C of G and MAC

[davidjh]

This may seem like a dumb question, but here goes anyway.

As CG can be expressed as a percentage of the MAC, does this mean that an aircrafts CG will always be located in the region between the LEMAC and the TEMAC i.e at the wings location? Another way of asking my question might be: does the FWD and AFT limit of the CG range correspond to the LEMAC and TEMAC positions, or is this not necessarily the case:confused:

Hope one of you boffs out there can help me.

[Mad (Flt) Scientist]

This may seem like a dumb question, but here goes anyway.
As CG can be expressed as a percentage of the MAC, does this mean that an aircrafts CG will always be located in the region between the LEMAC and the TEMAC i.e at the wings location? Another way of asking my question might be: does the FWD and AFT limit of the CG range correspond to the LEMAC and TEMAC positions, or is this not necessarily the case:confused:
Hope one of you boffs out there can help me.

The range of usable cg's is a function of many things - many of which are related to the relative position of the wing and the cg, since (simplistically) where the cg is relative to the wing determines the stability and controllability of the aircraft.

In practice, allowable cg ranges tend to lie anywhere between the 0% MAC (LEMAC) and 50% MAC points - but not for the same aircraft, since 50% of MAC is a HUGE range. So you might find that one aircraft has, say, 10% to 35% as the range, another has 20-40%, another has 15-35%, and so on. It all depends on the specific design.

Bear in mind that the MAC is just a reference chord - it doesn't necessarily correspond to a 'real' part of the wing, it's a kind of 'aerodynamic average' which is useful for various simplifications. Referring the cg range to the MAC is again just a convention; it would be just as valid to refer to Fuselage Station (location relative the fuselage reference geometry) - and this is in fact often the way that the cg is worked out when weighing the aircraft; it's then converted into MAC terms for pilot use.

There's also no reason why the cg might not, for an unusual configuration, lie outside of the MAC. I can conceive of a canard configuration having a cg forward of the LEMAC, for example.

So, to summarise to answer each question:
As CG can be expressed as a percentage of the MAC, does this mean that an aircrafts CG will always be located in the region between the LEMAC and the TEMAC i.e at the wings location?
No. Usually, but not always. Also, LEMAC to TEMAC isn't strictly "the wings location"

Another way of asking my question might be: does the FWD and AFT limit of the CG range correspond to the LEMAC and TEMAC positions, or is this not necessarily the case.
Not necessarily the case, and actually very rare. And never both at the same time (i.e if FWD LIMIT=LEMAC, I'd be astounded if AFT LIMIT=TEMAC)

[davidjh]

Thanks Mad Scientist,

At the end of the day I think I just wanted to know if the CG of an aircraft always lies in the region of the wings, thereby explaining why we can express it as a percentage of MAC. I'll re-read your reply a few times just to make sure that I (slow learner) understand it.

[Mad (Flt) Scientist]

OK, to answer that specifically.

By means of some useful simplifications, it's possible to express the amount of longitudinal (pitch) stability that an aircraft has by comparing the cg position to a 'neutral point'; both of these, in the theory concerned, are conveniently expressed in terms of percentage of mac, and this theory can be used to compare different aircraft.

So, in engineering terms, if I know the neutral points of two aircraft and their cg positions in mac terms, I can determine which is more or less stable.

There are also various design exercises one undertakes - such as deciding how big the tail needs to be - which are conveniently conducted using mac as a reference length.

BUT, once the designers have finished with the aircraft, it's just a reference, and not much special use. Unless you know more info, you can't JUST use %mac as a guide for anything much.

Also, for most conventional designs, 0-50% mac is going to contain the working cg range.