Increase FPE by 70% with Helium? The AirForce Texan HE Experiment!

[Rat Sniper (AKA: PaulT58)]

All this extra power and velocity is great, but what I want to know...  Does it make your rifle any lighter? 

[avator]

And to squirrels talk funny when you shoot them with it?   

[Scotchmo]

Hey guys, I know it's an old post but just for the sake of posterity, and if I'm not mistaken, I believe the increased velocity with helium has more to do with the faster speed of sound in helium versus in HPA.

Think about it, the gas can't travel down the barrel faster than its speed of sound. If you're wondering why can we get projectiles moving faster than the speed of sound at sea level with plain old air, it's because the speed of sound increases at heightened pressures and temperatures. For the conditions seen in airguns I've seen the figure of about 1800 FPS, which makes sense to me.

Maximum flux is limited by the speed of sound. So, yes, constant density air cannot “flow” faster than the speed of sound. However, air can also expand at approximately the speed of sound. So even if the column of air moving into the barrel reaches the speed of sound relative to the port, expansion can also happen at the air column front (behind the projectile). Given a barrel with enough length, or a projectile with a low enough mass, the velocity will approach the actual limit which is roughly 2x the speed of sound.

The higher speed of sound in helium gives it a higher maximum velocity limit, but the gains that we see in a typical airgun charged with helium are mostly due to the lower mass of the helium.

[Back_Roads]

And to squirrels talk funny when you shoot them with it?   

I have only tried it on Chipmunks

[Scotchmo]

All this extra power and velocity is great, but what I want to know...  Does it make your rifle any lighter? 

Yes. My ELR rifles with the 2L bottle would be about 1.5 lbs lighter if filled with helium, rather than air.

[rsterne]

the gas can't travel down the barrel faster than its speed of sound.

I'll have to tell my .25 Disco that.... I have shot an ultralight projectile (airsoft BB) at 2092 fps average for 3 shots with 3000 psi of air with it!.... Here is the local speed of sound for the gasses we use at various pressures.... Note that Mach 1 for air at 3000 psi is still only about 1360 fps....



Most of the improved performance with Helium is due to the fact the gas weighs 1/7th what air does.... This makes the projectile a far larger percentage of the total mass ejected, hence more of the energy goes into the projectile, and less is wasted accelerating the air....

Bob

[JohanG]

All this extra power and velocity is great, but what I want to know...  Does it make your rifle any lighter? 

Yes. My ELR rifles with the 2L bottle would be about 1.5 lbs lighter if filled with helium, rather than air.

Any bigger bottle and you'll have to tie it down lest it drift away..