pilotweb.aero | Pilot June 2018 | 43A second bite...
After a quick cappuccino,
I jumped into the next test
aircraft, an Aquila A211T, fitted
with a composite three-blade
MT constant-speed prop. This
machine is not an Aquila project,
but is being used by Rotax as a
flying test bed. In fact, and unlike
Bristell, Aquila Aviation does not
plan to offer the 915 as an option.
Interestingly (bearing in mind
the engines were identical) the
MT prop was very different to the
Bristell SW’s Hoffmann, being
much straighter and having a
much narrower chord.
The Aquila is a much heavier
aircraft (the MAUW is 25%
greater than the Bristell’s)
and it showed. Both the initial
acceleration and rate of climb
were−as you’d expect−not as
good as the SW’s. However, it
did seem significantly better
than the 100hp Aquila that I
tested back in 2014−although
unlike the Bristell, the increase in
horsepower meant that a lot more
right rudder was required to keep
the slip ball centred in the climb.
As with the Bristell, I
climbed rapidly up to 7,000ft,
set 5,500rpm and 37 inches
MP, trimmed forward and let
the aircraft accelerate. Again,
the turbocharger worked as
advertised, manifold pressure
was maintained and the IAS
finally settled on 131kt, a TAS
of 148 with a fuel flow of 34
lit/hr. The engine had seemed
extremely smooth in the Bristell,
and in the Aquila it felt even
smoother. This may be due to the
dissimilar propellers or even a
product of the different materials
used in the manufacture of the
two airframes (the Aquila is
predominantly of composite
construction, while the Bristell is
mostly made of metal).
From a quantitative
perspective, I’m reasonably
confident about the veracity
of the data gathered, as both
aircraft were fitted with a Stock
Flight Systems Engine Monitoring
Unit. Sometimes referred to as a
‘Stock Box’ this fully integrated
digital EMU was developed by
German engineer Michael Stock
in conjunction with Rotax, and
can display (and record) a widerange of parameters in a variety
of different units. The Bristell also
had a pair of G600s.
We finished a fun day’s flying
with a fine meal at a traditional
Austrian restaurant, hosted by
BRP Rotax GmbH & Co KG
General Manager Thomas
Uhr. Thomas proved to be a
most convivial host, and over
some excellent schnapps (I can
recommend the zirbenschnapps,
which is made with pinecones) he
indulged us with a Q & A session.
Bearing in mind both test aircraft
were fitted with C/S props and
that the 915 was specifically
designed with C/S props in
mind, an obvious question was
“Would fixed-pitch propellers be
an option?” He replied that we
should “have a look at all of our
other products−there are none
where we have let our fixed-pitch
customers down. But official
announcements are only possible
if a product is available, so:
no comment!”
Nor would he be drawn on the
launch customer for the 915.
He did however confirm that he
keeps a close eye on electric and
hybrid developments, and when
asked about the possibility of
developing an aerobatic nine-
series engine, his response was
“How many thousands will you
order? From an engineering
point, we would love to do so, but
we don’t see the market yet.”Tech Log: Rotax 915iS
ABOVE & BELOW: the
915-powered Aquila’s
MT propeller had narrow
blades and less blade
area than the Bristell’sA lot more
right
rudder was
required to
keep the
slip ball
centred