50 Issue No 22
21, 1930, Junkers-F 13 G-AAZK was carrying
four socialites back from a party in Le Touquet,
France, when it crashed at Meopham in Kent. The
aircraft’s fuselage, in plan, was Cayley’s well-fed
trout; a superb low-drag symmetrical aerofoil
with a t/c ratio of roughly 20 per cent. At the
point of maximum camber sat the wing, a 16 per
cent Göttingen 256 high-camber (curvy-topped)
aerofoil, similar to the Hurricane’s Clark Y.
The final report into the accident, in which
six people were killed (“four with titles”, as The
Times thought it important to relate), issued by
the Accidents Investigation sub-committee of
the Aeronautical Research Committee in January
1931, reported that “the aircraft, flying in clouds,
may have been thrown into an unusual attitude.
This resulted in buffeting of the tailplane, causing
the port tailplane to fail, and the aircraft entered
a dive”.^7 Flight, which campaigned for this and
subsequent air accident reports to be made
public, added: “Buffeting is attributed to the
eddying wind from the wings. As a rule, with a
given angle of incidence, the violence of buffeting
increases with the air speed”.
After years of placing bodies next to streamlined
shapes and recording the results, Ernest Ower
was ready to publish his own thoughts. In
January 1932 he began promoting Klein’s ideas,
and published a neat explanation of his thoughts
on the crash in an article in The Aircraft Engineer,
which is worth presenting here in full:
“The body surface was converging in the
direction of flow aft of the mid-chord of the wing-
roots. Hence, in order to follow both the wing
and the body surfaces, the airstream behind the
mid-chord would have to expand. Any such
expansion would, of course, be accompanied by a
positive pressure gradient in the direction of flow,
the static pressure increasing at the expense of the
speed. We have... conditions comparable with
those existing in the outlet cone of a venturi tube.
“With body/wing combinations the conditions
on the upper surface of the wing are more
unfavourable still, on account of the steep positive
pressure gradient that occurs on the upper surface
of a wing behind the section of maximum suction.
The flow has to overcome this gradient as well
as that [owing] to the geometrical divergence
of wing and body surfaces. When this occurs
the flow will detach itself from the upper wing
surface, and also, probably, from the adjacent
portion of the body surface, with a resulting
increase in drag and decrease of lift.”^8
This detachment also sets up buffeting. The
Junkers had an extreme version of this form
divergence; clearly, the USA’s National Advisory
Committee for Aeronautics (NACA) believed
the Meopham disaster was down to the Junkers’
wing roots. In reference to the official report,(Adverse) Pressure Gradient As air passes over
a shape it changes pressure. The rate at which
this happens, measured along the airflow, is the
“pressure gradient”. Normally this refers to the
region of deceleration and increasing pressure
over the rear part of a wing. Sometimes, if the rate
of change is too high, flow can locally reverse; air
is pushed forward along the wing surface (see
Boundary Layer) from the high-pressure to low-
pressure region. This causes problems (see Flow
Separation); this is an “adverse pressure gradient”.
Boundary Layer A region close to the surface
of the body where the air is not flowing as a free
stream but is instead slowed by friction created by
the surface.
Buffet The effect of turbulent eddies, usually
caused by flow separation, on other flying surfaces,
usually the tailplane.
Burble An early term for the turbulent eddies that
occur when flow separates from a surface, i.e.
ceases to be laminar.
IN THIS ARTICLE we are discussing parasite
drag, not induced drag (which is to do with lift).
Specifically, we are looking at the component of
parasite drag affected by the various shapes of
fuselage and wing etc, generically referred to as
“form drag”, as opposed to “friction drag”. In a
streamlined body, i.e. an aerofoil or a teardrop-
shaped fuselage, nacelle or pod, form drag
accounts for only around ten per cent of the total
subsonic parasite drag of the object. The rest is
largely friction.
A careless arrangement of these bodies into a
classic “aeroplane shape” can easily triple the form
drag of a “real” aircraft, as opposed to a simple
geometric body, even at low speeds, raising it to,
say, 30 per cent of the total. This additional element
is sometimes referred to as “interference drag”,
but the distinction is a false one; air molecules are
unconcerned whether the acceleration/deceleration
of the air and the resulting pressure gradient comes
from a wing, fuselage, droptank or any combination
thereof. It is a question of semantics.
It is this form-drag element which increases
exponentially as compressibility effects kick in,
although as shockwaves occur it becomes “wave
drag” — not a semantic choice this time, as the air
affected by the aircraft’s form starts to behave very
differently. A “real world” aeroplane, complete with
interference, begins this exponential rise from a
far higher form-drag base than a simple aerofoil or
body shape in isolation in a windtunnel. MB
UP TO SPEED?
BUFFET, BURBLE &
FLUTTER — A GLOSSARY
OF AERODYNAMICS TERMS