My Thumb's Rule of Airfoil Selection
Choosing an appropriate airfoil family for any given design is usually
simple. If the plane is to be a precision aerobat then a symmetrical
airfoil is most appropriate because it flies the same in any given attitude.
If the plane is to fly slowly or carry a load but is not intended to do
aerobatics then a flat-bottom or under-cambered airfoil should be considered.
By the way, when I say flat-bottom I don't mean a true flat-bottom
airfoil. Some airfoils are called "modified flat bottom." This
is an airfoil having a straight line from the main spar to the trailing edge
but curves up to the leading edge from the spar.
A modified flat-bottom airfoil is actually a semi-symmetrical airfoil, but most modelers consider it to be a flat bottom airfoil because
most of the underside isn't curved. I'm right, they're wrong, but if
you say semi-symmetrical then they'll picture something other than what
you're talking about.
Note: Any airfoil that is not
symmetrical is a cambered airfoil.
The terms "flat-bottom" and "semi-symmetrical" are not used by the
aerospace industry and they probably laugh at us when they hear us use those
terms. Nevertheless, this article is for us, not them, so I will
continue using these incorrect pretend names so you aren't shunned by your
fellow modelers who don't like it when you talk too fancy.
- Use for aerobatic airplanes - particularly monoplanes.
A design intended to be aerobatic should always have symmetrical flight
surfaces (wing, horizontal stabilizer and vertical stabilizer). Flat
surfaces (which are symmetrical sections) work well for tail surfaces to a
point but aren't as good as a true airfoiled section.
- Use for
secondary trainers, sailplanes and sport aerobatic biplanes. If
the biplane is intended to do precision aerobatics then a fully
symmetrical airfoil should be used.
Secondary trainer manufacturers make a big deal out of semi-symmetrical airfoils but
they are over-rated. If a beginner moves up too quickly and hasn't
mastered his primary trainer yet then a secondary trainer with this type
of airfoil is probably the best bad choice. Otherwise, a
lightweight, well behaved model with a symmetrical airfoil makes a good
Sailplanes often use highly refined and tested airfoils that
provide the best lift to drag so that they can scoot
across the sky quickly in their search for thermals and then climb easily
in the lift.
Sailplane designers tend to take a lot of care in their airfoil
selection. They have to because the airfoil is the only thing making
their plane fly. They don't have an engine to fall back on.
- Don't use true flat-bottom airfoils for anything.
- So called "modified" flat bottom airfoils are excellent for slow,
True flat-bottom airfoils are a poor choice for any design. They
are next to impossible to trim properly because they are extremely speed
sensitive. It may be possible to trim this trait out, but it means
spending hours tweaking the wing incidence, decalage and engine thrust.
I've never flown a model with a flat-bottom airfoil that could even
come close to being trimmed as it was built. I don't particularly
enjoy cutting the tail off my planes numerous times attempting to get it
The rest of this discussion refers to modified flat bottom airfoils.
Flat bottom airfoils are used for powered aircraft that are willing to
make the compromise of having more drag in exchange for slow flight or
high lift capabilities. They do not penetrate the air well but can
stay aloft at very low speeds. I have built a handful of models
having flat bottom airfoils that can hover right in front of me because
the aircraft's minimum flight speed was below the wind speed.
For example, if the model can fly at 10 MPH and the wind is blowing 15
MPH then the model can fly backward (relative to the ground) at 5 MPH.
As far as the air is concerned (which is the only thing the airplane cares
about) the aircraft is flying forward at 10 MPH.
An aircraft that is identical except for having a symmetrical airfoil
will have a higher minimum flight speed.
- Use for scale models, sailplanes and some high-lift situations.
I don't know much about under-cambered airfoils. They are mostly
used for scale aircraft from the dawn of aviation. They tend to have
high lift and are sometimes used in free flight models and some very small
radio control aircraft.
A reflexed airfoil has a trailing edge
that is turned up slightly. The airfoil shown above is extremely
exaggerated to get the point across. I haven't tested it but it
probably has five times the amount of reflex it needs. If you print
the image and scale it to your design, then don't hand launch the model
— it will probably loop, hit you in the
back of the head and kill you instantly.
Most airfoils have a natural tendency to pitch forward. If you
were to push a wing forward or just drop it, it would rotate or tumble forward
all the way to the ground.
The horizontal stabilizer prevents an aircraft from doing the same
thing. Flying wings don't have a stabilizer so the wing must be
self-stabilizing. The reflex provides this stabilization.
Note that a true reflexed airfoil isn't necessary and often isn't used
with flying wing model aircraft. A lot of designers fake it by
adjusting the elevons so they are slightly up.
Thwing! and my
both use faked "reflex" and fly very well.