Rc Model Aircraft Design
Lorin Mandaloniz
It is probably safe to say that everyone involved in this hobby has dreamt of some really cool model airplane designs. I've had ideas for glow engine powered FrisbeesTM, a canard sort of thing with a sleek migrating bird shaped fuselage and an inverted gull wing having twin ducted fans in pods slung underneath, a Bcker Triplane and a lot of things I can't remember.
The Frisbee thing may not work out but I once tried bolting a Cox .049 to the center of one. It was a resounding failure that revolved slowly all the way to the ground. The rest of the designs could be made to fly but I may never build them.
Most of my designs that I've built have flown. My early attempts were not stellar examples of anything other than what can be produced by a fledgling modeler with flawed design ideas. Nevertheless, I've always found designing model aircraft to be enjoyable. I have no interest in advanced aerodynamics so I do what many others do live by rules of thumb, learn from experience and apply the laws of aerodynamics that I do know and understand.
If any of my designs were ever subjected to wind-tunnel testing I would probably not be pleased by the results. I'm sure my designs have aerodynamic flaws that I don't know about. But that's why I don't care. I've learned a lot over the years and I'm very pleased with how my models fly for the most part.
Regardless of my lack of expert aerodynamic knowledge, I design most of my own R/C aircraft simply for the reason that it's fun and I can do anything I want. I began designing my own models because I had wood lying around that needed something done with it and I figured I'd try my hand at it.
I continued to design models because at the time kits weren't all that great. Most of them were poorly die-cut and often the wood was warped, too heavy or had other defects. Now I design because I enjoy it and to have models that are unique and are purpose-designed to do what they do better than anything commercially available.
It is not my intention to encourage everyone to design model airplanes. I simply want to pass along what I know for those who want to try it. These articles may help you learn how various design parameters affect a model even if you have no interest in designing your own.
This article describes the process I currently use to design a new model aircraft design. The information will be edited over time as I learn more. As I mentioned on the Model Aircraft Design home page, my early designs were nothing to brag about. I did not define goals for each model nor did I have a sound design philosophy. I had vague ideas and started building. This often resulted in having conflicting goals that weren't well thought out.
Most non-specific sport models such as Stiks and Super Sportsters are good middle of the road designs. They aren't exceptional at anything but they also don't have any particularly bad habits. They're easy to build and fly but that's as much as can be said for them.
When you purpose-design a model it will do what it is intended to do very well, but other flight characteristics may be precluded altogether. Additionally there may be the risk of one or more devastating flight characteristics such as vicious tip stalls under some conditions. If you understand these risks and don't fly your model in those realms you will be pleased with your design.
This is something you should be mulling over throughout. Always stay focused on your design goals, target loadings and finished weight. The structure should be strong enough for only the intended flight envelope plus a reasonable safety margin.
I can not stress strongly enough that this is the most important step to creating your new design. If you haven't read Creating a Design Specification for a Radio Control Model Aircraft then I suggest you do so before you begin your new design. Step 2 Choose a Powerplant I am firmly against designing a model for a wide range of engines. This practice results in a model that has too many engineering compromises. The model must be structurally designed for the largest engine in the range but it won't necessarily be the best aerodynamically. I hate to be the bearer of bad news, but you can't have it all.
For example, if the model is designed for a .25 - .40 engine then the model has to be strong enough for the largest engine in the range, a .40 in this case, which will result in an airframe that is too heavily built for the .25 engine.
What kind of climb do you want your model to have? Should it accelerate going straight up even from zero airspeed? Do you want it to have enough power to climb a few hundred feet before it runs out of momentum? Should the model loop from level flight or is diving to gain speed acceptable? Decide how heavy the model should be then design and build toward that goal.
Rate of climb and airspeed influence each other to the extent that they always compromise each other. If you want the model to fly fast and also be able to climb straight up indefinitely you will need to build the model lighter. A fast (high pitch) propeller doesn't have the same lugging power as a slow (low pitch) prop.
Ultimately you may need to compromise rate of climb to achieve the desired airspeed or vice-versa. Fill in the blanks Weight Ready to Fly ounces Minimum Airspeed mph Maximum Airspeed mph Rate of Climb
(description) Step 4 Design the Wing One thing I want to make very clear is that the wing is the airplane and by far the most important component. Many parameters must be considered at the same time and weighed against each other. Again we will need to make many compromises to determine what are we willing to give up to gain something else.
You can begin by choosing a family of airfoils that can conceivably meet the specification. Pinning down a specific foil will be dependent on which specific design goals are most important to achieve and which ones can be compromised.
For example, if aerobatics are a primary goal then you would certainly choose a symmetrical airfoil. The specific airfoil within that family will depend on other factors such as airspeed and desired stall characteristics.
Rule out airfoils that can not stall as desired. An airfoil that has a gentle, difficult to enter stall may be a poor choice, but an airfoil with an unpredictable or vicious stall could mean a short life for the model.
The table below is intended to demonstrate how easy it is to become bogged down in a quagmire of indecision. Note that each parameter in the chart affects every flight characteristic somewhat. Characteristics that are marginally affected are ignored here.
Let's wade ourselves out of the bog by concentrating on what's most important, taking it to the extreme of efficiency and then scaling it back as necessary so that the model is practical to build and to avoid creating a related characteristic that is devastating.
The wing loading is a compromise of several flight characteristics low speed flight, predictable landing approach, rate of climb (lift from the wing, not pull from the engine), control response, and how easily the plane is upset in flight.
The lower the wing loading, the slower the model can fly. The higher the wing loading, the more predictable the airplane is on landing approach. Light airplanes are strongly affected by pockets of rising and sinking air which makes it very difficult to spot land the airplane. By the same token a heavier model is less affected by wind but is also slower to respond to control inputs and must fly faster to stay in the air.
Designing to a light wing loading may restrict the plane to lower top end speeds to prevent wing failure during high-G maneuvers. For example, the wing may fold if you build a large, light wing and then yank the plane out of a dive. This is not because the aircraft is light but because it may be more frail due to the lightweight structure. But the lower inertia of a light airframe also imposes less load on the wing. It is very possible to build a wing that is both light and strong.
Wing Area is not included in the chart because it is virtually meaningless. All the wing area does is allow us to calculate the wing loading. It is better to determine the wing area based on the target wing loading which is based on target weight.
Often a kit comes in at a weight higher than the manufacturer specified. While I don't know this for a fact, I suspect the published recommended weight is based on a prototype built by the designer with hand-selected wood which isn't what comes in the kit.
A banked turn describes a cone. The lower wing tip (the pointy end of the cone) is moving through the air at a slower rate than the opposite tip (the open end of the cone). When the lower tip stalls, the other end of the wing is still lifting.
Most of us aren't terribly concerned with fuel efficiency, but for some specialized tasks this is a high priority concern. The most aerodynamically efficient wing in terms of lift to drag ratio will be one of extremely high aspect ratio.
A high aspect ratio wing has a better lift to drag ratio and is generally more efficient than a low aspect ratio wing. If the aspect ratio is too high the plane will have a sluggish rate of roll and is easier to break.
As the aspect ratio of a wing becomes lower the aircraft becomes more maneuverable in roll and less efficient in lift. That's why you never see fighter aircraft having high aspect ratio wings and you don't see bombers with low aspect ratio wings with some special exceptions.
If the aspect ratio is too low the plane may be twitchy about the roll axis and slow down excessively in turns. Low aspect ratio wings have tremendous drag as angle of attack increases. Low aspect ratio wings are inefficient and not good for load lifting.
Elliptical wings are very efficient but difficult to build particularly elliptical wings having elliptical thickness. Wood doesn't like compound curves. Some designs get around this by adjusting the airfoil (rib height) to create a straight taper in thickness from root to tip which never looks right.
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