View Full Version : Uncertainty in Definition

14th Aug 2012, 00:40
I understand the definition of center of gravity, but what do you call the manner in which mass is distributed throughout an aircraft.

An aircraft could, in theory, have almost all of it's mass right in the middle with little in front of or behind it; another aircraft can have a sizable amount of mass in front of and behind the center of gravity with the rest in the middle.

I can't think of a name for this.

Radu Poenaru
15th Aug 2012, 14:20
The way the mass is distributed (all in one big lump in the center or lots on the margins and nothing in the center) won't affect your CG; What will change is the moment of inertia (http://en.wikipedia.org/wiki/Moment_of_inertia) for the body... uhm...think of flywheels

15th Aug 2012, 18:04
Maybe the term "mass couple" might be appropriate. Some century series fighters, if at a high aoa, would tend to increase their aoa even more during rapid rolls, causing a rather unpleasant "pitch up". Radu's mention of the moment of inertia principle applies.

16th Aug 2012, 01:17
How about mass distribution?

16th Aug 2012, 17:42
In the Tipler physics manual there was a nice pic of two sea gull models hanging from a thread.

One of them had its mass evenly distributed, and its was hanging from its CG, in the middle of it. The other sea gull had its mass concentrated in the tip of one wing. It hanged from its CG which was in the wing, not in the center of the shape.

Distribution of mass will determine the CG. In fact, it determines the center of masses. For small bodies, this and the CG are coincident. For superlarge objects such as the moon, the CG and the center of masses will be close but not coincident.

16th Aug 2012, 18:15
So why does every large aircraft I have ever flown have a range of CofG and certainly for take off it is vital that the load distribution results in the CofG being within those limits and remains there throughout flight as fuel is used?

16th Aug 2012, 23:39
For small bodies, this and the CG are coincident. For superlarge objects such as the moon, the CG and the center of masses will be close but not coincident.

If the bodies are a sphere and are of uniform density (which the moon is not very close) then the CG and the centre of mass are the same, regardless of whether they are small or superlarge.

1106, normally the stab is set for the C of G. If that is not possible (eg, an aircraft that has no trimable stab), then the elevator trim tab is set to the correct setting.

I hear that the setting is computed for a V2 climb OEI with zero elevator force, though in the sim I've found it not to be all that close.

Radu Poenaru
17th Aug 2012, 16:16
What in the world is a "center of masses"? (or center of mass?) Do you mean the CP perhaps?

Second, if you have to use words like "small bodies" and "superlarge objects" to differentiate behaviour, your physics theory/understanding is wrong. Scale has no bearing on classical mechanics.

All that the OP was asking about was mass distribution. You can have 10 kg sitting on the center fuselage of the aircraft, or 5 kg sitting at the tip of each wing. Your CG will not have moved one bit. What will change is the moment of inertia (ie. in the plane with the weights on the wings will roll slighly slower because of the way the mass is distributed). There is no need to bring the Moon into the discussion.

17th Aug 2012, 18:55

When I used the term "centre of mass" I should have said "centre of the mass", or "centre of the shape".

Meaning, for a sphere, the point that is equidistant from any point on the circumference or surface. Not necessarily the C of G, as you have pointed out.

sorry for the confusion

17th Aug 2012, 19:17
CG location along the X-axis (i.e., fore/aft) impacts both static and dynamic longitudinal stability. Statically, the tail lift must be adjusted (stabilizer and/or elevator position) to trim the pitching moments. Dynamically, the further aft the cg goes, the lower the dynamic stability. As CG goes aft, it takes less control surface deflection to pitch the airplane, but the inherent damping of the short period reduces as well.

Moment of inertia impacts the angular acceleration that is generated when a control input is made. The biggest impact on roll moment of inertia is the amount of fuel in the wing tanks. Roll inertia is highest at takeoff and steadily reduces as fuel is burned. Pitch inertia depends on how the payload is distributed along the fuselage. Often the term "dumbbell" is used to describe load distribution with most of the weight either near the nose or the tail.

Handling qualities need to be assessed for conditions representing the range of both cg locations and moments of inertia that will be encountered. When stability augmentation control laws are involved, these variations in airplane configuration must be addressed during control system design.

17th Aug 2012, 19:38
Hawk 37
Thank you - I understand that but the earlier posts seem to suggest that while the centre of mass moved the CofG did not and this clearly is not the case. Maybe I misunderstood the argument.

As to V2 and take off trim settings, what you say is certainly what I was taught when flying the VC10 but, never mind the sim it never resulted in zero elevator force in the aircraft either! In fact if the tail trim setting for take off from the load sheet was less than 6 the usual response to the check 'TPI setting' was 'Noted - setting 6'. That at least ensured that you didn't have to pull too hard if aiming for V2!

Happy days


18th Aug 2012, 12:52
No need to get excited.:ouch:

So he asks about same CG for different mass distributions, right?

yes, the answer is that only the moments of inertia change.