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By Sooty25
#1778322
or more to the point, creating the calculation in the first place.

I'm having a spot of bother with a weight and balance calculation. The aircraft manufacturer has not provided arm lengths for crew and fuel tanks.

The original weight and balance sheet states for calculating the aft CofG, "Record scale readings with two "heavy people" in the aircraft (approx 215lb) and add fuel to reach MTOW, then measure wheel weights". Then calculate CofG and check it is in range.

My queries are;
is there a formal way of establishing the arm lengths for crew positions, where in the seat do you measure to?

and how do you establish the arm of a wing tank? Do you just measure to the centre of the tank?
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By GrahamB
#1778334
Can you not back-calculate the arm length of a station by measuring the mass on the scales with nothing/little mass at that station, and then again with it fully loaded, and work it out from the differences between the weights on the scales?

Simultaneous equations, or something like that?
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By Sooty25
#1778350
GrahamB wrote:Can you not back-calculate the arm length of a station by measuring the mass on the scales with nothing/little mass at that station, and then again with it fully loaded, and work it out from the differences between the weights on the scales?

Simultaneous equations, or something like that?


Possibly, but I'd have to get the scales back again. A tape measure is much easier and if there is a recognised way of measuring, it then provides a justified paper trail, rather than "this is what I did" !
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By GrahamB
#1778382
Sooty25 wrote:
GrahamB wrote:Can you not back-calculate the arm length of a station by measuring the mass on the scales with nothing/little mass at that station, and then again with it fully loaded, and work it out from the differences between the weights on the scales?

Simultaneous equations, or something like that?


Possibly, but I'd have to get the scales back again. A tape measure is much easier and if there is a recognised way of measuring, it then provides a justified paper trail, rather than "this is what I did" !

Ah, thought you might still have the scales.
#1778429
FWIW, when I was running a design office one of my experienced old contractors (CEng) had spent a lot of his life designing and certifying aircraft seats. His rule of thumb estimate was that the cg of the occupant of a seat was (and I quote) "at the tip of his erect peni5".

I always used this whenever stressing an aircraft seat and it was never queried by EASA or CAA - not that it was couched in those terms to them.

Allowances should be made for physical variations, and for females...
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By Sooty25
#1778570
Sooty25 wrote:
Flying_john wrote:I use middle of the seats if its not already been decided in manufacturers data sheet. Measured to the datum.

FJ

http://www.wingweigh.co.uk/


I'm now going to the hangar, I'll measure that and see which of the 3 other sets of numbers I have, matches!


I now have 4 different sets of seat arms! :lol: and the manufacturers method!
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By Sooty25
#1778572
Paul_Sengupta wrote:
Sooty25 wrote:"heavy people" ... (approx 215lb)


:shifty:


It's a microlight Paul, possibly not your first choice!

2 x 215lb is a stupid scenario for it anyway as it gives -19lb for fuel!
#1778586
Sooty25 wrote:"heavy people" ... (approx 215lb)


Thats not even 100kg. Thats me in clothes w/jacket and Im 6'3".
Im now officially "heavy people"... :shock:
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By Sooty25
#1778604
skydriller wrote:
Sooty25 wrote:"heavy people" ... (approx 215lb)


Thats not even 100kg. Thats me in clothes w/jacket and Im 6'3".
Im now officially "heavy people"... :shock:


It's also American, which makes it even more of a laugh! Although the design is over 30 years old. Tbh, 6'3" might be a squeeze as well.

Statistics suggest that UK "Mr Average" is 5'9" and 185lb (84kg), the clue is in the class name, "microlight", it's not an international tourer, but does also explain why limits are moving to 600kg. I blame McDonalds personally!

As it stands, it remains in balance whichever set of figures I use, it's just I can't formally justify any of them.
Last edited by Sooty25 on Tue Jun 23, 2020 1:25 pm, edited 1 time in total.
#1778622
Sooty25 wrote: I blame McDonalds personally!

:lol:
I'll have you know I have had no more than a half dozen Maccy Ds in my whole life... :twisted:

Any "heavyness" on my part is entirely down to the superb cooking of my GF/Partner. :mrgreen:

Regards, SD..