Scale Plans & Dimensions in Aircraft Modelling - Part 2

Draw large for accuracy

Drawings that appear in many aircraft modelling magazines are not necessarily in a style that is suitable for direct model working, as they are what is known as 'drawn for reproduction, having some lines thicker than others. Such lines may cover a scale two or three inches in their thickness. The drawings always try to pick out surface detail like skin joints and rivet lines.

But the thickness of the outer lines is something to be careful of, but they are necessary for a good reason. The use of very fine lines, which would be suited to model Working would in any event start to break up and vanish in places during the printing process.

But to draw large? This is a contradiction in many ways. It is quite easy to sketch out a small shape rather than draw out a large one. But in order to get a really accurate small scale plan one must start with a large, very carefully plotted out drawing which can portray all the various small changes in profile and section, by drawing in a fine line and then photographically reducing it down to a smaller scale. However to create the large drawing in the first instance, one must have plenty of manufacturers dimensional data to hand.



Having plenty of these dimensions means that ordinates can be accurately plotted in a large scale, which is not so easy in a small one, where the thickness of a fine pencil or pen could, again, cover several inches full scale.

Try drawing a "Spitfire" side elevation in 1:72 scale and compare it with one drawn to 1:12 scale and see the difference when the latter is reduced to the smaller scale.

Another aspect is that copying a works general-arrangement drawing can be dangerous as most of the outlines drawn are schematic ones, which means that only the dimensions shown on them are accurate. Very often too a subtle shape modification may have been made on the aircraft, but no effort will be made to alter a particular drawing. Dimensions too can be a problem and a recent case shows that mistakes can get through uncorrected on makers G.A. of Machine drawing, and it can, in some cases, be very easy to read a dimension wrongly.

Working recently on the Blackburn Skua - and this aircraft is no exception - and comparing G.A.'s with other published data, many confusions exist. Some are caused by model plan compilers miss-reading the data, for example, quoted dimensions have appeared giving the overall length as 34 ft 10/4 inches. Others have given 35 ft 8/s inches and 35 ft 7 inches - so which is correct without having a copy of the original drawing available to check?

The first dimension quoted happened to be the overall length tail-down, which was correct. The next figure was a misquote, as one copy which had been printed rather heavily showed distinctly 35.7, where the draughtsman had missed out the foot symbol and made such a short 'dash in between that it appeared as a 'dot, and the inch symbol had blocked up into what looked like a "foot symbol. Thus the correct Overall length as measured parallel to the line of flight (tail up) was ... the latter, i.e. 35 ft 7 inches.

What the modeller usually comes across are plans which contain quite accurate plans and elevations, which are nicely surface detailed but contain only about three or four cross sections. In the latter case that is not nearly enough, and when a kit manufacturer works from the same plan, the resulting model may look fine when seen directly in plan or side elevation, but looks entirely wrong when viewed from a different angle.

Even where manufacturers drawings show three or more fairly accurate plans and elevations with some component joints and skin/panel lines, the purpose of the drawing does not require sections to be shown. Thus, as has already been mentioned, we get cases time and time again where the kit makers model looks very convincing in direct plan or elevation but, in any other view, light and shade does not conform to the full size appearance because so many of the cross sections are far from accurate. Many kit Spitfire noses will confirm this situation.

Decimals or Fractions of a foot

Working from makers sub-assembly drawings which give both overall dimensions, cross sections and a complete idea of the internal structure, are the ideal reference to work from. But with these drawings often showing a profusion of dimensions in fractions or decimals of measurement, the modeller is put off by all the addition and subtraction necessary to convert to a Working scale drawing.

Most general engineering reference books show comparison tables of fractions to decimals and vice versa, together with very accurate tables for converting Imperial to Metric measurements. That's not too much of a problem and here it might be added that railway modellers, who are in many cases closer to scale model engineering than many aircraft modellers, regularly publish conversion data for all sorts of dimensional information and this is really something for the scale aviation modeller to adopt.

Some modellers find it easier to convert to metric working, as reading off millimetres is much easier than inches or fractions of an inch. But with today's pocket calculator to hand none of this dimensional activity should be a problem! Set up a scale for the conversion and, even if it came out at something like 12.6743mm to the foot, the calculator can easily handle any dimension. All the modeller has to do is to round off the figure to one or more decimal points.

For example: 20 ft 6 inches at 12.6743mm to the foot = 259.82315mm and rounding off at 259.8mm is a very easy dimension to measure, and probably is as accurate as most modellers would Want to go. Even so, many modellers (both aircraft and railway) continue to work in fractions but find it a hard struggle and, in railway terms, they enquired as long ago as the 1950s if there was some easy way to add and subtract fractions!

Easy Graphical Solution

It so happened that during the 1950s, Fairey Aviation Co. were going through a phase of repairing and updating their vast collection of models which dated back to 1916, many of which had been damaged during wartime storage.

They also required that construction drawings of past aircraft be redrawn (i.e. photographic copies) and have rounded off dimensions suited to the scale of the model. The task was a daunting one to undertake.

There had to be an easier answer than writing down sets of fractional figures each time and laboriously adding or subtracting a mixture of anything from eighths to sixty fourths every few seconds! As a memory aid, a vertical column of squares was marked off on a piece of graph paper, it then being much easier to total a given number of squares but it was still necessary to convert from a large number of sixty-fourths or similar number to get the final answer.
Having then found that the addition of a horizontal axis of squares enabled any denomination of fraction to be added to another by counting each amount. along a separate axis Scale, the answer was shown by running a diagonal from the outermost corner of the square or rectangle formed by the two fractions, to one or other of the axes, a complete fractions’ Square could be drawn, from which answers could be shown without the need to convert them.

The Decimal Foot Rule

Whilst with standard engineering tables, conversion of fractions to decimals has been shown to be relatively easy, finding aircraft drawings where all dimensions are given as decimals of a foot can cause problems. Dividing the primary number of feet by twelve, leaving a remainder which, added to the figure after the decimal point, gives an answer which can then be converted to fractions by means of the table, is a little time consuming.

This is where a foot rule, divided into tenths and each tenth into tenths again (and again if necessary) comes in useful as its edge can be laid directly against a fractions rule and the answer read off to the nearest sixty-fourth very quickly. Such a rule can be made by scribing on a piece of plastic strip. Rub some black paint over the scribings and clean off the surplus - a very useful tool will emerge.

Plans galore

Many requests are received from modellers desiring to make models of obscure or less well known aircraft types. The subjects requested can vary from a General Aircraft Owlet to a Martin Maryland or something far more obscure Such as a General Aircraft Monospar Universal. “No plans seem to be available, so how can I model this type?' is the usual request.

Mentioned before in this column, if you have a silhouette and, particularly, if it is a Ministry one, plus a set of basic dimensions, then the basis of a fairly accurate plan is there, albeit rather small in that particular form. It is a matter of getting it enlarged and then tracing off the outline and replacing the over thick white lines with very thin black ones.

What you will then have is probably a similar sized drawing which saw the creation of the silhouette in the first instance. In most cases they were drawn from fairly accurate G.A.'s by the manufacturer at Ministry request, who then transformed them into negative form of white shape on black background. By photographic means the image was then reversed and reduced into a small black shape with white lines and background. By setting up a scale on the enlargement and drawing all surface details in, and paying heed to published dimensions and other data to be found, the basis of a scale plan soon takes shape. Certainly many adjustments will have to be made in converting thick lines to thin, and some subtle cleaning up is often needed, but the end result is very much better than having to start from scratch - and guessing it at that.

Once the basic outline has been made, it is a matter of perhaps leaving the drawing on file but to keep coming back to it each time some new detail is discovered and adding to the slowly growing reference. Now that the plan and elevations have been established the next task is to start creating cross-Sections. Careful study of photographs is necessary and, of course, the full scale subject if one is available.

Aircraft modifications

Mike Stephens, from the Humberside area, faced many problems with the Westland Lysander, as some of the photographic references, model plans and dimensional details did not seem to tie up.
As it happened, a very early photograph he had was cross referenced to a tailplane and elevator root chord (on the aircraft centreline) that differed considerably from that shown on what appeared to be reasonably accurate plan. Was the plan wrong or were the dimensions wrong?

The problem was due to airframe modification work. Often a prototype Will fly in a certain configuration and will then be found lacking, controlwise, which results in a larger surface area being created. Or it may be that an already established type will, because of a change of role, need to have a larger fin, and so on.

The Lysander, with its short fuselage, had a few longitudinal stability problems when the engine was suddenly opened up as in the transition from slow to faster flight and vice versa. When the slats opened and with flaps down, nose-down trim was rapidly needed. So a larger tailplane was considered, increasing the chord by a fair amount and rounding off the tip. However that upset trim when gliding in to land. The remedy seemed to be a 'flying tailplane which could be trimmed to a large negative angle but again, there Were trim problems during an overshoot situation when on opening up the engine, the nose shot up violently. A problem which could not be easily overcome as it took time to retrim the machine quickly.

The new moving and larger tailplane was standard on the Mk.I's with the Pilot's Notes recommending that a slow throttle opening was made whilst winding the trimmer as quickly as possible at the same time. On the Mk. Il's and subsequent aircraft (plus retrospectively modified machines) a further increase in tailplane area was made, mainly by introducing a less rounded tip. In these three tailplane configerations the elevator remained almost the same overall shape apart from slightly blending the tip curve once, giving a slight change of area. For the purist this was something of a nightmare in attempting to find dimensional changes for each case.

The span remained at 12ft 6in. in each case, with the original total tailplane and elevator chord at 5ft 6in. The subsequent two modifications set a new chord of 6ft 4 in. in each case. But the main difficulty was in the subtle increased tailplane chord and tip curve dimensions, as the drawings reproduced in this article show! Notice however, that the dimensions are mixed up in feet and inches, fractions of inches and decimal inches, all in the same drawing.

This is where both conversion tables and the Graphmaster come in handy to arrive at some simplified scale dimensions.

With the Lysander too, the Mk.1 and Mk.Ill had the Bristol Mercury engine, whilst the Mk.ll had a change to the Bristol Perseus, the latter having a much shorter length cowling and did not have the 'blisters over each cylinder head in the former's cowling. Most of the more recent and highly detailed Lysander plans I have seen show well the fuselage stringer lines in the side elevation and underside plan, but do not show them on the cross sections which, admittedly, would require some very careful draughtsmanship.

However as the very fine 'clinker built effect is so readily seen on this aircraft it must be remembered that each cross section should show the same effect. But how to achieve that on a 1:72 scale model? Possibly the only answer is to carefully scribe some very fine lines along the fuselage length, to give an impression of Stringers.