Pilot – June 2018

48 | Pilot June 2018 | pilotweb.aero

Airmail

Slippery Sonex

Dave Unwin’s
comprehensive article
about the Sonex kitplane
(Pilot, April 2018) provides a
great insight into this sporty,
economical and chunkily
attractive kitplane. Enthusing
about the aircraft’s handling,
Dave queries the fact that,
uniquely, builders in the UK
are required to fit two kinds of
elevator trim system, and also
a restricted range of flap travel.
Seeing that, Pilot readers might
be excused for thinking that the
LAA had specified the second
trim system being needed in
a fit of over-zealousness, and
that duplicating the trim system
on a little aeroplane like this
was a pretty extreme and
bureaucratic measure.
In fact, when I first flight-
evaluated an early factory-built
Sonex demonstrator in Italy
back in 2001 at the start of
the LAA’s type acceptance
investigation, I found that the
aircraft was statically unstable
in pitch even when flown solo,
putting the C of G in the middle
of its range. Clearly it would
be even more unstable if flown
two-up, which would bring
the C of G further back. The
simplicity of the aeroplane was
appealing but the pitch handling
was not compliant with any
published requirements; in a
tight turn, the ‘g’ would rise
of its own accord and one had
to push forward to stop the
aeroplane tightening up and
stalling. There was no tendency
for the aeroplane to hold any
trimmed speed − hands off, the
speed would diverge away from
the trimmed speed in either
direction, at random, so it had to
be flown hands-on all the time.
The problem was raised
with Sonex, who came up with
a ‘fix’ in the form of moulded
fibreglass tailplane tips which
increased the tailplane area.
They also suggested moving
the C of G further forward
with the use of the heavier
six-cylinder Jabiru 3300 engine
rather than the flat-four 2200.
I flew a later demonstrator,

first and second stage, but the

problem persisted with full flap.

We might have experimented

with a bigger automatic tab to

counter the bigger nose-down

pitching moment caused by

full flap deflection, but instead,

with follow-on LAA members

eager to see the type approved

so they could get building, for

expediency decided to alter the

flap gate to limit the flap travel

to the second stage, which tests

had shown to be adequately

effective.

Dave describes the aeroplane

as a clean and slippery

machine, but as a glider pilot

I am sure he appreciates that

with an aspect ratio of only 4.9,

at the moderately high angles

of attack associated with the

approach and flare, the Sonex

wing generates huge amounts

of induced drag so, providing

the approach speed has been

properly nailed, an excess of

‘float’ really shouldn’t be a

problem, even with only two

stages of those huge flaps.

Well done, Dave, for giving

credit to the Sonex as a

budget-priced sportplane

with rewarding handling. I

hope this letter illustrates

how LAA HQ works with both

kit manufacturers and LAA

members in the field to bring

promising-looking aircraft into

the UK fleet and to sort out the

inevitable issues that crop up, to

bring UK pilots a varied, exciting

and safe range of aircraft to fly.

Francis Donaldson,

LAA Chief Engineer

Dave Unwin replies: It was

never my intent to denigrate

the excellent work done by the

LAA − and of course I did not fly

the original aircraft, so cannot

compare the static longitudinal

stability between the two.

Furthermore, I did point out that

the test aircraft’s longitudinal

stability was ‘relaxed’. With

regard to the flaps I actually

wrote ‘[I] can’t help but think

that if the flaps had been set

to 40° (as the manufacturer

intended) it would’ve sat down a

bit more promptly’. In my view,

that statement is irrefutable.

so configured, with Sonex’s

former CEO Jeremy Monett,

at their Oshkosh base in 2002.

Unfortunately, while this

example was an improvement

on the earlier model, when

flown two-up it was still

statically unstable in pitch.

Later, Sonex developed a new

trim system, the so-called dial-

a-trim system, which replaced

the existing cable-operated

elevator trim tab with a bias

spring arrangement, actuated

by a knob on the panel, which

applied a variable amount of

‘down elevator’ bias to the

control system via a spring

arrangement. This, we were

told, would provide enhanced

pitch stability, and indeed it did.

There are plenty of precedents

for such systems including the

venerable Tiger Moth which

also has a stability-enhancing

pitch trimmer.

When I tested the first

UK-built example, which was

modified to incorporate the

dial-a-trim system, I found

that in combination with the

enlarged tailplane tips and

heavier engine it did indeed

provide just positive static

pitch stability. The problem

was that, while the dial-a-trim

was also effective in trimming

the aeroplane to different

speeds, power settings and C

of G positions when flying with

flaps up, it couldn’t deal with

the big nose-down trim change

when flaps were lowered. On a

full-flap approach I had to hold

quite a significant rearward

stick force, even with full up

trim dialled in... not surprising

really as the spring bias works

by adding a variable forward

stick force effect, but (for good

reason) gives no ‘up’ bias. The

combination of the sensitive

elevator and the need to hold a

significant back stick force on

approach was not a happy one −

not least because inadvertently

releasing the stick during the

final stage of the approach

would result in a sudden, steep

lurch towards the ground.

Acting upon a suggestion

from the LAA, Mike Moulai, the

UK agent for Sonex, re-fitted

the elevator trim tab and its

operating Bowden cable, but

rather than attaching the front

end of the cable to a pilot-

controlled trim lever, linked it to

the mechanical flap system so

that the elevator trim tab would

move automatically downwards

when the flaps were lowered,

to balance out the nose-down

effect of the lowered flaps. We

can’t claim to have invented the

idea − the late model Austers

use a similar automatic trim tab

for just the same reason. Fitted

with the automatic elevator

tab working in conjunction

with the dial-a-trim spring bias

system, the aeroplane could

be trimmed out on approach

with the flap lowered to the

Write to: Pilot magazine, Archant Specialist, Evolution House, 2-6 Easthampstead Road, Wokingham, RG40 2EG or [email protected] stating town

26 | Pilot April 2018 | pilotweb.aero pilotweb.aero | Pilot April 2018 | 27

Flight Test: Sonex Aircraft LLC Sonex

S

ome aircraft look like they’re moving even when they’re standing

sitting still on the grass at Saltby still, and the Sonex is one of them. Even just

the aircraft looks good. In fact, it’s quite a striking machine−possibly because of the design of

the canopy which seems to flow from the tip of the small, sharp spinner to the top of the swept-

back fin in a single unbroken line. (At the time, the fuselage strongly reminded me of something, but

I just couldn’t quite place what it was. Take a look at the picture on page 31: doesn’t it look like

one of those fish-shaped plug-type fishing lures?) In complete contrast to the profile of the

flowing fuselage, when viewed in planform the wings look like they have been produced by an

industrial guillotine−there are lots of straight lines! Despite being a popular design

in America, the Sonex family of aircraft are relatively rare in the UK, with only about ten flyable,

although several more are under construction. In fact, the test aircraft, aptly registered G-SONX

and owned by Chris Rayner, is the only one I’ve ever seen in Britain, and I begin the walkround with a

heightened sense of anticipation. Construction is completely conventional: the primary

material is 6061-T6 aluminium alloy sheet, although some non-structural parts, such as the

cowling, wing tip panels and wheel spats are made from GRP. Stainless steel blind rivets are

predominantly used throughout the classic monococque design, with driven AD

rivets used for the main spar. Aluminium skins (primarily 0.025” and 0.032”) cover an

internal structure of ribs, spars and bulkheads. The first surprise is in the

engine bay, for unlike most LSAs, which tend to be Rotax-powered, this Sonex is fitted with a closely

cowled Jabiru J-3300 air-cooled flat-six, which produces 120hp at 3,300rpm. (Earlier J-3300’s

had a ‘max continuous’ limit of 2,750rpm, which produced 107hp, but this restriction was

removed for later engines.) It spins a wooden two-blade fixed pitch Sensenich propeller, and

is fed from a single fuselage tank−with a capacity of sixty litres−mounted between the

engine and cockpit. Access to the engine bay is adequate but not outstanding; it is held together

by six piano wire hinges and six Dzus fasteners. There is a small hatch on top of the cowling

which provides access to the oil dipstick and another for the fuel cap filler. The cantilever constant

Ripping through an English autumn sky at 125 knots while burning just 22 litres of fuel

per hour, or reefing around in a very tight turn the same question keeps bugging us –

why aren’t there more of these in the UK?

Words: Dave Unwin Photos: Keith Wilson

SUPER SONEX