Re: [Jsbsim-devel] alphalimits, hysteresis_limits

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

Hi Alan

I've taken a quick glance at the code.

The alpha limits are read in if they exist and exposed as properties:

  PropertyManager->Tie("aero/alpha-max-rad",   this, &FGAerodynamics::GetAlphaCLMax, &FGAerodynamics::SetAlphaCLMax, true);
  PropertyManager->Tie("aero/alpha-min-rad",   this, &FGAerodynamics::GetAlphaCLMin, &FGAerodynamics::SetAlphaCLMin, true);

The trim code makes use of the alpha limits in FGTrimAxis::FGTrimAxis().

  case tAlpha:
    control_min=fdmex->GetAerodynamics()->GetAlphaCLMin();
    control_max=fdmex->GetAerodynamics()->GetAlphaCLMax();
    if(control_max <= control_min) {
      control_max=20*degtorad;
      control_min=-5*degtorad;
    }
    control_value= (control_min+control_max)/2;
    control_convert=radtodeg;
    solver_eps=tolerance/100;
    break;

In terms of FGAerodynamics I only see it making use of alphaclmax to expose a stall warning property. I don't see any JSBSim code itself doing anything the stall warning.

double GetStallWarn(void) const { return impending_stall; }
PropertyManager->Tie("systems/stall-warn-norm", this, &FGAerodynamics::GetStallWarn);

FGAerodynamics::Run() - 

  if (alphaclmax != 0) {
    if (in.Alpha > 0.85*alphaclmax) {
      impending_stall = 10*(in.Alpha/alphaclmax - 0.85);
    } else {
      impending_stall = 0;
    }
  }

In terms hysteresis:

double GetHysteresisParm(void) const { return stall_hyst; }
  PropertyManager->Tie("aero/stall-hyst-norm", this, &FGAerodynamics::GetHysteresisParm);

FGAerodynamics::Run() -

  if (alphahystmax != 0.0 && alphahystmin != 0.0) {
    if (in.Alpha > alphahystmax) {
      stall_hyst = 1;
    } else if (in.Alpha < alphahystmin) {
      stall_hyst = 0;
    }
  }

Talking of hysteresis take a look at the example at:

https://jsbsim-team.github.io/jsbsim-reference-manual/mypages/user-manual-forces-and-moments/

Cheers

-----Original Message-----
From: Alan Teeder <ajt...@v-...> 
Sent: Saturday, May 25, 2019 1:43 AM
To: jsb...@li...
Subject: Re: [Jsbsim-devel] alphalimits, hysteresis_limits


On 24/05/2019 23:37, mik...@gm... wrote:
> Far as I have heard, alphalimits are used to indicate the modeled 
> range of AoAs

Thanks Mike.

That makes some kind of sense.

Does JSBSim do anything else with "alphalimits" or "hysteresis_limits", and are there flags which say when they are exceeded?  If not, do they serve any useful purpose?

This all came about because I am looking at a Datcom based FDM for Flightgear´s Piper Cub which I am working on. At the moment this simulation can easily get into a completely uncontrollable state. I am still checking , but it seems to happen if side-slip is allowed to build up.

Alan




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