Trans-sonic = Bad. Now I know why.

[JDub]

So was watching a YT vid on why commercial aircraft don't fly faster than the speed of sound...basically, it's too expensive. But they explained what happens when an aircraft traverses the speed of sound.

Basically, the drag increases exponentially while the aircraft is trans-sonic and the air flowing around the craft is supersonic in some places and subsonic in others which creates a drag differential which tries to tear apart the aircraft. I assume, in ballistics, this drag differential leads to instability.

Trans-Sonic.JPG

I always have know THAT this transition was bad but know I know WHY. Just wanted to share.

[diggler1833]

It leads to instability in a lot of bullets, but the ELR side of shooting sports has come a long way in bullet development.

[NugginFutz]

And THAT, boys and girls, is why bullets are not shaped like little airplanes...

[Klem]

JDub,

I'm going to agree with Nuggin on this one - the shape of an aircraft and especially its wings is very different to a bullet and the forces that act on it.

Aircraft need wings to impart lift as they travel through air whereas a spinning bullet is falling to earth from the moment it leaves the barrel. The diagram in your post is a teardrop which is the most aerodynamic subsonic shape. This means its the most slippery shape for a mass traveling subsonic through a medium like water or air. Slippery is good because it implies a higher BC. Teardrops do not generate lift however as there is no pressure differential created by the shape. You can however cut down the drag by shaping the body of your subsonic vehicles to mimic this shape. Wings look similar but have camber on the top which results in airflow over the top having to go faster to meet the air going under the wing and as Mr Bernoulli used to say, "Where speed is greatest pressure is least." With an aerofoil shaped wing there is low pressure generated above the wind and higher pressure below, which is lift.

Changing the angle of attack like in your photo increases drag but in a wing it also increases lift - like when you come into land. In your photo it is not an aerofoil, its a teardrop that is yawing. I wonder what it is representing.

Supersonic wings are a different shape to subsonic wings. They are more pointy at the front and back cut the air, to minimise formation of a forward detached bow wave. Same as supersonic bullets. So, separating the air is more important in a jet fighter and bullet than an airliner. Subsonic leading edges are more rounded which is more friendly for different angles of attack like when climbing and landing. An airliner looking for fuel efficiency can't have it both ways. As soon as it becomes supersonic its shape burns fuel, and fuel is money. Concorde is a good example of this, a pointy airliner designed for supersonic flight, but still burns fuel. Jet fighters burn fuel as well but cost is not an issue.

Supersonic bullets are of course not teardrop shaped; they are pointy and are supposed to be pointing directly at the path they are travelling, all the way to the target. Bullets have no need to operate in different angles of attack, or have a shape that generates lift. We are more interested in slipperiness, which means pointy, which puts the centre-of-gravity (COG) to the rear. Gyroscopic stabilisation (spin) prevent the COG from overtaking the front as it slows down, like a bike when you hit the front brakes only, or a car crash.

As the bullet slows the COG moves from the rear forward which increases the force (moment) that increases the influence of the heavier rear to overtake the lighter front/tip, to assume the most aerodynamic equilibrium shape, a teardrop. This means yawing increases as it slows down. Unless the spin stabilisation is still strong and has the yawing under control.

I am pretty sure this this is why shooters sometimes experiment with reversing bullets when seating for subsonic - they don't need to part the air and the COG is forward from the start making their flight more stable.

Lapua's 200gn 30cal bullet dedicated for subsonic is quite stubby in shape with a lock base. Again, a forward COG is more important than cutting the air.

Bryan Litz has studied the transonic effect on bullets. I have not read his book but I believe he recommends spin to keep the groups from blowing out. I can see how that will work, as long as the bullet points down to the target after the culminating point (2/3 of the trajectory to the target). If it is too stable then it will resist pointing down. This means it will be coming down to the target but still be pointing up and that is yawing. So, too little spin and it yaws, too much spin and it yaws. The BC also changes as it yaws so that will confound your ballistic predictions which are based on a muzzle velocity BC. Litz recommends shorter bullets with shallower boat tails. There's an advertorial for his book with summary on accurateshooter.com.

https://www.accurateshooter.com/ballistics/transonic-effects-on-bullet-stability-bc/

When I shot 1,000yd F Class the groups would be proportional all the way to 900, but then blow out by double on the 1K target. Very difficult to drive rounds on the target at 1K with any confidence - it was like they were drunk. The bullet crossed into transonic at 950yds (1,350fps) and went subsonic at 1,150.

In 50 years they will probably invent a bullet that reverses ends as it approaches subsonic, while maintaining perfect trajectory. Either that or phase plasma rifles in the 40 watt range.

[JDub]

Klem, thanks for the lengthy discussion. The subject fascinates me. One day, they be able to do all the fluid dynamic sims in real-time and by then the bullet will be composed of "mimetic polyalloy" (https://www.youtube.com/watch?v=Fa4Jq7gJvZ0) so they can change shape in flight.

[tdbru]

i think the COG (center of gravity) is always the same. that is based on shape and density. what changes with different shapes and Reynolds numbers is the aerodynamic Center of Pressure(COP). it's been too long since fluid dynamics. i can't remember if COP has to be ahead of or behind COG for stable flight. if i recall for aircraft designed for subsonic flight part of the issue with stability as you approach the speed of sound is that the COP starts to move forward quickly. but like i said, fluid dynamics was a long time ago. heck, when i was taking chemistry the periodic table was a piece of cake. earth, air, fire, and water. simple.....
-tdbru

[Dug Silverman]

Klem, thanks for taking the time to write that out. I doubt many here can say they already knew everything up there.

[GooseGestapo]

[NugginFutz]

Goosegestapo. Having participated in the actual shoot down of a foreign military asset with a MW Class laser, I can tell you 1 MW would be more than enough to dispatch anything less than a pick up truck in short order.

And since we are dipping our toes in the realm of Hollywood meets reality, a 40W laser / phase plasma rifle is definitely up to the job as far as flesh and blood targets go.

ETA: The Navy destroyer is fielding a 155 kW laser, not Milliwat.

[Klem]

Somebody needs to tell Arnie,

[Fess]

Klem, this is something that I like about this site. People go to the original sources to verify information.

edited to add: I am referring to the Terminator clip.

[Fess]

Tdbru is right. The COG, or CG (center of gravity) is fixed for a bullet*, but the COP (center of pressure) is not. The Accurate shooter link from Klem is an excellent one. https://www.accurateshooter.com/ball...-stability-bc/ . The COP does move foreword in the transonic range. The COP is essentially the same as COL (center of lift) of an aircraft wing. In the transonic range, the COP can change on one side of the bullet rapidly if the airflow is not exactly head-on. We know that even the most stable bullets fly at a slight angle to the trajectory, and a yawing or coning bullet at larger and varying angles. Once the COP moves foreword of the CG, it will try to pull the tip of the bullet away from the direction of travel, changing the drag and making weird shit happen ("weird shit" is a technical term, you know ).

Bryan Litz did a bunch of shooting and discovered some tricks to keep bullets more stable in the transonic range. Among other things, he discovered that a faster-than-previously-used twist creates a stronger gyroscopic force to help keep the bullet pointed in the right direction even when the weird shit in the transonic zone happens.

This was pretty important stuff, since for many years, match shooters tended to use the slowest twist that would stabilize a given bullet. This was a big deal when bullets were of lower and more variable quality. Many bullets were slightly out of balance and this would affect accuracy. The faster an out-of-balance bullet is spun, the more off-target it would tend to fly.


*well, until it deforms, but that is a different story

[biodsl]

When I shot 1,000yd F Class the groups would be proportional all the way to 900, but then blow out by double on the 1K target. Very difficult to drive rounds on the target at 1K with any confidence - it was like they were drunk. The bullet crossed into transonic at 950yds (1,350fps) and went subsonic at 1,150.

So where along the line I picked up that trouble starts are as bullets go into transonic, around 1.2 Mach. IIRC, for .308 shooters, going from a 1:12 twist to a 1:11.25 or a 1:10 helped in the transonic/subsonic transition and got the cartridge effect to 1,000 yards.

[Klem]

So where along the line I picked up that trouble starts are as bullets go into transonic, around 1.2 Mach. IIRC, for .308 shooters, going from a 1:12 twist to a 1:11.25 or a 1:10 helped in the transonic/subsonic transition and got the cartridge effect to 1,000 yards.

For me shooting F Class it was .223. Heavy 80gn bullets fired out of long 30" barrels, overpressured at 2930-2950fps. At the time no-one was discussing transonic, we were just happy to stay supersonic to 1K yards like the .308 guys. The .223 bullet would drop into subsonic at 1,050yds so it was a pretty close run thing. The .308 guys of course had no problems making 1K yds supersonic and their bullets were always more stable at that range.

From what I understand the transonic zone is between 890fps and 1,340fps. For .223 projectiles they drop to 1,340 fps at around 850yds. Which might explain why everything was fine up until 900yds when it turns to jelly.

It is interesting that the transonic range continues into subsonic down to 890fps, given the speed of sound is about 1,125fps. In that movie about Chuck Yeager and the X-1 breaking Mach 1 didn't the aircraft start to vibrate as it approached Mach 1 and then everything went smooth once it was through? Wouldn't a bullet be doing this in reverse as it slows down?

That's good info on the COP and COG, it makes perfect sense - thanks for that.

[Growler]