Airgun World – July 2019

http://www.airgunshooting.co.uk AIRGUN WORLD 77

case, misunderstand.
The pellet’s progress through the air is
maintained by its momentum (quantity of
motion), which is reduced as the pellet travels
further by drag, and the misconception is
either that drag in the immediate wake of the
pellet skirt (often called ‘base drag’) acting on
the rear of the pellet keeps the skirt in line,
and/or that drag along the side of the pellet
skirt keeps it in line. Both are wrong.
Drag is something that affects the entire
surface of the pellet, not just the skirt. It’s easy
to visualise the pellet head displacing air and
creating drag as it travels, and it’s also easy to
visualise a low pressure area immediately
behind the pellet case creating drag, but
there’s more to drag than that. At the
microscopic level, the surface of a pellet would
resemble a mountain range, with peaks and
valleys. The ‘valleys’ of the pellet’s surface are
filled with air molecules that travel with the
pellet, that are attracted to each other, and are
attracted to molecules that the pellet passes in
flight, and the attraction between air molecules
in the pellet’s surface and those the pellet
passes is what causes surface drag. At normal
sub-12 FPE speeds, though, base drag is
larger than the other parts.

GETTING TRICKY
In addition to drag, when a pellet yaws, the
pellet generates lift and this is where things get
tricky because lift is not amenable to
simplification, and when simplified
explanations of the mechanisms behind lift are
given, they only present one half, if not less, of
the story. So, let’s look at lift, and what it does,
whilst avoiding the fiendishly complicated
mechanisms that cause it.
We think of lift as the force that raises
aerofoils (aeroplane wings) against the force of
gravity, but lift in its wider sense can act in any
direction, up and down, left and right. Lift is

generated to varying degrees all over the

surface of the pellet while it is in flight,

according to the change in the diameter of any

part of the pellet. Where the diameter increases

• the head and skirt – positive lift is created,

and where the diameter reduces – the waist

• negative lift is generated.
  The combination of all the lift forces acting
  on the pellet produces a net force at what’s
  called the centre of pressure (CoP) which, in
  the case of the waisted airgun pellet, is situated
  well behind the CoG, and the distance between

TECHNICAL AIRGUN

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them, called the ‘lift moment arm’, allied to the

fact that the lift is acting at right angles to the

pellet axis, gives the lift huge leverage to

persuade the pellet back into line if it gets out

of alignment with the pellet’s direction of travel

• if the pellet yaws.
  The drag at the rear of the pellet, on the
  other hand, acts along the pellet’s axis and, if
  the pellet yaws at a small angle, the drag has a
  tiny amount of leverage against the CoG, and
  that is why it is incorrect to say, or think, that
  pellets are drag stabilised.

Lift has more leverage to keep the pellet on target than drag. Illustration
courtesy of Miles Morris.

This is how pellet stability is often described and depicted. It’s wrong.

Illustration courtesy of Miles Morris.

A pellet with dynamic stability is able to reduce the effect of yaw. Illustration courtesy of Miles Morris.

“Drag is something that affects the entire

surface of the pellet, not just the skirt”