Aero-Tips!

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 together.

Aero-Tips 05.13.06

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 reserve.

FMI: Aero-Tips