AOPA Pilot Magazine - October 2018 - 91

300

Continental IO-520 sea-level performance

280
260
240
220
200
180

24.5"
at 2,200

160

24"
at 2,100

140

"Oversquare" limit

120

23"
at 1,900

100
80

Recommended
cruise region

60

IT'S
YOUR
CHOICE
THE 2020 FAA MANDATE
DOES NOT REQUIRE
PILOTS TO USE
ADS-B WEATHER

40
18

19

20 21 22 23 24 25 26 27
Manifold pressure (inches mercury)

range of just 400 miles, but Lindbergh taught
the P-38 pilots how to stretch the range to
950 miles by operating them oversquare
and leaning them brutally. Some pilots were
reluctant to follow Lindy's guidance at first,
afraid that they'd damage the engines, but
ultimately the guidance became standard
operating procedure for the aircrews.
I tried to find out what Bellanca and
Continental had to say about oversquare
operation. The Viking pilot's operating
handbook proved unhelpful, but then I
discovered that Continental published an
IO-520 operator's manual full of charts
and tables. One of those charts, titled "IO520 Sea Level Performance," was especially
enlightening. It depicted a shaded envelope
labeled "recommended cruise region" that
depicted all the recommended manifold
pressure and rpm combinations. The lowerright boundary of that envelope showed
how far oversquare the engine could be
run without exceeding Continental's recommendations for continuous operation.
Turns out that Continental's recommendations included MAP/rpm combinations
such as 23 inches manifold pressure at

28

29

1,900 rpm (four inches oversquare) and
24 inches manifold pressure at 2,100 rpm
(three inches oversquare). Apparently my
CFI got it wrong and Lindy got it right. I
subsequently learned through experimentation that operating my Viking's engine at
three or four inches oversquare provided
significantly improved range during those
fuel-critical Flying Sam missions.
This isn't limited to Continentals. In
fact, Lycoming is even more liberal in allowing oversquare operation. The Lycoming
operator's manual for the O-360, for example, shows an envelope for recommended
continuous operation that includes settings
such as 23 inches at 1,800 rpm, 26 inches
at 2,100 rpm, and 29 inches at 2,400 rpm-
all five inches oversquare, and all with
Lycoming's blessing.
In 1987, I bought my third airplane, a
1979 Cessna Turbo 310. This was my first
turbocharged airplane (and my first twin),
and it took awhile to get comfortable with
the power settings recommended in the
POH. I found myself cruising at 29 inches
of manifold pressure and 2,100 rpm, a
whopping eight inches oversquare. In fact,
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AOPA Pilot Magazine - October 2018

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