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A good basic trainer has
several design features to ease this learning process. Most important is
that the wing usually has a flat bottom and a curved, or airfoil-shaped,
top section.
As far as I know, only one basic trainer—Hobbico's Hobbistar 60—uses a
semisymmetrical airfoil, and its wing is made overly large to
compensate. Without delving into total eye-numbing detail about why an
aircraft flies, this type of wing produces more lift than a similar
symmetrical wing with equal airfoil shapes on both sides.
The air flowing over the curved top section of a flat-bottom wing must
move faster to cover the longer, curved distance than the air flowing
over the straight bottom. According to Daniel Bernoulli's theorem of
gaseous density, the faster a given amount of gas moves, the less dense
it must become if all other conditions remain the same.
If the air above the wing has reduced density, the pressure it exerts on
the wing's top is lower. This means that there is a "low-pressure" area
above the wing. The wing tends to move upward into the low-pressure
area, taking the rest of the aircraft with it.
But then how does an aircraft with a fully symmetrical wing fly? Daniel
Bernoulli is on vacation where this wing is concerned, but, fortunately
for all sport and Aerobatics pilots, Sir Isaac Newton remains in the
house.
If the wing is pointed upward to the airflow even a few degrees—called a
positive angle of attack—much of the air striking the wing's LE is
redirected downward. But Sir Newton demands that for every action (here
the redirection downward) there is an equal and opposite reaction.
Therefore, the wing is "pushed" upward, creating lift.
This is all a gross oversimplification of why an aircraft flies. There
is still much debate about this subject even after 102 years. However,
this basic explanation serves to illustrate why a flat-bottom wing has
more lift per square foot than a symmetrical airfoil.
Of course, Newton's law also affects the flat-bottom airfoil--maybe even
to a greater degree than it does a fully symmetrical wing. But Bernoulli
and Newton work together on a flat-bottom wing, generating that extra
lift.
Extra lift means slower takeoff and landing speeds, lower stall speeds
(the speed at which the wing stops producing lift and begins resembling
an anvil), fewer bad habits such as snap stalls in tight turns, and
generally an all-round more rewarding learning experience.
It helps if the wing is placed on top of the fuselage (a
high-wing configuration) and has some positive dihedral; the wing is
bent in the middle so that the wingtips are higher than the
center-section. The fuselage below the high wing imparts a slight
pendulum effect—but every bit counts in a good trainer. The dihedral
reinforces this pendulum effect.
Click on photo to view large image with caption
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