Here is a set of air rules I threw together and tested a while back for my Pulp and WWII game. Seems to work pretty well. As always I'm no expert on actual vehicle stats but after checking out a few resources and other systems I've statted several basic planes. I will likely add more and tweak a few things in the future.
In a side tangent many Fighter planes were BIG. Look at them compared side to side with tanks. Tanks had the advantage in Heavy Armor of course. When I post those stats I have a mechanic for that: the Heavy Weapon vs Heavy Armor rule (pretty much like Savage Worlds.)
For use with my Pilot class or whatever you have of your own creations.
Normal surprise checks with bonus/penalty.
Normal + Dex bonus + Miscellaneous (Pilot class bonus, etc.)
+4 if a Maneuver roll was won the previous round.
Pilot vs Pilot roll (Dex checks)
Success and highest roll wins. Winner gets a +4 to hit and +4 to Initiative bonus the next round.
Tie equals no bonus.
Vehicle AC + Dex bonus + Faster Plane ( +1/25 mph faster)
Flying defensively (no attack attempt) adds +4
-4 non-Pilot class
+4 maneuver bonus if gained
Speed bonus to AC
+ linked weapon bonus if applicable
Targeting the Engine or Pilot
-6 to attack the engine or cockpit/crew (due to position and better armor at these locations.)
See Engine/Critical System damage below.
Damage to Aircraft
1. IF aircraft is at half Hp's or less, -4 to Piloting rolls.
2. A natural 20 is a critical hit.
Roll 1d6. On a 1-4 an engine/critical system hit. On a 5-6 1d3 crew are hit.
The damage done is the percentage chance on a 1d100 that the plane's performance is impacted. IF this roll is successful drop speed by 1/4 of full speed.
Each additonal hit and rolled percent roll drops the speed by another 1/4 of full speed until the plane reaches 0 speed and falls from the sky.
IF a natural 1-5 percent is rolled on any of the engine/critical system percentile rolls the engine stalls and the plane begins to fall from the sky.
The Pilot must make an Int check (+ Pilot bonus) - total engine damage to restart it.
The Pilot has 1d4 +2 rounds to get restart or get out.
3. Aircraft at 0 hp or less, roll a 1d6. On a 1 it explodes for 6d6 damage. On a 2 it bursts into flame causing 1d6 per round to crew. On a 3-6 it falls from the sky trailing smoke and impacting in 1d4 +2 rounds.
Multiple machineguns liked to fire at the same time and same target. Each extra weapon gives a bonus to hit or damage as seen on the chart below. If more than one caliber of weapon is linked choose the highest damage for the base damage.
Maximum three attacks per round, 10 bullets per burst ( unlike five per attack in my regular rules air combat requires more to hit). The pilot can choose to use only one or two bursts to conserve ammo.
AC Bonus and Air Speed
Airplanes move a LOT faster than ground vehicles, often going hundreds of miles per hour. The speed makes them harder to hit. I've found conflicting information on maximum air speeds for WW2 planes so I'm going to keep it as simple as possible keeping it in miles per hour (I'm taking the rough average of what I've seen and giving a rough miles per hour for the airplanes) and assigning an AC bonus for the mph speed. Therefore....
A soldier on the ground is firing at a P-51 mustang. The AC of the plane receives a +7 bonus as it roars by at over 400 mph.
In a plane vs plane combat the speed AC bonus of a lower speed plane will cancel out part of the higher bonus. (The faster plane is better but doesn't completely overwhelm it as it does a ground target.) A P-51 vs a Messerschmidt 110 has a +2 AC bonus instead of +7 (7 -5 = 2.)