Pilot – June 2018

38 | Pilot June 2018 | pilotweb.aero

and then tested in exactly the
same way. Most of the differences
seemed to be in the paperwork−
and here’s an interesting fact: you
can tell if an engine is certified
or ASTM-compliant at a glance,
as the data plates are colour
coded. Many people know you
can tell the power produced by
any given nine series Rotax from
fifty metres away, as the heads
are colour coded. Black for 80hp,
green for 100, red for 115 and
now blue for 141. However, what
you may not know is that if it’s a
certified engine, its data plate is
red, while if it is black it’s ASTM
compliant. See? Every day’s a
school day!

Proof of the pudding...
The next day it was off to the
historic Wels airport to fly some
aircraft fitted with the new
powerplant, but before going
flying we were thoroughly briefed
by Head of Flight Test Siegfried
‘Seigi’ Heer, who also revealed
one further fascinating fact: all
small GA airfields in Austria are
designated as nature reserves,
and this can help defend them
against voracious developers.
What a brilliant idea!
Back to the 915iS: to all intents
and purposes the engine’s
architecture is essentially the
same as that of the 912iS. A flat

four, it has the same combination

of ram-air cooled cylinders and

liquid-cooled heads and the same

displacement of 1,352cc (82.5

cubic inches). It also utilises ‘dry

sump’ lubrication, oil being fed

from, and returned to, a separate

reservoir, and FADEC (Full

Authority Digital Engine Control),

the electronic fuel injection and

ignition systems being controlled

by a dual-channel Rockwell Collins

ECU (electronic control unit).

There are also some

significant differences, such

as a reinforced crankshaft,

new pistons and a redesigned

gearbox with a higher reduction

ratio of 2.54:1, reducing an

engine speed of 5,800rpm to

a much more efficient−and

neighbourly−2,300rpm at the

propeller. The gearbox also

features an improved vibration

damper/overload protection

clutch, while a new quill shaft

(which twists between two to

three degrees) also plays a part

in reducing torsional vibration.

The drive has been designed

from the outset for constant-

speed propellers.

The big difference though is

the turbocharger installation. The

impellor has a pressure ratio of

3.5:1 and not only increases the

power available to 141hp for up

to five minutes, with a METO or

‘max continuous’ of 135hp, but

ensures the power stays constant

up to the engine’s critical altitude

of 15,000ft. This unit is a very

cleverly designed piece of kit. For

example, the temperature of the

compressed air as it leaves the

turbocharger unit is a remarkable

200°C, but once it’s been through

the intercooler it drops to only

around sixty to seventy degrees.

The entire installation is neat

and compact−except for the

turbocharger, it is very similar

externally to the 912iS and only

fifteen kilograms heavier.

Talking with Siegi before we

went flying, I opined that as

much as I admire the 912 series (I

have several hundred hours being

either pulled or pushed by them)

I’ve never liked the way they start

and stop with such harshness. “I

think you will see a significant

improvement in that area,” he

Tech Log: Rotax 915iS