reply to post by rezial666
rezial....the term "CG" means 'Center of Gravity'.
Trying to keep it simple here....there are considered four forces that act on an airplane, in flight. Thrust, Drag, Lift and Weight.
We will only concern ourselves, now, with Lift and Weight. Lift is, as the name suggests, the component that keeps an airplane in flight, to
counter-act Gravity. Every fixed wing is different, as to shape, dimensions, planform and airfoil....but each will have, for a specific Angle of
Attack, a particular point that is defined as the C/L (Center of Lift)...I tried to keep it simple, but you see, it needs some explanation.
The CG should be easily understood....it is the point of any mass, where the 'MASS' is considered to be 'centered'.
In conventional fixed-wing airplanes, the CG is ALWAYS forward, in 'front' of, the C/L. This is referred to as 'dynamically stable'. The greater
the distance, between these two 'points' on the Datum Line, the more downward force is needed by the Horizontal Stabilizer (that's the tail). That
is its function. To 'modify' the variance (the moment arm) between the C/L and CG, by providing a downward force, at the tail.
As the CG moves aft, and gets closer to the C/L, the moment arm gets shorter, and the HS (Horiz Stab) needs to provide less downward force....and the
overall drag decreases, and fuel efficiency increases....but only so far.
If the CG is AFT of the C/L, then the airplane is 'dynamically unstable'. this is not a scenario you want in a passenger jet.
The CG in a jet, as in any airplane, is dependant on these factors: The basic airframe, (empty). For modern commercial flight, the basic (empty)
weight is defined as OEW, or 'Operating Empty Weight'. OEW includes the weights of the normal crew, and normal on-board equipment, including galley
supplies, all emergency equipment, etc.
Starting from the OEW, we consider the fuel on board, by weight. Also, which tanks the fuel is in, since that has an effect on the CG. Then, we add
the payload....that is the passengers, and their baggage, and the cargo, and how it is distributed.
The purpose of those photos, purported to be of a 'spraying' airplane, were simply mis-used to support an unwarranted claim....that being,
The interior photos of the B-777LR were just flight test photos. It is what airplane manufacturers do, for FAA and ICAO Certification requirements,
and to verify their computer simulations by actual test data, for performance, fuel efficiency, duration, and.....to refine what is called the CG
Which brings me back to the CG question. When I say 'envelope' it refers to what is an acceptable range of CG, based on the airframe design.
As I noted earlier, a too far aft CG is dangerous in conventional airplanes.
A CG too far forward, while safe, is less fuel efficient. Flight testing, by the manufacturer, whether it is Boeing or AirBus, is mandatory to refine
their computer models, and then design charts and graphs to sell to their customers, the airlines.
Sorry if anything above got too technical....I'm just a pilot, not an engineer.