How to Trim Aircraft in JSBSim Plugin for Unreal Engine 5

[awdsaber]

Hello,

I am currently integrating JSBSim into Unreal Engine 5 and working on simulating the Cessna T-37 aircraft. I want to use the Trim functionality to trim my aircraft. I am unsure how to properly implement trimming in the Unreal Engine environment.
I calculated the a steady state level conditions but it's not working when set the command as initial conditions:
V = 77.16; % Velocity in m/s (150 kts)
Mass = 2157; % mass in Kg (converted from lbs)
alt = 4000; % Altitude in m
CL_trim = 0.5131
Trim conditions found at:
Alpha = 4.65 deg
Elevator = -1.80 deg
Throttle_Command_Norm_Trim = 0.4618

Could you please guide me on the correct approach

[bcoconni]

If you are getting output values from the trimming algorithm, this is meaning that it has succeeded finding a trimmed position. So what is exactly wrong in your example ? Have you some reasons to think that the trim conditions that it has found are incorrect ?

[awdsaber]

I want the airplane to be trimmed during flight (lift=weight, thrust=drag), but i can't find any blueprint/function that trim my AR during flight,
even if i put the above initial conditions an inputs using Engine Commands and Elevator Command, after 1 sec the airplane starts to pitch up then pitch down after 8 sec
I'm still junior in using JSBSim.I know that FGTrim.h trims the airplane but i can't find any way to use it as blueprint during flight.
Is there any blueprint/function that i can use to trim my airplane during flight?

[gallonmate]

Disclosure, this is my brief understanding and a brief test trying to use trim in active flight, it may not be 100% correct:

In UE5 we can call simulation/do_simple_trim, with value 1 to do a full trim through the blueprint command console node, seen in this image. Unfortunately it crashes the editor if trim fails (trim is not always successful). In addition there are also extra steps needed for Trim to successfully apply it's results to UE5.

-The editor crashes when/if trim fails because JSBSim throws an exception and UE5 editor does not handle exceptions, so it just crashes when using the blueprint node. The plugin can call Trim in c++ without crashing, so I'll have to see if I can apply some solution when using the blueprint node. Which I want to solve regardless because any JSBSim function we call through the blueprint node will cause editor crashing if JSBSim throws an exception.

The extra steps needed to apply trim are:

• You have update all the various Initial Conditions to the current conditions. I'm not entirely sure why it works this way for all Trim modes, but basically Trim appears to always use Initial Conditions and then reapplies Initial Conditions to the simulation. Since the UE5 plugin only sets Initial Conditions once at the start of play, we have to update Initial Conditions to current conditions if we want trim to work for the current location, etc. Otherwise Trim literally resets the aircraft's location, speed, etc.

• After Trim, you then need to get the various controls/commands back from JSBSim and update the controls/commands in UE5. This is due to the order of operations in the UE5 c++ plugin, the trim results will get overridden instantly on the next tick.

It might be much simpler to do just do all this work in c++. You should technically be able to do it all in blueprints, getting the current conditions and then updating each initial condition, etc. I added an image at the very bottom with an example of how to update the aircraft's initial location, unfortunately I've run out of time to fiqure how to get the trim controls to actually update the aircraft controls, so this elevator trim example is not currently working, or I'm doing something wrong with trim.

In addition, if you want to further test the example, there was a bug introduced in Jan 30th update, if you didn't update the plugin recently, then you don't need to change anything. If you did update/download since Jan 30th, the bug prevents use of -characters in the property inputs. I will submit the fix soon, but here is the issue locations by line and fix:

jsbsim/UnrealEngine/Plugins/JSBSimFlightDynamicsModel/Source/JSBSimFlightDynamicsModel/Private/JSBSimMovementComponent.cpp

Line 149 in e6208a3

if (!std::regex_match((TCHAR_TO_UTF8(*Property[i])), std::regex("^[a-zA-Z0-9.-_/]+$")))

jsbsim/UnrealEngine/Plugins/JSBSimFlightDynamicsModel/Source/JSBSimFlightDynamicsModel/Private/JSBSimMovementComponent.cpp

Line 113 in e6208a3

if (!std::regex_match((TCHAR_TO_UTF8(*Property)), std::regex("^[a-zA-Z0-9.-_/]+$")))

std::regex("^[a-zA-Z0-9.-_/]+$"))) needs to be corrected to
std::regex("^[a-zA-Z0-9.\\-_/]+$")))

Testing trim example:

[awdsaber]

Thanks too much for your help Mr. @gallonmate, you opened my eyes on other things too.
Can you help in how to create a callable blueprint function that update all the various Initial Conditions to the current conditions?

[gallonmate]

You're welcome. Unfortunately I do not have the free time to create the trim function. You can search for some of the other Initial Condition's property names through the Property Catalog seen in this picture. During play, pause the game and you can hover the mouse over the pink catalog icon and it will bring up a search box, you can type ic/ to filter the property names to just the Initial conditions. Or you can use a For Loop and Print Strings to the Log Output. And then filter the logout, also in the picture. Once you have the Initial conditions, you can search again for the corresponding current condition or value, without the ic/ in the name.

[awdsaber]

Thanks too much for your help.