Keep thy airspeed up, lest the earth come from below and smite
- William Kershner
12.1 Trimming for Flight
When modelers discuss their new creation they like to be able to
say,"It flew right off the drawing board."
Given the variables that are introduced by dimensional tolerances,
minor warps and weight distribution problems the chances of a perfect
first flight are slim.
The following ten steps for flight trimming a new rubber model are
taken from the Internet:
- Locate the CG
- Balance model without prop and motor to located CG
- Glide model
- Adjust for smooth glide
- Check stability by launching into slight dive and slight climb
- Readjust CG for stability if necessary
- Mark location of new CG (if changed in step 6)
- Install prop/motor and rebalance to established CG
- Test fly under power
- Adjust flight pattern with thrust line
Although the above steps were written for a rubber-powered model the
idea of separating the center of gravity adjustment and the thrust
line adjustment (for powered flight) applies to most model aircraft.
It is always best to make one trimming adjustment at a time. If two
or more adjustments are made at the same time, you won't know which
caused an improvement or poorer performance.
You will find each of the ten steps explained in much more detail at:
www.mindspring.com/~thayer5/modelhp.html and click modeling
tips and click 10 step.
12.2 Aerodynamics for Pilots
There is a valuable reference on the Internet entitled, See How it
Flies. The author makes the point that there is a difference between
aerodynamics books for engineers and aerodynamics books for pilots.
See How it Flies is written for pilots. Chapter 3 of that book
deals with airfoils. The book is on a Web site. See:
On page 14 of 29 we see that it is a misconception to believe that
wings must be curved on the top and flat on the bottom. Modelers
have often followed the practice of early full-scale designers in
making the bottom surface of the wing concave. It's called
undercamber. We can still have a cambered wing airfoil with both
upper and lower surfaces convex. Let's stop here for some
When viewing a wing cross section we see an airfoil. If we draw a
straight line from the leading edge of the airfoil to the trailing
edge, we have a chord line. If we draw a curved line from the
leading edge to the trailing edge, staying always halfway between the
upper surface and the lower surface, this is called the mean camber
line. The maximum difference between this and the chord line is the
amount of camber.
If the airfoil is symmetrical, where the top surface is a mirror
image of the bottom surface, it has zero camber.
On page 15 of 29 we see that under ordinary conditions the amount of
lift produced by a wing depends on the angle of attack, but hardly
depends at all on the amount of camber. On page 16 of 29 we see that
even a thin flat "airfoil' shape will produce lift, if the wind
strikes it at an appropriate angle of attack. But, we learned that
when flying the flat wing AMA Cub. (See 6.1) In a related Web site
NASA provides simplified definitions of a wing's geometry at:
12.3 History of Airfoils
It is interesting to note the variations in historical conventional
wisdom regarding airfoils.
In 1889 Otto Lilienthal published his findings on bird flight as the
basis of aviation. [Lilienthal, Otto. Bird Flight as the Basis of
Aviation. Hummelstown, PA: Markowski International Publishers, 2001.
(Unabridged facsimile of the original work)]
On page 45 we see diagrams of airflow over a flat plate at various
angles of attack. On page 53 we find that experimental results
showed that a "very slightly curved surface (analogous to the bird's
wing) possessed pre-eminently those qualities which we considered
essential for economy in flight." And on page 54: "Since it is
probable that the whole secret of flight is to be found in the
properties of such slightly curved surfaces, we must investigate them
more minutely later on."
If we fast-forward to a book about the Lockheed "Skunk Works" we find
the designers of the Stealth Fighter being required to design wing
shapes consisting of flat panels, needed to deflect radar. [Rich, Ben
R. and Leo Janos. Skunk Works, A Personal Memoir of My Years at
Lockheed. Boston: Little, Brown, 1994]. On page 30 Dick Cantrell,
Head of the Aerodynamics Group, was required to toss aside basics of
fundamental aerodynamics, "in deference to new technology understood
only by witches and mathematical gnomes." The airplane "would be so
deficient in lift-drag ratio that it would probably need a computer
the size of Delaware to get it stable and keep it flying." It did
fly, computer aided, one not the size of Delaware.
It is worthwhile to gain some understanding of how airfoils work
because they are used by wings, tails and propellers to move air. In
fact, they are used in compressors, turbines and other components
inside modern turbojet engines.
12.4 Related Web Sites