Aircraft C of G
 

Whilst reconciling the W & B/C of G of an aircraft,the Cof G Range is determined from a particular Datum point,in this case the wing LE,and expressed as `X`-Y`ins,Aft of Datum. Further investigation for similar models use a different datum point,ie propeller hub,and a further Weighing diagram for similar models expresses the Cof G range as betweem x-y % of Mean Aerodynamic Chord. Now, if all the points for Datums/ref. points are plotted together,we end up with different starting points,and different ranges,overlapping,but also extending the total range of Cof Gs. The Type Cert.Data Sheet gives only one Datum and Range.
So, my question is ,is there an argument for using the MAC range for Cof G,rather than just the other simple datum given in the TCDS. ?
The a/c is a tandem tailwheel trainer..

Of course there is, and anybody who has used a part of the propeller as the CG datum is a dangerous idiot who needs shooting, slowly - because an engine change, or slight change to the engine mount geometry will muck up everything.

I'm guessing that all of your drifting CG ranges and limits are down to various people re-inventing the wheel over the life of the aeroplane, and this can potentially be quite dangerous.

Many manuals do use %MAC, usually with conversions from inches/metres/centimetres from a fixed datum - wing leading edge at the root is usually most reliable, although mainwheel axle centreline works quite well to simplify the maths, or just a fixed bulkhead somewhere on the airframe. The BAe Hawk simply has a small plate under the belly with a line on it engraved "CG datum" - an elegant fix !

G

One needs to keep in mind that the datum position is entirely arbitrary - no more significance than a convenient place to hang the end of the tape measure on ie your datum preference is just as good as Fred's. Hence, any FS reference must always include the datum used otherwise it is a tad useless. It follows that, once chosen, the datum needs to be applied consistently.

A common datum trap is with trimsheet loading systems. Most of us define a trim datum somewhere inside the envelope, regardless of what the OEM might have defined. Generally, for larger aircraft, the OEM defines two datum positions - one for FS reference (which will be wherever that OEM fancies) and the other (often referred to as the trim datum) for loading calculations. Even then, a non-OEM trimsheet designer may well choose to use yet another datum for his/her own idiosyncratic reasons.

The best options for on aircraft datum positions are those which use an easily defined reference .. which is most aircraft. The DC9 type approach is another good fix where a plumb line sits over a grid to define long/lat level.

Use caution here Sycamore, While doing examinations of the C of G range of a number of Citabrias and Champs, I found differing C of G's and datums. Associated with this, were differing pitch control travel limits. A few of the C of G limits expressed for the floatplane were actually not achievable with possible loading configuations. On one occasion, I achieved the required aft C of G for test only by pouring water into aft float compartments.

I always work with the TCDS C of G data applicable to the specific model I'm working with.

Another useful datum is way outside the aircraft structure e.g. 100 inches fwd of wing root LE. This has the advantage of the arm ALWAYS being a positive number, providing less opportunity for calculation error.