Gun sight image size analysis

[AKDogg]

I have a question: does AH convergence take into account bullet drop or not?  (i.e. if I set the convergence for a long distance does it cause the guns to fire slightly upward so the bullet drops back down to the sight point at the convergence range?)

Yes it does

[Baumer]

I don't think it does, as an example here are two images of the 190A-8 with 4 20mm cannons.

For both tests the plane was on auto-pilot at 1000 feet and 350mph TAS. I tried to fire the same 1 second burst for both tests.

The first one the convergence is set to 650 yards and the .target command is set to 650.


In the next test the convergence is set to 275 yards and the .target command is set to 275.


I may be wrong so if there's some other explanation I'm all ears.

[EDIT] It may have to do with the mechanical limit on how far the guns could be set for convergence. But that shouldn't be an issue on the 190A-8.

[Ghosth]

Baumer, try that same test flying inverted. Then tell me there is no vertical coefficient to convergence.

[RTHolmes]

interesting, so the hangar convergence settings only tweak the guns horizontally, not vertically. this means that for hub mounted guns the convergence setting makes no differenceat all.

[Saxman]

Great job Baumer and devs! While we're at it, maybe you guys could figure out how to make the sights adjustable for target size, ie. being able to adjust base range/wingspan setting so we know what 200 yds looks like from any target, not just fighters. Range icons are for kids  

Gunsights are just a BMP image file. To make sights adjustable for wingspan and range--if appropriate (AFAIK the US Mk.8 couldn't be adjusted like this)--would require a fundamental revision in how sights are handled (not all sights were "just" made larger or smaller based on target wingspan. As Baumer indicated, the horizontal bar on the Barr and Stroud GM2/MkII changed size or position, but it sounds like the ring stayed the same).

Now if HTC were to eliminate custom gunsights and set all aircraft to use their HISTORICAL sight (something that I've WANTED to see for immersion) it would be possible to introduce things like this. Heck, you could then even give those Pony drivers with their K-14s their LCOS, assuming they want to try toying with that entire labyrinthine process.

It's something I'd certainly love to see, but would take a LOT more work to implement than what I understand is being done to correct the sight scaling now.

[Baumer]

I know what you're getting at Ghosth but I cant use auto-pilot inverted, and I don't trust my hand fling skills enough to us that as a definitive test.

[Bronk]

I know what you're getting at Ghosth but I cant use auto-pilot inverted, and I don't trust my hand fling skills enough to us that as a definitive test.

OK use a p38 set convergence to 200 and set target to 650.

Edit: better yet use a p-39 and use cannon only... you'll see the diff.

[715]

Edit: better yet use a p-39 and use cannon only... you'll see the diff.

OK.  I did that.  The main thing I learned is that the dispersion on the P39 cannon is dreadful.  However, sitting on the runway with .target 300, a convergence of 150 hits lower than a convergence of 650.  (Of course it hits way at the top of the .target because the plane is pointing uphill a bit.)  So it appears that convergence does change vertical as well as horizontal.  (The dispersion is so bad, however, that the two patterns overlap.)

[RTHolmes]

ok tested and they do converge vert too. the 650 yd 190 shot above must be because the drop is so bad at 650 the guns cant be adjusted up that far I guess.

[Bronk]

ok tested and they do converge vert too. the 650 yd 190 shot above must be because the drop is so bad at 650 the guns cant be adjusted up that far I guess.

 If you play with it long enough you will find that it will be on at 2 diff points... depending on convergence and ballistics.  

oooh and try it in opposite. Pull convergence all the way in and move target out 100 at a time.

[Ghosth]

.50's and .30's are the hardest to see, because they shoot so flat.  The difference vertically is less than the convergence cone.

[RTHolmes]

If you play with it long enough you will find that it will be on at 2 diff points... depending on convergence and ballistics.

sure, but im assuming baumer had the convergence and target distance the same, so the rounds should centre on the pipper (not the case in the 1st 190 d650 screenie where they are lower)

[Baumer]

 understand what you are saying Bronk, if you look at this chart for the 190A-8 you will see that ballistic arc for the inboard 20mm's should be at the sight line at 550m (601 yards).



so (in theory) if you set the convergence to 600 yards then you should get a good group in the gun-sight plane with a .target command at 600. However, in practice the pattern seems to be below the gun sight plane. Not enough of a major error in my opinion, but when ever I make a custom sight with a long convergence I always run tests to see where it hits.

This test with the convergence (and target) set to 600 yards shows a clear pattern of hits below the sight plane.


Now with the convergence (and target again) set to 200 yards this shows better distribution above and below the sight plane.


[NOTE:] both tests were flown on auto-pilot at 1000 feet and 335mph TAS.

Now I did the test with the P-39Q in a similar manner and got different results. The long range shots showed a consistent spread around the convergence point, so it appears that accurate convergence may be dependent on the specific aircraft model.


So the answer to the initial question may be yes, the important factor to remember is, test the sight with the .target command, to ensure the sight functions as expected.

[BulletVI]

Now after reading how the test was done i tried one of my own but with the Mk9 Spit.

Convergence was set to the closest at 150 yard's at a height of 2200. And the results are pretty diffrent to the Fw 190 and i believe that HTC may have left out a certain factor in the Fw190. the 190's cannons are not firing from the same height on the airframe as the machine-guns!!!! So maybe HTC has not alined the cannons slightly up ward's to converge at the same point as the machine gun's. Cos as in the test with the Spit 9 i got no indication of hits below the gun sight just a good old consistant group of shot's on target. But i did fire from the Zoomed in view and the normal view to eliminate any enforceable variables and both tests were almost equal in results.

[RTHolmes]

the 190's cannons are not firing from the same height on the airframe as the machine-guns!!!!

they shouldnt be - the mgs are on the engine cowling just under the sight line, the cannon are in the wing root way below that.

but that shouldnt make a difference, at convergence distance all of the rounds should hit the sight line (or disperse around it)