Scale Plans & Dimensions in Aircraft Modelling - Part 1

Scale aircraft modelling is perhaps not a matter of working to thousandths of an inch or millimetre, although a micrometer or other gauge might be used on wire or for checking the size of a drill, but again it may depend upon the scale of the model and the degree of detail being incorporated.
Usually we are dealing with Steel rules graduated in anything down to maybe sixty-fourths of an inch, or a metric rule with half-millimetre graduations. But whatever our measuring criteria might be, everything really depends upon a scale or dimensioned working drawing with which to set out the size of the model.

Queries about the accuracy of scale plans, and of actual aircraft dimensions is a matter that has occasionally cropped up over the past twenty years, but has gradually gathered strength during more recent times.

The general theme of modellers' questions has been on the lines of just how accurate are model kits, and how accurate are some of the model plans. Do they really represent true scale replicas of the original, particularly when the modeller may never come face to face with the real thing and only work with a plan as a guide, is it right or Wrong?


Don't trust either a kit or a plan

It is better to do a little research first so that simple corrections, if necessary, can be incorporated firstly into a modified plan or secondly, built into the kit - as it is better to do that at the start and not when the model is completed. People asking these 'scale questions vary from very experienced modellers who perhaps construct very large scale, Super-detailed models, to those less experienced but who have been modelling long enough to realise that accuracy plays a large part in a successful model. In these cases there are questions that I usually ask of modellers. "Do you use a scale plan as a reference for correcting or detailing a model kit, or are you using a plan to scratch build a model? Also, "do you make your own working drawings (perhaps based on a published plan) and detail it before commencing any model working:?" A further important question is “How much faith do you have in a published plan. Do you accept it as accurate or, having cast a critical eye over it, do you then make corrections and/or alterations to satisfy your standard of accuracy?”

Always check a plan first

Any scale plan should be carefully checked before accepting it as accurate. Modellers with no contact with the full scale aircraft ask how on earth it is possible to know whether a plan is accurate or not.
This is the catch of course. A lot of research should take place long before any wood, plastic or metal is cut or glued. The modeller should be very familiar with the real thing, even if only from a study of photographs or manufacturer's drawings, before starting the modelling cycle.

The standard between plans can vary greatly. Whilst some accuracy may result because of an existing museum example or for a machine which is still in service, both of which can have detail noted, what of the type for which no example survives? Can a true model result in that particular case?
Much really depends on the complexity of the full scale aircraft. For example, with WW1 types there often exists quite accurate original works drawings which, because of the simplicity of the basic structure and the fact that the overall size suited close-up photography which encompassed the whole machine in one plate, fine surface detail can be easily recognised and noted.

Compare that with a modern wide-bodied jumbo jet where, in photographic terms, a view of the whole machine becomes a mere speck on the horizon and close-up photos need to be so profuse in number as to form a huge mosaic before any real idea of surface detail can be obtained. The other point here is that with most modern aircraft there is no single works drawing that encompass the machine as a whole and which is both an accurate general arrangement and shoWS sections and surface details that would be suitable for accurate model making.

Sections galore - or there should be!

Usually such a drawing has to be produced by an enthusiastic model orientated draughtsperson who can translate many Works component drawings down into the type of overall drawing suitable for model making purposes. That is the sort of undertaking made by kit manufacturers, or individual modellers bent on producing a really accurate model. But no constructor always get the overall shape correct!

It is essential that any plan or elevation be carefully drawn out and as many cross sections as possible be created in order to capture the true shape of an aircraft. Most model plans fall down either from incorrectly plotted sections, or almost a complete lack of sections.

The aircraft manufacturer's side

The aircraft manufacturer initially spends as much time in preparing the actual physical shape of the intended product with what is known as the waterline drawings'. These are very similar to the drawings of ships hulls made by their builders, where closely spaced longitudinal sections are crossed by similarly closely spaced transverse sections of the proposed shape.

With such a drawing (or drawings) it is possible to plot any horizontal or vertical longitudinal section outline, or a vertical cross section at any given point on an airframe. Working sections such as at fuselage frame stations, or wing rib stations, are enlarged to full scale proportions, to form the basis of construction jigs, and for the detailed breakdown of each airframe sub-assembly, from which construction drawings are made.

The ‘waterlines’ are used to establish the true aerodynamic shape of the proposed design, and from Which a Scale model is constructed for wind tunnel testing. These skeletal line drawings show nothing more than profiles, outlines and cross sections, backed by hundreds of dimensions, usually to many decimal points. They are the only drawings which show the exact shape of the aircraft, or perhaps more correctly, from which the actual shape can be formulated. But they so not show surface detail, or any other detail suitable for modelling purposes.

Few aircraft are just a matter of true circles and Straight lines. Generally the more modern and larger they are, the more subtle the change in profile or section can become. The minute changes can take place a bit at a time, as every fuselage station, twelve inches or more apart, is passed. More generally, aircraft are formed from elliptical sections, which are out of Symmetry – the DC-3 (Dakota) nose is a typical case.

Relative to the fuselage profile, the full nose sections ahead of the cockpit windscreen are elliptical as they fair forward from the fuselage proper, but have the top section flattened off to form an approximate 'D' shape; flat surface up. Not many model Dakotas show this nose correctly. Similarly, whilst the general cross section of the fuselage is also a flattened ellipse along most if it's length, that immediately aft of the rear spar also forms a 'D' section, this time facing down to give an almost flat ventral surface at the wing trailing edge (matching the flat wing centre section under-surface), before coming gradually more elliptical the further aft travelled.