Weight and Balance - Why isn't the 'datum' located at the center of lift?
I understand that the datum location is arbitrairly set by the manufacture, usually located at the firewall or leading edge of the wing.  Wouldn't it make more sense to locate the datum at the center of lift?  It just seems to me that the CG numbers would be more meaningful if they were measured from a significant location and not an arbitrary location.  If the sign convention was such that CG forward of CL is positive, then the more positive would mean more stable, and negative CG would mean uncontrollable.
19 Replies
Ronald Levy
1516 Posts
My guess is because it's easier to measure the location of a component with respect to an obvious physical location than from the not-so-obvious center of lift.  This is important when alterations are made to the aircraft.
   The Reference Datum Line (RDL) is usually located at the firewall area or nose of the aircraft to make the weight and balance calculations and/or charts easier. Some aircraft RDLs are located in a point in space in front of the nose to allow for airframe/nose modifications.  
   If the RDL was located at the center of lift, then there would be positive and negative moments to be calculated when loading the aircraft, depending on item location. To simplify the math, the RDL allows for only positive moments in most aircraft (except the firewall-located RDL, then you have negative loading of fluids, like engine oil, equipment changes, etc.). Usually negative moments are “fixed” calculations for those particular aircraft and a factored into the aircraft basic weight and moments. 
   Finally, charts are much easier to graphically/visually track a center of gravity (CG) envelope on the positive integer axis points, and more user friendly to people who aren’t math fanatics.
I hope this helps.
CG and CL change even during flight as fuel is burned.  The RDL (such as the firewall) does not.  That's a great reason to use a fixed reference vs. CG or CL.  (CL changes based on AOA which changes based on load which depends on weight and/or G's pulled - too messy.  KISS: Keep it simple stupid and use a fixed reference.)
Also be aware that the center of lift changes with speed and  I also believe camber changes.

I understood your question when i saw this: "Doug Adomatis, Physics Instructor"  as an Engineer, i can relate, but what you are asking, although it _could_ simplify our math, (or at least make it look like something we are used to) cannot be physically measured in the field.  You pick a point that is easily measured physically and make the equations fit the physical, easily measurable, world.

The stuff about the CG and CL shifting is step two of this explanation, already covered.

@RafaelFlores --  You write that using CL "could simplify our math".   Hard to see how that works.  The established method and arithmetic involved is based on a single, known, fixed datum, for obvious reasons.  And, importantly, as you affirm, CL can, along with CG, change in flight.   Doesn't having two changing variables cause trouble and more complicated math?   Maybe you could clarify? 

Kudos to David, Michael, and Warren for lucid and practical, real world replies.  

Also, maybe you gentlemen could comment on the OP saying that "..
 positive would mean more stable, and negative CG would mean uncontrollable."    We use both "fore" and "aft" LIMITS for CG, both of which -- where limits are exceeded -- cause control problems in specific flight regimes.