Ah, I see what you mean. There isn't a single structure that
could resonate as such, just many, many connected ones which tend to damp each other out? That makes sense.
But do you see my point about leaving engines not synchronised?
It just seems the right thing to do, if it is possible.
I don't know if it has been mentioned, but during WWII, RAF crews always synchronised their engines - by ear I imagine. The Germans, on the other hand, deliberately left their engines to 'throb' in a effort to terrify the civilians they were about to bomb.
From what my Nana and Grandad used to tell me, all it actually meant was the average civilian, who knew nothing of beat frequencies and synchronisation, knew for sure when it was 'Gerry' and when it was 'one of ours' overhead.
Roger.