Wing bending measured in flight during turns
 

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.

Strain gauges have been used since the late 1930s in regard of flight load testing. You might have more luck in that direction.

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.

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?

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.

G

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.

Gaston

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.

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.

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.

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.

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?

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.

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.

"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.

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:
http://dynlab.mpe.nus.edu.sg/mpelsb/dts5121/Perf4n.pdf
The topic gets even more interesting when carried into three dimensions.

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.

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.

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"


Corner Velocity

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.

Gaston,

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!

Gaston444

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.