General Concepts for RC Aircraft Design

This isn't a "How to do everything" page, but will get you started.

Let's say you have a (very) general question like "What airfoil should I use for the best performance?"

If you're the excessively-scientific type, you can explore all the physics of flight in every potential flight regime, analyze every airfoil at exactly the appropriate Reynolds numbers, along with a CFD study and wind tunnel tests, use precision computer plotters and CNC machines to make your parts, etc.. Mostly, this is a matter of throwing time and money at a question.

Here's a group of design "factoids" that will take you pretty far, without such a scientific approach. I should point out that while many of these are the result of my own experience, many were obtained from several good sources that I trust. These sources are here. Just remember that the answer to any design question is "It depends..." There are a great number of variables that affect the performance of any aircraft, and so by the very fact that I can't see your design, these are generalizations.

Airfoils

Airfoils are as good a place to start as any. Here are some general rules: (See the "sources" link above for airfoil data.)

• There are occasionally exceptions, but for the examples below, it's safe to assume that symmetrical airfoils (i.e. Eppler 168 or NACA 0009) will be used for all tail surfaces.

• Use symmetrical airfoils with a thickness of 10% to 15% for most aerobatic planes. (i.e., an Eppler 168 or NACA 0012)

• Use a semi-symmetrical airfoil for more "scale" performance, including higher potential lift. (i.e., an Eppler 197 or Selig 8036)

• Use a thinner airfoil with more camber, for slower flight, such as a glider. (i.e. Eppler 193)

• Use a reflexed airfoil for a tailless design, such as flying wing or delta type. (i.e. Eppler 184)

• Canard aircraft have a number of variables that deserve a long explanation, but at the very least, make sure that the forward wing (canard) stalls before the main wing. This can be controlled in a number of ways. (more on that later)

• Try and keep the chord (width) of your airfoils at 5 inches or larger, for the best performance. There seems to be a fairly definite "break" in airfoil performance at that size. Below 5 inches, (in a typical modeling environment) the airfoil shape seems to matter very little. (Notice that in kits with tail surfaces this size, most are simple sheet balsa.)

• All things being equal, you get more lift per degree of angle of attack with a high aspect ratio (long & skinny) wing than a low aspect ratio (short & fat) wing.

• Properly designed high lift devices, such as flaps, slats, etc., are perfectly useable on model aircraft, but must be built lightly to reap the benefits they provide. See the High Lift page for more information.

Typical planforms

Obviously, there's no way for me to describe exactly how to create the ultimate design for every type of aircraft, in this little article. So, take these general statistics as a guidline, not as literal parameters. The goal here is simply to show how changing some design parameters affects performance. To keep the chart simpler, I've eliminated models at the two extremes of the spectrum, such as indoor free flight models, and huge, giant scale models. A 60-inch wingspan is the criteria to compare all these.

* "cid" in the "Power Loading" column stands for "cubic inches displacement". (a .40 engine is .40 cubic inches displacement) The idea of using "Power Loading" as a design criteria is mentioned in Andy Lennon's book, "The Basics of RC Model Aircraft Design". See the High Lift Page for info on how to obtain the book.

How to design around an engine you already have

This is an ultra-simple method that will give you an immediate idea of how you have to design, for a desired performance. First, see the chart above for some examples, then select your power loading.

Once you've decided on the performance you want, use examples like the chart above, along with your drawings, to determine how large, what shape, and what materials should be used in your design. (Remember to include the weight of the engine!) See Accurately Predicting Structural Weights for additional information.

How to determine basic performance specifications

The chart above shows the simplest form of comparison of several design types. When you want to get really specific, I suggest making some simple sketches, along with some notes about the flying qualities you want, then use the following method to check whether your design goals can be achieved. You can then edit them as required, to improve it's performance.

Using a spreadsheet, or piece of graph paper, make a chart something like the one above, but with more detailed design parameters. Make a chart for each type of plane you're interested in. (Sport, Pattern etc.) Include all the details you can think of, such as what airfoils were used, the root and tip chords of every flying surface, the power used, internal components, such as radio and battery, and even the kind of landing gear. Then, using whatever sources you like, including magazine articles, videotape, online catalogs, and so on, start collecting data for all the planes whose performance matches what you're looking for. Right away, you'll start to notice that among these "types", almost all the parameters for winning airplanes are very similar.

You can take the columns of data you have, and average them, which will give you a set of parameters that is very close. When you actually draw plans and build your design, you'll just be changing details. The comparing that you do may also give you hints as to how to improve your design.

Once you've been collecting information on different designs for a while, you'll be able to choose other criteria first. For example, if you have a need to design a model of a specific size, you might select wing span, or wing area as your first parameter. It's not that complicated, and after you've built a number of models, you'll know most of these things instinctively.

Testing and refining your design. (Trim, Thrust line, and Airfoil Incidence changes)

If you have a plane that has handling qualities you don't like, you may be able to improve them with some minor changes. The first thing to do is to go through the typical Pattern-style trimming process, to locate and isolate the problem. Thanks to the NSRCA, (National Society of Remote Controlled Aerobatics) for providing this very practical trim chart.

Test for	Procedure	Results	Adjustments
Control Neutrals	Test response to each control	Adjust trims for straight & level flight	Adjust clevises to center xmter trims
Control Throws	Apply full deflection of each control	Check for response; Aileron hi rate 3 rolls in 3 secs. Elevator, square loop corners Rudder, 35 to 40 Deg.	Change control horns, ATV, and Duel Rates as required
Center of Gravity Method 1 Method 2	1. Roll into a vertically banked turn 2. Roll into inverted flight	1. A. Nose Drops 1. B. Tail Drops 2. A. lot of down required to hold level flight 2. B. up elevator needed to hold level flight	A. Add tail weight B. Add Nose weight (see Note A at bottom)
Up/ Down Thrust, test 1	Fly model straight & level, then cut throttle Note Either change B or C requires retest of Decalage and Verticals	A. Model continues level flight with a gradual drop B.Model abruptly dives C. Model abruptly climbs	A. No Change B. Increase down thrust C. Reduce down thrust
Up/Down Thrust, test 2	Fly model straight & level, then pull up Note Either change B or C requires retest of Decalage and Verticals	A.Model continues straight up B.Model pulls to canopy C.Model pulls to belly	A. No Adjustment B. Increase down thrust C. Reduce down thrust
Decalage, Angle of Incidence	Power off vertical dive from high altitude (neutralize elevator) (see Note B at bottom)	A. Model continues straight down B. Model pulls to canopy C. Model pulls to belly	A. No change needed B. Increase wing or stab incidence C. Reduce wing or stab incidence
Knife Edge Pitch	Fly model on normal pass, roll to knife edge, left and right, use rudder to hold model level	A. Model does not change pitch B. Model pitches to canopy C. Model pitches to belly	A. No adjustment needed B. Either move CG aft; or increase wing incidence; or mix down elevator with rudder C. Reverse of B;
Tip Weight - Test1	Fly straight; level, roll inverted, release aileron stick	A. Model does not drop a wing B. Left wing drops C. Right wing drops	A. No adjustment B. Add weight to right tip C. Add weight to left tip
Tip Weight - Test 2	Fly model towards you / away from you, pull tight inside loop, repeat with outside loop	A. Model comes out with wings level B.Model comes out with right wing low C. Model comes out with left wing low	A. No adjustment B. Add weight to left tip C. Add weight to right tip
Side Thrust	Fly model away from you and pull up to vertical	A. Model continues straight up B. Model veers left C. Model veers right	A. No Adjustment B. Increase Right thrust C. Reduce Right thrust
Aileron Differential	Fly model toward you, pull into a vertical climb before it reaches you. Neutralize controls then half roll .	A. No Heading Changes B. Heading change opposite to roll command C. Heading change in direction of roll command	A. Differential settings OK B. Increase differential C. Decrease differential
Dihedral	Fly model on normal pass, roll to knife edge, left and right, use rudder to hold model level	A. Model does not roll B. Model rolls indirection of rudder C. Model rolls opposite to rudder	A. Dihedral OK B. Reduce dihedral C. Increase dihedral

Note A:These two methods for determining the C.G. of a model will give approximate results only. Start out with the C.G. where the Designer suggested, or somewhere between 25% to 35% of the Mean Aerodynamic Cord. The optimum C.G. for your model will require further testing while performing maneuvers. The results will only be an approximation at best. Note B:This portion of the trimming chart may be unclear for the following reason; In order to maintain level upright flight, the wing of a plane with a symmetrical airfoil wing needs to have a positive Angle of Attack (AOA, usually less than 1 degree). This positive angle provides the lift required to cause the plane to fly level. If the plane is balanced slightly to the nose heavy side (required for pitch stability), it will require a slight up elevator trim to hold level flight. A plane with a zero/ zero wing to elevator angle will also need a slight amount of up elevator trim to hold level flight. Therefore, a plane trimmed in this manner will have a tendency to pull to the canopy on a straight, thumbs off, down line because the elevator is controlling the AOA of the wing. This positive AOA may also be achieved by a positive incidence change, which requires an offsetting down elevator for level flight. Thus, a power-off down line should fall straight down, with neutral controls. There are significant interactions between wing incidence changes and CG, therefore it is most important that the C.G. of the airplane be established first. In the final analysis, flight trimming an airplane is a personal preference issue after you have taken care of the basic essentials.