78 AIRGUN WORLD http://www.airgunshooting.co.uk
TECHNICAL AIRGUN
HIT AND MYTH
So why has the myth that pellets are drag
stabilised arisen? One reason is that many
people have used the analogy of a shuttlecock
to justify their claims that the pellet is drag
stabilised, whereas in reality the shuttlecock
and waisted pellet have very little in common in
flight. The shuttlecock initially is almost entirely
drag stabilised due to flying backwards to start
with, and only affected by the lift forces that
stabilise the pellet when it has settled down to
small yaw angles.
The skirt of a pellet creates lift in the same
way as conical sections that are sometimes
built onto the rear of experimental (military)
projectiles to stabilise them, and these conical
sections are called ‘flares’, so the associated
stabilising mechanism is known as ‘flare
stabilisation’, and the term is equally correct
when used to describe the aerodynamic
stabilisation of skirted pellets.
DYNAMIC STABILITY
Again, avoiding the mechanisms at work
because they are very complicated and I don’t
understand them, it’s easiest to describe what
dynamic stability is, and what it does, by
describing dynamic instability.
When a pellet yaws in flight, the lift acting at
the CoP and the gyroscopic stability encourages
the skirt back into line, but if the pellet is
dynamically unstable, then instead of the pellet
stopping when it is in line with its direction of
travel, it carries on swinging in the other
direction to further out of line than it was in the
initial yaw, then reversing the process over and
over again, increasing the angle of yaw each
time. Unsurprisingly, a dynamically unstable
pellet would prove woefully inaccurate.
Happily, the pellets we use today are
dynamically stable except at long ranges or
high speeds where problems can arise.ACCURACY
The degree of accuracy that top shots in
outdoor competition disciplines can achieve
today has improved greatly when compared to
just a few decades ago, due in part to
improvements in the air rifle, but mainly due to
the higher quality pellets on the market, the
design of which has evolved to be able to make
the most of the three forms of stability. WIND DRIFT
This is a difficult subject to understand, and an even more difficult one
to explain, but Miles managed to explain it in a way that even I can
understand, and so I’ll attempt to explain it here in as few of my own
words as possible.
A pellet in flight in still air is travelling through stationary air
molecules, which are effectively approaching the pellet head-on. When
there is a crosswind, the pellet ‘sees’ a combination of this fast
approaching air, and much slower air from the side, and turns to face
the resultant combined air flow. As a result, the pellet is no longer
facing in its direction of travel, which moves drag to the downwind side
of its direction of travel, which pulls it in a downwind direction.
So, the pellet in a crosswind is NOT blown to one side; in effect, it is
pulled or sucked to one side by its own drag.A crosswind can cause a lot of
sideways drift, but the wind does NOT
blow the pellet.When you find a pellet batch that suits your rifle,
best to invest in a sleeve if you can.Pellets are not blown by crosswind, their own
drag is what causes wind drift. Illustration
courtesy of Miles Morris.