Re: [Jsbsim-devel] DATCOM+ released

[Ron F. <ron...@at...>]

----- Original Message ----- 
From: "Bill Galbraith" <bi...@ao...>
To: <jsb...@li...>
Sent: Wednesday, July 28, 2004 10:04 AM
Subject: RE: [Jsbsim-devel] DATCOM+ released


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

>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.

  I assume DCM files have siginficant AC dimensional data.  Smetana's low
speed applications probably use similar input.  Too many for me to tackle.
;)

    However, one of the guys I know is into this, and has used Smetana, etc.
to work out SD's for a few AC.  He also has quite a bit of Roskam, etc. data
from various sources.  Roskam's S&C Part 1 gives SD's for different AC,
depending on the edition.   If nothing else, a good way to check calculated
results.

    Data bases for AC that have been dimensioned. would be useful for most
any of the programs which calculate SD's.


    Smetana has some data files already worked out, such as for the C172
airframe.  It appears the FlightGear models were based on the resultant
SD's.

    ----

    "Theory of Wing Sections" gives wing polars for different Reynolds
numbers and generally one for 'standard roughness'.  It appears the later is
most appropriate for real AC.  Real AC may not even stay as smooth as
'standard'.   Flush rivets are used, so they don't generally have an effect.
However, there are all sorts of interference effects that increase drag past
what a simple calculation would give.  McCormick shows how cooling drag,
antennas, etc. built up a lot of extra drag in a WWII fighter.

    Rather than trying to calculate Cdo, known maximum speeds and HP/Thrust
allow one to set zero lift drag for actual performance.

    However, once one has done some AC, it may be that appropriate values
for 'roughness', etc can be entered in the DATCOM data to get a more
realistic end result.

    Control deflection limits are often given in the FAA TCDS files.
Aileron and Elevator deflections are typically asymmetrical.  Average L-R
aileron deflection determines the roll moment, while 'Adverse Yaw' is a
partly a function of asymmetrical deflections and accounted for separately.

    Up elevator is typically higher than Down Elevator.

    Even the + and - Rudder limits may be a bit different, but not by much.
+/- 30 deg is typical (but may drop to +/- 5 deg in jets at cruise speeds).

>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.

>Bill

    Further, one can use Excel to calculate most anything.  Such as the
required Cn_dr to cover an engine out. I think  'vpi-nasa.xls' has some
calculations for such matters.

    So, if one doesn't get Cn_dr from DATCOM, he could still try different
values in a SS to find what is required.  Even use Excel to iterate to the
needed value.  Any extra calculations could be added to extra SS pages, and
left out of  a data only SS.

    Ron