C of G Certification

[Wander]

Do any of you clever chaps/esses know how an a/c gets it's C of G certification?
I guess in these days of computer aided design the C of G is pretty well known prior to first build. Then there's the eventual weighing which will say where that a/c's C of G is but how is the C of G envelope defined from there?
Many thanks!:ok:

[747FOCAL]

There is a lot that goes into that kind of certification. Even if they do have an idea of what they think is the usable c of G it still must be flight tested.

[John Farley]

Wander

The forward and aft CG limits will be CHOSEN by the designer while the aircraft is just a doodle. The role of the aircraft largely determines how big a range is necessary to make the thing a practical aircraft to operate (ie how it will tolerate in your case pax moving around, where the freight is etc. the military tend to want to carry stuff and then drop it and so on)

Having chosen the range you then need to design the longitudinal aerodynamics so that the thing will handle satisfactorily at both extremes. Don’t forget to mention to your stressman what download you will need at the tail for the forward CG case because this will likely be his worst shear case at the backend of the fuse when pulling to the aircraft g limit.

JF

[Mad (Flt) Scientist]

Following on from John's post, once the target CG envelope has been CHOSEN by the designer(s) and the various systems and components all designed so as to meet that design goal, you then have to go and test the blessed thing and find out if all your guesses - err, detailed design calcs ;) - were right or not.

That's where all those strange cut-outs and kinks in envelopes often come from - if it's not a nice neat shape, chances are there's an 'oops' hiding somewhere.

[Genghis the Engineer]

The CG range is estimated by the designer, who will primarily be considering (at the aft end) minimum acceptable values of longitudinal static stability (that's pitch or speed stability in other words), and at the forward end both a maximum acceptable value (that is pitch inputs must not be estimated to be too great for the required manoeuvres) and the maximum acceptable tailplane loads in flight due to either high speed or steady turning flight.

Once it's been established on that basis, estimates will be made of lateral and directional stability (roll due to sideslip and yaw due to sideslip) which are also CG dependent, the latter in particular can sometimes be too low at aft CG positions.

But it's important to realise that however clever the design boffins are (and some of them are really quite bright these days) this is only an estimate.

Then, when the prototype reaches flight test, we start in the middle of the predicted CG range (on the grounds that the boffins estimate shouldn't have been that far out), start with handling evaluations at that point, then start moving outwards in small CG increments, re-evaluating handling at each stage.

The main players tend to be control forces and input becoming too high for the landing flare towards forward CG, or occasionaly the stall speed increasing and pushing the aircraft outside of the certification code (not a problem for a part 25 aeroplane, but can be for a part 23-marginal machine such as, for example, a PC21). At the aft CG it's usually one of:-

- Pitch forces becoming too light for proper speed control
- The aircraft becoming directionally unstable, particularly with full flaps.
- Stalling characteristics becoming poorer as the CG goes aft until they are deemed unacceptable (more likely a problem with lighter aircraft).


Once the handling limits are reached, either the CG limits are set in stone, or we go back to the designers and say "this isn't good enough, sort the aeroplane out!". They redesign something, and we start again around the critical conditions.

CG envelope cut-outs happen for one of two reasons. One is handling deficiencies in particular weight/CG/configuration combinations that we can't solve, or can live with the restriction so don't bother, the other is that we just don't need to. For example, the Islander has a heavy weight / forward CG cut-out for the simple reason that all the cargo capacity is up the back so it's impossible to get to a Fwd condition near MAUW.

G

[Wander]

Great stuff boys, many thanks.
Wander