I have almost no experience in P-51Ds.
However, I have some experience fighting them.
As a 109 flyer, I can say that the dumbest P-51D tactics against a 109 is a straight forward BnZ attack which comes down and immediately goes up after. All 109s, even the lesser Gs like the G-2 or the G-6, can neutralize the alt advantage the P-51D holds very quickly.
Lure a P-51D into a shallow spiralling dive down to 400mph, and roll out of his sights at the last moment. If he climbs immediately upwards, the 109 can catch him going up unless his attack pass was something higher than 500mph. About up to 70~80mph speed difference, the 109 can close the gap.
That means, the most fearful P-51Ds I've countered are those who don't do that. Particularly P-51D pilots with good gunnery, are a menacing experience to fight against.
Those P-51D pilots don't just come straight down and run straight up in a long, dull straight line. What they do, is they maneuver and carefully position themselves right on top of my head, pressuring me to a very uncomfortable state.
When I see a P-51D at 6Oc position, coming down to me in a shallow angle from distances more than 3.0k out, I actually feel comfortable with that.
But when I see a P-51D right above my head at 3.0k circling around, maneuvering so he always stays right on top of me, instead of my 6 or high 6, that freaks me out.
It's because I can't force anykind of meaningful overshoot when he comes down from that position and angle. Also, since he will be coming straight down to me from the top of my head, it's impossible to turn into him and force him into a high speed, high deflection front-quarter shot. Those shots are usually very hard to land. Aggressive tactics against a BnZing enemy plane usually requires the defender to turn and meet his every attack - I can't do that if he's coming straight down from the top.
..
So, I naturally try to gain some distance, and move away into a position where I can see him at my 6 or high 6 Oc. A good P-51D pilot knows that fact, and he will always try to deny me the chance to do so. Staying at the top of my head.
And when he sees an opportunity, he drops down. Because I couldn't lure him into a long spiralling chase, I can't neutralize the differences in E states. While I am squirming around, trying to lose him from the top of my plane(can't build enough speed), he dives in from a high angle(builds large speed).
The result is, I can't follow him up in that case. Every pass he makes, is like that - it's incredibly annoying and incredibly nervous to see an enemy who does that.
