page 45 - American Bonanza Society

www.bonanza.org
Turbos and Low Altitude Ops
by Bill Compton
This article is about comparison of like airframes with various
turbo setups and the advantages of turbocharged engines even
if operat ing at 10,000 feet and below, both in performance and
engine longevity. Close study of the data helps to appreciate the
fine points. The 36 Bonanzas were left out of these comparisons to avoid the
apples vs. oranges situation; they tend to be about 8 knots slower than the
35 and 33. This will concentrate on the normally aspirated V35 (V35NA)
versus its turbocharged (V35TC) and turbonormalized (V35TN) cousins
with 520 engines.
Bonanzas are flown at night, on
instruments, over all kinds of terrain.
Operators can, and do, increase
capabilities with avionics, anti-icing,
radar, SFERICs, standby alternators,
turbo-charging, but still hanging it all
out on a single engine. Every effort
might well be made to minimize the
risk of engine failure. Engine monitors,
tuned fuel injectors, oil analysis, borescoping,
and science-based operating
procedures from Advanced Pilot
Seminars have been advances in that
direction. The newer data does sometimes
conflict with POH advice.
Test cell research by Tornado
Alley Turbo (TAT), a turbo system
manufacturer and respected R&D
firm, has shown that operators can
extend engine life and reliability by
avoiding the “Red Box,”, defined as
EGTs between 40° lean of peak (LOP)
and 180° rich of peak (ROP) at 75%
power, and between peak and 100°
ROP at 65% power. At 60% and below,
there is no “Red Box.” This is controlled
with the mixture knob, referencing
EGTs and CHTs of all cylinders. Proper
mixture control slows the flame front
of combustion so that peak internal
Photo by Jay Burris
combustion pressures (ICPs) are less
and occur later in the power stroke,
where there is better mechanical
advantage and less heat being passed
to the cylinder walls. Going from
ROP to LOP reduces cylinder head
temperatures (CHT) by about 30°F.
All this knowledge has changed the
way many operate their fuel-injected
Bonanza engines, especially the
turbocharged models.
The V35NA POH prohibits 2300 RPM
with MP higher than 24.5", as reduced
RPM brings peak ICP closer to Top Dead
Center (TDC) of piston travel. However,
when operating LOP, the speed
of combustion is slowed, so many run
the TN conversion at 2300 RPM with
full throttle and maximum manifold
pressure, knowing that the slower ICP
peak is kept further from TDC by LOP
operation, and are reassured by normal
cylinder head temperatures.
Beech produced the V35TC from
1966 through 1969, powered by a turbo -
charged IO-520 engine with a reduced
compression ratio to improve detonation
margin. The airplane earned a rep
as fuel hog and cylinder trasher, but
in the mid 1990s, operators of the
V35TC found they could mostly avoid
those problems by operating LOP. It
does well with LOP cruise climbs, and
will do 200+ KTAS up high.
More recently, TAT engineered “turbo
normalizing” (TN) for 520 and 550
Bonanzas, using an intercooler rather
than a reduced compression ratio to
maintain detonation margin. The V35TN
performance is superior to the V35TC,
as it can maintain power to higher altitudes
at lower fuel flows. The V35TN
can do 210+ KTAS above 20,000 feet.
Illustrated in Table 1 are power
choices for IO-520 Bonanzas. Nauti -
cal air miles/gallon (Nam/gal) is a
refer ence for range performance.
36 AMERICAN BONANZA SOCIETY MAY 2013