My chronograph reads 1060fps at the muzzle, but I hear a sonic crack. Is the pellet still accelerating a few yards from the muzzle?
Is a pellet still accelerating after it leaves the barrel?
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No.. What you are hearing might be correct tho. You can have air moving at supersonic speed around diabolo shape even if your pellet isnt at above speed of the sound.
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No it's starts to slow down as soon as it exits the barrel.
Sonic crack depends on altitude and temperature relative to sea level.
At sea level it's roughly 750mph or roughly 1100fps under 60 degrees F and at 70 degrees it's 1130fps and 1150fps at 90 degrees F.
Air pressure and temperature (atmospheric conditions) play a role as well.
The colder the temperature the lower speed needed to crack.
The higher the altitude the lower speed needed to crack.
The thinner the air the lower speed needed to crack.
My guess is you barely reached that speed to crack. Likely due to high altitude and colder temperature where you live.
You could try heavier pellets to slow the speed down.
To reiterate the speed if sound to make things easier to understand in layman's terms-
With k (kappa) = cp / cv or the ratio of specific heats = 1.402 for air.
The ratio of P / ro (pressure over density) is always the same for a given temperature by the formula P / ro = RT with R = individual gas constant for any given gas ( 1716 (ft lb / Slug R for air) and T = absolute temperature in Rankin). Pressure falls out of the equation. So if you want speed of sound find air temperature in F (t) then add 492 to get it in Rankin (T) then drop into THIS EQUATION-
SPEED OF SOUND = sqrt (1.402 x 1716 x T).
Sonic crack depends on altitude and temperature relative to sea level.
At sea level it's roughly 750mph or roughly 1100fps under 60 degrees F and at 70 degrees it's 1130fps and 1150fps at 90 degrees F.
Air pressure and temperature (atmospheric conditions) play a role as well.
The colder the temperature the lower speed needed to crack.
The higher the altitude the lower speed needed to crack.
The thinner the air the lower speed needed to crack.
My guess is you barely reached that speed to crack. Likely due to high altitude and colder temperature where you live.
You could try heavier pellets to slow the speed down.
To reiterate the speed if sound to make things easier to understand in layman's terms-
With k (kappa) = cp / cv or the ratio of specific heats = 1.402 for air.
The ratio of P / ro (pressure over density) is always the same for a given temperature by the formula P / ro = RT with R = individual gas constant for any given gas ( 1716 (ft lb / Slug R for air) and T = absolute temperature in Rankin). Pressure falls out of the equation. So if you want speed of sound find air temperature in F (t) then add 492 to get it in Rankin (T) then drop into THIS EQUATION-
SPEED OF SOUND = sqrt (1.402 x 1716 x T).
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Probably slowing in the barrel.
It is not a mistake the length of barrels that airgun designers use.
For example..the lowly .22lr.
18” is just about right to use the last grain of powder as the bullet reaches the muzzle.
The Beeman R1 (HW80) rifled barrel is only 9.5”.
Ditto for the TX200..
It is not a mistake the length of barrels that airgun designers use.
For example..the lowly .22lr.
18” is just about right to use the last grain of powder as the bullet reaches the muzzle.
The Beeman R1 (HW80) rifled barrel is only 9.5”.
Ditto for the TX200..
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The pellet is going 1000fps but the air going around the dome of the pellet is super sonic and that’s what you hear.
With most pellet dome pretty close to a sphere or circle the math is relatively simple:
Circumference of circle = π x Diameter of circle
basically the air going around the dome or half of the circumference it needs to go 1.57x the speed of the dome itself give or take a little given the shape but at least 1.2-1.5x speed.
So if the pellet is going at 1000 FPS then the air around the dome has to move or be displaced at 1200fps-1500fps which is super sonic. If a pellet is going at 850-900 FPS optimal speed for domed pellets the air going around the doom will be between 1000-1300 FPS. This explains why pellets with traditional bigger domes like to be shot at 850 FPS and the redesign pellets with less pronounced domes can be shot at 900fps or a little faster because the air doesn’t have to travel at super sonic speed around the dome to destabilize it.
With most pellet dome pretty close to a sphere or circle the math is relatively simple:
Circumference of circle = π x Diameter of circle
basically the air going around the dome or half of the circumference it needs to go 1.57x the speed of the dome itself give or take a little given the shape but at least 1.2-1.5x speed.
So if the pellet is going at 1000 FPS then the air around the dome has to move or be displaced at 1200fps-1500fps which is super sonic. If a pellet is going at 850-900 FPS optimal speed for domed pellets the air going around the doom will be between 1000-1300 FPS. This explains why pellets with traditional bigger domes like to be shot at 850 FPS and the redesign pellets with less pronounced domes can be shot at 900fps or a little faster because the air doesn’t have to travel at super sonic speed around the dome to destabilize it.
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From wiki on the topic of vapor cones...(with my additions)
Since the local Mach number is not uniform over the (pellet) aircraft, (air flow over parts of the pellet) of the air may be supersonic while others remain subsonic—a flight regime called transonic flight.
The pellet itself is not supersonic, but the pellet is accelerating the air around it to super sonic speeds.
Original text....
https://en.m.wikipedia.org/wiki/Vapor_cone
Since the local Mach number is not uniform over the (pellet) aircraft, (air flow over parts of the pellet) of the air may be supersonic while others remain subsonic—a flight regime called transonic flight.
The pellet itself is not supersonic, but the pellet is accelerating the air around it to super sonic speeds.
Original text....
https://en.m.wikipedia.org/wiki/Vapor_cone
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Thank you for all the replies. I have installed a lighter hammer spring and now get 860 fps. There is no more "crack". The stock spring with Pile Driver pellets didn't make the cracking sound, the weight must have really slowed the pellet down.
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I recall some testing (Harold Vaughn?) showing that a rifle bullet was still being pushed (though not necessarily accelerated) for about 20 diameters past the muzzle. I can see this being true for air as well, though the coefficient may vary. Not to say that it's going to add velocity, just that it may yet influence the flight.
GsT
GsT
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As "airflow" goes, what Gene says is basically true. There will be a smaller diameter column of air than the barrel diameter (boundary layer will be slower than near the center of flow), of air that will continue to exert a small force outside of the barrel. BUT this column will dissipate very quickly and have little actual effect on pellet speed, more like one or two pellet diameters past the end of the barrel. As Gene says, more of a continuation of pressure than an acceleration of pressure.
Mike
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Yes pellets will keep accelerate after leaving the barrel because they have build in after burner that keep accelerating the pellet till it reaches the target. So, the longer the distance you are shooting, the faster the pellet gets. If at muzzle your gun is shooting 1000 fps. At 100 yards it will be 1300 fps, at 200 yards it will be 1500 fps and so fourth.
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Not only are pile drivers heavy, they are barrel shaped, so there is not the diabolo shape behind the head affecting the pressure zones down the length of the projectile.
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Not unless you got a rocket motor attached to it. In my 50 plus years of shooting, I have had a lot of shooters think that all bullets speed up. Or a bullet rises when it leaves the muzzle, those trajectory charts were not understood. I would like to have a pellet that did not drop any in 100 yards. then for a day , I would be happy. Oh, better got check on the Ham, don't want to burn that.
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Keeps accelerating for about an inch past the barrel, then the air pushing it is dispersed and gravity/air resistance takes over and it starts slowing. Moderators, baffles, and air strippers will end the acceleration at them.
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