Direktlänk till inlägg 16 februari 2012
Hastighet och BC
Min gode vän Harry Fuller skrev ett mycket intressant och tänkvärt inlägg som jag tänkte dela med mig av här på bloggen:
"As with the other calibres there are pellets which have poor BC and so poor wind bucking ability.
It happens that a small number of designs have relatively high BCs within their calibre. Kodiak/Baracudas, JSB Exacts and Eunjins generally top the BC lists in each calibre.
Now, the sectional density of the .25 cal also should give it some advantage but this must be combined with a good design profile, and be able to settle into a stable flight. There is overlap in wind bucking ability between the various pellets of the different calibres.
For example .22 Kodiaks, Eunjins and JSB Exacts have better BCs and wind bucking ability than just about any .25 pellet excepting the same models in .25 cal. .177 Kodiaks have better BC than some .25 cal pellets that are light for calibre and poorly designed. So when folk say .25s buck the wind best, then they must be referring to the good .25s or they are referring to the same design pellet across the calibres.
There is one important issue often overlooked. The .25s generally (no not always) set out at MVs below Mach 0.80 and often below Mach 0.70 This can give a drag coefficient that is closer to ideal when compared to .177 and .22 pellets starting at velocities closer to the speed of sound. The outcome is a better * "delay" or * "lag" time which is the real culprit related to wind deflection. When velocities are pushed into the Mach 0.90+ zone lag time increases, BC goes down and wind deflection increases.
An example from my current research into .25 cal pellet ballistics:
Same pellet produced a BC average over 50 yards of 0.0275 when shot at average 924 MV but 0.0313 when the MV was reduced to 900 fps; both corrected for sea level.
In this example for the MV 924 fps, Mach 0.82, there would be a wind deflection of 13.77 inches at 100 yards from a constant 10 mph wind at 90 degrees; whereas when the MV was dropped to 900 fps, Mach 0.80, the deflection would be 12.04 inches. (Chairgun GA). ... Initial further tests at 880 fps seem to lift the BC another notch by dropping to Mach 0.78 for another slight wind deflection advantage( all at my test altitudes ).
When wind deflection is to be considered, lag time effects seem to be best generally between 870 fps and 900 fps for our best pellets. Above and below that velocity range wind deflection increases.
So a heavy, well designed, .25 pellet set off at 870 to 900 fps will deflect less in wind than any smaller calibre pellet blasted out at 950 to 1000 fps - and way less if the smaller calibre pellets' velocity is lifted to 1200 or 1300 fps. ( Same as LV target sub sonic ammo versus HV hunting stuff ).
* The difference between actual time of flight and time of flight in a vacuum.
Kind regards, Harry."
Här kan man se en del intressanta iaktagelser som Harry har gjort, inte bara att BC varierar med hastigheten utan att den dessutom har ett gyllene hastighetsfönster som han specificerar till runt M08. Med kaliber 25 så innebär detta 870-900 fps då man uppmår optimal BC, en lägre hastighet ger ett lägre BC och detta är även fallet om man väljer en högre hastighet - något som många tyvärr missar.
Att försöka nå bättre prestanda på längre avstånd bara igenom att skruva upp hastigheten och därmed minska bantiden kan med andra ord vara helt kontraproduktivt. Det högre effektuttaget rubbar alltså harmonin, "balansen", hos vapnet. Man får en högre luftförbrukning, en högre ljudnivå, mer vapenrörelse och dessutom en projektil som blir mer känslig för sidvind samt levererar en mindre flack projektilbana. Ingen bra lösning.
Bättre då att "växla upp" vapnets drivkälla igenom en grövre kaliber och därmed vinna bättre BC. Värdet av just BC får absolut aldrig underskattas vid skytte över längre distanser och att vapnet då råkar prestera en lägre utgångshastighet i det register då man erhåller bäst BC blir sådeles helt underordnat.