TECHNICALLY SPEAKING
Small wedges fitted to the leading edge of the inboard wing section generated
prestall buffet and created a progressive stall starting at the wing root.On the other hand, wing fences are intended to minimise
spanwise flow and are set parallel to the aircraft centreline.The “missing” outboard 15 inches of the flap “vane” is the “fix”
to eliminate pitchdown on flap extension. The addition of
SMURFs allowed the replacement of the deleted section.wing drop. The second, not really relevant on
a model, is to generate prestall buffet which
affects the stabiliser and can be felt via the
control column, a classic warning of the stall,
which has to be simulated on many aircraft
with power controls, via a stick shaker. On a
model, however, they may be very useful in
minimising any wing drop, or snap, as the
stall is approached. Tip stalling is, of course,
to be avoided, as on a swept wing aircraft ifthe rear fuselage (B737), take a look next
time you are a passenger!
The second aerodynamic dressing on the
basic Hawks are the “Toblerones”, two wedge
shaped devices measuring about 8 inches
long and about 1 inch wide. These serve two
purposes: One is to precipitate a stall at the
root section BEFORE the tip by causing ear-
lier flow breakaway, so that aileron control is
retained at the stall, as well as minimisingThe SMURF is used in place of the deleted flap vane section to smooth airflow onto the stab.the tip section stalls first it causes the centre
of lift to move forward with the associated
tendency to create pitch-up, the opposite of
what is required at the stall, and may well
lead to a departure from controlled flight. If,
however, the root section stalls before the
tips the centre of pressure will move AFT with
the associated tendency to cause pitch down,
a desirable and safe characteristic at the stall
when a reduction in angle of attack is requi-
red for recovery.
Another feature of the Hawk, and indeed
many other swept wing aircraft, is the wing
fences. The purpose of the wing fences is to
prevent, or at least minimise spanwise flow,
and the fences are installed parallel to the air-
craft centreline - minimising spanwise flow
can also reduce the possibility of stalling of
the outboard wing sections.
In addition the above “fixes” one other aspect
of the basic Hawk is the flap vane. On an
early test flight with the late Duncan Simpson
in command, the Hawk pitched severely nose
down on flap extension. This behaviour was
fixed by deleting the outboard 15 inches of
the flap vane, something which I find difficult
to understand as the gap is outboard of the
stabiliser, but the aerodynamic “boffins” at
Hawker Siddeley came up with a simple
answer which worked. Later Hawks have thisGladwin_Layout 1 08/03/18 10.49 Pagina 3