Flight Training - July 2021 - 12

PREFLIGHT WHAT AM I?
BY WILLIAM E. DUBOIS
PROP TALK
Spiraling slipstream (aka propwash):
A twisting column of air thrown off
the propeller that at low airspeeds
and high power can strike the tail and
yaw the airplane to the left.
Gyroscopic precession: The prop is
an excellent gyroscope, and when
you push on a gyro, it deflects 90
degrees ahead in the direction of
rotation. Taking off in a taildragger
can trigger a left yaw motion when
the tail is raised.
P-factor: At high angles of attack the
downward-turning propeller blade
has a higher angle of attack and
produces more thrust, and that tends
to push the nose to the left (assuming
the engine turns the prop in the
standard direction).
Torque: Blame Sir Isaac. His Third
Law commands the airplane to move
in the opposite direction-with the
same force-as your engine and
prop, rolling the airplane on the
longitudinal axis.
BLADE RUNNER
THERE'S MORE TO PROPELLERS THAN MEETS THE EYE
IT'S RIGHT IN FRONT OF YOUR NOSE.
Well, sometimes above your head. Or
to your left and right. It might be made
of metal. It might be made of space-age
composite. For purists, it's still made of
wood. But you rarely look at it or think
about it unless it stops moving, which
isn't really its fault. If it stops moving, it's
the engine's fault.
Yep, it's your propeller.
Aviation propellers have advanced
from hand-carved mahogany to metal-clad
laminated hardwoods, to
aluminum, to traditional composite,
and finally to resin transfer carbon fiber.
They commonly feature two, three, four,
five, six, and even eight blades; but the
Antonov An-70 is the propeller superstar
with 14 blades on each of its four engines.
Among pilots, the preferred number
of blades spurs passionate debates. The
more blades you have for a given engine,
the smaller they can be in diameter,
which reduces the tip velocity and thus
propeller noise. But increasing the number
of blades reduces overall efficiency.
Any combination of airframe, engine, and
prop arrangement is a compromise.
12
FLIGHT TRAINING JULY 2021
Like the number of blades, the shape
of propellers sparks lively conversation in
hangars when the ceiling is too low to fly.
Propeller tips come in pointy, rounded, and
square. What's the difference? Generally,
the sharper the tip, the faster the airplane's
top speed, but the less lifting thrust you'll
get; while the squarer the tip, the better
the climb, but at the cost of speed.
The choice of propeller of-number
of blades, tip shape, size-can have a
dramatic impact on the performance of an
airplane. For instance, Hartzell's Scimitar
Top Prop three-blade conversion kit for
the Cessna 182 family boosts climb performance
over the factory prop, increases
cruise speed, and is quieter. It's sexy on
the ramp, too. But what most pilots don't
realize is that there's nothing new about
the asymmetrically swept scimitar blade.
It was the most common propeller shape
in the early 1900s.
Speaking of the early days, airplanes
weren't the first flying machines to use
propellers: After trying to steer hot air
balloons using rowboat oars, hand-spun
props were introduced in 1784. These
first props were designed more like boat
propellers and didn't work all that well.
It was the Wright brothers who first
developed the correct shape of propeller
to move an aircraft through the air, essentially
a perpendicular spinning wing.
In fact, a prop really is a wing. The
spinning airfoil creates a low-pressure
area at the face of the prop and a
high-pressure area at the back of the
airfoil, generating thrust to draw (or push
in the case of a rear-mounted engine) the
airplane forward. The Wright brothers'
props were heavily twisted, but even
today's propellers maintain a twist, having
a bigger bite at the hub and flattened tips.
Why the twist? The blade tips rotate
faster than the hub, as they have farther to
travel as the prop spins. To avoid a stall-
being airfoils, props are subject to all the
same aerodynamic factors that your wings
are-and to maintain uniform " lift, " a constant
angle of attack along the span of the
blade is needed. This is accomplished by
twisting the blade from the hub to the tip.
WILLIAM E. DUBOIS is an aviation writer who
blogs his personal flying adventures at
PlaneTales.net.
CHRIS ROSE
http://www.PlaneTales.net
Flight Training - July 2021

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