|Quoting MrChips (Reply 1):|
As for passenger arms, I believe it goes one of two ways - either each class has an average point (like the arm you use for fuel), or there is one average arm for the entire cabin
It does indeed go one of two ways, but not exactly like that.
For most planes
a certain seating order can be assumed. For example, the assumption for coach may be that the passengers sit window-window, filling in from front to back, then they sit aisle-aisle filling from front to back. When all the window and aisle seats are filled they begin to fill in the center seats.
So let's say coach hold 120 pax in 20 rows of 6 arranged are 3-3 on each side of the aisle. Well the arm of each row is known, so if we have 90 passengers on board, we can assume that all the window seats are filled (two passengers in each row) all the aisle seats are full (two more passengers in each row) and ten more passengers are distributed throughout the cabin in the center seats - based on some assumption about in which rows. This gives us a total weight (standard weight times 90) and a total moment based on the above assumptions.
Now the exact assumption might not be the one above, but it will be something like that.
For more CG-sensitive airplanes
-80 might just be such an example) there will probably be an actual seatmap produced as part of the weight & balance.
The W&B for a larger airplane should not intimidate you in any way. The system is well worked-out and just has a few more fields in which to enter data than does your Cessna. (and it does not normally use negative numbers)
Most of us use an indexed system by now, where the last two digits, plus one decimal place are not pounds, but trim index numbers. So, for example the basic operating weight of a particular plane might be something like 72502.4
which means that the BOW
is rounded off to 72,500 pounds and the empty-airplane trim setting starts at 2.4 units nose-up. Each item we add ends with the trim index value for that; for example fifteen thousand pounds of fuel might translate, or be reported to us as 14998.5
which means that while it is still 15K lb. it has the effect of (adding algebraically) giving us 1.5 units nose-down trim from where we'd be without it.
That might sound confusing but all we do is add up all these weight-index numbers and the last digit plus the decimal is the trim setting. Simple.
So there we are climbing out and two flight attendents unlatch a five hundred pound service cart and drag it eighty feet forward or aft! We sit there watching the elevator trim wheel spin.
Happiness is not seeing another trite Ste. Maarten photo all week long.