only best 10 units will deal dmg. So in example of 20 arty it will check the attacked stack units, and use the best arty to deal the most dmg. If enemy stack consists only of infantry, it will use 10 sp arty. If there is 10 enemy armoured units, it will use 10 regular arty. If mixed, will use 5/5 or something like that, depending on proportions. So is the case with the bombers

Not really. When a stack of 10 attack bombers and 10 tacs attacks a stack of 1 tank and 2 infantry, 33% of the stack damage goes to each enemy unit. For each armor class, the best 10 units are selected. So the 10x attack bombers do their damage to the tank, and 10x tacs to the infantry (and each is reduced to 33%, because there are three units in the target stack)

If the air stack had been 8 tacs and 8 AB's, damage to the tank would be (8x AB damage + 2x tac damage) * .33 and the other way around for the infantry.

Regardless of which are the best 10 for doing damage, if you are stacking 20 units in a single stack you are wasting units. Breaking the 10 SPA and the 10 SPRA into 2 stacks of 5/5 SPA/SPRA and use BOTH of them to attack thus the DAMAGE OF ALL 20 units is done and therefore you are doing the max damage.

Same with the aircraft, my aircraft stacks are always 5/5/5 ints/AB/TACs (and maybe 5/5/5/5 ints/ab/tacs/NB) (and the only reason to add planes in 5/5/5 stacks is for dividing damage between all units in the patrol stack) If you have question on aircraft use read the Air combat Guide.

That may be true when you don't get return fire, and your enemy is sitting like cattle at the slaughterhouse. In most battle situations, that is not the case; there are concern about artillery getting rushed or bombed, planes meeting AA or interceptors, etc. Here's some numbers for an L1 bombing operation with Axis against Axis doctrine against a mixed stack. Actually, this might be a slaughterhouse situation.

Available attack force: 10 AB, 10 tac.
Bombing: 5 LT, 5 infantry.
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When combined into one stack, the planes do:
10x tac damage against infantry = 52
10x AB against LT = 44
In the first round, the infantry takes 26 damage (50% of 52) and the LT's 22 (50% of 44), for a total of 48 HP.
Ground stack fires back at your planes, total damage is (LT: 5x 0.6) + (inf: 5x 1.8) = 12 HP.
Killed/lost ratio = 48/12 = 4.0
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When using two 5/5 stacks, each air stack deals:
5x tac + 5x AB damage against infantry = (5x 5.2)+(5x1.3)=32.5
5x tac + 5x AB damage against LT = (5*3.2)+(5x4.4)=38
In the first round, the infantry takes 16.25 damage (50% of 32.5) and the LT's 19 (50% of 38), for a total of 35.25 HP per stack; since you have two stacks now you deal 70.5 HP damage (which is indeed more than the single stack)
HOWEVER: since the ground stack now has two opportunities to fire back at your planes, your own stack now takes 2x12=24 HP damage itself.
Killed/lost ratio = 70.5/24 ~= 2.94
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Conclusion: yes, you indeed deal more damage when you use two stacks, and you should use it when you're blitzing your enemy and are on a tight schedule. When you're in a protracted attrition battle though (or if you just want to take as little losses as possible destroying a set amount of enemy units), the one stack solution is better.

Actually just to point out the flaws in your math. First the return damage of 12 would actually only 6 as planes on patrol only take 50% damage in return. Secondly after the first stack of aircraft damage the 5/5 LT/Inf stack the second will NOT take the same damage as the first stack did as there are now fewer units firing back. Finally to point out if used in stacks of 5/5/5, the return damage would be divided by 15 and rounded down for even less damage to the aircraft which is why planes are the only units that should be overstacked and always be used in groups.

Also from personal experience in attacking with planes, many times I will make one pass of both (5/5/5) air groups and destroy an enemy force while a single 10/10 group will take upto 3 passes to destroy the same sized unit. Thus the 5/5/5 group will take far less damage over the course of a game than a 10/10 group will.

Here's me tryna find a way to avoid math in CoW, and it keeps screaming at me in my face ._.

Wow, you're really introducing some fine fuzzy logic there! Smoke screen specialist, eh? Well, lets try to put a ventilator on that...

1) When you're patrolling ALL damages are divided in half compared to a direct strike. So the entire calculation still stands, as EVERYTHING is halved; you can see it as a calculation for two consecutive patrols if you want.

2) It is true that the ground stack has been slightly damaged by a first patrol when the second comes in, so it doesn't do 12 damage anymore. It is pretty insignificant on the first round though, I'd estimate it stays well above 11.
But since you brought up the subject, HP loss is yet another argument for the one-stack solution.
When you leave your planes overhead for a second round, the one stack lost 12 points, but the two stacks each lost (lets say) 11.5 points. That means that their attack values have gone down more than the one stack (remember that all those damages were taken by the same total of 20 planes). The two-stack damage output is going down faster than the one-stack output. The damage output of the ground stack is also going down faster of course, but RELATIVELY slower (namely, the difference in killed/lost ratio of 4 versus 2.94). So as the battle progresses, you're doing worse and worse.

3) Please don't confuse the argument by adding interceptors as well. You're comparing a two 5/5/5 groups to a 10/10 group now. When you want to do that, compare them to a 10/10/10 group and you'll get similar results as my calculation above.

4) "Rounding down" doesn't matter in the long run, since there is also an X factor involved, and all damages are sometimes higher, sometimes lower than the textbook tables. You'll have a lucky result once and a bad result the next time; in the end it all evens out.

5) Your "personal experience" doesn't correspond to mine. It is completely counter-intuitive that two patrols with 5/5/5 groups would do the same damage as three patrols with a 10/10/10 group, and I didn't see any results like that on any battlefield, except maybe some strange fluke of the dice.

Fuzzy logic? Really? So when somebody tells you that you are comparing apples and oranges you select bananas.
or are these 50% calculations because of the 10/10 stack they are only doing 50% damage or that only 5 units of the 10 are attacking??? So MY POINT of 2 stacks doing 100% damage vs 1 stack doing 50% is proven valid. Also if you are not using meat shields for your artillery you should reevaluate how you are playing.

I read it as if you were patrolling and not direct attacking. If you are direct attacking with aircraft you really are not using them up correctly and (not to be insulting) tbh have no clue on how to effectively use aircraft nor how effective patrolling aircraft are. Your math would be even MORE incorrect if you are using aircraft to direct attack then if patrolling.

I think you're the one introducing the banana's.

Of course I'm patrolling on the actual battlefield (though not always; there aqre several good use cases for direct attack). The point is that there's no difference for a calculation about patrolling or about direct attack since it is simply "half a direct attack" on BOTH ends. Without disrespect, you seemed to imply that the planes did full damage while the AA did half damage (only six; your post). BOTH sides do half damage so for clarity sake it is easier to calculate direct attacks; you get the same result when dividing everything in half. That's how calculus works, you know. No disrespect if you don't understand that of course.

The 50% is included because half the stack is infantry and the other half is armored. Did you actually know that you do less damage to the infantry when there's also other units (in this case, tanks) with them? Only half your planes attack the infantry now, the other half the tanks. Hence 50%.

It has nothing to do with any "proof" for your fuzzy logic. I don't see what "meat shields for artillery" have to do with a theoretical case about trying to explain to you how damage actually works; is it yet another effort to hide that you're losing an argument?

@S Schmidt, A 10/10/10 plane stack is better than 2 5/5/5's in general. A 10/10/10, being a 30 stack has better damage distribution than a 15 stack and so less damage efficiency deterioration.

Yes, with the 2 5/5/5's you will kill enemies faster but you also die faster - you get to attack twice but you also get counter attacked twice. And though you're correct that the second counter attack will deal less damage as damage efficiency of units do go down with their hp, that damage efficiency reduction isn't a straight parallel to damage sustained.

Damaged units punch well above their weight. If I'm not mistaken, the formula for damage efficiency is -> X • 0.8 + 20 where X is the current hp percentage.
If my math is correct, a unit at 50% hp would still have a damage efficiency of 60%, not 50%. Ignore this If that unit has a damage and hp value of 100, it would deal 60 damage despite having only 50hp. That's an extra 10 damage taken on your second 5/5/5 stack, 5 on the collective hp pool of the 2 stacks. In percentage, that's an extra unnecessary 20% more damage taken on the second 5/5/5 stack, 10% more on the collective than if you just stacked them in a single 10/10/10, yeah it'd be a slower kill but it'd be less self destructive.

Keep in mind that the lower your damage efficiency gets as a result of unnecessary damage taken, the longer it'll take you to kill a unit, and as result of that the more instances it can inflict damage on you and repeat.

Lastly, the lower a unit's health is, the bigger the disparity of it damage efficiency will be in proportion to its hp percentage. As mentioned above, a 50% hp unit will have a damage efficiency of 60% - it deals 20% more damage in proportion to its hp. A 20% hp unit on the other hand will have a damage efficiency of 36% which is 80% more in proportion.

Let me just start by saying that I was distracted by the
and counter with
what you think and what reality is, are two different things.

As I pointed out I assumed (I know this makes an ass out of u and me) the 50% of your math was because I thought it was on patrol, thus it was my error. But I will ΚISS (Κeep It Simple Stu) it this time just for you Κ Rok.

The question the OP asked was "Artillery for max damage"
and while you answered that all 20 units will do damage your math shows that 2 stacks will do the max damage. Thus I will stand behind my answer
as the right answer to his question. Regardless of damage TAΚEN by a stack, the damage DONE by a stack of 20 will never equal the damage done by 2 stacks of 10. If you wish to continue the discussion in another thread please be my guest.

GAME, SET, MATCH.
Nope its just time to move on. Have a good day.

It is kind of hard to say GAME SET MATCH back to you because you're probably say we weren't playing tennis anyway; and if we did, that the first two points in a game are more important because they score 15 points each while the third only scores 10 SO they are more important; or that you should play a racket three feet wide because you have a bigger chance to hit the ball.

I'm always a bit surprised when people simply try to deny plain logic, start introducing irrelevancies and try to obfuscate arguments on purpose, but yeah, you're not the first one that I meet.

Now you're introducing artillery which doesn't get return fire in the same way air does; actually you are right in the sitting duck situation for ARTILLERY ONLY. I gave an argument about the air combat since that was what OP also asked. And on AIR COMBAT, my first post containing the math example clearly shows that net kill/loss ratio for one stack is better. It is a real pity that you apparently can't read a pretty simple calculation.