From: Avin <blist@concentric.net>

Newsgroups: warbirds.training

Ok, this came up in another group, and I find I don't really know the answer.

In a plane with low-velocity cannon and high-velocity mgs, with the convergence set to 300 yards, what happens if you're pulling major gs?

Do the mgs and cannon still converge?

I would have thought not, as the mgs will be above the cannon 300 yards out when you're pulling g. Correct? Incorrect?

avin

From: Wells

Newsgroups: warbirds.training

G forces shouldn't affect convergence much as the G-force on the projectile itself is zero as soon as it leaves the barrel. However, the G force would increase the weight on the projectile (before firing) and may affect muzzle velocity a little (or alot). That would affect convergence. I don't think we see anything like that in WB.

From: Avin

Newsgroups: warbirds.training

Wells Sullivan wrote:

G forces shouldn't affect convergence much as the G-force on the projectile itself is zero as soon as it leaves the barrel.

Yes, this is what was pointed out to me elsewhere (on the mac group, no less <g>). I'm going to head back and tell that "technically-challenged mac guy" he got it right.

The thing is - while flying the P39D, my feeling has been that I need more lead in high-g situations to hit with the 37mm than with the mgs, even at convergence. I guess I was imagining it? That low ROF 37mm is tough to hit with under any circumstance, and I could easily be wrong.

Thanks, Wells.

avin

From: Doug Hansen

Newsgroups: warbirds.training

Avin wrote:

Wells Sullivan wrote:

G forces shouldn't affect convergence much as the G-force on the projectile itself is zero as soon as it leaves the barrel.

Gravity affects convergence a great deal if you are trying to get projectiles with different muzzle velocities to meet at the same point in space ahead of your cockpit in the convergence zone (do you hear the music ? do do do do . . . do do do do . . . "just ahead. . . the convergence zone" ). But you are right, the projectiles do not experience any affect from the G's you are pulling once they leave the muzzle. In physics-speak, the shells experience projectile motion or follow a ballistic flight path.

Yes, this is what was pointed out to me elsewhere (on the mac group, no less<g>). I'm going to head back and tell that "technically-challenged mac guy" he got it right.

The thing is - while flying the P39D, my feeling has been that I need more lead in high-g situations to hit with the 37mm than with the mgs, even atconvergence. I guess I was imagining it? That low ROF 37mm is tough to hit with under any circumstance, and I could easily be wrong.

Maybe not. See below.

WE NOW CHANGE THE CHANNEL TO THE MAC SUPPORT GROUP AND JOIN A DISCUSSION ALREADY IN PROGRESS:

Doug Hansen wrote:

But you aren't changing g for the world outside your cockpit. Once the bullets have left the muzzle of your guns, they are experiencing g at the same acceleration as they do if you are not pulling g's.

Hm. That does sound reasonable.

I still think I might be right, but I'm having a hard time showing I'm right for the right reasons ;-). We need a good hard scientist - I'll find one. I'm a biologist, and it's showing.

Ahhhh... Just the opening I've been waiting for. I have a little training in physics (convinced a university once to give me a Ph.D in physics :-) ) So, I can't be trusted, but listen in and see if any of this makes sense.

Now what does happen, on the other hand, is that if you are pulling a turn, or some other maneuver in which the wings are banked as you fire, then the convergence will be off because convergence was set for g being oriented straight down. When you fire on a tilt, the offset that works for straight down orientation is now going to place the MG shells and the cannon at different points with probably both horizontal and vertical offsets, unless the different guns have essentially the same muzzle velocity I think this is true for planes with wing-mounted guns. When I find a hard scientist, I'll ask about the case where the weapons are all mounted in the nose. That should simplify things.

Let me get into some more details. To achieve convergence on the MG's and cannon for any aircraft, you have to give the gun some deflection. In other words, the guns in you aircraft are not firing straight ahead, the are aimed to send the shell initially directed up with respect to the sight line of your gunsight, and presumable the level flight attitude for the aircraft.

On the wings, they are also aimed towards the centerline of the aircraft to bring all the shells together at a point in space some distance ahead of you. For a center mounted gun, it seems to me that this side-to-side aiming might have been superfluous, but if it was done, the amount of deflection would be significantly less. And, of course, all this aiming and deflection was dialed in assuming an up-right attitude for the aircraft with gravity parallel to the lift vector and the wings horizontal.

If you now bank the aircraft, you change the entire geometry. The upwards deflection designed to counter the drop caused by gravity is now altered into a combination sidewards and upwards (or even downwards if you are inverted) deflection compared with the sightline of the gunsight and the gravity vector. This means that for shells with equal velocity and shape (to account for differences in drag caused by differences in shape) the shells will still all converge at some distance, but the point of convergence will no longer be on the line defined by the gunsight but off to the low side and above this line as viewed from the cockpit.

The convergence distance may also change because the side-to-side deflection of the left side and right side guns is now also affecting the drop of the shells.

If you further complicate this with shells with different velocities and shapes, you now get a series of convergence points, one for each type of gun you are firing. This occurs because the guns with lower muzzle velocity will be set with a higher deflection and will therefore have a convergence point further above the gunsight line as viewed from the cockpit than the higher velocity shell. Each convergence point will also occur at a different distance below the gunsight line to the side (or in the direction of gravity) because the convergence for each gun will occur at a different time-of-flight, with the longer time of flight to convergence leading to a bigger drop below the gunsight line to the side.

I have no idea if WB's gets into this level of detail, though if they have truly modeled bullet ballistics in 3D, then it should be automatic. But it raises a couple of questions.

One is are you truly better off with a short convergence distance if you are a turn fighter?

If you have center-mounted guns, the higher upwards deflection of a longer convergence distance will translate to a little bit less lead you have to pull to get guns on target in a turn-fight.

I don't know if this is enough difference in the amount of lead required to be valuable, but it is something I have been wondering about. The second question is if you have two different guns mounted in different banks, can you truly get both on target when in a hi-g turn fight because of the differences in convergence distance and location relative to the gun-sight line? I don't know. If anyone has observed guns hitting two different spots on the target in a turnfight, I would be interested.

AND NOW TO RETURN TO OUR ORIGINAL PROGRAM . . .

In response to the question about P-39 cannon and high-g turns, there is another factor that matters. Muzzle velocity and time-of-flight to the convergence and aiming point are directly related. This also matters in a turn-fight because you have to lead your target, pulling lead, to hit him or her. If the shell is slow, then the time it takes from your plane to arrive at the aim point is longer, which means that target also has more time to move in his flight path, resulting in the need to pull more lead to get your slow but hard-hitting cannon shells to arrive at the same place at the same time as your target.

Thats why lasers and phasers are so nice. Since the time of flight is essentially zero for nearby targets, you just point straight at him and shoot. (Though I have always wondered about Phasers. Remember that one episode where these aliens made Kirk speed up real fast so he could dodge the phaser beams? What is the true propagation velocity of phasers? Any body know?)

Doug Hansen

"warmut"

From: warmut

Newsgroups: warbirds.support.mac

warmut wrote:

(snipage Of WONDERFUL stuff....but it made my head hurt! I hate guys this smart! )

The following is absolutely HOUNDING me. I really believe that long conv is best in this version of the game, vecause d6 shots seem the most assured kills. But I love to be in d2 or less! When I have short conv though, it doeswn't seem to make a ding-dong's worth of difference in close in fighting.(Sorry for the technical wording there.... Warmut's deep thinking may be helping my IQ (G). I notice the good pilots can kill me at will in a turn fight, but I struggle to get a few pings close in. It's the mystery of the ages for me.

Flapz

There are a couple things to keep in mind here about convergence and what that is actually doing to path your bullets are following. The advantage of short convergence is two fold. 1. If you are good at getting in close, you put the pip on the target and blast away without worrying too much about accounting for the sighting of your gunsight for distance. 2. Again, if you are good at getting in close, it gives you a big punch in a small area on the target when you fire.

Now for the disadvantages. 1. If you are a close-in fighter, but also need to hit with shots at d4 or d5 occasionally, you want to be careful with your convergence setting. The closer you set your convergence, the faster the shells diverge after that distance. Remember, twice your convergence distance away, your shells will be distributed on the target just as your guns are on your aircraft. This probably increases your chances of hitting at longer distances, but reduces the punch, because if some are hitting, some are probably also missing. For a wing-gun fighter like most of the american ones, I would set my convergence for a minimum of 250 so I can still do reasonable damage at d5, and beyond d6, I wouldn't waste many shells taking pot shots. Setting it for 300 or even 350 might be better because then your shells don't diverge as fast after the sweet spot. For a fighter with center-mounted guns, this is much less of a concern, because even though the convergence distance is shorter, the divergence is not as fast because the convergence angles are much smaller. A discussion of these features can be found elsewhere also, such as Fletch's pages, etc.

One thing I haven't seen discussed much, on the other hand, is what happens inside the convergence zone. If you set for 300 for example, and shoot at 150, what happens? Well, your guns won't be maximally concentrated at 150, but they will be closer than together than they are on the aircraft. but the other thing that is of interest is how fast the concentration changes with changes in distance. If you set convergence for 200, the angles on the guns will be larger, and the triangle your shells describe in flight will have smaller angles so the triangle is shorter.

This means that the sweet spot isn't as "long" when your guns are set to a shorter distance. In other words, with convergence at 200, the sweet spot (the distance in which the pattern diameter changes 10% from its minimum, say) is about 40 yards in length, from d1.8, to d2.2. With a 300 convergence, it is now 60 yard long, from d2.7 to d3.3. It is further away, but it is longer. What this means is up to the pilot, how you like to fight, and your level of capability.

What I think it means is that if I have a nose mounted gun-set, such as with the 190, the P38, etc. I set my convergence on the long side and I will get a tight grouping on the target for a long ways out there. The only thing I have to do now is remember that if I have set convergence for 400, that at d2, my shells will arrive on target a bit above the crosshair. If I have wing-mounted guns, I need to be more careful, but I would set my convergence a bit beyond where I like to actually shoot my victims, so I can also get good effect for the longer shots, since I have some length in the sweet spot. In other words, if I like to do my killing at d2.5, I would set my convergence for probably about 275 to 300. I won't get much reduction in the punch at my preferred distance by doing so, but I will get better capability for the longer shots.

Of course, your mileage may vary. If you are getting 10% of your shells consistently on target, or better, I wouldn't change a thing, even if what you are doing contradicts everything I said.

"warmut"