Pull up trim problem?

[shenmoxie]

In FGTrimAxis.cpp，when I see getstate function：

case tNlf: state_value=fdmex->GetAuxiliary()->GetNlf()-state_target; break;

but when I See GetNlf()：

double FGAuxiliary::GetNlf(void) const
{
if (in.Mass != 0)
return (in.vFw(3))/(in.Mass*slugtolb);
else
return 0.;
}
Is there a lack of gravity here? And will this cause pull up trim problem?

[seanmcleod70]

What gravity are you expecting to see in GetNlf()? It's simply returning the Normal Load Factor.

So simply the ratio between the normal force in the wind axes in.vFw(3), i.e. the lift force divided by the weight of the aircraft.

So if you want to perform a pull up trim of 3g then you would specify a Normal Load Factor of 3 and the trim routine will find the relevant AoA required for the given airspeed and the given CL vs AoA of the aircraft etc.

[shenmoxie]

I understand what you mean. But I see that Nlf's state_target defaults to 1, with no units. However, GetNlf returns xxx g. This doesn't seem to match up? Or should I change the state_target unit to g?

[seanmcleod70]

However, GetNlf returns xxx g

Where do you see it returning this? As I mentioned above:

It's simply returning the Normal Load Factor

Which is dimensionless, since it's the normal force in the wind axes divided by weight which is also a force, so it's simply a scale factor.

[shenmoxie]

Meaning in.Mass is already mass*g, not mass?

[seanmcleod70]

No, in.Mass is mass, but note the multiplication.

Maybe you're confused about the non-SI units?

[shenmoxie]

I understand what you mean. Thank you very much！