Model Aircraft Design Step-By-Step

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.

There are three things you should get from this discussion if nothing else:

• Compromise is a recurring theme in model aircraft design.
• A model aircraft designed to excel in every flight category won't.  It simply isn't possible.
• Before you do anything else you should work out specific goals and a specification for your design.

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.

With Each Step — Establish Engineering and Construction Methods

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.

Challenge your building skills but stay within the realm of what you can accomplish

You may have an idea for a beautiful blended-body design, but can you actually build it?  Maybe you can't now but as your skills increase it might be very possible in the future.

Step 1 — Create a Specification

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.

Step 3 — Establish Vertical Performance and Airspeed Envelope

Rate of climb is determined by the powerplant, propeller and the aircraft's ready-to-fly weight.

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.

Airspeed (minimum and maximum) is determined by the engine, propeller and wing.

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.

¨ Affects characteristic

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.

Wing Loading

How to Calculate

Wing loading is a goal you should design and build to — not a surprising discovery at the end of construction.

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

How to Calculate

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.

For this example we're building a model to weigh 7 lbs with a wing loading of 20 oz./ft².

Plug those numbers into the wing loading equation to find the wing area:

Given:

Wing Loading = 20 oz./ft²
Target Weight = 7 lbs

Find the Wing Area:

Wing Loading = (Weight x 2304) ÷ Wing Area
(Note that Weight is in pounds)

Rearrange the equation to find the wing area:

Wing Area = (Weight x 2304) ÷ Wing Loading

Plug in given parameters:

Wing Area = (7 x 2304) ÷ 20

Wing Area = 806.4 in²

Now we know what to do — build a 7 lb airplane having about 800 square inches of wing.  When building a kit you have to hope and pray that the design and included materials make it possible.

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.

You should easily be able to stay at or under the target weight of your design because it's your engineering and you select the materials.

Aspect Ratio

How to Calculate

In the chart above you can see that aspect ratio strongly affects nearly every flight characteristic.  It is one of the most important decisions you will make so consider it well.

The aspect ratio of the wing affects several areas of flight such as roll rate, lift-to-drag ratio and pitch sensitivity.

Not the least important flight characteristic strongly affected by the aspect ratio is the wing's propensity to tip stall — particularly in banked turns.  Wermacht explains in the sidebar.

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.

For example, short, stubby wings are not even considered an option for these types:

• Sailplanes

  Go a long way using very little fuel.

• Airlines

  Money spent on fuel is less money in the bank.

• Voyager by Burt Rutan.

  It's sole purpose was to fly non-stop around the world without refueling and then retire.

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.

Sounds like we need to compromise again.

Wing Taper

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.

The way to build a wing easily while approaching the efficiency of an elliptical wing is to build a tapered wing.

The Taper Ratio of a wing is simply the Tip chord divided by the Root chord.  High aspect ratio wings with low taper ratios (tip chord much less than root chord) are extremely prone to tip stalls so it is best to avoid using both on the same wing.

If you want a highly tapered wing then keep the aspect ratio down.  If you want a high aspect ratio wing then keep the taper ratio closer to 1 (same root and tip chord).

Knowing the taper ratio, aspect ratio and wing area allows you to calculate the root and tip chords assuming the wing does not have multiple tapers.

Wing Sweep

I am told that sweeping a wing rearward is equivalent to adding dihedral (each 2-1/2° of sweep is equivalent to approximately 1° of dihedral).  Presumably it works even when the aircraft is inverted.  Sweep also makes an aircraft more stable because it causes the aircraft to pitch down in a stall.

Sweep somewhat broadens the CG range of an aircraft as well as moving the CG rearward.  Lastly, sweep makes the aircraft appear sleeker.

I have never built a forward swept wing and don't expect I will ever design a model that uses one.  You'll have to find another source of information if you want to build a forward swept wing.

Dihedral

Dihedral has two distinct aerodynamic purposes that immediately come to mind:

• Increased stability
• Allows an aircraft to be steered with the rudder alone (no ailerons)

As a corollary to the second bullet, dihedral can also add undesired control coupling.  Control coupling occurs when one control causes the aircraft rotate about a different axis than intended such as pitching or rolling when rudder is applied.  Aerobatic ships in particular should have as little coupling as possible.  Often the dihedral needs to be adjusted to remove roll coupling caused by the rudder.

Unfortunately there is no way to know in advance how much dihedral will be necessary so we make our best guess based on previous experience.  As Don Lowe will tell you, if you want your ship to be tuned for competition you need to be willing to cut the wing apart to adjust the dihedral.

I have no interest in competition or precision aerobatics and I don't cut my wings apart, but you might need to.

I've found that approximately 5° of dihedral works very well for a rudder-only model.  Too little dihedral will make turns sluggish.  Too much dihedral will make the wing inefficient.

Washout

Washout is a deliberate warp built into the wing so that the wing tips fly at a lower angle of attack than the wing root.  The purpose is to delay or prevent tip stalls.

I have never found washout to be necessary for sport models — especially constant chord wings which by their nature are very resistant to tip stalls.

Washout on an aerobatic ship is a bad move because these types of models should fly as neutrally as possible.  An aircraft having washout will have wash-in when the model is flying inverted.

Cases where washout will be beneficial are aircraft such as those with high aspect ratio wings (sailplanes, airliners), heavy scale models, utility aircraft (camera platforms) and other models not intended to perform precision aerobatics.

Aileron Style and Area

Strip ailerons are easier to build and in my experience have a better roll rate than barn door ailerons.  Tapered strip ailerons tend to work best with the least possibility of flutter.

Aileron area is usually 10% - 20% for strip ailerons and up to 25% for barn doors.  Again, it depends on what you want the plane to do.  Don't do anything too radical for your first designs.  Once you have some time on the prototype you can adjust things on the second prototype.

Fill in the blanks

Wing Loading	oz./ft2
Wing Area	in2
Aspect Ratio	:1
Taper Ratio
Root Chord
Tip Chord
Wing Sweep
Dihedral	°
Stall performance (description)
Washout
Airfoil (root)
Airfoil (tip)

Step 5 — Determine Pitch and Yaw Rates

The fuselage is nothing more than a lever to mount flying surfaces, the powerplant and various systems.

As far as I'm concerned the only consideration for the length of the fore or nose moment is so that the aircraft will balance as closely as possible without adding ballast.

The length of the pitch and roll moments determines the rate at which the aircraft rotates about these axis.  The moment itself does not make a model more or less aerobatic, but determines the rate and size of the maneuver.

Longer moments make for larger, smoother aerobatics than a shorter moment.

Most people will tell you that the combined area of the fixed stabilizer and elevator should be about 20% of the wing area.  You can't go wrong with that number but it is a very conservative and safe area that doesn't always make the most efficient setup.

The vertical stabilizer, including rudder, are usually about 8% - 15% of the wing area.  As with the horizontal stabilizer, the size can decrease as the moment increases and vice-versa.

A good starting point for ratio of moveable surface to fixed surface is that 10-40% of the overall surface should move.  You can adjust that however you like.  3D aircraft often have moving surfaces that are larger than the fixed surface as well as extreme control throws.

Even though I don't like doing it, I will cut off tail surfaces and replace them with new ones to improve a design.  Many people consider a design successful just because it flies, but frankly that's not much of a feat.  If you want your designs to be excellent then consider the prototype to be a test bed and be willing to cut it up.

Step 6 — Lay it Out on Paper

At this point we don't care about the outlines of the design.  We can make it pretty later.

Start by drawing a reference line on your paper.  Locate the wing as desired.  For a more stable design, move the wing higher.  For a neutrally stable design place the wing on the reference line (as well as the stabilizer and engine).

Determine the horizontal stabilizer location and draw it.

At this point I start sketching outlines.  If you want your design to achieve high speeds then the fuselage should be aerodynamically clean and have as little frontal area as possible.  Fillets help prevent turbulent, high drag airflow.

If you want the fuselage to support knife edge flight then it should have plenty of side area with a large percentage of that area toward the front of the aircraft.

If you want to prevent speed build up then the best way is to design a thicker wing.  You can also increase the drag of the fuselage by designing more frontal area or adding a blunt wind screen and cowl rather than a streamlined cowl.  These items create turbulent drag which is why they are not as good of a choice as building thicker flying surfaces.

I honestly have no way to determine in advance how long the nose moment should be.  I normally flesh out the design from the wing to the tail and then visualize it in 3D in the bare construction.

From there I locate the engine where it looks like it will balance the model.  I'm usually pretty close although my models tend to come in nose heavy.  Another consideration is that I try to make the fuel tank fit entirely in its compartment rather than allowing it to extend into the radio compartment.

Step 7 — Build It

Then fly it a lot.  Learn what you can, make practical adjustments and then build another.  When the aircraft flies as you originally specified then it's ready for prime time.

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Copyright © 2005 Paul K. Johnson