No radios. Voice comms will be cheating. Or simply allowed.

What would be neat and maybe only used offline is the model pilots give and respond to arm signals and wing waggling. That would be a big realism feature, moving, signalling pilots and crew.

There is also the colored flare or lights guns.

And friendly arty firing shells off to the side of your group to signal direction to enemy planes and relative height. And yes, at least the German Army did this.

Also to get the AA smoke colors right; black for German and white for Allied.