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Written by Paul Kohlmann
Learn how to correctly read your plans.
Featured in the Construction Series in
Model Aviation May 2015.

One of the best parts of writing this series of articles has been discussing modeling with some notable builders in our hobby. Pat Tritle is one of these people, and when I asked what topics he would like to see covered for new builders, his reply was immediate. He suggested a piece on how to read plans.

This led to some interesting discussion on how often problems with a build can be avoided by closely reviewing the plans before the building starts. It is often useful to study all of the notes and views and then “prebuild” the project in your mind.

This article will cover the basics of how to read a technical drawing. It will focus on the “correct” methods that drafters normally use in industry, but it is important to recognize that model designers often work to a much looser standard.

Title Block and Notes

This should be the builder’s first stop. Adding notes to a drawing can be time consuming. With that in mind, you won’t find many notes on plans that the designer didn’t think would be important to you.

At a minimum, a title block with the subject, designer’s name, and date should be located at the lower right hand corner. The date is handy because the equipment available at the time the plans were drawn probably influenced the construction method. Some designs withstand the test of time while others—well, not so much.

A designer often provides additional blocks of notes. One common example is a specification table that lists the model’s wingspan, length, and expected weight. Wing area and projected wing loading may also be provided.

The specifications may include the power system used by the designer. This is useful because it gives the builder an idea of the designer’s goals. If the plans call out a 370-size motor, then you will probably need substantial modification if your goal is to use that spare Power 15 in your drawer. That’s not to say that you are stuck with whatever the designer used.

For many, straying from the plans to create something new is part of the reward of scratch building. Just be aware that a modification that significantly changes the model’s power or weight is likely to require modification to the airframe itself.

Designers often include assembly instructions on the plans when they spot potential problems. By calling out an assembly order, the designer may help you avoid a fit or accessibility problem. Again, the designer’s notes aren’t gospel, but consider the possible consequences before you choose to go your own way.

One of the most important notes on the plans is the placement of the center of gravity (CG). By marking this on the plans, the designer is telling you that this aircraft will fly when it is balanced as shown. Disregarding this is the most common source of poor performance. It is possible that you will fine tune the CG to better suit your flying style, but this is best done after the maiden flight.

Finally, a bill of materials (BOM) is handy. When fully developed, the BOM is the complete shopping list for wood, electronics, hardware, and power systems for your project. The BOM is often partial because designers understand that builders normally use the hardware and electronics that they are familiar with.

Drawing Views

Drafters have traditionally used the “glass box” to characterize the standard views on their drawings. These standard views are arranged in a specific order so that fabricators can rapidly construct a 3-D mental image of the subject. Adhering to these rules avoids problems such as building the mirror image of the intended part.

Engineering drawings of this type are commonly called three-views, although more or fewer views can be incorporated. The basic three-view layout will show the front view, the top view, and whichever side presents more detail.

Model plans often use a looser standard. The main reason for the departure is that the views most needed by builders are the ones that they will be building directly on top of. A side view of the fuselage is much more useful than a head-on view. The side view gives the builder the overall size and appearance of the subject, as well as the position of critical components such as formers. A similar top view of the wing provides the correct angles of ribs, spars, and other elements. Separate views for the layout of the tail group parts are also handy.

Fitting all of these “building” views onto reasonably sized plans normally requires the builder to abandon the glass-box arrangement. The views may be rotated and moved around to maximize clarity within the allotted real estate.

The "glass box" encloses the subject of the drawing.

Special Views

Building an airplane often requires more than the limited number of views previously described. Two types of special views are worth mentioning. The first is the detail view.

The detail view focuses into a specific area by stripping away other components, by changing the viewing angle, or both. The detail view can be either two-dimensional (2-D) or three-dimensional (3-D).

A 2-D detail view may show the alignment of certain parts, such as two keel halves that must be preassembled over the plans before installation. A 3-D example might show the arrangement of an assembly of many parts that is tough to envision from the 2-D plans views.

As the name suggests, detail views show only part of the picture. For this reason, they must be isolated in some way. 2-D details are often cut off by a jagged line. 3-D details might be enclosed within a circle or fade away at the edges.

An exploded view is often used to show how a collection of parts is fitted together. This view is normally a 3-D representation.

Detail views may be 3-D such as the inset on the left, or 2-D like the setup template to the right.

Line Types

The type of line used by a drafter is also subject to rules. Each type is selected for a specific purpose. The most common line types are:

Visible: This heavy, solid line is used to show the feature outlines of parts that are visible from the angle shown.
Hidden: This medium, regularly dashed line shows the positions of features that are internal or behind the features that are visible from the angle shown.
Center: This light line is broken into alternating long and short sections. It is used to show the axes of holes and shafts, and the centers of circular features.
Section: Collections of these light lines are used to show cut surfaces as hatched or cross-hatched areas.
Dimension: Light lines that flag the points measured by a dimension. Lightweight arrows are used perpendicular to the dimension lines to bracket the numerical dimension.
Break: May be heavy or light, depending on the style. These lines enclose a portion of a drawing that has been cut away, like a detail view.

As with views, model designers may take some liberties with the rules on lines. Usually these departures are limited to using a fancy line type to draw attention to a feature.

Wrapping It Up

Reading a bona fide blueprint can require a degree of skill and knowledge. Fortunately, most model plans are simple forms of technical drawings.

Even with that noted, everything on the plans was put there for a reason. Taking the time to review the plans until you clearly understand them before the build starts is the best way to avoid problems with the build.

The next time we meet, we’ll discuss some detailing techniques that can help transform your project from functional to fantastic.

—Paul Kohlmann


Grumman Goose Plans

Paul Kohlmann


how do you interpret the scale on the plans.
I see some different builder ways they display the scale.
I am use to 1/4=1" etc.
Thank you for any help you can offer.

Hello Chester! We have an answer for you from AMA Plans Service that we hope helps. Here is the answer: This was always left up to the plan designer. Some state 1 inch equals 1 foot, some say ¼ scale, and some just give the wingspan. I will usually look up the wingspan of the full-size plane and divide by the wingspan of what the builder wants. Good luck with your project!

There are several common types of errors on model plans.

The fuselage top view and side view are drawn to the same length, as would be expected in a 3-view. However, the true length of the longerons should be longer than shown on the side view, because they are curved. You can see the greater length of the curved longerons in the top view. Model plans rarely show true length on the side view which is built upon. After building to side view plan length and curving the longerons, the built fuselage will be slightly shorter than the plan. Rarely will modelers draw a properly developed building plan showing the longerons at true length.

This is not a problem on Guillows and similar keel and former plans because they show the keel true length flat on the center line. You must be careful to not cut the stringers to plan length, though, they should be bent into place and cut to match the curve defied by the formers.

Another common error is in the representation of dihedral. The front view will show 1" under each wing tip. The build diagram will show one wing flat on the table and the opposite tip raised 2". Each of these produces a different angle for the dihedral under each wing; they are not the same. Assume each wing panel is 12", flat. The first representation produces a dihedral angle of 4.780 degrees ( arcsin(1/12) ) under each wing. The second produces a dihedral angle under each wing of 4.797 degrees ( arcsin(2/12)/2 ). In this case, the angles are small enough that the difference of 0.017 degrees may be ignored to simplify construction. No one is going to cut a dihedral jig to exactly 1.993" high to get the exact 4.780 degrees dihedral indicated by the front view.

Another common discrepancy is in the wingspan. The span of the built wing with dihedral will be less than the flat span on the building drawing. Boxes, title boxes and plans often give a nominal span, so a 30" P-30 might have an actual span of 28" or 29". If you want to get the full 30" span to fit the rules, your wing panels should be lengthened to compensate for the dihedral foreshortening.

The plan for a sheet balsa wing will call out a 3" wing chord made from 3" sheet. When the camber is formed in the wing, the chord will be shortened by about 1/16". That means the wing saddle should be 2 15/16" long, not 3", as shown on the drawing. Ignoring this discrepancy can result in the wing incidence being off, because the wing can slide back and forth 1/16" on the saddle. If the wing is glued to the saddle so the leading edges coincide, the incidence angle will be less than if the trailing edges are made to coincide. In a duration contest, this much discrepancy in wing incidence can make a difference.

The differences are small enough that most modelers, and scale judges, ignore them.

In a professional drawing room, those errors would not be permitted. The side view would be developed to full size.

Scale is represented in several different ways. A common scale description is something like 1/4" = 1'. At first this seems absurd. One quarter inch is not one foot. This is read as 1/4" on the model corresponds to 1' on the full size airplane. It is sometimes called "quarter inch scale". This is not the same as "quarter scale" in which the model is one quarter the size of the prototype. This type of scale designation is common on architectural and ship model drawings (which are often 1/8" = 1" or 1:96 scale) and is facilitated by the use of an architects scale in which integer feet are labeled at 1/4" intervals on the scale. These scales will often have an initial foot divided into 12 scale inches. There are 48 one quarter inch (1/4") divisions in a foot (4 x 12 = 48), so this is also represented as 1:48 scale. The model is 1/48 the size of the full scale airplane. This also corresponds to the engineer's 40 scale, in which there are 40 divisions per inch. Each 10 of those divisions is 1/4", there are four sets of 10 divisions per inch. In quarter inch scale, each of these divisions corresponds to 1/10 foot. Such drawings are often called "forty scale". The engineer's 60 scale corresponds to 60 divisions per inch, six sets of 10 divisions per inch, 10 divisions per 1/6 inch. This scale can be used for 1/6" = 1', or 1:72 scale (6 x 12 = 72).

Engineer and architect scales come in three sided forms in which there are six scales, one on each edge of the scale. Between the architects scale and the engineers scale, the most common model scale ratios may be found. These different ways of designating scale can be confusing at first, but after working with them for a while, you will be able to shift from one to the other with ease.

This article is useful to anyone wanting to interpret and use construction plans for model aircraft. After adhering to provided plans and building and successfully flying several model aircraft, the serious modeler may want to take the next step- making their own plans instead of building model aircraft to someone else's ideas.

If this process continues, and the modeler stays enthused and gets sufficiently hooked on the wondrous hobby of model aircraft, there will come that magic time when the model aircraft builder becomes a designer of successful model aircraft.

It may progress to an even more wondrous stage, where the modeler becomes an "intuitive aircraft designer", using no plans, photographs, calculations, previous aircraft designs, or other outside influences to confine their imagination, and using materials and construction techniques across many applications to inspire truly unique model aircraft.

Fantasy or impractical you say? Successful musicians and visual artisans have mastered this intuitive creative process, and it works equally well for all who take their creativity seriously.

Frank Horine

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