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Equations/formulas for Aircraft

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

Equations/formulas for Aircraft

Postby kjadam » Fri Nov 01, 2019 2:13 pm

Hello,

I am modelling an Ikarus C42 and am very new to Flight Gear. Would someone mind pointing me in the right direction for a step by step process of the formulas/equations for writing the aircraft files as I would like calculate all of these:

Pitch
CMih
CMde
CMde due to propwash
CLq
CL adot
CM adot
CMq
CMq due to propwash
CMo
CM flap
CM delta trim
CM gear

Side Force
CyB
CyP
CyR
Cy Delta Rudder

Roll
ClB
ClP
ClR
Cl Delta Aileron
Cl Delta Rudder

Yaw
CnB
CnP
CnR
CnR due to propwash
Cn Delta Aileron
Cn Delta Rudder
Cn Delta Rudder due to propwash

Any help would be greatly appreciated!

Thank you in advance :-)
kjadam
 
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Re: Equations/formulas for Aircraft

Postby D-ECHO » Fri Nov 01, 2019 3:18 pm

Good day kjadam and welcome to the forum!
The equations that are used for these various coefficients are located in the JSBSim configuration file of each aircraft in FlightGear. Looking at other aircraft or generating a generic (base) model for your aircraft using aeromatiC++ should give you a way to get into how to calculate these properties. The relevant section you would be looking at is <aerodynamics> of the aircraft's JSBSim .xml file. Also the wiki http://wiki.flightgear.org/JSBSim_Aerodynamics might be a good start point or reading through the JSBSim manual. To get a very realistic flight model in FlightGear, you can either use real-life wind-tunnel measured coefficients (quite seldom available) or you might want to model the aircraft in programs like openVSP.
Regards and glad to see another aircraft developer coming on board ;)
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Re: Equations/formulas for Aircraft

Postby kjadam » Sun Nov 03, 2019 3:30 pm

Hello D-ECHO!

Thank you ever so much for your truly detailed and helpful response, very much appreciated! I have not been able to find the exact location of the <aerodynamics> file, is that within the Flightgear programme?
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Re: Equations/formulas for Aircraft

Postby legoboyvdlp » Sun Nov 03, 2019 3:50 pm

Hi,
Yes, it's within the JSBSIM.xml file. For instance, c172p.xml is the JSBSIM file associated with c172p-set.xml. this is found in the root directory (e.g. Aircraft/c172p).
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