_____
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Jon S.
Berndt
Sent: Saturday, July 21, 2007 12:48 PM
To: 'FlightGear developers discussions'
Subject: Re: [Flightgear-devel] Epsilon,alpha_tail and independent tail
contributions
Fabian wrote:
Currently , as far as I understand, JSBSim does not support the
buildup of the aerodynamic characteristics by taking the wing-body alone and
adding the tail contribution. This is essential for example in investigating
an aircraft behaviour during ice contaminated tailplane stall, a "hot"
subject nowadays.
For example, the total pitching moment would be: CM_wing-body +
CLtail * tail_volume.
Actually, JSBSim supports exactly that kind of thing. [You might want to
move this part of the discussion to the JSBSim list.] In JSBSim we use the
coefficient build-up method to define the forces and moments about each of
the axes. There is also the possibility of creating functions that calculate
whatever you want, which can then be applied to the coefficients as needed.
For example, multipliers into tables or other functions. You could use
DATCOM+ to calculate the wing/body effects, then add other effects as
needed, unless you already have data. For information on DATCOM+, see
<http://www.holycows.net/datcom> www.holycows.net/datcom.
Of course, the aero model should then need to include the downwash
(Epsilon) at the tail (as a function of alpha, flaps, etc.) and CLtail vs.
alpha_tail.
My questions therefore are:
1. Is it possible to introduce Epsilon tables in the aircraft
properties file?
To my knowledge, nobody has tried this, yet. However, it is certainly
possible.
2. Is it possible to have an option to evaluate alpha_tail
(alpha_aircraft + horizontal_stab - Epsilon + dynamic components) whenever
an Epsilon table is present in the properties file? This would be essential
to calculate the tail lift and its contribution to the overall pitching
moment.
Yes. That's one of the reasons that we created the function capability. One
of our newsletters (see <http://www.jsbsim.org> www.jsbsim.org) has
information about defining functions in the aerodynamics section of a JSBSim
aircraft config file. Also, there is this (somewhat formal) definition of
JSBSim-ML:
<http://jsbsim.sourceforge.net/JSBSim/JSBSim.xsd.html>
http://jsbsim.sourceforge.net/JSBSim/JSBSim.xsd.html
Some information on the function definition can be found here:
<http://jsbsim.sourceforge.net/wiki/index.php/User%27s_Manual#Configuration_
Files>
http://jsbsim.sourceforge.net/wiki/index.php/User%27s_Manual#Configuration_F
iles
For more information, as on the JSBSim list!
Jon
A couple problems that I've been addressing lately is that Datcom doesn't
produce downwash angles when you drop flaps. Also, the elevator sweeps are a
function of free-stream angle of attack (i.e., AOA at the wing, not the
elevator). Another important omission from Datcom is that you can't drop
flaps then do elevator sweeps.
To me, these are important factors, since most of the crashes happen near
the ground, when you have flaps and gear down. The FAA testing is heavy on
near-ground performance, because that is where you get into the most
trouble, such as losing an engine during take-off or landing.
One approach that I've been considering is to calculate what the airfoil
looks like when you lower the flaps, and input THAT as the airfoil, then do
all of the normal calculations. For something like a 737 where you have
leading and trailing edge flaps, you could easily get into 20 cases, with
each case generating all of the coefficient tables (Cd, CL, CM, etc.). For a
twin-prop aircraft, you just have trailing edge flaps, but you really need
to run power effects at each flap setting, and you really should split the
aircraft in half, since you could be flying on one engine. Engine thrust
should be run at several settings, such as off, idle, cruise, and max, as a
minimum. Interpolation between those power settings should be fairly
accurate.
Let me give you a good example. Twin engine prop aircraft, drop the flaps
fully, you lose the right engine, and push the left one to max in order to
climb. On the right side, you just have freestream airflow over the right
wing, flap, rudder and elevator. On the left side, you have a prop blast
hitting the wing, part of the flap, left side of the rudder, and most of the
elevator. Ignoring the engine torque issue, you are going to experience
different lift (higher on the left side) which creates a rolling moment. You
will also experience different elevator control effectiveness right versus
left. Rudder dynamic pressure might actually be different left versus right,
creating a yawing moment, which is in addition to the thrust different left
versus right.
Now, is this a little overkill? Maybe, since most trainers that I've seen
don't go into such details. It is possible to generate data for these cases,
and I think it would yield higher fidelity trainers than anything seen
before, simply because that kind of data hasn't been available before. The
question I'm wrestling with is whether it is worth all the headaches. After
all, we could end up with 60-80 times more data than what we currently deal
with now.
Any thoughts?
Bill
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