This month’s question comes from a long time
member and glider guider Bob Film. This type
of thing is near and dear to my heart!
Since there are 100’s of airfoils, and each one is
usually efficient at an air speed, I would assume
that the thicker the airfoil & under camber airfoils,
would be for slower air speed, so is there a simpler
formula to determine each one’s angle of
incidence (not dihedral). So, for instance, if you
put different wings on a plane; how would you set
its incidence for max performance. I assume that
angle is always in reference to elevator being
zero degrees.
Since I am a sailplane “NUT”, this becomes more
critical.
Thanks – Bob Film
Thanks for the question Bob. You have hit the
nail on the head here. Airfoil selection and
setup (incidence, planform, washout ect.) is
the heart of how the plane flies. A simple
incidence change sometimes can turn a dog
into a thoroughbred. Unfortunately there is no
simple formula to figure this out, but there are
some guidelines that I like to use. There is
much, much more, that can be added to this
discussion, so I’ll try to keep it in the context of
your question. Please feel free to contact me
for further discussion if you wish.
Up front I’d like to state that I generally am not
a big fan of using incidence and/or decalage
to trim an airplane. These are compromises to
make a plane behave a certain way at a
certain airspeed, and my feeling is airfoil
selection plays a big part here and if done
correctly, you won’t need the incidence bandaid.
More often than not, my designs will be
setup with zero incidence. They certainly have
their place, Trainers and Old Timers for
instance, but since you asked about max
performance, I’m going to assume you equate
that to efficiency. It’s all about trim drag. If you
need up or down elevator trim at certain
airspeeds, it means your incidence, CG or
both are wrong and there is unnecessary drag
from trim detracting from performance.
Climbing under full power is a classic
symptom of this.
First, you must define the purpose of your
airfoil selection and its use. An Old Timer type
aircraft with an undercambered section will
likely be flown more slowly and benefit from
some positive incidence. All airfoils have a
natural pitch down tendency, but UC sections
react more than most. This positive incidence
coupled with a necessary more forward CG
makes for a very sedate flying aircraft.
For a high performance TD (Thermal
Duration) sailplane, Racer or even an
aerobatic plane you’ll find that incidence
needs to be at or a fraction of a degree of
within zero.
So how do you measure it? You are correct
that on a sailplane the reference line, “0”
degree or datum is often the elevator, and the
wing is measured in degrees positive from
there. Power planes are a different story, with
their thrust angle, positive incidence, decalage
(tail incidence), they often use a line through
the center of the fuselage for reference.
For most airfoils you can find a published set
of ordinates that will include useful information
such as chord line, camber line, zero lift line
and other fun things like lift vs. drag polars
and laminar bucket information. In most
instances you can use the chord line to
measure from which is often simply a line from
the leading edge to the trailing edge. For
highly cambered sections, the chord and
camber line can actually exit the bottom of the
airfoil making measurement a bit trickier.
There are many online and printed references
out there. For printed references I like Airfoils
at Low Speeds (Soartech series) available
from Carstens Publications SoarTech, and online there is the
UIUC’s webpage from Professor Michael
Selig. With this reference material it will tell
you specifically at what angle of incidence it
will likely perform best.
Bottom line for me is I will set up a plane with
the least amount of incidence possible (often
zero) and move the CG aft incrementally until
the trim change with speed goes away and I
still have acceptable stability.
Email Red at
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with your comments and suggestions.
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