AOPA Pilot Magazine - August 2022 - 87

RUDDER & WRENCH
Throttle and prop
It would be easy to conclude that achieving
optimum cruise is simply a matter
of throttling back to Carson speed. But
our piston aircraft engines convert fuel
into power most efficiently at wide-open
throttle (WOT). Pulling the throttle back
chokes the engine's air supply, reducing its
volumetric efficiency. The engine wastes
energy pumping air past the restriction
created by closing the throttle. These
" pumping losses " are minimized at WOT.
For best efficiency, we always want to
operate the engine at WOT-or as close to
WOT as possible without exceeding the
engine's operating limitations. Normally
aspirated and turbonormalized engines
usually can (and generally should) be operated
continuously WOT in cruise. Highly
boosted turbocharged engines may need to
be throttled back to remain within operating
limitations, but shouldn't be throttled
back any further than absolutely necessary.
If the airplane has a constant-speed
propeller, best efficiency is generally
achieved by pulling the prop control back
to the lowest rpm that the book allows. Low
rpm dramatically reduces friction losses,
improves volumetric efficiency by reducing
pumping losses, and usually improves
propeller efficiency. Low rpm also gives
the engine more time to convert the heat
and pressure of each combustion event
into mechanical energy before the exhaust
valve opens and discards what's left.
Operating at WOT and low rpm will
make some pilots uncomfortable, because
they had a CFI who taught them never
to operate the engine oversquare. That's
what my primary flight instructor taught
me in the 1960s when we didn't know any
better. This notion that " oversquare is bad "
is another one of those myths that refuse
to die. It's dead wrong. For three decades,
I've operated the Continental TSIO-520s
on my Cessna 310 at 32 inches of manifold
pressure and 2,200 rpm in cruise-that's 10
inches oversquare if you're counting-and
achieved legendary longevity.
If your airplane has a fixed-pitch prop,
you can't control rpm from the cockpit, but
you can maximize fuel efficiency by making
sure that you have a cruise prop rather
than a climb prop installed. The cruise prop
has greater blade pitch and will operate at
lower rpm than a climb prop, and give you
better performance and efficiency in cruise
(at some sacrifice in climb performance).
Altitude
So, to achieve optimum fuel efficiency, we
want to cruise at Carson speed (which is a
relatively modest indicated airspeed), but
at the same time we want to operate the
engine at WOT with low rpm and a LOP
mixture. Is it possible to do all these things
at the same time?
It is indeed, but to do this you need
to cruise at a relatively high altitude that
allows you to be WOT without exceeding
Carson speed. In a normally aspirated
airplane, this generally means cruising up
around 12,000 feet. In a turbocharged airplane,
it usually requires climbing up to the
high teens or low flight levels where these
airplanes are most efficient.
Bargain hunting
When planning a long cross-country flight,
websites and apps like AOPA's Airport
Directory, AirNav, and 100LL.com make it
easy to find the lowest fuel prices along your
route. It makes sense to refuel at airports
and FBOs that offer the best fuel prices.
However, it almost never makes sense
to make an extra fuel stop in order to load
up on cheap avgas. The additional fuel
consumed in making the descent, landing,
takeoff, and climb usually costs more than
the savings from the lower-priced fuel.
Also, loading up on lower-priced fuel and
tankering it to your destination extracts a
weight penalty on aircraft performance
that can at least partially offsets the cost
savings of the lower-priced fuel. So, while
can be excruciatingly painful to purchase
high-priced fuel, it sometimes turns out
to be the optimum thing to do. Ouch!
mike.busch@savvyaviation.com
savvyaviation.com
Ask the A&Ps /
Podcast hosts Mike Busch, Paul New,
and Colleen Sterling answer your
maintenance questions on Ask the
A&Ps. See the show live, Tuesday
July 26 at 11 a.m. at EAA AirVenture
at the AOPA Program Pavilion.
AOPA PILOT / August 2022
87
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AOPA Pilot Magazine - August 2022

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