Plane & Pilot - August 2018

56 AUGUST 2018 ÇPlane&Pilot

what you may not know is that the

data plates are also color-coded. If it’s

a certiied engine, its data plate is red,

while if it’s black, it’s ASTM compliant.

So, what exactly is a 915iS? For all

intents and purposes, the engine’s

architecture is essentially the same

as a 912iS. A lat four, it has the same

combination of ram-air cooled cylin-

ders and liquid-cooled heads and the

same displacement of 1,352 cc (82.5 ci).

It also utilizes dry sump forced lubrica-

tion and FADEC (Full Authority Digital

Engine Control), as the electronic fuel

injection and ignition systems are con-

trolled by a dual channel Rockwell

Collins ECU.

here are also some signiicant dif-

ferences, such as a reinforced crank-

shaft, new pistons and a redesigned

gearbox with a new reduction ratio

of 2.54:1, reducing an engine speed of

5,800 rpm to a much more eicient—

and neighborly—2,300 rpm at the pro-

peller. he gearbox also features an

improved dampening and overload

protection clutch, while a new tor-

sion shaft (which twists between 2 to

3°) helps reduce vibration. It has been

designed from the outset for constant

speed propellers.

he big diference, though, is the

turbocharger installation. his has a

compression ratio of 3.5:1 and not only

increases the power available to 141-hp

for up to ive minutes, with a METO,

or max continuous of 135-hp, but also

ensures the power stays constant up to

the engine’s critical altitude of 15,000

feet. his turbocharger 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 60° to 70°. he entire installa-

tion is neat, compact (except for the

turbo, externally it’s very similar to the

912iS) and only 15 kg heavier.

he irst aircraft I sampled was a

Bristell SW, itted with a wooden three-

blade constant speed Hofmann pro-

peller. he prop was quite interesting

in its own right, being of the slightly

curved “scimitar” type and having really

quite a broad chord, bearing in mind

the horses available.

Cranking the engine to life revealed

that this aspect of the nine series engine

has been signiicantly improved. he

combination of the “soft-start” system,

torsion bar and improved clutch really

has made starting the engine feel softer.

he engine also seemed to tick over

more smoothly, but one lesson I took

away from my visit was that, from cool-

ing to vibration, the proof of the pud-

ding is in the installation. Nine series

engines are used in over 260 diferent

types of aircraft, and it is inevitable that

the installation of some is probably

“perhaps not quite as good as it could

be.” As I taxied into position, I briely

reviewed our weight and the ambient

conditions. With almost full fuel and

two POB, we were within 30kg of the

600kg MAUW, while the combination

of unseasonably high temperatures

and Wels airport’s 1,043-ft elevation

gave us a density altitude of around

2,500 ft. he wind was 7-10 kt straight

down runway 27’s 4,500 fet of concrete.

Up at 97.8 percent

power, the 915 in

this evaluation hop

is burning just 27.2

liters per hour, or, in

American, less than

8 gallons per hour,

while putting out a

high percentage of

rated power. The

increase in power

is enough to give

some LSAs an

additional 20-25

knots at cruise,

too fast to be LSA

legal in the U.S.,

but surely tempting

nonetheless.

So what exactly is a

915iS? For all intents

and purposes, the

engine's architecture

is the same as a 912iS,

though there are,

of course, signiicant

diferences.