plots don't peak
as close to CLmax at 45 deg.

    I think the shape of the CL curve is important in modeling spins  (also
Cm vs alpha).  I suspect CL  gets stuck a bit past the bottom of the dip at
20 degs in a stall/spin.  Something has to keep Alpha relatively constant
since a spin tends to stay in a constant state.

  Ron

On Wed, 21 Jul 2004 18:03:02 -0500
  "Ron Freimuth" <ron...@at...> wrote:
>
>     I think the shape of the CL curve is important in modeling spins 
> (also
>Cm vs alpha).  I suspect CL  gets stuck a bit past the bottom of the 
>dip at
>20 degs in a stall/spin.  Something has to keep Alpha relatively 
>constant
>since a spin tends to stay in a constant state.

Ron:

Given what you know about simulation, let me state these questions to 
you (and anyone else that wants to answer):

1) What do you think is the value of providing some modeling of high 
alpha lift/drag characteristics - even if the data is sort of generic, 
providing perhaps (at best) only "plausible" dynamics?

2) Do you have any insight or suggestions about modeling high alpha 
lift/drag characteristics, given the use of the JSBSim coefficient 
buildup method?

Jon

> Given what you know about simulation, let me state these questions to=20
> you (and anyone else that wants to answer):
>=20
> 1) What do you think is the value of providing some modeling of high=20
> alpha lift/drag characteristics - even if the data is sort of=20
> generic,=20
> providing perhaps (at best) only "plausible" dynamics?
>=20
> 2) Do you have any insight or suggestions about modeling high alpha=20
> lift/drag characteristics, given the use of the JSBSim coefficient=20
> buildup method?

I'll throw out some words on the subject, since I know a little bit =
(maybe
not as much as I think, though). This is just intended to be
thought-provoking, not the start of any flame-war.

One of the places that we'd see high AOA is in a flat spin, a maneuver
prohibited in a lot of aircraft. Normal straight and level flight, with =
the
minor variances from that state, that we generally encounter use a =
linear
set of equations, and we assume the airflow over the wings to be the =
same,
and we generally don't model the airflow over each wing. In spin =
modeling,
your modeling has to switch from linear to angular equations to fully =
and
properly describe the airflow and dynamics of the aircraft. The wings =
are
not seeing the same airflow and thus have to be modeled independently.

Spin testing is done on a new aircraft, just to see how easily it goes =
into
a spin, and how to recover from that spin. If the aircraft doesn't enter =
the
spin nicely, or can't be recovered predictably, the manufacturer might =
slap
some more surfaces on, such as dorsail fins, strakes, etc. Predicting
behavior in a spin is well beyond the scope of the JSBSim, and most of =
the
people that will use it. (I wouldn't do it myself).

Okay, what about high AOA without the rotational aspects? Well, in all =
but
the most acrobatic aircraft, you probably won't see high AOA. The
manufacturer is unlikely to provide data much past stall, and you =
probably
won't see data anywhere near 90 degrees.  Having the model handle AOAs
higher that stall is a good idea, and preventing the model from =
exploding if
a high AOA is encountered is a good idea, but maybe that protection is
merely upper limiting the AOA to 15 degrees over stall, or some other =
value
determined by the roll of the dice.

So, I guess I should ask the question, to what ends are you modeling =
high
AOA?

Bill Galbraith

> So, I guess I should ask the question, to what ends are you modeling high
> AOA?

All the way to where the ballistic trajectory intersects the ground.

;-)

Then, there are these:

http://sukhoi.xiloo.com/dianping/su-27-05.jpg
http://aeroweb.lucia.it/~agretch/RAFAQ/cobra.html

It's Pugachev's Cobra, and it goes to 120 degrees. With fly-by-wire aircraft that are
neutrally or negatively stable, and where the FLCS can have pitch limits overridden,
modeling high alpha becomes important.

One can also ask the question, what can we do better?

Jon

Yeah, I knew that you'd bring that up. Okay, EXCEPT for that =
aircraft....
What is your return on your investment?  Would those efforts be better =
spent
somewhere else, such as adding helicopters to your modeling...   (Oooo,
sounds like a challenge from the new kid on the block!)

> > So, I guess I should ask the question, to what ends are you=20
> modeling=20
> > high AOA?
>=20
> All the way to where the ballistic trajectory intersects the ground.
>=20
> ;-)
>=20
> Then, there are these:
>=20
http://sukhoi.xiloo.com/dianping/su-27-05.jpg
http://aeroweb.lucia.it/~agretch/RAFAQ/cobra.html

It's Pugachev's Cobra, and it goes to 120 degrees. With fly-by-wire =
aircraft
that are neutrally or negatively stable, and where the FLCS can have =
pitch
limits overridden, modeling high alpha becomes important.

One can also ask the question, what can we do better?

Jon

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> Yeah, I knew that you'd bring that up. Okay, EXCEPT for that aircraft....

There's this one, too:

http://www.dfrc.nasa.gov/Gallery/Photo/X-31/HTML/EC94-42478-3.html

> What is your return on your investment?  Would those efforts be better spent
> somewhere else, such as adding helicopters to your modeling...   (Oooo,
> sounds like a challenge from the new kid on the block!)

It's a step along the way to modeling off-nominal conditions better.

Jon

Okay, it's been a long time in the works, but I have the first release =
of
DATCOM+ ready for general release.

What is DATCOM?  Datcom is the USAF program for predicting aerodynamic
coefficients based on the design of an aircraft, either in the design =
phase
or an existing aircraft. Where could I find a program like this? =20

On my web page....    http://www.holycows.net/datcom

I did some comparisons to a couple of existing flight simulators, and =
the
match was impressive. There is a report with those finds available... =
WHERE
???

On my web page....    http://www.holycows.net/datcom

Now, wouldn't it be nice if it cranked out the coefficients in JSBSim =
XML
format?  IT DOES!!!  Okay, the XML file isn't complete, as it needs
propulsion and control information, but it's a decent start.

DATCOM+ is a modified version of DATCOM, with a new front end, and a new
back end. The front end allows the input file to be more user-friendly, =
and
the back-end is tailored to output data in JSBSim-friendly format.

Let's consider this an Alpha-Beta release. The DATCOM code itself is =
already
proven. I've added a front-end to it, which is already proven. I've also
added a back-end to it, to output the data in JSBSim XML format. That's
still a little shakey, as it hasn't been pumped through JSBSim yet. =
Soon,
though.

The program runs under a CMD window from Windoze, or in Cygwin. There is
also a plotting routine to allow you to plot the coefficients easily, =
just
so you can see what they look like. Someday, we might have a plotting
routine wthat will plot directly out of XML format.

If you are thinking that in 10 minutes, you can have your aircraft =
defined
and crank out a flight model, think again. This is a serious tool, and =
you
are likely to spend a few hours diging out aircraft parameters or =
measuring
them off a good 3-view drawing.

Much of the data that you'll need can be found in Jane's All the World's
Aircraft. If you need a page or two scanned in, I can do that for you.

I'm also going to be trying to put more user's instructions together, to
make it easier for the novice to use this tool. There are some half-way
decent instructions (IMHO) to get you going, and the package comes with =
a
Citation model already in place, which should help you building your =
model.

If you are interested in using this, help yourself. If you'd like, drop =
me
an email at bi...@ho... and let your intentions be known. Maybe I =
can
help you out, and if nothing else, I can notify you if there are serious
changes made. I'm also open to suggestions that would make it better or
easiler to use.

Thanks,
Bill

> Okay, it's been a long time in the works, but I have the first release of
> DATCOM+ ready for general release.
>
> What is DATCOM?  Datcom is the USAF program for predicting aerodynamic
> coefficients based on the design of an aircraft, either in the design phase
> or an existing aircraft. Where could I find a program like this?
>
> On my web page....    http://www.holycows.net/datcom

<blink>

<blink> <blink>

<somewhere off in the distance, a dog barks>

Well, at least *I* think this is cool Bill. Thanks for you efforts. I can't wait to try
this out. But I'm working on a little PowerPoint presentation now, for ...

August 16th is creeping ever closer.

Jon

Okay, so I guess I was in a pretty goofy mood this afternoon. I've been
juggling lots of strange things, and... Well, too late, huh?

Had 10 hits... Or one guy that was bored, and ran my counter up....

> -----Original Message-----
> From: jsb...@li... 
> [mailto:jsb...@li...] On Behalf 
> Of Jon Berndt
> Sent: Tuesday, July 27, 2004 10:54 PM
> To: jsb...@li...
> Subject: RE: [Jsbsim-devel] DATCOM+ released
> 
> 
> > Okay, it's been a long time in the works, but I have the 
> first release 
> > of
> > DATCOM+ ready for general release.
> >
> > What is DATCOM?  Datcom is the USAF program for predicting 
> aerodynamic 
> > coefficients based on the design of an aircraft, either in 
> the design 
> > phase or an existing aircraft. Where could I find a program 
> like this?
> >
> > On my web page....    http://www.holycows.net/datcom
> 
> <blink>
> 
> <blink> <blink>
> 
> <somewhere off in the distance, a dog barks>
> 
> Well, at least *I* think this is cool Bill. Thanks for you 
> efforts. I can't wait to try this out. But I'm working on a 
> little PowerPoint presentation now, for ...
> 
> August 16th is creeping ever closer.
> 
> Jon
> 
> 
> 
> -------------------------------------------------------
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> FREE Java Enterprise J2EE developer tools!
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_______________________________________________
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_______________________________________________
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http://www.JSBSim.org _______________________________________________

Bill Galbraith wrote:
> Okay, so I guess I was in a pretty goofy mood this afternoon. I've been
> juggling lots of strange things, and... Well, too late, huh?
> 
> Had 10 hits... Or one guy that was bored, and ran my counter up....

You got another hit ...
:-)

I think this is a really great addition to JSBSim/FlightGear, esp. for 
someone who wants a more realistic flight dynamics model for their 
modeled aircraft and takes the time to gather all the right data.

Thanks for supporting JSBSim BTW.

Some day I'll try this and see if I can port it to IRIX.

Erik

> Some day I'll try this and see if I can port it to IRIX.
>
> Erik

At this time I think Bill is not releasing the source code. But, at some point a version
for IRIX and Linux would be good...

Jon

> But, at some point a version for IRIX and Linux would be good...

Okay, there is a version for Red Hat 9.0 Linux up on my web page, or you =
can
just click

http://www.holycows.net/datcom/datcom_040728.tar

(No sense running that Hit counter up, huh?)

Sorry, the plotting routine hasn't been ported over yet, but I'll work =
on
that.

I wouldn't hold your breathe for an IRIX version.

Bill Galbraith wrote:

> I wouldn't hold your breathe for an IRIX version.

Get me the source and it will be a matter of days (if not hours).

<prepares for a solid time without breathing/>

Erik

----- Original Message ----- 
From: "Bill Galbraith" <bi...@AO...>
To: <jsb...@li...>
Sent: Tuesday, July 27, 2004 10:28 PM
Subject: RE: [Jsbsim-devel] DATCOM+ released

> Okay, so I guess I was in a pretty goofy mood this afternoon. I've been
> juggling lots of strange things, and... Well, too late, huh?
>
> Had 10 hits... Or one guy that was bored, and ran my counter up....

    Well, I looked at your comparison pdf file.   Looked like pretty
good.data. Though, I don't know what AC. ;)

    It would be interesting to do a C172 airframe and see if you get about
the SD's that Smetana's apps get.  I imagine the input would be similar:
lots of measurements.  However, he already worked out the data for the 172,
so one can just look at the output.

   McCormick publishes some SD's for 'his' Cherokee 140. Cmq is about half
that of the 172 airframe.

    There is an Air Force report on measured SD's for the T-37B,  up to
about 10 deg alpha.  The T-37 is slow enough so compressibility shouldn't be
a factor.

    Ron

>     There is an Air Force report on measured SD's for the T-37B,  up to
> about 10 deg alpha.  The T-37 is slow enough so compressibility shouldn't be
> a factor.

Compressibility is not necessarily a problem with DATCOM. Specifically, I believe the
transonic regime can be problematic, though. However, if you can supply some "seed"
values, DATCOM will fair the data and give a better output. This is all from what I've
read - I haven't modeled anything supersonic using DATCOM, yet.

I'm excited about DATCOM+. One it gets put through it's paces (and once we find a way to
produce rudder coefficients, which DATCOM does not do), then I think it might be possible
to write a script or something that could somehow have Aeromatic and DATCOM+ cooperate.
The ultimate goal would be to craft a tool that made it as easy as possible to specify the
relevant geometric, mass, and planform data about an aircraft, have DATCOM+ produce the
aerodynamic JSBSim output, have Aeromatic produce what it does, and then end up with a
file that defines a high fidelity aircraft flight model.

Jon

> >     There is an Air Force report on measured SD's for the=20
> T-37B,  up=20
> > to about 10 deg alpha.  The T-37 is slow enough so compressibility=20
> > shouldn't be a factor.
>=20
> Compressibility is not necessarily a problem with DATCOM.=20
> Specifically, I believe the transonic regime can be=20
> problematic, though. However, if you can supply some "seed"=20
> values, DATCOM will fair the data and give a better output.=20
> This is all from what I've read - I haven't modeled anything=20
> supersonic using DATCOM, yet.

Jon is correct. DATCOM will model transonic and supersonic aerodynamics, =
but
I haven't done anything there either, since I don't have any data to =
compare
it to. You would probably also have to figure out a way to blend the
subsonic, transonic, and supersonic data.

For now, I was just concentrating on the subsonic regime, because that =
is
where most aviation is happening (and where I have truth data). I'd be =
more
inclined to work on the helicopter issue before doing supersonic... Of
course, then I'd be answering mail on why we can't do tilt rotors, =
right?
... And no, you can't have supersonic tilt rotors either  ;-}

Bill

----- Original Message ----- 
From: "Jon Berndt" <js...@ha...>
To: <jsb...@li...>
Sent: Wednesday, July 28, 2004 8:52 AM
Subject: RE: [Jsbsim-devel] DATCOM+ released

> >     There is an Air Force report on measured SD's for the T-37B,  up to
> > about 10 deg alpha.  The T-37 is slow enough so compressibility
shouldn't be
> > a factor.

> Compressibility is not necessarily a problem with DATCOM. Specifically, I
believe the
> transonic regime can be problematic, though. However, if you can supply
some "seed"
> values, DATCOM will fair the data and give a better output. This is all
from what I've
> read - I haven't modeled anything supersonic using DATCOM, yet.

    There is a pdf report at Mason's univ Virginia site that compares some
SD's between DATCOM, Wind Tunnel, and the LDstab results.  However, I can't
find it right now.  I think the LDstab program gave better Lateral
Derivatives.  However, that Fortran program had to be 'adjusted' to match a
747-100, and it was suggested that reasonable results would only be obtained
with similar AC.  ;)

    LDstabdoc.pdf mentions DATCOM and includes the .f source code and a list
of SD's it gives for the 747-100.

 ConfigAeroHiAlphaNotes.pdf is a useful document for one of Mason's classes.
It has info on pitching moment, Rudder Moment, etc. curves necessary to keep
an AC (typically a fighter) controllable at abnormal attitudes.

 There is also ConfigAeroHiLift.pd: typical curves on Lift augmentation
devices, lots of references.

>I'm excited about DATCOM+. One it gets put through it's paces (and once we
find a way to
> produce rudder coefficients, which DATCOM does not do), then I think it
might be possible
> to write a script or something that could somehow have Aeromatic and
DATCOM+ cooperate.
> The ultimate goal would be to craft a tool that made it as easy as
possible to specify the
> relevant geometric, mass, and planform data about an aircraft, have
DATCOM+ produce the
> aerodynamic JSBSim output, have Aeromatic produce what it does, and then
end up with a
> file that defines a high fidelity aircraft flight model.
>
> Jon

     I skimmed Bill's notes on what SD's DATCOM[+] generates.

    In MSFS, I generally set control moments by experiment.  I think the
published values would be for small deflections; but they drop in effect at
maximum deflections.  Maximum control moments are important in determining
Roll Rate, Pitch Rate and ability to stall and AC (and, recover with down
elevator).  And, finally how much Beta the rudder will create.  I generally
set Cn_dr to get 15 to 20 deg Beta.

    I set Cl_da to get an appropriate p_max.  Easy to calculate, but since
p_max also varies directly with V, it's better to adjust  experimentally.
Of course, Cl_da/Cl_p determines Roll Helix Angle, and one can estimate that
from the AC's roll performance.

    While I keep Cm_de/Cl_de  constant; based on the normalized tail moment
arm distance. Tweak Cm_de to the final value experimentally, but always keep
that ratio constant.

    I figured the same thing applies to Cn_dr and CY_dr.  Only the moment
arm is normalized to b rather than c_bar.

    I have noted that elevator vs horiz_stab lift and moments are typically
in a 1:3 ratio in MSFS.  That suggests the elevator is 1/3 of the H Stab
area.   Higher than the typical physical ratio.

    However, the total H. Stab needs to be considered as a unit.  Curves in
McCormick, etc. suggest the elevators have a greater effect than simple
consideration of their relative area gives.

    The Vertical Fin and Rudder would be similar.  Though,
d(epsilon)/d(alpha) typically reduces the horizontal tail effects more than
the corresponding effect for the vertical stab.

    Further, the vertical stab is generally asymmetrical so it adds some
roll moment, not just yaw. Regardless, CY_Beta is generally the most
important Side Force, I see I set CY_dr to only 1/6 the former in my 727.
Based on keeping Cn_dr/CY_dr at about 0.4.

    Complicating matters is how q effects the control moments.  Either due
to limits on pilot strength, purposeful limits on hydraulic pressure, and
control system scheduling.

    On top of that, all MSFS SD's can be modified by Mach Number.  However,
the effect is limited in jet transports.  Even Cd_m (Cd_p(M)) is small at
typical cruise Mach,  It's the Lift Related component that mostly determines
increase in CD and the optimum flight level at a given weight.  Of course
Cm(Mach) pitches the nose down in the transonic region, but some curves I've
seen have a fine structure that would be impossible to calculate without
CFD.

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

    To sum up Control Moments and associated 'lifts': I've found  2/3 the
published value appears to be 'about right' as far as sensitivity and
available control goes. That suggests lower than calculated values may also
be desirable. Cn_r should generally be able to create about +/- 15 deg of
Slip.  This is required for X-Wind landings!    Multi engine AC have to have
enough rudder control for engine out conditions.

    While the control surface forces are pretty much dependent on normalized
moment arms.  Though, just what point one measures them from  isn't exactly
the CG or aerodynamic center.

     I've verified many of the MSFS SD's by calculation, so I'd imagine they
would apply to simulators with similar  flight model approaches.

    Ron

On Tuesday 27 July 2004 15:39, Bill Galbraith wrote:
> Okay, it's been a long time in the works, but I have the first release of
> DATCOM+ ready for general release.

Looks great!  Maybe we're going to need a repository for the input files 
(*.dcm) we create?

Dave
-- 
****************************
David Culp
dav...@co...
****************************

> Looks great!  Maybe we're going to need a repository for the input files
> (*.dcm) we create?

There is a datcom cvs directory, IIRC. That's where they would go. I have a P-51 there
already, I think.

Jon

>=20
> On Tuesday 27 July 2004 15:39, Bill Galbraith wrote:
> > Okay, it's been a long time in the works, but I have the=20
> first release=20
> > of
> > DATCOM+ ready for general release.
>=20
>=20
> Looks great!  Maybe we're going to need a repository for the=20
> input files=20
> (*.dcm) we create?
>=20
>=20
> Dave

I've thought about that issue. The .DCM files allow comments in them, so
wouldn't one large files with ALL the models in it be useful (okay, up =
to a
certain point). There are some parameters in the DATCOM input file which =
are
real headscratchers, and being able to look at the values that other
aircraft used might be nice. No, I'm not talking about wing span, but
surface roughness, and how to input surface deflections correctly.

This would require a couple of scripts to be written, one to merge an
outside script into the Master script, and one to extract a particular =
model
out of the Master script..... (Ooo, can someone say 'DATABASE'???)

Jon and I have kicked a couple of ideas around about making it easier to
build a DATCOM model, etc. One possible idea was a fancy Excel =
spreadsheet,
with questions and blanks, and examples of other aircraft values. All =
those
could be stored on a second sheet of the spreadsheet, one column per
aircraft.

Your suggestions are always welcomed.

Bill

Thread: [Jsbsim-devel] Aitfoils at high alpha

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