Issue No 22 51
NACA set up a series of tests in 1934 on the sole
McDonnell Doodlebug, with similar geometry
to that of the Junkers, and reduced the tail buffet
(and drag) with wing fillets.^9
Closing the gap
Perhaps one explanation for ongoing “root blind-
ness”, despite all the contemporary evidence
relating to the issue, is the term “interference
drag” itself. This has been explained in several
ways, most of which describe an interference
between two bodies. Few recognise the problem
as two properly streamlined bodies working
together — not “interfering” with each other —
to create the conditions for aerodynamic failure.
Today, aerodynamic notes still commonly read
like this example from a test pilot’s flight manual:
“Interference is caused by the airflow over the
wing meeting the differing airflow over the
fuselage in the area of the wing root. The effects
of this collision can be reduced by allowing a
smoother merging of the two air currents by
installation of a fairing at the fuselage/wing
root junction”. This is like a long expert treatise
Compressibility Essentially, all gases are
compressible, but in low-speed aerodynamics air
is presumed to be incompressible for simplicity’s
sake. In other words, it will not change density.
It takes a great deal of energy to compress air,
but as a body moves faster those energy levels
are eventually reached. Well before the speed
of sound is reached air can be compressed and
decompressed — but at the speed of sound it
compresses by default as in essence it has no
time to do anything else. This is “compressibility”.Critical Mach The forward speed at which the
air accelerated over some part of the travelling
body begins to move at a speed faster than sound
relative to that body.(Expanding) Fillet A fillet is a specific type of
fairing designed to smooth airflow at a junction,
particularly wing/fuselage. The most effective
examples are “expanding fillets”. Of increasing
radius, these increase in surface area as they
move aft over the wing and then continue to fair
into the fuselage aft of the wing. This eliminates a
“pocket” effect, especially with low-wing designs,
reducing the severity of the local pressure gradient.Fairing Any surface designed to allow air to pass
over a shape more smoothly.Flow Separation The state in which the airflow
over a surface starts to breaks down, and instead
of being laminar, with straight streamlines, forms
whirls, eddies and vortices. A lifting surface will
cease to be efficient in the separated region.
Flow separation can negate the effects of control
surfaces, and creates considerable drag. It may be
caused by the presence of an adverse pressure
gradient (see above).Flutter The physical movement of an aircraft
component under the effect of a disturbed airflow
when the disturbance matches some fundamental
frequency of the system. Control surfaces may
develop flutter independently, or whole flying
surfaces may flex in a cyclic motion depending
upon the aeroelasticity of the design. The ultimate
result of flutter is catastrophic structural failure.Induced Velocity As well as changing pressure,
air passing over an object changes its speed, in
inverse proportion to pressure. Near the thickest
part of an aerofoil, the air is at minimum pressure
and at maximum velocity relative to the wing. This
extra component is called “induced velocity”.Shock The behaviour of a gas when it is forced to
compress or decompress by transition to or from
a supersonic relative velocity. The compression/
decompression travels out from the point of
transition in a “shockwave”. The strength of this
wave may be altered considerably by changing
the conditions, but in particular by reducing the
pressure gradient. MBABOVE The wreckage of Junkers-F 13 G-AAZK at
Meopham, Kent, in July 1930. The port tailplane of
the aircraft had separated before impact and was
found some distance away. The German investigation
blamed freak conditions in cloud, while British
investigators looked at flow separation and buffeting.
PHILIP JARRETT COLLECTION