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Detecting Propeller load variations on the vertical plane

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Detecting Propeller load variations on the vertical plane

Old 29th Feb 2020, 17:20
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Detecting Propeller load variations on the vertical plane

What I am interested in is a question of basic prop physics, for a configuration like that of WWII fighter types: How would a vertical (to fuselage) prop load variation be detected, since, on take-off, any vertical tendency would be held by the ground one way, and the airplane's weight the other? The exact same holds true in straight level flight, or vertical pull-outs: Wing lift being then substituted to the "ground": These much larger vertical forces would "hide" any vertical variations in the propeller load...

Maybe steeply banked horizontal turns would be more sensitive to this vertical prop load variation, since some types are known to require stick pushing while turning at lower speeds, to maintain the turn, which indicates some kind of anomaly in the vertical trim of the aircraft. One that seems only detectable in sustainable speed (lower speed) steeply banked turns...: Interestingly, lower speed turning is where wing lift is weaker, but propeller load is at its highest... This effect usually disappears at higher speeds.


What I am specifically interested in, related to this, is what is the current flight physics consensus on the effect of the wing's presence in altering the uniformity of the vertical loads on the propeller. What I mean is that, relative to the turn's curved airflow, the wing in effect drops in relation to the prop disc in a turn: Would this not disrupt, vertically, the uniformity of the loads inside the propeller?

The reason I am asking is that all of the well-known prop load variations (P-factor, torque, splistream spiral on tailplanes) are either lateral or rotational in nature, but purely vertical prop load variations would be far more difficult to detect, since the ground or wing lift is "holding" them one way, and the aircraft weight the other...

Exactly what is known about the wing position's effects in disrupting the vertical uniformity of the prop's thrust, since even large effects would be completely undetectable to the pilot, except maybe as raw comparative turn rate data?

Gaston
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Old 2nd Mar 2020, 12:28
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I'm not sure exactly what you mean by "detected", but the bulk of the research on the forces normal to the propeller axis of rotation was done in wind tunnels by groups such as NACA. The usual reference is "Propellers in Yaw" by Herbert Ribner, NACA Report 820. The forces were "detected" by the wind tunnel balances.

A slight variation to this would be the work done on the effect of downthrust (tilt down of the propeller axis) on longitudinal static stability, eg "Effect of tilt of the propeller axis on the longitudinal-stability characteristics of single-engine airplanes" by Harry Goett and Noel Delaney, NACA Report NACA-TR-774. You could argue that the impact of vertical force affects long-stat, and thus is "detected" in that measurement.

Not sure if this helps you.


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Old 2nd Mar 2020, 17:52
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You've mentioned vertical a few times, but which vertical do you mean? Are we talking relative to aeroplane axes, perpendicular to prop shaft or vertical in relation to the ground reference? The first one is very much dependant on engine installation, as that is not always completely in line with the aeroplane's horizontal axis, the second one would be mostly irrelevant for situations where local airflow is lined up with the prop axis, but becomes interesting when you start moving the aeroplane to disturb this relationship. As for the third one, with all the different attitudes possible within even a normal category aeroplane it wouldn't be very useful to start on discussing this.

When talking about loadings on a prop disc in a turn, I'm not sure you can say that the wing influences the prop in any way. The wing influences the balance of forces on the entire aeroplane, which in turn influences the aerodynamic loading on the prop and it is this aerodynamic loading, together with gyroscopic forces and the torque of the engine that mostly determines the forces within the prop. P-factor is one way of modelling this aerodynamic loading due to AoA (could be used for side-slip as well). Torque is an input from the engine to the prop. Rotational slipstream is an effect from the rotationg prop, affecting the rest of the airframe. I wouldn't describe this as an influence on prop loading.
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Old 2nd Mar 2020, 22:43
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Thank you stressmerchant and Jhieminga: I will try to look up these documents.

The problem I see with wind-tunnel testing is it would not duplicate the air's curvature in a turn... The vertical I refer to is always to the fuselage. Even low speed sustained turning could be close to a 80- 90 bank, as pilots can keep the ailerons deflected during the turn, this to increase bank angle beyond "normal" parameters (catching the wing drop, in effect),.. And low speeds is the area of maximum prop load. So variations vertical to the fuselage, or close to it, can be assumed here for simplicity, even during low speed sustained speed turns.

The effect on the prop I would see is that, in a low wing position, the prop's exit spiral is, by necessity, "split" into above wing or below wing airflows. Increasing the engle of attack, while curving the incoming air upward, might causethat "split" to change in its above-below wing distribution, some of the "below wing" air "shifting" to above the wing. If that proportion of change is significant, this air would then be forced into a kind of "dogleg" path, which would lenghten its path, and so accelerate this portion of the air, depressurizing the corresponding area of thrust within the prop disc (in this case the below wing area).

The basic reason why I am asking this is that 25 years of reading WWII fighter combat accounts has me convinced that the prop, in these particular low-wing types, is "turn averse" to a significant extent. All you hear about, constantly, is the use of reducing the throttle, not using full power in combat, using less than full power in combat, except strictly for straight lines. Even more significant, increasing the throttle to increase the turn rate is never mentionned, not even once (given the tens of thousads of accounts I have read, I find this absolutely astonishing)... There are numerous accounts that are cristal-clear about lowering power to increase the turn rate at extremely low speeds, very near the ground... All this implies prop power is turn-averse to a certain extent, especially at low speeds: To be turn-averse, there must be an uneven (opposite to turn) bias within the prop disc... The arrival of jets in 1945 radically changed this power-averse pilot behaviour, which is yet another clue. The behaviour of front-line pilots regarding power management bears no resemblance at all between the jet era ad the prop era: Prop era pilots always behave as if their aircrafts had too much power, and that this was detrimental to combat maneuvering, no matter how low the speed or altitude, and especially at low speed and altitudes...

The general impression I get from known propeller effects is that their aversion to turn is only mild and yaw-related, but nothing about vertical to fuselage turn-aversion. Hence my question.

Gaston
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Old 2nd Mar 2020, 22:48
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Thank you stressmerchant and Jhieminga: I will try to look up these documents.

The problem I see with wind-tunnel testing is it would not duplicate the air's curvature in a turn... The vertical I refer to is always to the fuselage. Even low speed sustained turning could be close to a 80- 90 bank, as pilots can keep the ailerons deflected during the turn, this to increase bank angle beyond "normal" parameters (catching the wing drop, in effect),.. And low speeds is the area of maximum prop load. So variations vertical to the fuselage, or close to it, can be assumed here for simplicity, even during low speed sustained speed turns.

The effect on the prop I would see is that, in a low wing position, the prop's exit spiral is, by necessity, "split" into above wing or below wing airflows. Increasing the engle of attack, while curving the incoming air upward, might causethat "split" to change in its above-below wing distribution, some of the "below wing" air "shifting" to above the wing. If that proportion of change is significant, this air would then be forced into a kind of "dogleg" path, which would lenghten its path, and so accelerate this portion of the air, depressurizing the corresponding area of thrust within the prop disc (in this case the below wing area).

The basic reason why I am asking this is that 25 years of reading WWII fighter combat accounts has me convinced that the prop, in these particular low-wing types, is "turn averse" to a significant extent. All you hear about, constantly, is the use of reducing the throttle, not using full power in combat, using less than full power in combat, except strictly for straight lines. Even more significant, increasing the throttle to increase the turn rate is never mentionned, not even once (given the thousands of accounts I have read, I find this absolutely astonishing)... There are numerous accounts that are cristal-clear about lowering power to increase the turn rate at extremely low speeds, very near the ground... All this implies prop power is turn-averse to a certain extent, especially at low speeds: To be turn-averse, there must be an uneven (opposite to turn) bias within the prop disc... The arrival of jets in 1945 radically changed this power-averse pilot behaviour, which is yet another clue. The behaviour of front-line pilots regarding power management bears no resemblance at all between the jet era and the prop era: Prop era pilots always behave as if their aircrafts had too much power, and that this was detrimental to combat maneuvering, no matter how low the speed or altitude, and especially at low speed and altitudes...

The general impression I get from known propeller effects is that their aversion to turn is only mild and yaw-related, but nothing about vertical to fuselage turn-aversion. Hence my question.

Gaston

Last edited by Gaston444; 3rd Mar 2020 at 00:45.
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Old 3rd Mar 2020, 09:23
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A couple of quick points:
  • Ailerons are not kept deflected during a sustained turn. Once the needed bank angle has been attained, you keep the ailerons neutral to kill the roll rate.
  • Wing drop is a behaviour associated with the stall. Once his occurs, you have exceeded your critical angle of attack on that wing and aileron input won't solve that.
  • If I'm reading your second paragraph right, you're assuming that a local flow at the wing is influencing prop loading upstream of that wing. I can't see that happening. Without delving into it, I'd hazard a guess that the influence of the wing is negligable at the prop. See my previous post, but also, consider the relative size of the speed vectors involved. A prop is essentially a bit of wing on a circular path, and you can look at the local speed vectors for that bit of wing to see what's happening.
  • Adding power to increase the turn rate is never done because adding power will increase your speed, which will decrease your turn rate. That's basic physics and has nothing to do with a prop or a jet. See here: https://aviation.stackexchange.com/q...d-rate-of-turn
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Old 4th Mar 2020, 00:29
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Gaston,this may help with general `turning performance` but primarily for level flight turning. A significant difference between piston prop/and turboprop aircraft and jets is that the propwash can give you better yaw and pitch control at lower speeds than a jet aircraft,as a generalisation,and usually have lower stalling speeds..
If you have as an example two aircraft,same,weight piston aircraft stallspeed 80 kts,jet stall speed 100kts,both limited to 4G,,both start turning at their limiting G,the piston will be at 160 kts at 75* bank,with a turn radius of 611ft and turn rate of 24*/sec; the jet will be at 200kts at 75*bank,turn radius of 955ft,and turnrate 20*/sec.But real combat is not like that,so one has to employ other tactics...
Try-csgnetwork/aircraftturning performance.com
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Old 4th Mar 2020, 23:43
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Originally Posted by Jhieminga View Post
A couple of quick points.

Ailerons are not kept deflected during a sustained turn. Once the needed bank angle has been attained, you keep the ailerons neutral to kill the roll rate.
That is not what actual WWII pilots did... You waited for the wing drop then deflected the ailerons, and kept them deflected. In addition, the FW-190A had 3 different types of aileron chords available, and turn-oriented pilots chose the widest chord to have the greatest "bite" at low speed, precisely to "catch" the stall at low speeds. One pilot whose account I've read went further, and added spacers at the hinge to get the ailerons to "stick out" of his wing even more... The idea that a given speed and given bank angle will give a rigid and uniform turn rate is simply false...
  • Originally Posted by Jhieminga View Post
    Wing drop is a behaviour associated with the stall. Once his occurs, you have exceeded your critical angle of attack on that wing and aileron input won't solve that.
He did not fully wait for the wing to drop: He must have been able to anticipate the warning of the behaviour properly... Maybe the above is true on some types, but not all, for the above reason.

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  • Originally Posted by Jhieminga View Post
    If I'm reading your second paragraph right, you're assuming that a local flow at the wing is influencing prop loading upstream of that wing. I can't see that happening. Without delving into it, I'd hazard a guess that the influence of the wing is negligable at the prop. See my previous post, but also, consider the relative size of the speed vectors involved. A prop is essentially a bit of wing on a circular path, and you can look at the local speed vectors for that bit of wing to see what's happening.


I posted this very question on the Physics Forum, and this is the answer I got:

Quote: Question: "It seems odd that something behind the prop could affect, asymmetrically, the airflow through the prop... Or, at least, the ability of the prop to generate thrust evenly, throughout its surface.


Answer: "The short answer is, yes, the changed position can change the airflow. Everything is inter-related. Pressures and flows behind the prop must come from somewhere before the prop. The flow from the prop must fit in with the other airflow around the wing. So changes in airflow behind will almost certainly change things before."


Source https://www.physicsforums.com/thread...irflow.972086/


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  • Originally Posted by Jhieminga View Post
    Adding power to increase the turn rate is never done because adding power will increase your speed, which will decrease your turn rate. That's basic physics and has nothing to do with a prop or a jet. See here: https://aviation.stackexchange.com/q...d-rate-of-turn
I am of course aware that the radius is always square root smaller with less speed, that is basic physics... But the highest sustained speed turn rate is the goal, which is the same as the lowest speed at which it can still be achieved, (I'll ignore unsustained speed turn rates)

Adding power does not necessarily increase your speed...: It can simply prevent your speed from dropping further... More power WILL allow you to maintain a higher G at a given speed, because it prevents speed decay falling under the minimum allowable speed. That is true of jet fighters, and is the basis of energy tactics in general. The problem is that prop fighters, at similar minimal speeds, seem to achive higher turn rates withless power...

WWII dogfighting was often described as a race to be slowest, but more power should allow you to be slower still, until you reach 1:1 thrust to weight, and can hover and spin in place...: More power should not be detrimental to turn rate when the minimum speed for the maximum sustainable turn rate is achieved,

Gaston



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Old 5th Mar 2020, 02:11
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but more power should allow you to be slower still
A little bit, yes.... To reduce turn radius... not really. Yes, you can drag in on a wings level approach, and more power will allow you to fly a little slower, to a point. But, the same technique won't work well in a tight turn, as torque will become a factor, and handling will become terrible. You'll be so close to a stall, that any disturbance will cause a stall, and perhaps a roll off. There's enough asymmetry already in a tight turn, that adding more power makes it much worse.

Transport Canada had me flying aggravated stalls in a Cessna Grand Caravan last December: 30* bank power off stalls, then 30* bank at 75% power in both directions, then again, 30* bank at 75% power one ball out, in both directions. The addition of power degraded control, one ball out, more so. And the stall/spin entry was quite different left and right entry. I sure was impressed with the Caravan's tolerance to spinning though! (And I passed the flight tests!).

During towed "bird" flight testing I did in another Grand Caravan many years ago, I found that the P factor was really upsetting the winch recovery of the bird, and often I would glide the plane, and occasionally feather the prop for bird recovery, as then the airflow around the plane was very symmetrical around the plane.

I did spin testing for an engine change in a Lake Amphibian many years ago, and found that spins entered at 75% power either refused to enter (against torque), or simply snap rolled (with torque). It only requires one torque induced snap roll in a Lake Amphibian to tell you that you should not keep doing it! But, even with only 210 HP, torque was a huge factor at very slow speeds.

I agree that WWII propeller powered fighters might have differing handling, which perhaps would not be compliant to today's certification standards, and probably the experience base for the WWII types is small.
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Old 7th Mar 2020, 18:48
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Originally Posted by Pilot DAR View Post

I agree that WWII propeller powered fighters might have differing handling, which perhaps would not be compliant to today's certification standards, and probably the experience base for the WWII types is small.
That is a very interesting comment, as I usually hear the exact opposite... I would also suspect the "experience base" is not only small, but quite old, using old measuring protocols and instrumentation...

One of those questionable test data results would be this: The 1940s flight manual of the P-51 claims the minimum speed for a P-51D to briefly "touch" 6 Gs is 255 mph ias (240 with flaps), but when the SETP in 1989 did a test with 4 old warbirds at METO power, the minimum speed needed for the P-51D was nearer 300 mph ias at 10k, and they could not touch 6G under 276 mph even when spiralling down... (The manual, as I said, claims 255 mph ias, presumably including level flight...): The SETP tests were during actual 6G level turns, and I think this made the difference...

My theory for this is that the manual's data was extrapolated from doing dive pull-outs at 6 Gs (an easier maneuver to execute consistently while "upright"), and that this was assumed to be the same limiting speed for horizontal turns (in theory, there is nothing about the horizontal that would change the data, except -in my view- that the prop blades are unloaded in a dive)...

You also mention power in the 210 hp range: I suspect data at this power level might not linearly translate ("scale up") to low wing single engine types in the 1500-2000 hp range.

I would like to ask you this question concerning my theory, given your experience: I'll quickly recap it here:

"The effect on the prop I would see is that, in a low wing position, the prop's exit spiral is, by necessity, "split" into above wing or below wing airflows. Increasing the angle of attack by turning, while curving the incoming air upward to the fuselage, might cause that "split" to change in its above-below wing distribution, some of the "below wing" air "shifting" to above the wing. If that proportion of change is significant, this air would then be forced into a kind of "dogleg" path, which would lenghten its path, and so accelerate this portion of the air, depressurizing the corresponding area of thrust within the prop disc (in this case the below wing area)."


Although you might think weakening the outer turn (lower) prop half would yield a nose down trim while banked, I believe in low-speed turns (thus at high prop load) the effect is actually to cause a turn to "self-tighten" on high power low wing monoplanes, this due to the CL shifting forward in response to the turn-aversion of the prop... (This would explain the horizontal turn "self tightening" at low speeds -presumably causing mild stick pushing-, but not high speeds, an odd phenomenon that is described on some WWII fighter types)

My question would be this:
Are thereknown differences of handling in horizontal turns between low wing and high wing single engine aircrafts? I note nearly all WWII fighters are of low wing design, while most post-WWII civilian/utility single engine prop aircraft are of high wing design. These differences are probably mostly related to stability in flight and the lenght of take-off, but I wonder what is said about the difference in steep turn handling, since a high wing obviously "splits" the prop spiral differently?

I'll note that this is not an insignificant issue, since one of the the rare WWII mid-wing fighters (converted from a floatplane), the N1K1, had a mysterious tendency to go in "auto-rotation" during turning combat (flipping on a vertical or vertical axis, sometimes alternating both unpredictably), this being severe to such an extent that the entire aircraft was redesigned with a conventional low wing...

Wing height on a prop-bearing fuselage seems to be a truly major handling issue...

Gaston

Last edited by Gaston444; 10th Mar 2020 at 20:20. Reason: spelling
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Old 8th Mar 2020, 20:27
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I believe that the factors you suggest probably are genuine factors, however, in the context of all of the other factors and variabilities would not show themselves as clearly as you suggest. When I consider the differences in handling between two otherwise identical airplanes I have observed, I don't attribute any one factor, I consider them all.

Yes, it would be unwise to extrapolate a 210HP engine to a multi thousand HP engine installation, entirely different planes. The certified general aviation airplane world does not have many 1000+ HP single engined planes.

the N1K1, had a mysterious tendency to go in "auto-rotation" during turning combat (flipping on a vertical or vertical axis, sometimes alternating both unpredictably)
Okay, but that sounds like a standard spin entry from too tight a turn, 'happens all the time with careless aggressive maneuvering of light planes. I would be surprised to learn that the manufacturer chose to reposition the wing on the fuselage because pilots were spinning the airplanes in combat. But, being an uncommon WW2 type, for which data is probably not held in the western world, I think in depth investigation can be a mind exercise, but otherwise has less relevance for modern airplanes.

Wing height on a prop-bearing fuselage seems to be a truly major handling issue...
I don't share that opinion. Hundreds of each type in civil use have found great success with suitable (and certifiable) handling - they just do not have 1000+ HP.

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Old 9th Mar 2020, 10:03
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The effect on the prop I would see is that, in a low wing position, the prop's exit spiral is, by necessity, "split" into above wing or below wing airflows. Increasing the angle of attack by turning, while curving the incoming air upward to the fuselage, might cause that "split" to change in its above-below wing distribution, some of the "below wing" air "shifting" to above the wing. If that proportion of change is significant, this air would then be forced into a kind of "dogleg" path, which would lenghten its path, and so accelerate this portion of the air, depressurizing the corresponding area of thrust within the prop disc (in this case the below wing area).
The flow behind the propeller is anything but orderly. Many years ago I had to do some work with the stability of a twin turboprop. The simplistic view of the propeller slipstream may envisage a cylinder of high speed air trailing back from the prop disk. From the research that I did at the time, it appears that the air is split above and below the wing, which each half of the flow forming its own smaller "cylinder", although now a bit squashed. Due to a bit of sideflow, the two cylinders don't meet again in the same vertical plane - the lower one is usually displaced towards the wingtip, so the cross-section now looks like a figure 8 with a diagonal slant. The prop rotation can push them a bit as well, so if both your props are turning in the same direction, the left and right cross sections are not symmetric about the centreline. Add to that the fact that the"cylinders" are being subject to fluid shear at the boundary with the relative wind, and you can imagine that, even before reaching the tail, the shapes have started to break up. Now add to the the effect of nacelles, fuselage interference, etc....

So yes, I'm sure that the wing position and fuselage shape do affect the nature of the airflow. However, the stability impact of wing height alone is probably one of the smaller variables. It may be instructive to look at the Cessna Caravan and then some of the military trainers, all with similar engines. Are their varied handling characteristics solely a function of their wing height, or do planform and inertia play a bigger role?

By the way, why don't you have a look through the old NACA data and see what they have on the Grumman Wildcat stability.

Last edited by stressmerchant; 9th Mar 2020 at 10:06. Reason: Fix up position of quote marks
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Old 10th Mar 2020, 22:54
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Originally Posted by Pilot DAR View Post
I believe that the factors you suggest probably are genuine factors, however, in the context of all of the other factors and variabilities would not show themselves as clearly as you suggest. When I consider the differences in handling between two otherwise identical airplanes I have observed, I don't attribute any one factor, I consider them all.
And what about identical airframes, of the same weight, same power, but with a different lenght of nose from a different engine?:

- 1946 US evaluation of FW-190D-9: "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."

- Donald Caldwell wrote of the FW 190 D-9’s operational debut in his "The JG 26 War Diary Volume Two 1943-1945" (pages 388 – 399): "The pilot’s opinions of the “long-nosed Dora”, or Dora-9, as it was variously nicknamed, were mixed. The new airplane lacked thehigh turn rate and incredible rate of roll of its close-coupled radial-engined predecessor."

Of note is that the reverse path to the above, that is to say, from long-nose inline to short-nose radial, so a significantly shorter nose on otherwise identical airframes, yielded massive gains in handing for the Ki-100 over the 300 lbs lighter K-61-I, which was not only lighter but very slightly faster as well...: The gains in performance were all in slow speed turning, and they were massive in favour of the heavier radial engine conversion, to the point of it being considered the best Japanese fighter of WWII by its users... A similar story with the (again heavier) La-5 radial engine conversion over the inline Lagg-3, but this time with a speed gain making the handling issue less obvious than in the Ki-100's case.

If we were to assume a relationship between this and and the distance of the propeller to the leading edge of the wing (which would affect the way the outgoing prop spiral is split for a given AoA change), then the initially visible conclusion would be that a shorter nose is less turn-averse than a longer one, but that turn-aversion is a similar factor for all. The other thing that would follow from this is that pilots should feel a nose down trim with more power during a turn, but they don't: It seems the effect is mostly neutral, or exactly the opposite...: They feel a slight nose up trim at low speeds during turns, when the prop is more heavily loaded by the turn.

If the prop was turn-averse on the vertical to fuselage plane (lower half unloaded/weaker), then the only way the pilot would not feel a nose down trim is that the wing is compensating with a more powerful nose-up trim... If that was true, then it means there would be a relationship between propeller load, wingloading and CL position, causing the wing to somehow produce this nose up trim without the tremendously favourable leverage of an 8 to 10 foot nose pulling down...

No relationship is currently recognized between the prop load and the wing load or CL position, at least not at a fixed speed that is unchanging.

Again, assuming these assumptions were all true, then the low-speed sustained turn performance should be increased by reducing power, and maybe even re-loading the turn-averse, de-pressurized part of the prop with a coarser prop pitch, this while reducing power...:


Hanseman (505 sq.) combat report, 24 May 1944 (Merlin P-51)
"Dogfight at 500 ft. (with a second higher aircraft,afterclimbingfrom130 ft., having closed to 50 ft. on a wheel down 109G that was landing)"--"At first he began turning inside me. Then he stopped cutting me offas I cut throttle, dropped 20 degrees of flaps and increased prop pitch.Every time I got close to the edge of the airdrome they opened fire with light AA guns."(Meaning several successive 360 turns near the same airdrome)--"GraduallyI worked the Me-109G away from the fieldand commenced to turn inside of him as I reduced throttle settings."


Gaston

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Old 10th Mar 2020, 23:21
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Originally Posted by stressmerchant View Post
The flow behind the propeller is anything but orderly. Many years ago I had to do some work with the stability of a twin turboprop. The simplistic view of the propeller slipstream may envisage a cylinder of high speed air trailing back from the prop disk. From the research that I did at the time, it appears that the air is split above and below the wing, which each half of the flow forming its own smaller "cylinder", although now a bit squashed. Due to a bit of sideflow, the two cylinders don't meet again in the same vertical plane - the lower one is usually displaced towards the wingtip, so the cross-section now looks like a figure 8 with a diagonal slant. The prop rotation can push them a bit as well, so if both your props are turning in the same direction, the left and right cross sections are not symmetric about the centreline. Add to that the fact that the"cylinders" are being subject to fluid shear at the boundary with the relative wind, and you can imagine that, even before reaching the tail, the shapes have started to break up. Now add to the the effect of nacelles, fuselage interference, etc....

So yes, I'm sure that the wing position and fuselage shape do affect the nature of the airflow. However, the stability impact of wing height alone is probably one of the smaller variables. It may be instructive to look at the Cessna Caravan and then some of the military trainers, all with similar engines. Are their varied handling characteristics solely a function of their wing height, or do planform and inertia play a bigger role?

By the way, why don't you have a look through the old NACA data and see what they have on the Grumman Wildcat stability.
Very interesting stuff! I cannot thank you enough for providing all these additional details, as they are very valuable in visualizing what I am theorizing about...

The thing I was concerned about is the splitting action of the main wings, so this is definitely before the "shapes have broken up".

As to the Wildcat, you are absolutely right that it is a mid-wing I had forgotten about... It had superb handling in turns, so that doesn't appear to have hurt it...

Gaston
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Old 11th Mar 2020, 14:11
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Factors affecting flying qualities of WW2 fighters may not be a firm basis for making aerodynamic assumptions relative to today's certification efforts. If the research of the handling of the WW2 fighters is out of interest, excellent, but it is what it is, and the data is there, there's no more to be had. I expect that a lot of WW2 airplane development was rushed. Although detailed investigation of flying qualities was certainly made, airplanes were certainly put into service with handling defects, so when improved, improvement could be noticed. When dealing with airplanes of such high power to weight to wingloading ratios, I think the rules diverge somewhat. Again, aside from a few military trainer types, there not much development going on in the realm of 1000+HP single propeller planes.

In terms of changing the station location of the prop on an otherwise same plane, my closest experience would be certification test flying I have done on a deHavilland Beaver, the Beaver with the 9 3/4" engine mount extension, and the Turbo Beaver, all of which I flew a floatplanes. In each case, though I was specifically looking for differences in handling resulting from the change in propeller station location, I was unable to notice a difference I could attribute to this. All three versions fly delightfully (just the Turbo Beaver smells of exhaust in the cockpit!)
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Old 11th Mar 2020, 21:58
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If my premise is true, this line of inquiry has major historical significance, to say nothing of of the future conception of innumerable simulation games...

If there is a significant undetected imbalance in prop thrust -on the vertical to fuselage plane-, this has enormous implications on our basic understanding of how these particular aircrafts functioned at the most basic level...: If unknown vertical variations are true, we go from prop power being largely neutral to sustained turns to adverse to sustained turns (especially if assuming nearly 80 of bank in many cases)... This would explain the constant WWII obsession with reducing power in slow speed sustained turns, at the lowest limit, even after many consecutive 360s at very low speeds. And no one ever using "Emergency Power" in turning combat, only when wanting to go straight (innumerable interviews to that effect)...

In fact, the entire notion of "speed is life" would be entirely turned on its head, since WWII gun firepower was usually not effective enough to tolerate high bisecting angles (in non-expert hands at least, which is why it is mostly aces who pronounced dogfighting "dead" at the time), and this explains the increasing obsession with slow speed turn-fighting right up to 1945, especially in Europe, where opponents had matching top speeds, and thus could "rope in" each other into a "locked" decelerating turning contest. This is entirely contrary to the usual narrative of dogfighting becoming obsolete as power increased, at least during the gun/prop era...: In fact, the presumed "obsolescence" of dogfighting was widely assumed as early as the monoplane era was established (1930s), causing the entire worldwide late-1930s obsession with the failed concept of the twin engined "heavy" day fighter, real-life firepower turning out to be insufficient, at high bisecting angles, in the hands of most pilots...

If the prop is turn averse by a significant amount, on the vertical-to-fuselage plane, the implication is phenomenal: It means a faster sustained speed turn rate, from minimal speed, will be achieved by reducing power.

This is how I interpret the quote I posted above (there are innumerable other hints going the same way, with nothing to the contrary, in thousands of combat accounts I have read on this topic). I use it here only because it is so exemplary, and combines with the use of coarse prop pitch at low speeds.

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Hanseman (505 sq.) combat report, 24 May 1944 (Merlin P-51)
"Dogfight at 500 ft. (with a second higher aircraft,afterclimbingfrom130 ft., having closed to 50 ft. on a wheel down 109G that was landing)"--"At first he began turning inside me. Then he stopped cutting me offas I cut throttle, dropped 20 degrees of flaps and increased prop pitch.Every time I got close to the edge of the airdrome they opened fire with light AA guns."(Meaning several successive 360 turns near the same airdrome)--"GraduallyI worked the Me-109G away from the fieldand commenced to turn inside of him as I reduced throttle settings."
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So this is my interpretation: Assuming the Angle of Attack (of about 6-7) lowering the wing does force a vertical dogleg (longer trajectory) in the lower prop half's air, then this air "shifting" above the wing would force an acceleration of that air: Acceleration of that air would have several major effects, even if the overall loss of thrust is only 6%.

1-A 6% loss of outer turn disc half thrust would cause an increase of 3% on the inner turn disc half load, through asymmetry of load: I assume half of the loss of thrust would get expressed as a deceleration, since the upper to fuselage disc half is unaffected by air acceleration, but the whole aircraft is decelerated by the 6% loss, half of which would affect the upper disc half by adding a load.

2-So we are now at a 9% disc imbalance, to which we may add 1% from the turn curvature: A 10 foot prop over a 1000 feet turn radius is roughly a 1% imbalance. So -6% outside and + 4% inside: 10% of total imbalance. At high load, a figure I read was a 3000 lbs of overall prop load, so we could have 300 lbs of adverse pitch load at the end of a ten foot nose...

3-A very important point is that such a load should feel to the pilot as a heavy nose-down trim, yet it obviously doesn't at all... (In some WWII types there is even a nose-up pitch in low speed turns only): Something is obliterating that effect in pitch, yet the effect is still there, since the turn rate is increasing with less power...

4-The important thing is the acclelerated air: In the pilot account above, he finds it advantageous to use a coarser pitch, despite being at a very low speed: This use of a coarse prop pitch indicates the presence of accelerated air inside the prop disc, in a way that is adverse to the turn (so outside the turn): A coarser blade pitch would mitigate the accelerated air unloading the outside turn/lower prop half. A coarser pitch would, in effect, "reload" the "unloaded" outer half.

5-He also reduces power: This would simultaneously unload the now overloaded inner turn/upper disc half.

The overall description is clearly that of prop power being adverse to sustained turns at minimum speed, after several consecutive 360s, so it is not a sitution where he has too much speed at all...

I will go into other points later.

Gaston

Last edited by Gaston444; 11th Mar 2020 at 23:09.
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Old 12th Mar 2020, 03:51
  #17 (permalink)  
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as I cut throttle, dropped 20 degrees of flaps and increased prop pitch.
'Careful how you interpret this. Yes, "increase" prop pitch would mean to make it coarser. However, if you read the propeller control "increase" means move toward fine pitch, increase RPM. This is for slowing down quickly, and is somewhat abusive to the engine and prop - particularly geared engines! 'Same poor wording as "full" throttle. Fulling throttling something means cutting off the airflow. "Open" throttle, or "full power" convey properly. I'll say "full power", or "open the throttle", but I never say "full throttle". I'll also say "prop fine pitch" or "increase RPM". There is no flying condition where you'd reduce power, lower flaps, and coarse the prop pitch. If you've closed the throttle, the next thing you'll be doing is opening it again sometime, for which you'll want the prop fine pitch, so we plan ahead.

So, if we infer increased RPM - fine pitch - these are things a pilot would do to slow down quickly, hopefully with a liquid cooled engine they really didn't care too much about. Yes, if you want to change direction more quickly in a turn, reduce the airspeed.

Assuming the Angle of Attack (of about 6-7)
If you're flying with that low AoA with any power, the propeller airflow path will hardly be changed from a normal cruise flight AoA. A a single propeller pilot flying a maximum performance rate turn will be flying a higher AoA, closer to a stall AoA - that's where the G comes from. In such case, the torque of the prop will have more effect on the handling of the plane than a change of airflow over or under the wing. This effect is increased right up to the stall, where the plane will roll with torque if the power is up. The tendency of one wing to stall before the other (and at least induce a roll, if not a spin) will be much more subject to uneven AoA of each wing, and imbalanced aerodynamic forces, than airflow over or under the wing. I'm not saying it's not a factor, it's just not a major factor.




You can judge the AoA I'm flying here. This is wings level, flaps up, level flight, full power just approaching the point of the stall. But, this is sustained, stable, one G flight. Now, if you were turning, you'd be pulling G, and would have stalled, so not this AoA. And, doubtful that WW2 fighters had the low speed airfoil that a Cessna 182 has, so could not achieve this AoA anyway.

The only plane I've flown that comes near being a WW2 fighter would be the Harvard.




Note that at the top of a loop, with lots of power, a bunch of AoA, and not a lot of airspeed, I have the ailerons wings level, 'not overcoming propeller airflow change effects.

If my premise is true, this line of inquiry has major historical significance
Okay... but I don't think the outcome of WW2 will be affected. Interesting thinking point, and perhaps a peripheral aerodynamic characteristic, but not very relevant to airplanes in use today.

the future conception of innumerable simulation games..
.

Okay, if you have a piloting game which models the forces affecting airplane handling so accurately that this factor could affect things, well done!
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Old 12th Mar 2020, 21:26
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Originally Posted by Pilot DAR View Post
'Careful how you interpret this. Yes, "increase" prop pitch would mean to make it coarser. However, if you read the propeller control "increase" means move toward fine pitch, increase RPM. This is for slowing down quickly, and is somewhat abusive to the engine and prop - particularly geared engines! 'Same poor wording as "full" throttle. Fulling throttling something means cutting off the airflow. "Open" throttle, or "full power" convey properly. I'll say "full power", or "open the throttle", but I never say "full throttle". I'll also say "prop fine pitch" or "increase RPM". There is no flying condition where you'd reduce power, lower flaps, and coarse the prop pitch. If you've closed the throttle, the next thing you'll be doing is opening it again sometime, for which you'll want the prop fine pitch, so we plan ahead.

So, if we infer increased RPM - fine pitch - these are things a pilot would do to slow down quickly, hopefully with a liquid cooled engine they really didn't care too much about. Yes, if you want to change direction more quickly in a turn, reduce the airspeed.
I had carefully researched this term before coming to this conclusion. What you suggest is not how the higher or lower pitch term is commonly used in "pilot parlance" (and especially not in the even less-specialized terms of a WWII combat report), because the usual reference of pitch is the blade angle, which is higher when rpms are lower: The useage you suggest is counter-intuitive and confusing, referring to the increasing pitch of the blade trajectories if they were screwing through wood like screws, which may be how they are oriented on the lever (engineers being fond of counter-intuitive abstractions), but not something easy to visualize if by increasing pitch you mean LOWERING the prop blade angle... I'll just point to some aviation articles I quickly found, where the meaning is clearly on my side, and never explained or specified, because in this case the intuitive has obviously won out over something that would be severely confusing:

Quote: "The Mustang is so aerodynamically clean that it glides more efficiently than most general aviation airplanes, but only when the propeller is set to high pitch. Blade pitch affects glide ratio so much that it can be modulated during descent to vary glide performance as necessary during an engine-out approach."

Quote:"Should you need high RPM (low blade pitch) oil pressure on the back of the plate is reduced slightly. This allows the plate to slide back, pushing the shaft backwards and rotating the beveled cam clockwise. As a result, the blades also rotate clockwise which sets low blade pitch. Because of this plate arrangement the Hamilton can't be feathered; there isn't enough travel room for the plate. In any event of oil pressure loss the prop had no means of control and would usually "run away." The blades would go to high RPM (full low pitch) while producing no thrust and a ton of drag."

I could find many more such quotes, but suffice it to say that the use of a term in a not generalized way, which is also highly counter-intuitive, is, I think, extremely unlikely...

To this you have to add the circumstances the pilot describes: He is in a low speed turn battle, after slowing to 150-200 mph to intercept, from 50 ft behind!, a landing aircraft at 150 ft, having climbed 350 ft afterwards to counter a new challenge from above, at 500 ft.... He is now "locked" in a slow speed turn battle with a lighter aircraft, with no real possibility of diving or spiraling down, and so highly pressed from behind that he is forced to stay in the turn as the circle carries him, repeatedly, into the AAA fire clearly located within the airdrome...

Are you saying in this condition, with flaps down and his engine deliberately reduced in power, after multiple 360s on the deck, that his aim would be to impose on himself severe drag, after completing multiple complete level circles he did not want to do?

If you are saying he wanted to increase engine rpm, then why would he deliberately lower his throttle? Please do not tell me you think this guy had too much speed, because he was forced to turn into the same AAA multiple times...: There is no way he would choose to approach that airdrome multiple times, unless he was forced to make consecutive full circles near the ground... His immediately previous altitude was even lower, and his speed was already low as well.

So my criticism is that you interpretation involves a highly unlikely use of the term "increasing pitch", and one that cannot fit in any way the situation as it is presented.

What would be your interpretation of what the pilot did, assuming my version of the prop pitch is the correct one?



Originally Posted by Pilot DAR View Post
If you're flying with that low AoA with any power, the propeller airflow path will hardly be changed from a normal cruise flight AoA. A a single propeller pilot flying a maximum performance rate turn will be flying a higher AoA, closer to a stall AoA - that's where the G comes from.
That is precisely what I wrote my entire previous post about... WWII fighter pilots did not care about maximum unsustained Gs, because for one, as the SETP found out in 1989, these aircraft could barely even reach far above those speeds in a straight line: The minimum for 6Gs being around 300 mph ias minimum (276 mph spiralling down, and of course much lower in dive pull-outs: 255 to 240)... They could, horizontally, complete maybe 90 or 180 of turn above 5-6 Gs, briefly, and that was useless in combat, because unless the opponent matched this value exactly, you had no "pepper time". My entire previous post was about how the pre-WWII assumptions of firepower and bisecting trajectories were wrong... Only a few Ace pilots ever managed more than light damage at bisecting angles, given the more usual 1% hit rate...

The G levels that mattered in WWII combat where the maximum sustained speed Gs, so around 3 Gs: Because this was sustainable, this was where the real damage was done.

As to what AoA angle turning at 3Gs represent, I assumed around 7, since 6Gs is 14, not including the wing camber: If that is wrong I would be glad to be enlightened.


Originally Posted by Pilot DAR View Post
In such case, the torque of the prop will have more effect on the handling of the plane than a change of airflow over or under the wing. This effect is increased right up to the stall, where the plane will roll with torque if the power is up. The tendency of one wing to stall before the other (and at least induce a roll, if not a spin) will be much more subject to uneven AoA of each wing, and imbalanced aerodynamic forces, than airflow over or under the wing. I'm not saying it's not a factor, it's just not a major factor.
If the wing effect is 6% of the thrust, leading to a 9% prop imbalance, would you agree that this is around 300 lbs of nose-down trim trim at the end of a 10 foot nose?

Since this effect would be purely vertical, is it inconceivable the wing's lift would take up the slack, and you would be completely unaware that they are doing so?

Here is some calculations I made as to what these -imperceptible- 300 lbs of prop imbalance would do: a 120 inches long nose would need to be countered by a forward movement of the CL, in front of the CG. How far in front of the CG can the CL move? I have no way to know... It could be a foot, it could be one inch....

If the figure is one inch, then it is costing the wings 36 000 lbs of lift to conceal, to the pilot, that the prop is pulling down... If that is indeed what they are doing, how would you know the difference? Only by measuring/comparing raw turning performance, where energy outcomes are affected by the intensity of conflicting internal forces (since the wings have to bear the internal conflict)...

All of a sudden, the notion of a 45 lbs/square foot FW-190A out-turning, at slow sustained speeds, a 30 lbs/square foot Spitfire, is not so outlandish...:

Johnny Johnson (top Spitfire ace of WWII) "My duel with the Focke-Wulf": (At about 1000 ft. above seal level) "With wide-open throttles I held the Spitfire V in the tightest of vertical turns [Period slang for vertical bank]: The Mk V was also known as better turning than the Mk IX and all later models]. I was greying out. Where was this German, who should, according to my reckoning, be filling my gunsight? I could not see him, and little wonder, for he was gaining on me: In another couple of turns he would have me in his sights. I asked the Spitfire for all she had in the turn, but the enemy pilot hung behind like a leech. It could only be a question of time..."

Originally Posted by Pilot DAR View Post

The only plane I've flown that comes near being a WW2 fighter would be the Harvard.




Note that at the top of a loop, with lots of power, a bunch of AoA, and not a lot of airspeed, I have the ailerons wings level, 'not overcoming propeller airflow change effects.
How would you know, if 300 lbs of force is cancelled out by an extra 30 000 lbs from the wings? It bears repeating: These forces are cancelling each other out, whatever is the value that they have. To know their value you would have to measure how much you wings are bending in actual turning flight: The reality is that they might bend a lot more than is assumed, since bending in flight measurements were never done for these types.

There is two ways to detect the effect I am talking about: 1-Measuring prop blade load uniformity (during each rotation) in a level turn 2-Measuring wing bending in a level turn.

It really is quite simple, but so far I have never seen data that suggests this was ever done for the 1000+ hp low-wing types we are talking about (at least not in sustained level turns, where the prop is at its highest load, so of course not dive pull-outs): That data, if it existed, would instantly prove my theory wrong...


Originally Posted by Pilot DAR View Post
Okay... but I don't think the outcome of WW2 will be affected. Interesting thinking point, and perhaps a peripheral aerodynamic characteristic, but not very relevant to airplanes in use today.
Oh please...

.
Originally Posted by Pilot DAR View Post
Okay, if you have a piloting game which models the forces affecting airplane handling so accurately that this factor could affect things, well done!
If the wing lever is 1 inch, and the nose lever is 120 inches at 300 lbs, then the forces involved on the wing are 36 000 lbs, on top of what is curerently assumed: 30 000 lbs at 3 G for a P-51, plus 36 000, so possibly 66 000 lbs. Obviously that is too high, but yes, they do have the structural margin to take it, since the effect tapers down at high speeds (It obviously has to, since the prop load goes down at higher speeds.). Mustang wings easily take 70 000 lbs, and their maximum rating is around 120 000 lbs (or 12 Gs).

And since these are all forces whose total sum is zero, this is perfectly within accepted physics. Because as a pilot you are at the balance point of much larger forces, there is no way you can tell just how large these forces actually are. You can just gauge the residual energy outcomes in side by side fly-offs... And in the case of the FW-190A, it does breach and greatly exceed a 20 000+ lbs of wing load difference, because of a lower leverage ratio being less taxing to its smaller wings (the only way this observable gap could be overcome, in my reckoning).

Gaston

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Old 12th Mar 2020, 22:54
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If you are saying that he wanted to increase engine rpm, then why would he be deliberately lowering throttle?
It is possible to move the throttle toward closed, and increase RPM by moving the prop control to the "increase" position, which will be forward. The word "increase" on the throttle quadrant is in the forward control direction, which is the fine pitch position. Though nearly no WW2 fighters were "certified" in the context of today's plane, were they to be, they would have to comply with the following design requirement:

Sec. 23.779

Motion and effect of cockpit controls.

Cockpit controls must be designed so that they operate in accordance with the following movement and actuation:
........
(b) Powerplant and auxiliary controls:

.....Propellers----------------------------------- Forward to increase rpm.
Perhaps the pilot wanted to slow down very quickly to assist in positioning the plane for turn he wanted to achieve (not to overshoot the turn) and used fine pitch to create drag to slow the plane down. I do this gently in one of my planes on short final, when I want to loose the last few knots and settle into the flare. If I want to glide that plane, I select full "decrease" for drag reduction. I don't "modulate" a propeller control, it invites engine or propeller damage. Some planes (Cessna Caravan, for example) are set up for the propeller to move into a very high drag, flat blade angle when the power is selected to near idle, to aid a more steep approach. Reverse may be used on the surface.

Fine pitch may be selected for more thrust when combined with higher power, or fine pitch may be combined with very low power to create drag for slowing down. A more coarse pitch will be selected for extracting optimum power from the engine in cruising flight or otherwise for more efficiency.

It is possible to overspeed some engines by moving the propeller control toward "increase too quickly, even at low power - and doing so creates noticeable drag. It's a horrible thing to do to a geared engine.

As for WW2 pilot combat techniques, well, I'm not a WW2, nor combat pilot, so I'll step back from the discussion.
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Old 12th Mar 2020, 23:33
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Originally Posted by Pilot DAR View Post
It is possible to move the throttle toward closed, and increase RPM by moving the prop control to the "increase" position, which will be forward. The word "increase" on the throttle quadrant is in the forward control direction, which is the fine pitch position. Though nearly no WW2 fighters were "certified" in the context of today's plane, were they to be, they would have to comply with the following design requirement:



Perhaps the pilot wanted to slow down very quickly to assist in positioning the plane for turn he wanted to achieve (not to overshoot the turn) and used fine pitch to create drag to slow the plane down. I do this gently in one of my planes on short final, when I want to loose the last few knots and settle into the flare. If I want to glide that plane, I select full "decrease" for drag reduction. I don't "modulate" a propeller control, it invites engine or propeller damage. Some planes (Cessna Caravan, for example) are set up for the propeller to move into a very high drag, flat blade angle when the power is selected to near idle, to aid a more steep approach. Reverse may be used on the surface.

Fine pitch may be selected for more thrust when combined with higher power, or fine pitch may be combined with very low power to create drag for slowing down. A more coarse pitch will be selected for extracting optimum power from the engine in cruising flight or otherwise for more efficiency.

It is possible to overspeed some engines by moving the propeller control toward "increase too quickly, even at low power - and doing so creates noticeable drag. It's a horrible thing to do to a geared engine.

As for WW2 pilot combat techniques, well, I'm not a WW2, nor combat pilot, so I'll step back from the discussion.

"Overshooting the turn" seems plausible in the context of an initial turn entry. Seems unlikely in the context of multiple 360s on the deck... There is nothing to suggest there was any kind of speed in the encounter. "Increasing prop pitch" to describe a lower blade angle is very rarely used by pilots.



If someone could find wing bending measurements for these types, while in level turn, not dive pull-outs, it would resolve the issue.

Gaston
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