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
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
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.
Wing loading is a goal you should
design and build
— it should not be 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.
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:
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
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.
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.
The stalled tip falls from lack of lift but lift from the wing that's still
flying flips the plane over. The aircraft may enter a spin from which
it may not be able to recover.
This slick maneuver is an especially entertaining way for someone else's
plane to crash.
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
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
Go a long way using very little fuel.
Money spent on fuel is less money in the bank.
Voyager by Burt Rutan.
Its sole purpose was to fly non-stop around the world without refueling and
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.
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
The Taper Ratio of a wing is simply the Tip chord divided by the
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
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 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 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
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
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
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