A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.") It's part of what makes aviation
so exciting for all of us... just when you think you've seen it
all, along comes a scenario you've never imagined.
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators, and as
representatives of the flying community. Some of them, you may have
heard before... but for each of us, there will also be something we
might never have considered before, or something that didn't
"stick" the way it should have the first time we memorized it for
the practical test.
It is our unabashed goal that "Aero-Tips" will help our readers
become better, safer pilots -- as well as introducing our
ground-bound readers to the concepts and principles that keep those
strange aluminum-and-composite contraptions in the air... and allow
them to soar magnificently through it.
Look for our daily Aero-Tips segments, coming each day to you
through the Aero-News Network. Suggestions for future Aero-Tips are
always welcome, as are additions or discussion of each day's tips.
Remember... when it comes to being better pilots, we're all in this
Running out of gas is one of the leading causes of engine
failure...and it's often fatal. Thursday and Friday we discussed
considerations that go beyond the guidance in FAA Advisory Circular
61-23C, the Pilot's Handbook of Aeronautical
Knowledge. Let's look at another factor in fuel
burn... propeller pitch and rpm.
What 61-23C says
The rate of fuel consumption depends on many factors...
[including] propeller pitch [and] propeller RPM...
How can propeller
settings affect fuel consumption? Think about propeller settings in
three ways: how they affect airspeed, how they contribute to
airframe drag, and how they set the speed of combustion events.
Props and speed
Propellers turn engine power into thrust by generating "lift"
with airfoil-shaped blades. High propeller speeds -- a function of
prop pitch with controllable propellers -- logically generate more
lift, er, thrust because the blades are moving through the air (in
the direction of rotation) more rapidly. So it seems a higher prop
speed would propel the airplane forward faster for a given fuel
burn. With prop speed, though, there are tradeoffs…
What a drag
The faster a propeller is spinning, the flatter it presents to
the relative wind. If you've ever seen a multiengine airplane's
propeller feathered, twisted "skinny end" forward, consider high
rpm to be "feathered in reverse". At some point the resulting drag
is greater than any additional thrust created at the higher rpm.
Consequently, the engine/propeller combination becomes more
efficient (in terms of fuel to cover a distance) at lower propeller
speeds…you might take a little longer to get there, but
you'll do so using less gas.
Burn, baby burn
With a direct-drive (i.e., not reduction gear-driven) propeller,
the speed of combustion is directly affected by propeller speed.
Fewer revolutions per minute (or hour) mean fewer combustion events
in the same time. Fewer combustion events means less fuel burned.
If the airplane is almost as fast at lower prop speeds because it
is aerodynamically more efficient (despite the power reduction),
then you may arrive slightly later but with a dramatically reduced
fuel burn by cruising at a lower rpm.
Note: Observe all engine/propeller
Limitations and engine temperature limits when experimenting with
different propeller settings.
You see it in cars that have instantaneous Miles per Gallon
displays -- keep the tachometer speed down and fuel efficiency goes
up. The same holds true for airplanes.
Aero-tip of the day: Handbook cruise fuel flows
are dependent on a specified propeller speed from the Performance
charts. Any change in prop settings will affect fuel burn one way
or another—another factor in arriving with a healthy fuel