Diving unloads the prop blades because the air is hitting more the front of the blades: The fact that the prop is constant speed, that is, gets coarser pitched as speed goes up, does not change the fact that the prop contributes less of the percentage of forward movement force in a dive...
As diving speed goes up, the prop is still somewhat turning at constant speed, but increasingly being turned instead of turning itself from the given power... Which is why it pays to downthrottle when diving steeply, so as to not overstress the engine...
Interestingly, many US P-51 pilots mention not only downthrottling at very low speeds, to reach maximum turn rate values in sustained speed horizontaL turns below 200 MPH, but they also specifically mention putting the prop blades on
full coarse, at reduced power and low speeds, to get the full beneficial effect of reduced power in slow speed turns (along with flaps down 20°: A procedure I call "the triple trick": Power down, flaps down and prop on coarse)...
This reduction in power allows
immediate gains in turn rate,
no slowing-down delay: This immediate effect inclines me to think the benefit is related to the load on the prop blades, and how this leverages down on the wings, causing them to bend more, hence my interest in finding out if any actual tests were done to verify if the wings bend differently in horizontal turns at different levels of power (on old prop fighter types).
Again, to bend the wings more, the prop power cannot just press down on the wings (which I think it would do because of the
assymetrical inflow of air, of a kind specific to a roughly level turn, which would make the thrust axis more nose-down than the expected 90° to the disc: The lenght of the nose becoming a major leverage multiplier): Pressing down more is no good if the wings do not simulataneously lift up
more by the same amount, in effect erasing the extra nose-down load...
I don't yet know how the wings would do that, but the only measurable trace of this happening would be wing bending measurements in level turns...
Please note that what follows is just an exercise to determine what would be required to observe the actual results of a FW-190A out-turning a Spitfire in low speed level turns, and yet not doing so at higher speeds (again as observed in real-life)...
Even the obvious loss of speed due to the extra drag of this "theoretical" nose-down tilting of the thrust axis (in effect creating an "artificial" higher angle of attack) might also be "hidden", because I think that what happens in a turn is that
the inside half of the prop disc gets slower incoming air in a turn,
creating a greater void in front of the blades, and thus greater thrust on the inside turn half of the prop disc.
In effect, the incoming air speed assymetry, inherent to a curve, creates extra thrust on one half of the prop disc, making the extra drag of nose-down thrust angle tilting less noticeable...
I figure the extra thrust effect could be as high as 100 lbs + of thrust assymetry per degree of angle of attack, which in sustained turns would add up to 700 lbs of assymetrical resistance at the nose at 7°.
Why do the tailplanes not collapse at the other end from lifting such a load at the front? Why does the pilot not even feel an initial effort in the stick? I think the only explanation to that would be that the CL shifts in front of the CG, the initial elevator action being simultaneous with incoming airflow assymetry, both of CG and CL now suddenly acting in tandem in a "scissor action" to lift the nose into the inside of the turn: This would create an instant "pulley", proportional to pilot stick effort, making the pilot completely unaware of the real effort involved in bringing the prop "higher" "into" the turn...
Even so, 700 lbs is way too little to explain how a FW-190A turns tighter and faster in slower turns than a Spitfire (but it does explain why shorter noses seem to often display an advantage in level turns, the FW-190
D losing much of this advantage when the D's nose became longer): To bridge a 50% wingloading gap, the leverages must be as high as 30:1: The Spitfire's CL moving 4 inches in front of the GG to now fight a 10 foot nose: 700 lbs over 10 feet requires 21 000 lbs to beat with a four inch lever of CG/CL acting in tandem...
All that extra CL effort can only come from an (undoubtedly complicated) increase in the void above the wing, simulatneous with the increase of the void in front of one half of the prop disc, just as the turn begins.
But that is
still not enough to bridge the wingloading gap for the FW-190A... Even with a two foot shorter nose, the only chance for it to even match the Spitfire's "real" wingload is to have its CL move dramatically more in front of the CG: One whole foot to an 8 foot nose, meaning the same 700 lbs is only adding 5600 lbs of wingload compared to the Spitfire's 21 000 lbs "add-on"...
At 3 Gs, a 9000 lbs FW-190A becomes: 27 000 + 5600= 32 600 lbs "real" load.
At 3 Gs, a 7400 lbs Spitfire Mk IX becomes: 22 200 + 21 000= 43 200 lbs "real" load, and that
finally takes care of the Spitfire's bigger wing advantage...
I think the sheer enormity of what is required for a FW-190A to beat a Spitfire in slow speed level turns becomes apparent now...
Don't doubt the airframes could take those extra loads: They were all designed with a factor of two to resist 12-14 Gs: The "added" load value of my theory probably diminishes at high speeds, so it is still the same value or less at 6 Gs (+ say 3 Gs, for 9G's worth total of
wing bending load at only 6 Gs felt by the pilot), destroying the FW-190A's advantage at high speeds/high Gs just like in real-life, the aircraft being comparatively very poor at high speed handling, its weight finally becoming the dominant factor as the Gs multiply...
Anyway, this is what I had to come up with to bridge the wingloading gap between these two: Have no doubt that the gap
was bridged in real-life:
First, I have never read an account of a Spitfire out-turning
at low speeds a FW-190A, or even in any kind of multiple consecutive 360° turn encounter. For a 50% wingloading advantage over hundreds of combat accounts, it sure is mighty discrete... Pilots could probably get to within 1-5% of the limit when bullets were flying... On the other hand, WWII-vintage flight tests appear much less reliable and consistent than actual real combat for turn comparisons, or so it would seem...
Second, the opinion of someone who was actually there and had first hand experience should always cause "theoretical correctness" to question itself, especially when the rest of the combat record agrees in spades...:
This is a quote from Hurricane pilot John Weir:
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"A Hurricane was built like a truck, it took a hell of a lot to knock it down. It was very manoeuvrable, much more manoeuvrable than a Spit, so you could, we could usually outturn a Messerschmitt. They'd, if they tried to turn with us they'd usually flip, go in, at least dive and they couldn't. A Spit was a higher wing loading..."
"The Hurricane was more manoeuvrable than the Spit and, and the Spit was probably, we (Hurricane pilots) could turn one way tighter than the Germans could on a, on a, on a Messerschmitt, but the Focke Wulf could turn the same as we could and, they kept on catching up, you know."
And there's plenty more in that vein, including from top Allied ace Johnny Johnson himself...
I can't post everything I found here, but it goes way beyond the anecdotal... That being said, if the wing-bending in
level turns was actually measured in flight in those types of machines, and did not show any bending beyond the loads expected, it would of course immediately blow my attempt to "bridge" the theoretical vs observed level turn gap to pieces...
Hence my interest in finding out if such level-turn wing-bending tests ever happened on these particular configurations of aircrafts...
Even data from dive pull-outs could still be useful: After all, the P-51s did break wings or tails in dive pull-outs, when North American engineers initially claimed this was impossible given the 13 G structure...