http://www.airgunshooting.co.uk AIRGUN WORLD 5NEWS & OPINIONS / KIT REVIEWS / FEATURES / TUTORIALS / TECHNICAL
58 Go Mad for Nomad
Dave Barham is impressed by an excellent
new air compressor from Air Venturi60 One for the Pot
The editor’s young mate, Alan, shows us
how he preps a rabbit for the table71 Small Claim
Is a reduction in scope size a good thing?
Phill Price debates80 DIY Tuning
Pete Evans offers some ideas on how you
can improve your airgun at home85 Rosie’s Recipe
Rosie cribs a recipe from a friend -
Pigeons in Paper, with a Greek t wis t90 Subscriptions
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94 Top Value Guns
Tim Finley enjoys a plinking session with
a Daisy 415 Powerline pistolCONTENTS
MICK GARVEY
p6524-HOUR CAPABILITYJOHN MILEWSKI
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Technical
Airgun
http://www.airgunshooting.co.ukAIRGUN WORLD 77
case, misunderstandmaintainedmotion), which is reduced as the pellet travels The pellet’s progress through the air is by its momentum (quantity of.
the rear of the pellet keeps the skirfurther by drag, and the misconception is either that drag in the immediate wake of the pellet skirt (often called ‘base drag’) acting on
and/or that drag along the side of the pellet skirt keeps it in line. Both are wrsurface of the pellet, not just the skirt. It’s easy Drag is something that affects the entire ong.t in line,
behind the pellet case cto visualise the pellet head displacreating drag as it travels, and it’s also easy to visualise a low pressure area immediately reating drag, but cing air and
there’s more to drag thanmicroscopic level, the surface of a pellet would resemble a mountain range, with peakvalleys. The ‘valleyfilled with air molecules that travel with the s’ of the pellet’s surface are that. At the s and
flight, and the attraction between air molecules in the pellet’s surface and those the pellet pellet, that are attracted to eacattracted to molecules that the pellet passes in h other, and are
passes is what causes surface drag. At normal sub-12 FPE speeds, though, base drag is larger than the other parts.
In addition to drag,pellet generates lift and this is where things get tricky because lift is not amenable to GETTING TRICKY when a pellet yaws, the
simplification, and when simplified explanations of the mgiven, 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 echanisms behind lift are
mechanisms that cause it.aerofoils (aeroplane wings)gravity, but lift in its wiWe think of lift as the force that raisesder sense can act in any against the force of
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,
- negative lift is generated.on the pellet produces a net force at what’s and where the diameter reduces – the waist The combination of all the lift forces acting
called the centre of pressure (CoP) which, in the case of the waisted airgun pellet, is well behind the CoG, and the distance between situated
TECHNICAL AIRGUN(^) n dir v y ff th h ll ill i h id P hld bbl
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pellet persuade the pellet back into line if it gets outthem, called the ‘lift moment arm’, allied to fact that the lifaxis, gives the lift huge leverage to t is acting at right angles to ththe e
of alignment with the pellet’s direction of travel
- if the pellet yaws.other hand, acts along the pellet’s axis and, if the pellet yaws at a small angle, the drag haThe drag at the rear of the pellet, on the
(^) ca cause he pellets are drag stabilised. that is why it is incorrect to say, or think, that tiny amount of leverage against the CoG, and s a
Lift has more leverage to keep the pellet on target than drag. Illustcourtesy of Miles Morris. ration ThisIllustration courtesy of Miles Morris. is how pellet stability is often described and depicted. It’s wrong.
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”
http://www.airgunshooting.co.ukAIRGUN WORLD^75
UP IN THE AIR
Tnormally shy away from because it encompasses areas of physics that are close to impossible for the layman to comprehend, he science of projectiles in flight is called ‘external ballistics’, and it is a subject I
and much of the subject defies explanatioway of simplification. The great problem is that some people have written about it, and HAVE attempt to over-simplify it, often using n by
flawed analogies, with the result that a lot of what has been written on the subject, and is contained in videos available on the Internet, is plain wrong.
I was fortunate enough to be approached
TECHNICAL AIRGUN
Jim looks at how on earth our tiny lead pellets manage to hit distant targets
»
last year by retired ballistician, Miles Morris, who was keen for me to publish an article that set the record straight, which meant that between us we first had to educate me (no
small task). Three months on, with much help from Miles, and many hours studying the NASA and Hyperphysics educational web sites, I feel ready to give it a go.
OVERVIEWIdeally, the pellet would point in its direction of travel throughout its flight from the muzzle to the target, although for a number of reasons,
the pellet won’t constantly be pointing quite in
its direction of travel, and the difference between its direction of travel and the direin which it is pointing is called ‘yaw’. Yaw can cause the pellet to deviate from its intended ction
trajectory, and the potential ill effects from yaw are mitigated by the mechanisms of stability.air rifle benefits from three types of stability The waisted, or diabolo, pellet shot from an
that help to keep it on course to the target, and these are; spin, aerodynamic and dynamic stability. In order for the pellet to stand any chance of hitting the desired pellet
point of impact (POI), all three forms of stability must contribute; two out of three
It’s incredible that a tiny 8.4 grain lump of lead can travel 40 yards at high speed and land where it was intended to land.
Basic Tech p
Jim Tyler gets to grips with a
complicated subject -
external ballistics - and
translates it forthe layman