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Practical Intro to RotorLib on a snowy Saturday Morning



163277.jpg OK, so its snowing in parts of the UK.  And for a change its actually stuck.  Which means this country is in total Chaos.  I don't know what it is about the UK population and snow that they lose the ability to drive, but that's just the way it is here.  So in response to this embarrassing situation, I'm staying in doors for the duration.

Which in turn means I either paint my hallway (which isn't going to happen) or i spend the day trying to work out how to accurately calculate all the necessary config values for a Helicopter in ArmA3 without the proper SDK from RTDynamics.

Love it or hate it the ArmA3 Advanced Flight Model is here to stay.  And once mastered its actually a lot more fun than the basic flight model.  Far more satisfying to prove you have the real skill to fly properly than just fake it?

Before we go any further lets address the aerodynamic representation of the Cow in the room.  What does it mean?  Why is it relevant?

The Answer?  It just made me laugh.  Its the inspiration for writing this article simply because the absurdity of the image gels nicely with my feelings toward generating a practical config for the ArmA3 Advanced flight model.

So for the last year or so I've been banging my head on my desk trying to work it all out and get all my helicopters up and working with it.   This has led to some ...interesting craters on the various A3 Islands.  But hey, its all leading towards progress. 

I have not failed. I've just found 10,000 ways that won't work.

rock-efa.jpgI used to be, I'm going to say, a pretty decent Aerospace Engineer back in the day.  But even way back when I did get my hands dirty in CADDS5 and Catia some of the maths related to Aerodynamics scared the crap out of me.  

This article is not an all out Tutorial.  Not even close.  Lets think of it as the start of discussion.  I'm going to share some things I've found along the way.  If you have a better way to do something, or If I've gotten something wrong.  SHOUT it out and we can all move on and learn together.

For those that don't know much about the flight dynamics of ArmA3 helicopters, this isn't going to make much sense.  For this that do know something about ArmA3 Helicopters this may make some sense...possibly. 


Step 1 - In your quest to lose hair and go gray early in your life.

My best advice is find a class of helo; eg: small, med, heavy and modify these the stock values.  There are some values you can calculate and others that require some mysterious and complicated voodoo incorporated into the RTDynamics SDK that we are not privy too.Simulink-Attitude-Hold-Cropped-Wide-e1314192224119.png

Step 2 - Research, maybe this should come first?

You will need to find some basic facts about your aircraft.

  • Dimensions,
  • engine specs,
  • mass,
  • rotor blade dimensions. 

Other values we are simply going to try and make an educated guess.

Step 3 - The joy of XML

So we've got our XML file open. You are greeted by a wall of text, figures and variables that  are both terrifying and baffling to all concerned. Some of the variables are easy to explain and find.  Others are a little more esoteric and mysterious.


You are going to see long tables of values as shown above with variable names like:

  • SideForceDueToSideslip
  • LiftDueToAoA
  • DragDueToAoA
  • DragDueToSideslip
  • itchingMomentDueToAoA - 
  • YawingMomentDueToSideslip
  • LiftDueToSideslip 

This is a calculated table of values.  My best advice is find a class of helo, small, med, heavy and use the stock values.  These are calculated by a RotorLib Plugin for MATLAB that on licensees have access to.  Since we don't have all the tools we are guessing.

BIS has provided a breakdown of the RotorLIB values as well as some of the formulas to create the values on the BIKI but it still isn't the most accessible read.


I'm not going to repeat everything in that BIKI article.  I dont see the point.  But some things need de-mystifying...and I don't have all the answers but i am willing to share 

We are going to focus and the values we can find online or in manuals or calculate from known values. MOM-diagram.JPG

  • Mass - Obvious really. This is the (I believe) empty weight of the helicopter.  I'm basing this off values seen in the Littlebird and KA60 configs etc
  • MomentsOfInertia - Ixx, Iyy, Izz, All these are unique to your helicopter type.  They will change based on the dimensions and mass of the airframe relative to the "allup" Center Of Gravity (CoG)
  • Engines:
    • emergencyPowerHP
    • maxTorque
  • DriveTrains
    • TorqueLimits emergency
    • Load gearRatio Main Rotor
    • Load gearRatio Tail Rotor








I'm in the process of writing an excel spreadsheet to help calculate all the values we are going to need.  But its a long slog


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