[Jsbsim-devel] F-35 VTOL Engine Modeling

[Sean M. <se...@se...>]

Hi

In terms of modeling the engine layout of an F-35 or a Harrier for that matter in JSBSim can an engine have multiple thrusters defined? With some mechanism to control what percentage of the current thrust is diverted out each thruster and a property to change the thruster's orientation during flight?

Here is a typical definition for a single (per engine) thruster with a fixed orientation. Pitching and yawing moments will be calculated based on the thruster's relative location to the cg during flight.

    <propulsion>
        <engine file="CFM56">
            <location unit="IN">
                <x>  540 </x>
                <y> -193 </y>
                <z>  -40 </z>
            </location>
            <feed>0</feed>
            <feed>2</feed>
            <thruster file="direct">
                <location unit="IN">
                    <x>  540 </x>
                    <y> -193 </y>
                    <z>  -40 </z>
                </location>
                <orient unit="DEG">
                    <roll>  0 </roll>
                    <pitch> 0 </pitch>
                    <yaw>   0 </yaw>
                </orient>
            </thruster>
        </engine>

Looking at the F-22 example included with JSBSim I was expecting to see a thruster with a property for the thruster's pitch angle orientation that could be modified during flight but instead the thruster has a fixed orientation of (0, 0, 0) and instead a pitching moment is added to the aerodynamics section to model the thrust vectoring.

               <function name="aero/coefficient/CmDtv">
                   <description>Pitch_moment_due_to_thrust_vectoring</description>

Having a quick look at the code in FGEngine it looks like it only supports a single thruster?

And having a quick look at FGThtuster it looks like it automatically creates the following properties.

propulsion/engine[0]/pitch-angle-rad
propulsion/engine[0]/yaw-angle-rad

So in theory the F-22 model could make use of these instead of creating a pitching moment in the aerodynamics section to model to the thrust vectoring?

So at the moment it looks like for the F-35 VTOL case one option is to add 4 engines each with a thruster, one of which swivels in pitch, the other 3 are fixed, and then have a mechanism in the FCS to map the throttle command to 4 individual throttle commands?

Or is there a better/easier mechanism to model this in JSBSim currently?

Cheers