View Full Version : C of G

29th May 2002, 19:12
On light aircraft the C of G is usually quoted as a distance from the aircraft datum, whereas on large transport aircraft it's usually quoted as a percentage of the MAC (Mean Aerodynamic Chord).

Anyone know the reason for the difference in presentation ?

Weight and Balance
30th May 2002, 00:47
Just some theories:

Theory one: small airplane pilots can't afford/understand pocket calculators. They can afford tape measures, and they can reach the datum plane (usually the firewall).

Big airplane pilots can afford/understand calculators, but can't find the datum plane (often an imaginary surface some odd distance in front of the airplane).

Theory two: little airplane designers can't afford/understand pocket calculators, but big airplane designers can.

It's probably theory two, since most pilots don't really know where the c.g. is: that is, they couldn't point to it on the airplane. It's just a number, that is either good or bad.

30th May 2002, 01:13
Actually it is far simpler than W&B's humorous solution.

There is no essential difference in presentation ... the two are looking at the same thing through different eyeglasses and, as is generally known, are related by the simple equations

%MAC = (distance to CG from LEMAC * 100) / length of the MAC

CG from datum = distance to LEMAC + (%MAC * length MAC) / 100

where LEMAC indicates the position of the leading edge of the calculated MAC.

Typically, for any aircraft, positions on the aircraft are given with reference to some convenient datum position (and it matters not one iota where the datum is located .. except in the case of graphical loading systems such as trimsheets .. when it is quite important) so that people can refer to and find things ...

When it comes to questions of handling and so forth, the %MAC measure of CG location gives a good indication of what to expect from an aeroplane of a given configuration. The aerodynamicists and flight test people work in %MAC for this reason.

In most manufacturing organisations, the aerodynamicists also generate the permissible CG envelopes and loading systems and, being not much different to anyone else ... why bother changing the method of measurement ? .... so the normal preference is to present the data in terms of %MAC.

For larger aircraft, engineering weight control data associated with the final envelope calculations usually is presented in terms of %MAC because there is no pressure anywhere to change it to something which is somewhat more useful for Pilot Bloggs .. ie simple distances from the datum position.

In the case of light aircraft, this would be the aerodynamicist's preference as well.

However, the majority of light aircraft come out of the GAMA organisations and there are reference documents which give guidance on the development of pilot handbooks and, more specifically, the style and presentation of information in such handbooks. This is the reason that, for these aircraft,

(a) if you pick up one manufacturer's POH it is pretty similar in style and layout to that from another

(b) in the weight control section, final calculations and envelope data usually is specified in terms of distances from the datum rather than %MAC.

30th May 2002, 19:00
Thank-you gentlemen, W & B for your amusing response and John for your informative one............I knew someone on PPRuNe would have the answer ! Best regards. virgo.