Wing bending measured in flight during turns

[Gaston444]

Well if turn rate is not proportional to Gs, that is not what this source seems to says: It is from a Kirtland air force base military magazine (link doesn't seem to work for me though):

http://safety.kirtland.af.mil/magazi...8/pullout.html

"Take me at my word when I tell you that turn radius is proportional to the square of the airspeed and inversely proportional to the number of Gs pulled [R V2/n]. When the airspeed goes up, the turn radius increases very quickly if we hold Gs constant. Hold the airspeed constant and the turn radius decreases when the load factor goes up. Contrast this to the relationships for turn rate. Turn rate is inversely proportional to airspeed [* n/V] and proportional to Gs. As you go faster, turn rate decreases. Conversely, the turn rate increases as you pull more on the pole. "

I always assumed the "Corner Speed" was the lowest speed to pull maximum structurally safe Gs, and so also the point of the highest possible rate of turn, since it is also the point of the smallest radius at this uppermost G level... Since lower speeds means a smaller radius but also lower Gs, that should also mean a lower turn rate, as you cannot pull as many Gs below Corner Speed.

If you say Gs and turn rate are not proportional, then that would mean a higher turn rate than "Corner Speed" is available below Corner Speed, which doesn't make sense to me.

Basically, my understanding of sustained turns was that they should be done as close as possible to the "Corner Speed", while knowing well that in WWII that meant sustaining low-speed turns as fast as possible, with as much power as possible, given that the structural Corner Speed limit was way above in Gs what those weak WWII engines could keep the speed constant at...

A gap of about 50-100 mph + it seems, from 200 sustained to the 250-300+ MPH lowest speed to reach 6Gs...

The "Fighter formation qualification program", in "Fighter formation fundammentals", also seems to agree:

"Corner Speed is the airspeed at which the highest bank angle can be achieved at the minimum airspeed. Thus maximum rate of turn will be realized at this speed."

[Gaston444]

Very interesting perspective on wing bending by Linktrained...

I'll just ask a simple question outright: Is it at all possible that wing bending in level turning flight was never formally measured on any WWII-era fighter?

Is it possible that it was never done in turning flight before the jet age?

I would appreciate a perspective on this, or a pointer as to where to look for the answer to the above questions.

[John Farley]

Gaston

I think you may have a hang up about 'level turning flight'. There is no structural or aerodynamic reason to be worried about level turning flight in particular.

Most issues associated with structural loading are related to g. For all formal flight tests where these are measured they are normally done during woigs level pull ups from either level flight or dives according to the amount of power or thrust available.

Rolling under g tests are another and quite separate subject as these involve not only wing bending but wing twisting at the same time.

I know this does not answer your original question as such but I want to get you away from this level flight turns issue into the more general case of applying load via g.

[Gaston444]

Quote: "Most issues associated with structural loading are related to g. For all formal flight tests where these are measured they are normally done during woigs level pull ups from either level flight or dives according to the amount of power or thrust available."

Actually, this above part almost answers my question, as it clarifies quite well the way these wing bending tests would usually be done: If that assumption is made, and the tests are typically done this way, then this does mean it is possible that wing bending measurements in level flight turns has never been done on these particular types of old fighters...

I'm sure a whole range of wing bending tests have been done since on modern jet fighters while in flight, but maybe not on less "advanced" single engine piston-propeller aircrafts that are low-wing monoplanes, as that kind of instrumentation seems uncommon on such small types, or of such vintage, with similar power and wing position at least...

The reason why I believe dive pull-outs might not give a meaninglful indication of level turning wing loads is because diving would unload the prop disc during the pull-out phase.

I know very well (in great detail) that the prop disc's load is not seen as affecting the wingload during level turns at all (at least not independantly of forward speed), but I think the location of thrust origin so far down the nose could be wrongly assumed to have no effect on wingloading. (I realize for it to do so would mean something very complicated is happening)

There are two indications that make me lean towards this no-effect assumption being entirely wrong: First, the repeated mention by WWII fighter pilots of superior sustained turn rates with reduced power. This far, far below their "Corner Speed", which makes no sense, as CS is the speed of the highest possible turn rate, and you would want to be as close as possible to it in a turn, which inevitably means all the power available for these old types.

Second, I have reasons to believe, from numerous first hand accounts, and general condensed conclusions (from several frontline's worth of combat experience over several years), that the Spitfire, of all marks, at around 140/150/160 lbs per square feet of wingloading, actually has no hope to catch the FW-190A, at 215/230 lbs per square feet of wingloading, in sustained and level low-speed turns at medium-low altitudes: Contrary to widely accepted lore, the margin in favour of the heavier fighter over the lighter aircraft is in reality quite noticeable, and commented on by some pilots (many other pilots having evidently trouble believing it, even including, it seems, a few of its users) as long as speeds remain slow and the turning continuous, but that advantage is reversed when speeds are high and especially when the turning is abrupt, where the Spitfire then turns inside it quite easily until speeds again get slower.

Another observation is that the dive pull-out performance of the FW-190A is extremely poor: A 40° dive from 1200 m will, upon levelling off, produce a further 220 m (660 ft.) loss of altitude with a slight nose-up attitude, with a tremendous loss of speed as the aircraft decelerates violently "nose up" while still going down for a considerable distance, this with a "tendency to black-out the pilot" if speeds are high enough, yet Gs and the nose-up attitude (and thus the amount of deceleration while "mushing"), still responds to stick inputs...

High-speed turn performance is similarly poor but not quite as bad, with sometimes a similar "mushing" or a violent wing drop. Low-speed sustained turn performance is, on the other hand, excellent to superior, especially with reduced power, flaps and below 220 mph.

As can be seen, the correlation between dive pull-out performance and low-speed turns seems extremely poor.

All of these observations makes no sense according to current accepted flight physics, hence my question about actual wing bending measurements in level turns for these particular types of aircraft.


P.S. Of note is that the exact same type of relationship can be observed between the P-47D at 17 000 lbs and the Me-109G at 7 000lbs (about 15-20% lighter wingloading for the Me-109G), except that, in this case, the slow-speed sustained turn superiority of the P-47D is far larger compared to the Me-109G than is the case with the FW-190A vs the Spitfire (all Spitfire Marks being roughly similar on that aspect).

Note there are a lot of tests that conclude otherwise (except for Geman tests, which confirmed the P-47D's superiority in sustained turns to the Me-109G, and that of the FW-190A to the Me-109G as well), but I preferred large numbers of combat accounts because most controlled tests were contradictory among themselves, while combat seemed perfectly consistent in comparison.

[Lumps]

I tried to follow this but got a bit lost... I cannot see how wing bending relates to anecdotal evidence of aircraft with higher wing loading being able to out-turn another.

Also please explain how pull out from a dive 'unloads the prop disk'. You are talking about aircraft with constant speed propellers, so this is not power related, are you referring to precession?

Btw have you considered the fact that the 109 had leading edge slats? I dont know much about that particular installation but on the Helio Courier high power / high AoA the slipstream from the propeller can force the slats closed (the inboard on one wing due slipstream rotation), reducing the stall AoA of that part of the wing. Reduce power and the slats pop out again.

[Gaston444]

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-190D 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...

[barit1]

The P-47 was mentioned in earlier posts. You may be interested to know the earlier P-47B had a bit shorter forward fuselage, and at some point was extended 8" IIRC for the majority of P-47 production. Not sure how this might affect this study.

[Gaston444]

The B never saw service, but I did note the earlier Razorback Ds or Cs did better in sustained turning combat than the later Paddle-Blade equipped Razorbacks, and more so the Bubbletops.

Unlike the shorter B, here the nose length from Razorbacks to Bubbletops was the same, but the prop was now always a paddle-blade prop on the Bubbletop, and the Bubbletop was also heavier with more power.

On this point, it should be noted some P-47D pilots are on the record stating the Paddle-blade prop and the extra power of water-injection did improve their relative turn rate vs the Me-109G, but the actual combat reports generally leave me sceptical on this point, as they were doing quite a bit better in combat doing sustained turns in late 43 and early 44 than in early 45...

One aspect that might give them this perception is if you have more climb rate and acceleration, you can then climb higher easier, to then make a prop-unloading descending turn, or you can reach a higher speed in shorter straight lines, which then allows you to pull more Gs while burning more speed.

For sustained speed level turns, however, the less powerful needle-tip prop Razorback seemed to offer a more significant advantage margin: An underpowered Razorback P-47D with needle-tip prop was tested by the Germans of KG 200, and their conclusion was unequivocal: "The P-47D out-turns our Bf-109G" (Source: "On Special Missions, KG 200")

In paralell to this, on the issue of nose length, I have a link to a 1946 test of a "long-nose" FW-190D-9, and the results are surprising considering the reputation of this aircraft:

http://www.wwiiaircraftperformance.o...d-fw190d-9.pdf

Quote: "1-The FW-190D-9, although well armored and equipped to carry heavy armament, appears to be much less desirable from a handling standpoint than other models of the FW-190 using the BMW 14 cylinder radial engine."

Any advantage this airplane may have in performance over other models of the FW-190 is more than offset by its poor handling characteristics."


By contrast, notice the huge and seemingly unexpected jump in handling performance of shortening the nose in the two following types:

LaGG-3 to La-5 and Ki-61 to Ki-100...

Contrary to popular lore, the shorter-nose radial conversions were in both cases heavier than the longer nose type they replaced, by 200 lbs and 100 lbs respectively...

Yet it was widely acknowledged there was no comparison in handling performance... The Ki-100 was considered so highly performant (by the Japanese), in turning and climbing combat, that it could take on up to three Ki-84s and still have a chance of winning in a mock dogfight...

The longer-nosed Spitfire Mk IX vs the similar shorter-nosed Spitfire Mk V is a bit more difficult to pin down: Wartime RAE tests have them as identical in sustained turn rates at all altitudes, with the Mk IX exhibiting only a very large advantage in climbing and diving attacks, this advantage increasing with altitude.

However, I have from first-hand knowledge from Warbird operator "Planes of Fame", who have compared both over a long period of time, that the Spitfire Mk IX cannot turn with the Spitfire Mk V in sustained turns, although how large the difference is was not specified.

Gaston