Re-scaling a plan

A reader emailed me a question about a plan he had downloaded.  The plan didn’t have any size dimensions on it, so he didn’t know how big to build the airplane.  I’ve never had this problem myself as a beginner, because until recently my plans were all on paper.

Traditionally, model plans are drawn on paper at a 1:1 scale, meaning what you see on the paper is the exact size of the finished model.  You don’t have to wonder how big it’s supposed to be because it’s right there in front of you.  Material thickness, engine size, control throw, and other data will be noted on the plan as needed.  In fact, on a traditional paper plan everything is drawn to its exact size, so even if something isn’t labeled you should be able to measure it on the plan to determine the intended thickness of wood or wire, or the size of wheels.

When a paper plan is scanned and saved as a PDF file, the size of the print is encoded in the file.  By default it comes out at the correct size when printed, unless you tell it to print at some other size.  So the answer to the question “how big do I build the plane shown in the PDF” is that the plane is built exactly the same size as the drawing when it is printed.  That’s why you have a drawing, so you can lay the paper out on a table and build the plane on top of it.

That’s a pretty simple question, which makes for a short article, so let’s talk about another issue that comes up pretty frequently regarding plans.  Because scanning paper plans and turning them into computer files is common nowadays, and because you can tell the computer to print at different sizes, it’s easy to change the size of a model.  In earlier times it was customary to turn a resizing into a new project complete with reengineering and new test flights because the plan had to be redrawn anyway if a new size was desired.  But now you can run a paper plan through a scanner and print it at any new size.  Is this a good idea?  And if you’re doing it, how do you make sure the plane will be built properly?

Because of the square-cube law, there are different amounts of stress on airplanes depending on their size.  For instance, the Q-Tee just isn’t big enough to exert a lot of destructive force on whatever part hits the ground first.  The landing gear doesn’t have to take a lot of stress, which is why it’s made of a single thickness of 3/32″ wire sewn onto the bulkhead with copper wire or carpet thread. If you were to build a 120 size Q-Tee with a 10 foot wingspan, you would have to totally redesign the wing struts and the landing gear, put some doublers in the nose section to distribute the vibration loads, and reinforce the firewall to prevent it being pulled out of the fuselage by the engine.  At a much larger size, each part of the plane is carrying heavier loads and is subject to more stress.  When you scale a small plane to a larger size you have to add a lot of strength to carry the extra loads.

Conversely, it’s sensible to eliminate extra structural pieces when scaling a big plane to a smaller size.  On the Miss Texas plan the landing gear, firewall, and other parts are designed to carry heavy loads and distribute them properly.  What if you want to reduce the Miss Texas to the size of a Q-Tee?  The redundant landing gear mounts with dual wire struts would only add unnecessary weight.  There wouldn’t be any point in having full size doublers, complex stabilizer construction, or big firewall reinforcements.  Essentially, you would redesign the entire structure to be about as simple as the Q-Tee.

This all seems sensible enough, but whose job is it to figure out how much structure is needed?  It’s yours.  You accepted that job when you resized the plan.  The whole thing needs to be reengineered for the new size.

Generally, the way to get the right amount of structural strength is by emulating established designs.  So if you’re resizing a plan, copy the structural details of a popular airplane that was designed in the target size.  After a while you will start to get an instinct for how planes are built at various sizes.

Having said all of that, I’ll tell you a big secret.  Rescaling plans is actually pretty pointless because there are already more designs than you can build in a lifetime, at whatever scale you like.  No matter if you like unlimited aerobatics, training, soaring, sport aerobatics, flying from water, flying old timers, or even building and flying a scale model, you can most likely find a well engineered magazine plan already available in any size from park flier to giant scale and everything in between.  If you have a really great 60 size plane that flies just right, and then you move to a smaller flying field and try to recapture the magic in a 20 size airplane, your best bet is to find a new plane in a 20 size.  For instance, if you love the Super Kaos but you don’t have room to land it, try the New Era III, which was engineered from the start as a 20 size pattern plane.  The configuration, wing loading, etc, are substantially different from the Kaos, but the 20 size New Era III flies just like a 60 size pattern plane because of very good engineering.  If you’re going to insist on a Joe Bridi design because you’re a die hard Kaos fan in need of a 20 size plane, there’s still a better answer than a scaled down Kaos.  Old Joe designed the Sun-Fli 4-20 and the Tweedy Bird just for you.

There are lots of planes already in existence to serve this purpose.  The 76″ Lazy Ace and the 40″ Sporty Forty biplane fly almost the same.  The 62″ Bingo and the 44″ Wicked Wanda fly almost exactly the same.  The 96″ Senior Telemaster and the 50″ Goldberg Eaglet 50 fly almost exactly the same.  You don’t have to do any engineering, because somebody else already did it for you.

I’ve gotten over the urge to resize plans.  The original designers did a lot of work building, testing, redesigning and rebuilding their planes.  It’s hard to get results that good simply by running a piece of paper through a copy machine.

But what if there’s a plane you really like, and you just have to have one in a vastly different size?  In that case you need to be a designer rather than just a resizer.  Put some thought into it, and pay attention to the airfoil and the scale of the wing, and the wing loading, because these wing design factors will make or break the flight performance.  Use the right thickness of sheets and sticks, and put the right amount of weight and structure into the landing gear and wing mounts for the weight of the plane.  (For more info on performance at different sizes, check out my article about flying different size airplanes.)  Then test the plane, redesign it, and test it again.  For an example of the same design at three different sizes, check out these three Outerzone links.

Barnstormer Baby

Barnstormer 52

Mighty Barnstormer

These planes were all designed by David Boddington, a highly respected designer who was notable for producing models in a wide range of sizes.  If you study the plans you will see the different design philosophy employed in each of the three sizes.  Take note of the cowling and firewall, the wing strut mounts, and the construction of the fuselage sides.  I guess what I’m trying to say is that electronic resizing is not a magic wand.  You have to do some design work.  If you know how to design, that’s great.  And I hope I was able to help at least a little bit by providing these examples.  If you’re a beginner, I really think you’re better off finding an existing design that flies just like your plane, except in a different size.