Airfield Models - Introduction to Radio Control Model Aircraft Design

January 21, 2009

Introduction to Radio Control Model Aircraft Design

At the risk of stating the obvious, the first step in creating a model aircraft design is to decide what kind of aircraft you are designing. Stating that you want to design a trainer would be meaningless if it weren't for the fact that everyone knows what a trainer is.

Mention the word trainer and most of us picture a model that looks pretty much like any trainer we've ever seen and we think it's obvious that's what a trainer is and should be.

But what if you were to design the first R/C trainer ever? If what any given aircraft type "should" be is immediately obvious then our less successful ancestors would not have chosen to cover their arms with feathers and jump off a cliff. Trainers haven't changed much over the years, but other types have.

For example, look at precision aerobatic models. If everyone knew what it should be then it would not have taken so long to develop them to the point where they are now (and there is much more development still to come — look back to today 20 years from now).

If you want to design an exceptional model then you can't just choose a type of model and expect everything to fall into place.

Generic goals such as, "I want an aircraft that flies well and is aerobatic" sound good but are meaningless.

A Sig Kadet flies well and is aerobatic. Coincidentally, this also describes a Sukhoi, a Piper Cub, an F-15 and almost anything else that can fly in a controllable manner. It's easy to create a successful design if you let the model define the design after the fact even though it might not even be close to what you originally wanted.

Additionally, the commonly asked question, "How big should such and such model be for <insert engine size here> be?" is fundamentally flawed. A much better approach is to choose a powerplant, then set a target weight based on various factors. From there a target wing loading is chosen, again based on whatever flight characteristics you want your model to have. The size of the wing is calculated from this information.

Sorry, but that don't count

If you want a standard sport model, then copy a good standard sport model and change the outlines to suit your tastes. If you don't think anyone does this, just look at all the "new and original" designs in some of the R/C magazines that are nearly exact copies of 100 other "new and original" designs published by the same magazines over the past couple of years.

I don't consider changing outlines to be "designing your own." At best it should be considered re-styling. But it will get you started so it is not a horrible thing to do either. However, please be courteous enough not to submit it to a magazine. There are more than enough Kadet, Ugly Stik and Kaos clones out there already that people are taking credit for even though they don't deserve it.

I read a web page a while back entitled, "How to Design a Radio Control Airplane." As I read through the page I realized that the title is misleading and wrong. The author of the article provides a set of cookie-cutter parameters for one design — you guessed it — an Ugly Stik or Kaos clone depending on the wing location and whether you decide to taper it (Kaos). A better title for the page would be, "Copy my design please."

All moments, areas and ratios are pre-determined leaving the "designer" only to determine the size of the aircraft and choose an airfoil. All aircraft built from the given parameters will basically be the same aircraft.

As far as I'm concerned that's not really designing an airplane. It's simply copying someone else's design. When you think about it there are very few new ideas in our realm. We are standing on the shoulders of giants. That doesn't mean we can't mix it up a little to design something somewhat out of the ordinary or just to further improve an already existing design.

I believe that if someone is going to say this is how to design a model airplane then he should cover how things actually work and how each parameter affects the airplane. A set of pre-packaged numbers doesn't do that.

I encourage you to build your original design. There is absolutely no reason why you can't. Build a canard, a flying wing, something out of Star Wars or whatever. You will realize it is not a black art and it doesn't require an aerospace degree when you see your creation take to the skies. Even if your design doesn't fly (which is unlikely) you'll still learn a lot. An honest attempt is never a failure regardless of the outcome.

The one thing that you need to know is that if you design something that looks right it will fly. I can almost guarantee your success if you build the airframe straight, strong, light and get the center of gravity close to correct. If the model is a standard planform then put the CG between 27% and 33% MAC to start.