Point mass to vary CG

[gasilva]

@seanmcleod see if this way of varying CG is better than changing the aircraft XML

# Developed by JSBSim Team and
# Guilherme A. L. da Silva - aerothermalsolutions.co
# GNU Lesser General Public License v2.1

# Varies point mass weigh and location to vary CG

# To install jsbsim module in Python
# pip install jsbsim
# Or conda install jsbsim (if you have anaconda, this is the best way)

import jsbsim
import os

# Put your path here
PATH_TO_JSBSIM_FILES="/Users/gasilva/Documents/GitHub/jsbsimNew" #my is a macbook
# Object with JSB executable
fdm = jsbsim.FGFDMExec(PATH_TO_JSBSIM_FILES)
# Load script

fdm.load_model('global5000')

fdm['propulsion/engine[0]/set-running'] = 1
fdm['propulsion/engine[1]/set-running'] = 1
fdm['ic/h-sl-ft'] = 30000
fdm['ic/gamma-deg'] = 0
fdm['ap/alpha_hold']= 0
fdm['ap/altitude_hold']= 0
fdm['ic/vc-kts']=280

# Run JSBSim
fdm.run_ic()
fdm.run()

cgx=fdm["inertia/cg-x-in"]
print("------------------------------")
print("Original X location {:.1f} in".format(cgx))
print("------------------------------")

newx=cgx*2
neww=1000
fdm["inertia/pointmass-location-X-inches"]=newx
fdm["inertia/pointmass-weight-lbs"]=neww

fdm.run()
print("X location {:.1f} in".format(newx))
print("point mass {:.0f} in".format(neww))
print("New CG {:+0.2f} %".format((fdm["inertia/cg-x-in"]/cgx-1)*100))
print("------------------------------")

newx=cgx*.5
neww=2000
fdm["inertia/pointmass-location-X-inches"]=newx
fdm["inertia/pointmass-weight-lbs"]=neww

fdm.run()

print("X location {:.1f} in".format(newx))
print("point mass {:.0f} in".format(neww))
print("New CG {:+0.2f} %".format((fdm["inertia/cg-x-in"]/cgx-1)*100))
print("------------------------------")

Results example

------------------------------
Original X location 790.8 in
------------------------------
X location 1581.6 in
point mass 1000 lb
New CG +1.36 %
------------------------------
X location 395.4 in
point mass 2000 lb
New CG -1.34 %
------------------------------

[seanmcleod]

Yes this is what I was thinking of. If you take a look you'll see inertia/iyy-slugs_ft2 also changing as you move the point mass.

[gasilva]

It is really more realistic doing this way.

[seanmcleod]

First prize would be configuring various point masses, fuel tanks included that match the actual layout of the various masses loaded in the aircraft in order to achieve your mass target and your cg target. As opposed to a single point mass in your example above.

For example if you take a look at the 737 model in JSBSim you'll see that there are 3 fuel tanks specified at different locations.

jsbsim/aircraft/737/737.xml

Lines 167 to 196 in 24f41de

	<tank type="FUEL"><!-- Left wing tank -->
	<location unit="IN">
	<x> 520 </x>
	<y> -80 </y>
	<z> -18 </z>
	</location>
	<type>JET-A</type>
	<capacity unit="LBS"> 10200 </capacity>
	<contents unit="LBS"> 10000 </contents>
	</tank>
	<tank type="FUEL"><!-- Right wing tank -->
	<location unit="IN">
	<x> 520 </x>
	<y> 80 </y>
	<z> -18 </z>
	</location>
	<type>JET-A</type>
	<capacity unit="LBS"> 10200 </capacity>
	<contents unit="LBS"> 10000 </contents>
	</tank>
	<tank type="FUEL"><!-- Center tank -->
	<location unit="IN">
	<x> 480 </x>
	<y> 0 </y>
	<z> -18 </z>
	</location>
	<type>JET-A</type>
	<capacity unit="LBS"> 15000 </capacity>
	<contents unit="LBS"> 4000 </contents>
	</tank>

So if your target mass was say TM, and the empty mass is say EM, then you would load in TM - EM of fuel, and partition it between the fuel tanks to achieve your target cg.

What is a bit suspicious about the Global 5000 aircraft is that all 3 fuel tanks specified coincide exactly with the aircraft's empty cg, here is the empty cg.

jsbsim/aircraft/global5000/global5000.xml

Lines 87 to 91 in 24f41de

	<location name="CG" unit="IN">
	<x> 790.80 </x>
	<y> 0.00 </y>
	<z> -29.07 </z>
	</location>

And here are the 3 fuel tanks.

jsbsim/aircraft/global5000/global5000.xml

Lines 235 to 263 in 24f41de

	<tank number="0" type="FUEL">
	<location unit="IN">
	<x> 790.80 </x>
	<y> 0.00 </y>
	<z> -29.07 </z>
	</location>
	<capacity unit="LBS"> 8097.63 </capacity>
	<contents unit="LBS"> 8097.63 </contents>
	</tank>
	<tank number="1" type="FUEL">
	<location unit="IN">
	<x> 790.80 </x>
	<y> 0.00 </y>
	<z> -29.07 </z>
	</location>
	<capacity unit="LBS"> 8097.63 </capacity>
	<contents unit="LBS"> 8097.63 </contents>
	</tank>
	<tank number="2" type="FUEL">
	<location unit="IN">
	<x> 790.80 </x>
	<y> 0.00 </y>
	<z> -29.07 </z>
	</location>
	<capacity unit="LBS"> 8097.63 </capacity>
	<contents unit="LBS"> 8097.63 </contents>
	</tank>

For additional realism and flexibility take a look at the following 737-800 model, which in addition to specifying 3 fuel tank locations they also include a number of cabin areas and cargo areas as point masses.

https://github.com/zakalawe/737-800/blob/d1f2b32031ea7c7993ac3c8ed994ef93b4e8886f/738.xml#L43-L139