View Full Version : Wing bending measured in flight during turns
30th Jan 2012, 16:52
Hello everyone. I cannot find my original posting: I wanted to know if for low wing single engine prop aircrafts there had ever been measurements done while in flight of the actual wing bending during sustained turns...
The types I am interested in would have to be similar in size and power to a typical WWII fighter.
I am guessing a simple camera looking down the wing leading edge could have been used...
Thanks in advance.
30th Jan 2012, 17:19
Strain gauges (http://en.wikipedia.org/wiki/Strain_gauge) have been used since the late 1930s in regard of flight load testing. You might have more luck in that direction.
Genghis the Engineer
30th Jan 2012, 19:55
Many of the WW2 era aircraft that are still flying are quite well instrumented - the usual strain gauges, accelerometers and the like - as part of their ongoing airworthiness oversight programmes.
So, find a friendly Spitfire owner, and they may well be able to point you at actual data.
31st Jan 2012, 15:19
Thanks very much for the pointer!
I wonder exactly how do these WWII strain gauge work and how are they calibrated? What is the gradient value used? Do they give instant "live" information during flight?
Genghis the Engineer
1st Feb 2012, 06:27
They work the same as any other strain gauge, aren't WW2 vintage, they're modern, and they feed a data logger, not a cockpit display. Just google it to find out how strain gauges work.
5th Feb 2012, 20:24
Oh ok: So they are not the usual WWII vintage...
I wonder if specific turn tests, using their values as a reference, were documented during WWII?
I'll contact some operators to see how they use them...
Thanks again very much.
21st Nov 2012, 00:34
It seems typical Warbird-operating outfits do not have strain gauges installed, nor do they need them to be certified for flight...
I contacted one of the head engineers of Vintage Wings in Ottawa, which operates about ten WWII fighters near my home, or various other 1930s single engine aircraft types (including one 1950s Sabre), and it seems wing strain gauges are not on their radar screen for any type of use or certification.
I wonder if any such strain gauge in-flight data exists for horizontal turns in WWII fighter aircrafts, either contemporary or more recent: The only data I have been presented with has been wing bending measurements that are for static on-the-ground tests...
It seems the use of strain gauges on the wings of these old machines is quite a bit more esoteric than I thought... As a practical matter, the limited access to the main spar might be a reason.
21st Nov 2012, 08:12
The only data I have been presented with has been wing bending measurements that are for static on-the-ground tests...
If that data includes deflection for a given bending moment, which it presumably does, then you are almost home and dry.
Go back to your warbird operator and ask them what load factor they would typically limit their aircraft to in a turn, or coming out of a loop, from which it's not difficult to work out the wing bending moments involved.
Then apply the static test data to see what the corresponding deflection would be.
10th Dec 2012, 10:02
The data would have to be in-flight.
There are "anecdotal" suggestions out there that there might be something different going on with the wings besides the "normal" bending moment on some low-wing single-engined nose-pulled types that are beyond a certain weight and power (like WWII fighters).
I specifically want to know if a WWII era, or thereabouts, flight test exists that shows that the wing deflection was actually flight-tested on a single-engine prop fighter type and matched to the "calculated" wing-bending moment for a given amount of in-flight horizontal Gs. This specifically in horizontal turns, not dive pull-outs or other vertical maneuvers.
The reason I am asking this is that some WWII pilots claim they achieved their fastest prolonged and continuous turn rates at much reduced power levels, way below maximum power, because they were sustaining much smaller, slower in speed but quicker in rate, horizontal circles over very, very long periods of consecutive maximum-rate sustained turning (up to 45 consecutive 360s non-stop at ground level).
So it seems strange, as reducing power below maximum here was clearly not a matter of killing excess speed.
I am pretty sure these pilots were mistaken in thinking this was fastest in sustained turn rate, but as I said, I am only interested in knowing about the existence of an actual WWII-era wing-bending measurement vs in-flight G horizontal turning test.
11th Dec 2012, 04:31
I think you're looking in the wrong direction. Wing bending in a balanced turn will be no different than for a similar g force pulling out of a dive or into a climb. It sounds like what is happening is that the best turn rate is occurring at a moderate speed, which is to be expected. Go too fast and the turn rate reduces, go too slow and you stall before you can pull the required g to get a higher rate of turn. So why are they finding this is happening at a lower power setting? Shouldn't they be flying the turn at max power and using the wing loading to keep the speed back? I think what is happening is that if they genuinely flew max power max g, they'd be pulling more g's than they are comfortable with, either from the perspective of looking after an old aeroplane or from the perspective of staying conscious.
Edit: I think, reading your post again, that you're talking about actually WWII pilots, not modern pilots of WWII aircraft. In that case it'd be a matter of them staying conscious, they probably didn't care so much about looking after the aircraft beyond keeping it together enough to get home.
11th Dec 2012, 05:32
The reason I am asking this is that some WWII pilots claim they achieved their fastest prolonged and continuous turn rates at much reduced power levels, way below maximum power, because they were sustaining much smaller, slower in speed but quicker in rate, horizontal circles over very, very long periods of consecutive maximum-rate sustained turning (up to 45 consecutive 360s non-stop at ground level). Once upon a time, I flew an aircraft that you could start a low altitude continuous horizontal turn at 420 knots, pulling maximum g, and with maximum power, the thing would accelerate up toward Mach 1. Clearly to sustain the maximum turn, you would have to use less than maximum power.
Isn't this the very effect you are wondering about?
11th Dec 2012, 11:47
With any aircraft, for a given bank angle in a coordinated turn, higher IAS yields two results: 1) greater radius of turn, AND 2) lower turn rate (degrees/second).
It's basic physics.
11th Dec 2012, 14:05
Clearly to sustain the maximum turn, you would have to use less than maximum power.
A fighter pilot would use a little less bank and holding the speed and g use the excess power available to gain height in an upward spiral.
A test pilot might do the same to establish steady state data at a range of powers that included more than you needed in a level turn.
23rd Dec 2012, 02:10
"Go too fast and the turn rate reduces, go too slow and you stall before you can pull the required g to get a higher rate of turn. So why are they finding this is happening at a lower power setting? Shouldn't they be flying the turn at max power and using the wing loading to keep the speed back? I think what is happening is that if they genuinely flew max power max g, they'd be pulling more g's than they are comfortable with."
This is exactly the question I have been asking myself: However these aircrafts could not sustain more than a very reasonable 3.2-3.4 Gs in continuous sustained speed turns, so this was well below what would be unsustainable for the pilot, especially in a combat situation: Given the vital need to gain on the opponent, why would less power be used?
Even a broader circle, if it is completed faster, is still a gain in sustained turn rate, and thus a gain towards an opponent's tail.
There is other corroborating evidence to the existence of this puzzle: One Me-109G-6 ace mentionned that the best sustained turn speed on his machine was an incredibly low 160 mph, barely 55 mph above stall, specifically mentionning reducing the throttle (and opposing that to what most other wartime pilots were doing).
When giving out full power (for a maximum of around 400 mph straight), this German aircraft type cannot turn hard enough continuously lower its sustained turn speed much below 200-220 mph, which is a good indication of how much the throttle was reduced by the Finnish ace to achieve what he considered the "best" sustained turn speed of 160 mph.
The problem with these types of aircrafts is that they were always short on power, so more power in turns for these prop types should not be a bad thing that would put the sustained turn rate outside of the pilot's indefinite endurance: 3.2 Gs is barely half the maximum the pilot could tolerate in unsustained high speed turns in those machines...
If that was so, why reduce the throttle, and maintain it there, when speed is already low?
Furthermore, even making a smaller circle at a lower speed will cause the same exact disconfort in applied Gs if the same turn rate is to be maintained: Turn rate and Gs are correlated it seems to me: Except for slight gain in gunsight lead from being "inside" in a smaller circle, there is no advantage in G confort to reducing the speed and still be completing the smaller circle in the same amount of time as a broader but faster-speed circle.
Since these WWII aircrafts lacked the power to black-out the pilot in sustained level turns (surrendering altitude in turns being generally a tactical no-no when "locked" in sustained low-speed turns, or a moot point when near the ground anyway), the only reason to reduce the throttle and find an advantage is if the smaller circles at a slower speed (and thus at a lower power level), were actually completed faster, because they were so much smaller in diameter compared to the loss of speed.
But reducing the power alone to achieve this doesn't make sense, as one would assume the reduction in circle diameter would be proportional to the reduction in speed, nullifying any circle completion rate advantage or worse, especially when speeds get as low as the above-mentionned 160 mph. (At a very low near-ground altitude, so no huge IAS-TAS discrepancy)
The only way to produce an advantage in sustained turn rate at a lower power level, it seems to me, is if the lower power actually reduced the "real" in-flight wingload in some way: This would allow making the circle disproportionately smaller compared to the loss of speed of the reduction of power, thus gaining a sustainable advantage (where the different thrust location of jets might not cause the same effect, hence the absence of this tactic in the jet age)...
Hence my interest in finding out if any variations in wing bending vs power output in level turns, for nose-driven aircraft types, has ever been observed and measured, or if any wing-bending stress gauge tests of this kind has been done on similar-configuration aircrafts.
23rd Dec 2012, 05:14
Go too fast and the turn rate reduces, go too slow and you stall before you can pull the required g to get a higher rate of turn.Each aircraft has a "cornering speed" which gives you best turn rate. Best turn rate occurs at the airspeed which just gives you maximum structural g (permitted) without going any faster. This is a well defined point on the V/n diagram where the stall speed curve intercepts the maximum permitted g limit line.
You can read on the topic of energy maneuverability on this paper:
The topic gets even more interesting when carried into three dimensions.
28th Dec 2012, 06:56
I am talking about the best sustained turn rate, that is the best turn rate that can be maintained indefinitely without loss of speed, and that is usually what is referred to as "out-turning" or "matching turns" in WWII lingo because the target often had to be peppered steadily from a fixed distance, leading slightly accross a circle, for quite a while before going down from an average 2% hit rate.
High closure rates in straight dives, or brief snapshots, were more demmanding on the pilot's aiming skills, and more suited to less common centralized armaments or really fast-firing and flat-shooting guns.
The sustained turn rate has nothing to do with "Corner Speed", which was a concept defined after the war, and is the minimum speed at which maximum safe G can be reached. (During the war, this was typically referred to as the "minimum radius of turn" for 180° for a given starting speed followed by speed decay, as opposed to "out-turns" or "best turn rate" for full indefinitely sustained consecutive 360s)
In WWII "Corner Speed" would typically be the minimum speed to reach 6Gs, and that is emphatically NOT a sustainable turn rate...
In any case, even the more modern concept of "Corner Speed" is poorly understood in vintage WWII fighters: A 1989 "Society of Experimental test Pilots (SETP) evaluation of four US WWII fighter types (P-51D/P-47D/F4U/F6F-5) revealed the 6 G "corner speed" on all to be an extremely high 320 MPH IAS in flat level turns, or close to their maximum level speed at 10 000 ft....
Previous calculations assumed this was around 240-270 MPH, but 6 G at METO power could not be reached in level turns at these speeds without stalling, yet this apparenty can be done in dive pull-outs at reduced power...
In any case, the results were not what they expected, and it makes me wonder how the "well defined" limits of these types were actually tested, particularly with what instrumentation, especially regarding the in-flight wing-bending in level turns.
28th Dec 2012, 10:17
Speed has a massive effect on turn rate. If you leave aside max rate for moment and look at rate one (3 degrees per second ) the angle of bank required increases with speed. If you a flying at low speeds this will always be 1g or thereabouts. If you keep increasing you reach a point where you will be pulling some serious g hence the normal limit for transport aircraft is 25 degrees angle of bank. If the procedure is designed to cater for high speed operations then it will assume a turn rate of less than rate 1.
In terms of combat manoeuvring speed is still a key factor. The classic modern example is fighter vs helicopter. A helo can sustain a very high turn rate at low speed. It won't be pulling much if any g to sustain this. If turn rate is what you are after then going faster does not help you. There are practical reasons why you don't want to be plodding around the battlefield at low speed in a fighter though so, lie so much else in life it is all about compromise.
When I was learning how to chase another aircraft in a spread formation my instructor called it the egg (small radius at the top, wide at the bottom). If you needed to tighten the turn pull up in the vertical, trade speed for height, at the lower speed you make up the turn you need and then dive back on the speed. You could adjust you position relative to the other aircraft without adjusting the throttle. Not everything was abut pulling more g.
29th Dec 2012, 06:49
The comparison to a helicopter is a useful point, especially if it can match the turn rate with lower Gs.
But in this case, the the Gs are lower because the helicopter is in effect rotating on itself, continuously changing its directional axis inwards into the turn, which relieves the Gs experienced: Gs are from the trajectory vs speed alone, but if you rotate on yourself as you turn, this "extra fake turn rate" rotation -gained with the foot pedal I would assume- will gain you "turn" rate at no cost in Gs, as the pivoting is within your own CG: This feels like "nothing" in effect...
Except by working-in some momentary slip with the rudder, an aircraft cannot fly like that in continuous uninterrupted turns where maintaining speed at the highest possible sustained turn rate is paramount (side-slipping continuously would cause drag).
So my assumption, in the Finnish Me-109G pilot's case, is that if he claims to beat in turns aircrafts flying around in a flat circle (near the ground) at full or near-full power and 200-220 mph, pulling say 3.2 Gs, and he can beat that by going at an extremely slow speed of only 160 mph at partial power, then that means he is sustaining, all things being equal, at least 3.3 Gs at partial power and 160 mph, while the other can only sustain 3.2 Gs without losing speed, which his faster plane could not tolerate.
If the two aircrafts are assumed equal (the Finnish pilot considered his own aircraft's inherent turn rate to be inferior, given the extra weight/drag of two optional underwing 20 mm gondola guns weighting 180 lbs each, the sole performance difference in his mind being his continuous "downthrottling" tactic, with no mention of "upthrottling" ever), how can the other aircraft be threathened with a stall at 200 mph and 3.2 Gs (Quote: "He made a mistake and his aircraft warned, forcing him to widen his turn momentarily") while he himself, on an inferior aircraft, is not stalling at partial power, 160 mph, and yet gaining slightly at say 3.3 Gs in this apparently inferior state?
I do know about gravity-aided turns, but here, as in many WWII dogfights, the two are barely hanging on in consecutive flat turns: Gravity-aided turns have no relevance to sustained speed turns in level turns.
It seems to me there is no way an aircraft can match turn rate while actually pulling lower Gs, even if it is going slower: For an aircraft, a given sustained turn rate means a given amount of Gs: x degrees per second of turn rate means X Gs, no matter what the speed is.
Yes at lower speeds you can turn tighter than at high speeds, but you can only complete turns faster because you produce more degrees per second and at the same time more Gs.
My problem with what the Finnish pilot (27 kill ace Karhila) is saying is that basically, while being barely 50 mph above stall, he could still pull more sustained Gs than a faster flying aircraft, which, at 200 mph (40 mph faster than him) one must assume was closer to its "Corner Speed", since the corner speed on these things was measured as being in the 300 mph range, at that height, in 1989, by the SETP.
None of these fighters at the time had the power to sustain turns at their Corner Speed, so one would assume (for these old machines) the more power the better for the available sustained turn rate and thus available Gs at low speeds
So by lowering power away from his Corner Speed, he was mysteriously gaining slightly in his wing's available lift it seems, since he could produce more degrees per seconds on an inherently inferior-turning aircraft.
A quote might be useful to show I do understand the concept of "Corner Velocity": From "The art of the kill"
KCAS — knots, computed airspeed
You may think that slowing down to minimum airspeed and pulling as hard as you can is the best course of action in order to achieve a high turn rate. Not so fast. There is a relationship between airspeed and Gs. At lower airspeeds, you have less G available or, in other words, you can't pull as many Gs as you get slow. Less lift is produced by the wings of an aircraft at slower speeds, and as a result, there is less force available to turn the aircraft. If you get going really fast (above Mach 1, for example), you also lose G availability. For every fighter, there is an optimum airspeed for achieving the highest turn rate. The airspeed where the jet has the quickest turn rate with the smallest turn radius is called corner velocity. In most modern fighters, it is between 400 to 500 KCAS. The F-16 has a corner velocity of about 450 KCAS.
29th Dec 2012, 09:04
A lot of what you are assuming is incorrect. Its a long time since I have delved into this type of aerodynamics so explaining all the science is beyond me at the moment but from what I remember you are heading down some incorrect paths.
Your assumptions about a helo turning in forward flight are incorrect in that the aircraft does not turn around the mast. The aerodynamic of how the rotor works can and do fill many text books but for the turn performance we are talking about here, in practice it turns the same way as an aeroplane. You also mention that turn rate is directly proportional to g. This is also incorrect. If you are really interested in this topic you will need to do some deep reading. 'Aerodynamics for Naval Aviators' is always a good start. I am sure others on here can recommend some other high quality texts. Good luck and happy reading!
29th Dec 2012, 16:07
On a slightly similar thought... I flew aircraft with nearly rigid wings, like the Hermes and Britannias which were fitted with periscopic sextants, this was in the 1960-70s.
Today's aircraft have flexible wings and no sextant mounting, unfortunately, but it might be of interest to measure the amount of wing flexing at a variety of weights and speeds ( Mach and CAS, I'm not sure which would be better) I suppose that this would give the actual wing loading during the course of a flight as fuel is burnt off. If the aft fuel trim tank was full, some of the total A.U.W. would be carried by the tailplane, allowing the wings to operate at a higher, more efficient flight level, sooner.
I recall that the B707's Optimum cruising level increased by about 1000ft per hour (restricted by other factors, of course).
When possible, as SLF I try to see the wing-tip lift (between V1 and VR, I estimate) through a window on the opposite side of the cabin. But it is not calibrated, sadly ! And sometimes the curtain has been drawn.)
On a Britannia baggage was normally stowed 2/3 forwards, 1/3 aft. It ought to have been the other way round with a full load of passengers.
30th Dec 2012, 06:31
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):
"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."
1st Jan 2013, 07:54
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.
1st Jan 2013, 10:01
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.
5th Jan 2013, 11:00
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.
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.
11th Jan 2013, 13:20
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:
Page Not Found (HTTP 404) - Veterans Affairs Canada (http://www.vac-acc.gc.ca/remembers/s..._101/SF_101_03)
"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...
11th Jan 2013, 14:14
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.
14th Jan 2013, 10:13
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:
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.