sellener

Im trying to understand how the Datum is chosen?(by the manufacturer)

I understand what the Datum is, but cant find a good explanation of how it is chosen. I also believed incorrectly untill i began studying weight and balance more that the CG was the Datum.


The Datum has to have something to do with where the CG is. Cause the CG is determeined based on the moments/weights and moments are determeined by arms and arms are determined from the datum???


Ok just had a thought. Would the Datum be the CG for the empty aircraft prior to loading?? That would make sense to me right now.

Then you use WAM to get the "new" CG

I dont know.... any ideas?

Rustee

Datum is simply a reference point designated by the manufacturer to base all the arm lengths off of, and subsequently to determine the "inches aft of datum" in the computation of the aircraft's CG. I'm sure there's some logic to where the manufacturers pick their datum point; such as the nose of the aircraft so all numbers are positive or a prominent structural feature (e.g., the firewall in a Cessna 172). Regardless, it's simply a reference point.

No, because each aircraft technically has it's own unique BEW and Moment (ergo it's own unique CG), the datum would theoretically be different for every plane. If that were the case, the arm lengths would be different, so the CG computation would need to be tailored to each aircraft's POH. Not really a viable method of selecting a datum point.

NoyGonnaDoIt

Rustee is correct. It's really just an artificial starting point for measuring station locations (arm). It's usually forward of locations that are variable so that the numbers are generally positive, but they don't have to be. But the location is chosen by the manufacturer and is not a uniform standard. They could have had someone walk blindfolded along the side of the airplane and told him to drop a coin anywhere and that would be as valid a datum as any other.

For example, the moment of the front seats in their middle position may be 35" if the airplane manufacturer chose the firewall as the datum, but -87" if they chose the front of the baggage compartment as the datum.

If you want to know how it was chosen, you'd have to write to Cessna, Piper, Mooney, Boeing, etc... and ask "how did you choose the datum for your airplanes?"

rickair7777

Like others have said, it can be anywhere. On some military planes it used to be a point far forward of the nose...not even on the airplane itself. The exact location was a secret, this way the W&B sheets did not have to be classified until filled out.

It doesn't matter where it is, except I suppose that on a non-swept wing airplane with symetric wing tanks you could locate it at the center of the fuel tanks. This would simplify calculations as any changes in fuel would require only a weight change, as the arm would be zero.

usmc-sgt

Since I built my airplane I was able to pick my own datum and just used the leading edge of the wings. I could have used the fuel strainer or the tail wheel or the leading edge of the exhaust, it didnt matter.

Cubdriver

Airplane manufacturers also name a global origin for each airplane. This because all the parts on the airplane need to be located precisely and they do not wish to just bolt one thing to another and let things go where they happen to fit. The global origin is ahead of the nose located on centerline a few feet in front of whatever the foremost part of the airplane might be. It can be quite a few feet ahead of the airplane because pitot booms and other gear are fitted for testing which they want in a positive region. The global center is located below ground also, because no part or assembly is negative if it is placed there.