Not long ago the question was asked, "How do you build so light"? Well, this isn't the first time the question was raised, so I thought it might be a good time to put "the Philosophy of Light" on paper so that the less experienced modelers among us can get an idea of why's and wherefores of the Light is Right mindset. The design style found in my Short Kit designs is a culmination of many years of trial and error, employing those things that have proven themselves over the years.
"How do you build so light?" is a great question, and one I wish more "modelers" would ask. Rather then just throw down a simple answer like "Oh, it's easy", I'll first offer a little background since the "Why" is actually more important then the "How". Here's how it goes;
First off, my background is in free flight, NOT R/C, and in the free flight realm, a model simply won't fly well -- if at all -- when it's too heavy. So the design philosophy in more along the lines of a "free flight model with a radio" then it is an “R/C model”. That being the case, a light wing loading is always the goal, and in any design, "flying qualities" are the primary goal, and "fidelity to scale" is secondary.
In today's world, the New Conventional Wisdom is that "weight doesn't matter, you can buy all the horsepower you want". No truer statement has ever been spoken, you can buy all the horsepower you'll ever need. But what you can't buy is a lower stall speed which dictates not only the strength required for the model to stay together under the normal flying and landing loads, but also plays a huge part in the models' level of stability. A lighter model is always more stable, and will always be easier to fly -- unless it's grossly over-powered.
Another consideration you must keep in mind is that regardless of the size of the airplane, be it a full scale C-5 or a 22" Fokker D-VII, it will occupy the very same air mass when in flight. The trick is, you can "scale" the model down, but like it or not, the air mass remains the same, and the un-repeal-able law of physics is that smaller wings are less efficient then larger ones. It's just the way it is, and that being the case, your only defense is a lighter wing loading.
Building Light is a State of Mind
By that I mean that you have to think light all the way along. When building a model, the first thing to consider is that every item that goes into the model during construction will have to be picked up by the model when it's committed to flight. That’s why I rarely if ever use hardware such as clevises or EZ-Connectors or plastic control horns in my designs -- they're heavy, and there's no point carrying a connector when a Z-Bend will do the job, or plastic control horns when a small plywood part or a toothpick will suffice. I also use pull/pull cables to control the rudder since string is much lighter then plastic tube and steel wire.
Now, as the airframe gets lighter, the amount of power required to fly the model decreases, meaning that you can obtain the same performance and flight duration with a smaller motor, ESC, and battery, and control the model with smaller, lighter, less powerful servos and lighter linkages. But here's the big one; The landing gear is typically the heaviest single component on the model, and as the weight goes down, the need for a big beefy LG -- and mounting system -- goes away. With a lighter wing loading, the model will stall slower, meaning lower landing speed, and less stress on the landing gear system.
I'm sure you've heard the statement, "the whole is greater then the sum of all its parts". Well it's true, no part of any model airplane needs to hold up on its own. On an airplane that uses lift struts, tail brace wires, or as with a rigged biplane, those components on the model can be built lighter because they don't need to stand up on their own. Each is supported by something else that's lighter then a self-supporting platform could be built. And that includes everything right down to the cover. A wing doesn't need to be built rigid since the cover will offer the rigidity not found in the structure itself.
And finally, this will also fly in the face of the New Conventional Wisdom, but a lighter model tends to hold up better in any reasonable mishap then a heavy one because a slower model doesn't hit anything as hard as a heavier faster one will. And, a light structure tends to be more flexible then a heavy, rigid one, and will give rather then break. Now don't get me wrong, no "good" airplane will "crash" well. But that's OK, airplanes are built to fly, not to crash. Crashing is what tanks are for -- they'll knock down all kinds of things, but they will always fly poorly. So the bottom line is to build the model to "fly", not to "crash". And there’s one final consideration too, and that is that electric powered models can be built significantly lighter then wet fueled models because there's no engine vibration, no need to guard against fuel soaking, and you don't have the sheer brutality of the engine start.
The thing is, this whole "light is right" mindset didn't happen overnight. This campaign toward light R/C models started many years ago when I got tired of chasing guided missiles around the flying field. A just couldn't warm up to a civilian high wing "2 seater" flying more like a P-51 then a Cub, so I went to work designing lighter models, just to see what would happen. Then as the models got lighter I noticed that they were not only slower, but also much easier to fly overall. From there it was a steady lightening campaign until it reached the point where the model wouldn't hold up under normal flying conditions, at which point I would go back a couple of steps and stay with what worked. A good example of the results would be the 60" J-3 that weighs in at 22 oz. and flies exactly like a Cub should. The model is now 10 years old and has logged just shy of 500 flights and at least 3,000 touch and go's, and is still my front line weekend flyer.
Handling a Light Weight Airframe is a State of Mind Too
When building, and flying lighter models, they must be handled gently. You just can't "manhandle" a light electric airframe like you do a big beefy gas model. It does take practice in learning what to grab, and where, and above all, how hard to grab it. But as with every other building technique, it takes practice to get good at it. But once you master the art of building light, you'll be rewarded with some terrific flying models that are generally nothing but fun to fly.
So, that's pretty much it in a nutshell. And the best bet to learn the "art of light" is to watch the guys that are successful and emulate what they're doing. But above all, live the 3 basic rules of modeling;
1- Light is Right
2- Thin is In
3- When in doubt, Copy