Distinctions Between Class C, B and A Modeling Requirements

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Editor: Graham Bullock B.Ed. Des Tech (CNAA)

I am always being asked to define these 3 modeling standards

and I can only do that in general terms, as all OEM’s have different ideas and standards for class C, B and A modelling. I meet the Class C, B and A guys on a regular basis , so here’s a bit of incite into how major OEM’s make the distinction between the 3 concepts:

Class C coordinator –

They emphasize EVEN CV distribution, recommending that modelers use  5×5 deg. surfaces, as if you need to go beyond 5×5 this might be an indication that the area is not fully resolved. Modelers always have to build a sharp closed volume model first with the theoretical intersections (TPI Theoretical Point of Intersection.Sometimes referred to as;  TLI, Theoretical Line of Intersection , then fillet. At this stage they are not interested in G3 continuities, and even ball corners can be split into several smaller surfaces.

NB: Some OEM’s try to enforce a regime of 3 x3 / 3x 5 deg. srfs. IMHO. There are a few parent srfs on an exterior that can be easily formed using 3 x 3 deg. (eg; roof and main side-wall)

Class B definition –

The Project is halfway frozen (Parent surfaces have been approved), panel gaps are then generated, some surfaces are further smoothed.

Class A definition –

The Project is totally frozen, continuities are all resolved to G3 wherever possible. All Engineering edicts have been incorporated , no zebra-plot “Jaggedness” throughout the model. Everything must run smoothly. Ball corners must all be made out from one single Bezier surface. Now the final touch, what some OEM’s refer to as Profiling /tolerancing, which is the nominal offset between two adjacent surfaces designed to match, say, the fuel cap cover/lid, which is initially just a panel gap. The surface of the cap is intentionally offset inwards around 0 .2 millimeters, just to compensate for some process tolerance, as expected all fillets must then be rebuilt.

kind regards

graham bullock

Hi Guys

My name is Manohar and I studied with LA, scince then I have gained several years of Class A experience with a leading OEM. I would like to add my point of view to this open article and hope that some of you other guys out there will chip in too in order to help the next generation of Aliaas modelers coming thru:

“Here I want to say that I am not clear with class B & C, I know only Class A & Non Class A. Sounds funny but Really I never tried to search for that till now.

Here In  Our Studio general practise try to use Class A only, which includes, 3*3 or 5*5 degree Surfaces without spans & with uniformly distributed CV’s. Also surfaces wit h proper lead in so as to achieve G2 & G3 continuities. In concept modelling also we try to make it with class A ,but yes Some time it ends with non-Class A or  will say like lil Less Class A.
So our approach is Class A only. only in some cases where we don’t have any choice like Engineering driven areas then we are using Non-Class A, it may be Class B or Class C, but I am not sure about what is class B and what is Class C exactly is. So I won’t comment on it. But sure I will get some info from your website about it in coming days..  🙂

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