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Written by Paul Kohlmann
Getting your model to the right size
How-to
As seen in the July 2017 issue of
Model Aviation.


Times have never been better for scratch builders. Numerous plans services are publishing plans dating back to the 1930s and beyond. Many of the print wood kits from yesteryear are now available as laser-cut kits. And just a little surfing on the internet can produce a drawing of virtually any aircraft imaginable.

One thing about builders is that we seem to thrive on personalizing our projects. Maybe we need to add a little more detail than what is found on the plans. Or perhaps we want to convert a scale model to a different variant.

One of the most common modifications is to change the scale of the plans. Maybe that 19-inch Earl Stahl Grumman Wildcat got your attention, but it’s too small for the spare motor and servos you’d like to use. Or maybe you’d like a 1/8-scale Douglas Devastator to fit in with other models, but the best plans available are a different scale.

I often hear from builders who are uncertain how to resize plans. It’s actually quite simple, but with that said, there are a few guidelines that come in handy.


What Scale Is This Model?

A common question that builders ask is what scale are these plans? This is particularly true when working with designers who group their designs by wingspan instead of by scale.

Builders have good reasons for asking what scale their project will be. A key reason is to make sure that detail parts will be the right size. For example, pilot figures are usually described by their scale. Putting a 1/8-scale pilot in a 1/12-scale cockpit will look odd.

As an example, I’m currently working on a Curtiss Hawk 75. The wingspan for this project will be 60 inches, but what scale is that? The math needed to answer this question is simple.

We only need to know the wingspan of the model and the wingspan of the actual airplane.





In the case of the Hawk, the full-scale aircraft had a wingspan of 37 feet, 4 inches. As mentioned, the model has a 60-inch wingspan. The first step is to get both of the wingspans into the same units—in this case either in feet or in inches.

Let’s start by working in inches. There are 12 inches to a foot. With this knowledge, we can convert the full-scale wingspan into the same units as the plans. We need to convert the 37-foot part of the wingspan to inches. That extra 4-inch part of the wingspan is already in the right units, so we’ll add that bit on after the feet are converted.

Here is how the conversion looks mathematically:

Full-scale Hawk 75 wingspan = (37 feet x 12 inches) + 4 inches = 448 inches.

Now that the full-scale units match the model units, all we need to do is find the ratio of the big airplane to the little airplane. We do this by dividing the full-scale wingspan by the model wingspan:

Ratio of full-scale aircraft to model = 448 inches / 60 inches = 7.47.

So, the scale is 1/7.47. This just happens to be 7% more than 1/7 scale and 7% less than 1/8 scale. There’s not a lot of 1/7-scale stuff out there, but 1/8 scale is pretty common. Is that a problem when we are looking for a pilot? Perhaps, but when you consider that full-scale Hawk pilots ranged in size from somewhere between 5 feet, 6 inches and 6 feet, 3 inches (coincidently a variation of +/-7%), we can probably make a 1/8-scale pilot work.

A note on units: It doesn’t matter if the units are in feet, meters, or furlongs. The same process applies. First convert as necessary to get both the full-scale and the model into the same units, and then find the ratio.


Scaling Plans Up or Down

A different scaling challenge is changing the size of the project. For scratch builders, that normally means changing the size of the printed plans and the parts outlines.

Digital printing has made this a simple thing. As an example, let’s adjust that aforementioned 19-inch Stahl Wildcat to an even 30 inches. The ratio of the 30-inch model to the original 19-inch one is 1.58. All we need to do is print our plans 158% larger.




The Earl Stahl Wildcat is a FF classic, but with only a 19-inch wingspan, it might be smaller than some builders are looking for.


Most of us don’t have a large-format printer of our own, but many printing resources do. These include copy centers, local blueprint shops, and online printing services. Most can change the scale of your printout if you explain your goals. The AMA Plans Service can also provide plans that are larger or smaller than the original.

For larger plans, blueprint shops can be your best bet. They often have larger machines than copy centers have and they are more focused on technical drawings rather than flyers. My local shop costs slightly more, but its service is excellent. The 36 x 48 plans for my last project were less than $10 a copy.

Even if you don’t have a large-format printer at home, you can still print your own plans on a standard printer. This is done by tiling the file. Tiling simply breaks the large page down into standard letter-size sheets. The sheets are then taped together to make full-size plans. I use Adobe Acrobat Pro for this, but there are several tools available online.

Some file types scale more easily than others. CAD formats such as DWG and DXF are designed to be scaled up and down. An important benefit of these formats is that their line widths and resolution don’t degrade as the scale is blown up. A PDF file works nearly as well, with high levels of resolution, but the line width will vary with the scale.

Graphics files such as JPEG and TIFF are a little trickier. The resolution of these formats degrades as the scale goes up. That means that a letter-size sheet gets mighty fuzzy by the time it is blown up to big plans.


Scaling a Design Up or Down

Now that we know that changing the size of the drawing is simple, we can make any size model from any plans, right? Well, not exactly. There are a few things to work out first.

The first thing to consider is the thickness of the materials you will be working with. When the scale changes, so does the thickness of the stock.

To get this section started, here is a list of standard wood thicknesses in inches. As a side note, please be aware that the actual thickness of balsa varies.

Fractional Decimal
1/32 0.031
1/16 0.063
3/32 0.094
1/8 0.125
3/16 0.188
1/4 0.250
3/8 0.375
1/2 0.500

Some designs and scale combinations work better than others. For instance, the 45-inch Bf 109D that was in the October 2016 issue of Model Aviation, began life as a 30-inch model. We find the ratio of the two by dividing the new project from the old one:

Ratio of new model to original = 45 inches / 30 inches = 1.5.

Bringing the 30-inch model up to 45 inches meant that everything needed to get bigger by a factor of 1.5. The 30-inch model was built up from 1/16 and 1/8 wood. So what sizes of wood do we need for the 45-inch model?

1/16 x 1.5 = .063 x 1.5 = .095 and 1/8 x 1.5 = .125 x 1.5 = .188
On the table, the new numbers match standard wood thicknesses of 3/32 and 3/16 respectively. Perfect—that makes this 1.5x jump a simple conversion.

So what happens if we want to scale the little 109 to some oddball size like 1.79? Let’s do the math:

1/16 x 1.79 = .063 x 1.79 = .113 and 1/8 x 1.79 = .125 x 1.79 = .224

When we check our table, we see that the wood that we need isn’t found on the chart of standard wood thicknesses. Are we doomed? Not really. We have a couple of options to keep us in the game.

The first is to pick the closest things on the list and keep going. The most common problem to result from the wood sizes being off is that any notches in the design will be either too tight or too loose. Tight notches can be opened up with a file. Loose notches might be filled with shim stock.

Alternatively, we could adjust the size of the wood itself. This is most practical when making strip wood for stringers or the parts for a built-up tail. By using a stripping tool, we can cut the strips to any size we need. If necessary, we can use a sanding bar to sand oversize stock down to a custom size.

So even in the case where we convert the plans by a strange ratio, it will still be buildable, although it might take slightly more work to get the parts to fit properly.


How Far Can We Go?

As a rough guideline, the size of most plans can be doubled or cut in half and still made to work. The limiting factor is whether the structure will still be effective at the new size.

I’ll illustrate my point by driving this to an extreme. Let’s consider scaling the Stahl Free Flight (FF) Wildcat up to a 1/4-scale monster. We’ll do a little more math to figure out the scaling factor:

• Stahl Wildcat wingspan = 19 inches
• 1/4-scale wingspan = 38 feet (the full-scale wingspan) / 4 = 9.5 feet.
• Convert 1/4 wingspan to inches = 9.5 feet * 12 = 114 inches.
• Scale factor = 114 inches / 19 inches = 6.0.

The fuselage of the original Stahl design uses eight 1/16-inch formers spaced roughly 2 inches apart. These formers are tied together by six 1/16-inch square stringers that appear to be reasonably close together in the model.

After multiplying these values by 6.0, our upscaled Wildcat will be built with chunky 3/8-inch thick formers spaced more than a foot apart. A better plan would be to use a greater number of thinner formers.

As for the stringers, the upscaled version now has bulky 3/8-inch square stringers that are spaced inches apart. The structure that worked well as a tiny FF model now looks like a loosely associated collection of lumber.




Here’s a look at the tiling and scaling functions in Adobe Acrobat Pro.


Another complication comes when fitting motors and other hardware. Builders who choose to make large-scale changes on a set of plans need to consider whether the framework is designed to handle the loads it will see as a larger or smaller model. Modifications to motor mounts, battery trays, etc., could also be required.

These concerns aren’t meant to rule out radical adjustments to existing plans. They are simply offered to show that builders who successfully take on these projects do more than print a bigger piece of paper and buy thicker wood.


Winding It Up

I began this conversation by talking about how the number of plans available to builders is nearly limitless. Now that we can change the scale at will, our options are truly infinite. It’s a great time to be a builder.

—Paul Kohlmann
ptkohlmann@aol.com


Sources:

AMA Plans Service
(800) 435-9262, ext. 507
www.modelaircraft.org/plans.aspx






5 comments

Nice article! What you don't mention is changing the type of wood where is makes sense. When do you know you must go from balsa to plywood or vise versa? Scaling up to a larger size model, where balsa might have been fine in the smaller scale, now requires lite ply or aircraft ply to take the flight loads. This is where good common sense, and plenty of building experience comes into play. If you don't have either, then ask questions in your club, or on the online forums, RC Groups, etc...

Yes it is an era to be a builder. We just upscaled a miss America to 109”. Quite a bit of engineering to make a 2 piece wing and stab. I have ventured into CNC and CAD so model building has never been so rewarding! Great article and thanks
For sharing.

JJ.

Really good subject. Thank you for taking the time to write it.

I made a mistake a few years back: I started a "scale" kit project without doing the scale documentation first. At 48", the wingspan suggested the plane was 1/8 scale, the fuselage it turns out, was an almost perfect 1/7. I'll finish that plane some year, after I learn how to fly...

Great article. Thank you. It's nice to know there are still scratch builders out there.
Thank you for putting the logic and the math for scaling plans down in print. MH

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