OK, I sat CAA(NZ) exam for Rotorcraft today, 94% (yay!), but one question bothers me. Lateral Centre of Gravity calculation had me guessing.
So, arms left of centreline are negative, arms right are positive. I have loads and arms for left and right fwd scales, rear scale is on the centreline.
The way I figure, lateral CoG arm is
left weight x left arm(-)=left moment
right weight x right arm(+)=right moment

LW+RW=WT, LM+RM+MT, CoG(lat)=WT/MT.

But it isn't. For some reason I need to use the aft scale reading aswell,even though that lies on the centreline (ie; 0 arm=0 moment),to give a total weight, and divide that by the total L/R moment to get the lateral CoG position. Why?

ladened or unladened? is it an african helicopter?

Depends on your favourite colour

Random question = random answer?

The fore/aft ballance only applies if you are attempting takeoff/landing from a treadmill.

So basically you've been given these weights and distances:

|————–|————–|
Wl…………Wa…………Wr

You need the position of the CoG ie. the position that the total weight Wl + Wa + Wr acts through.

So yes, you need all three weights to compute that.

(Note that if Wa is much larger than the other two, the CoG must necessarily be close to that position regardless of any imbalance between Wl and Wr. If not, the CoG can be further out.)

So Wa influences the arm of the lateral CoG, because the L/R weights are taken ahead of the longitudinal CoG?

Nothing to do with longitudinal balance, it's purely a 2-d problem.

CoG is where the total weight of the helicopter acts through, and Wa is part of that total weight.

What you're looking for is the point where the combined (lateral) moment of *all three* components is zero.

Of course Wa has a moment of zero about the centreline. But if the CoG is not on the centreline…

Bing! The light goes on "Of course Wa has a moment of zero about the centreline. But if the CoG is not on the centreline… "

Thanks. I'll sleep better tonight!

BTW, I did get the question right, but I couldn't figure out why.

No probs!

So do you get to fly them now..?

Bagsy first on the uplift day.

Ain't allowed to fly them – just get to fix them!