Plane & Pilot - August 2018

58 AUGUST 2018 ÇPlane&Pilot

Unfortunately, although I’ve lown

several Bristells, I’d not lown the SW

(shortwing) version, and although the

acceleration certainly seemed stronger,

the rate of climb didn’t seem to be

as improved by as much as a 40 per-

cent increase in power would suggest,

though this can easily be explained by

the higher wing loading. Even before

getting in, I’d noticed that the engine

is ofset slightly, so that the thrust line

isn’t exactly straight down the fuselage

centerline. Consequently, although

some right rudder was required to

keep the slipball centred in the climb,

it wasn’t as much as I’d anticipated,

bearing in mind that this aircraft had

40 percent more power than the last

Bristell I lew in 2015, and the in and

rudder didn’t look that diferent.

he initial climb rate was almost

2,000 fpm at 70 knots, which (bear-

ing in mind the density altitude) was

pretty respectable, while at 7,000 ft

MSL the increase in performance was

very noticeable. he turbo worked as

advertised, with no discernible reduc-

tion in manifold pressure and, having

trimmed forward and set max cruise

of 5,500 rpm and 37 inches of manifold

pressure, the IAS soon settled on 135

kts for a TAS of 150 while burning just

under 9 gallons per hour. Pretty impres-

sive numbers, and although if you’re

a long-term Rotax pilot you might be

thinking that 9 gph is quite thirsty, I’d

counter that 150kt TAS is quite fast!

Pull the power back a long way to,

say, 4,800 rpm and 17 inches, and the

engine is now just barely sipping just

over 2.5 gph at 80kt TAS, while a good

compromise (Bristell calls it the ECO

setting) of 5,000 rpm and 36 inches of

manifold pressure still give a TAS of

around 145 kts at 7,000 feet.

As the primary purpose of this light

was to evaluate the engine, I didn’t

get the opportunity to explore the

envelope completely, but it certainly

seemed as if the slightly heavier engine

had (as you’d expect) shifted the CG

slightly further forward. Stopping the

engine back on the ground at Wels also

seemed smoother than with previous

nine series engines of my experience.

I then jumped into the next test

aircraft, an Aquila A211T, itted with

a composite three-blade MT constant

speed prop. his machine is not an

Aquila project, but is being used by

Rotax as a testbed. Consequently

(and unlike the Bristell), Aquila does

not plan to ofer the 915 as an option.

Interestingly, bearing in mind the

engines were identical, the MT prop

was very diferent to the Hofmann,

being straight and with a much nar-

rower chord. he Aquila is a much

heavier aircraft (the MAUW is 25 per-

cent greater than the Bristell’s), and it

showed. Both the initial acceleration

e initial climb

rate was almost

2,000 fpm at 70

knots, and at 7,000

feet the increase in

performance was

very noticeable.

LEFT: The Rotax 915 is slightly larger and slightly heavier than the 912,

but it will likely fit in most cowlings with little or no modifications.

BOTTOM LEFT: What will pilots love most about the 915iS? I'm guessing

it'll be two things, the additional power, of course, but also the FADEC.

ABOVE: Sharp eyes will be able to spot the 915iS in some installations by

the prop with a wider chord to make better use of the additional power.