To comment first on what another poster added: The correlation between wing bending loads and slow speed turning at various power levels was likely never done on WWII fighter aircrafts. As he points out:
all WWII fighters wing bending data is likely static in nature...
Two, I finally think I understand what you are trying to say when you talk of low-speed turn stress. See if I am right. You are thinking that if the aircraft is at a relatively low speed, say … 180 mph, and in a level, say … 4g turn, then the power in the engine must be increased and this, in turn, applies more stress to the airframe than a level, 4g turn at, say … 300 mph. Is that it? .
Yes that is exactly it: I'll go back to the power level issue just below...
To bridge the gigantic gap in wingloading between the Spitfire and the FW-190A, and to still give a significant sustained low-speed turn advantage to the FW-190A, I figure the Spitfire's wing at a 3G turn at full power is
really being bent as if it was almost at a "theoretical" 6G in actual in-flight wing bending force, while the FW-190A's wing at the same 3 G would be bent as if it was at say 4G: The gap between the 4G bending stress on the FW-190A wing (at 3G of "true" turn felt on the pilot) and the 6G bending stress on the Spitfire wing (also at 3G of "true" turn felt on the pilot) would overcome the Spitfire's theoretical wingloading advantage: In fact the Spitfire's wing
real in-flight wingloading seems higher than a FW-190A, as RCAF pilot John Weir observed when he said "The Spitfire has a higher wingloading": It was observable experience to him, not theoretical numbers: I think those Spitfire wings are much more heavily stressed in low-G sustained turns than current flight physics theory considers, because there must be a loophole or error in basic flight physics that may not be scaleable to lesser power (or high-wing) prop aircrafts...
At 6Gs the Spitfire's wing would, according to this, be bent like if it was at 9G, but the airframe is rated around 12-14Gs so there is room in there for the effect...
The root of the phenomenon (that I presume to exist) is something I call now the "CL collapse", but is in fact a CL displacement down and forward of CG as the elevator tries to raise the nose.
This displacement is caused by the prop's resistance to be forced below its potential forward speed in an assymetrical way (which prop "resistance" insures continuing directional stability, even with the CL suddenly displaced forward of CG).
A valuable counter-argument to that is that common drag, like a simple external mirror, also puts the whole prop surface below its potential forward speed, but linearly, and thus to my mind in a benign way: My argument is that the real prop resistance starts only with the assymetrical necessity of a turn. In other words the prop doesn't react strongly to being slowed down
as long as it is done evenly on its whole surface, but leverage-wise
there is hell to pay if you want to slow it down unevenly, as in a turn, because the length of the aircraft's nose suddenly intrudes, with leverage, as soon as any tilt to the trajectory is introduced...
If tail and nose are close in lenght, as in a P-51, then the CL collapses
more down and moves
less forward. On a longer tail and shorter nose type, like a FW-190A, the CL collapse is
less down but moves
more forward: That is the engine of the system. It is largely an issue of how taxing for the CL is the leverage of the tail vs the nose: A long tail with a short nose is less taxing to the CL: The CL will go less down but strangely enough it will consequently want to move more forward, giving the CL more leverage to lift the nose as the turn increases the CL's lift.
CL collapse is a very small, perhaps micro-second event. The "pivot axis" of the CL collapse I situate in the prop face: It "sets" everything that follows: The more the CL collapses, the lower the prop's "pivot point" for that collapse, the lower the prop's "pivot point", the larger the prop surface brought below its forward speed potential: The larger the prop surface brought below its forward speed potential, the greater the disparity in incoming air speed between the outside turn of that area and the inside turn of that area. The larger that disparity in incoming airspeed, the greater the amount of thrust slanting for 1° of "normal" pilot-induced AoA increase.
In effect, for a micro-second the prop is "pulled" back assymetrically from the top as the CL collapses a very minor amount, then the prop rises as it rotates up, as the now forward CL
relieves the pilot of the effort of lifting it in an upward movement, as the turn actually begins to tighten... There is a "scissor" action, with the CL being now forward of the CG, that is favourable to lifting the nose as the turn tightens (unlike if the CL had not moved forward of the CG).
This is why huge prop forces at the nose
cannot be felt by the pilot in the elevators: The collapsed CL moved
forward and its action took over like a pulley, still responding to the pilot's touch but nullifying 90-99% of the effort to lift the nose, and thus lightening the load just like a pulley does.
Say the pivot point of the CL "collapse" is set in the lowest 10% of the prop, then 90% of the prop moves back compared to the trajectory: This may be equivalent to +0.9° of thrust slanting for each 1° of "pilot induced" AoA increase: 1.9° worth of
lift for 1° AoA, 13.3° for 7° AoA, but probably after that no more than 20.6° for 14° AoA.
There must be an extra cost in drag in all that extra "added" thrust slanting angle, but if the rule is the same for all, how do you know there is an "anomaly"?
So the "kind" of CL collapse ("deep" and "short" vs "shallow" and "long") sets the "boost" to the AoA by lowering the initial pivot point in the prop's face (which pivot point may exist for barely a micro-second to "set-up" what follows), and it also sets the amount of extra void above the wing needed to actually bend the wings beyond what the "visible" AoA could in theory accomplish.
Now I have to say I don't know how that extra void above the wing balances itself with the thrust slanting, as the AoA is gradually increased or decreased by the pilot, and even how it remains in place: How can that extra void (initiated by the CL collapse) not be washed away by the airflow?
The only explanation I can come up with is that as the aircraft turns, the difference in the speed of the air above and below the wing is exacerbated, and some of that faster air below the wing "leaks" around the trailing edge as the CL collapses, this faster airflow moving then forward(!) and under the boundary layer on top of the wing, keeping the boundary layer higher and thus the void above the wing proportional to the "boosted" value of the AoA + thrust slanting.
The proportional aspects would be as such: A tighter turn would make a greater air speed disparity on the top and bottom of the wing, "leaking" comparatively faster air from below the trailing edge, and lifting the boundary layer higher, creating a larger void. A slower turn would "leak" comparatively slower air and cause less lift of the boundary layer, for a lesser void.
That is a stretch, even for me, but if it has remained ignored for so long it cannot be that simple to figure out...
This is the only way I could figure how the engine's power could in fact increase or decrease the bending of the wing independantly of the turn's G force.
How many WWII types report faster
sustained turn rates with seriously lowered power? The P-51D is most prominent, as is the FW-190A, with just one Me-109G pilot (Karhila) also mentionning the best sustained turn speed for the Me-109G-6 as being 160 mph with a strong emphasis on reducing the throttle to achieve this best sustained turn performance.
Absolutely no mention of reduced throttle sustained turning for the P-47D and Spitfire, and relatively little for the Me-109: Why? I think the convenience of the flap "set up" plays a big role in how pilots will risk experimenting with lower throttle settings: The flap set-up for the P-47D must not have encouraged exploring lower speed turning performance at reduced power, and neither did the size and weight... The Me-109G could not really use its laborious flaps in combat, and the Spitfire had only full up or full down flap positions, a huge limitation in my view...
Above corner speed, the pilot CAN fail the wing, but above corner speed the aircraft is not in a "low speed turn" and the situation does not apply..
The SETP "Society of Experimental Test Pilots" tested the corner speed of 4 WWII US types in 1989, at maximum continuous power ("normal power") and found the P-51D's "Corner Speed" in
horizontal turns to be a surprisingly high 320 MPH, close to the maximum level speed at this power level at 10 000 ft for ALL four types: P-47, P-51D, F4U, F6F: This was due in my opinion to the loading of the prop in horizontal turns...
At War Emergency Power, the "Corner Speed" (minimum speed allowing 6Gs) in level turns would probably have been even higher: 350 mph?
Did you ever achieve 6 G in
horizontal turns at full power below 300 mph? At normal power? Remember you cannot unload the prop by spiralling down: It has to be a true
level turn.
Gaston