Detecting Propeller load variations on the vertical plane
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"Increasing prop pitch" to describe a lower blade angle is very rarely used by pilots.
In reading the original report, if that were being reported to me by a competent pilot, who was probably still justifiably excited reporting such an encounter, I would interpret what the pilot meant to say to be that he moved the propeller pitch control to the "increase" position - which is a lesser blade angle. Interpreting that the pilot rapidly reduced power, and extended some flaps, there is no plausible reason to then coarsen propeller pitch. I bet that if you asked the pilot did he mean to say that he "increased the propeller RPM?, he's say: "yeah... that's what I meant". If power is being reduced in maneuvering, or anticipation of landing, you want the prop to be in fine pitch. If not, if/when the power is increased, perhaps quickly, and to a high power setting, a constant speed propeller would govern to a lower RPM, and cause damaging overboost of the engine.
For the P-51, the propeller control is labelled: "RPM" "Increase" (forward), "decrease" (rearward). This is because that control controls the propeller governor, so the pilot selects RPM, not pitch - the governor controls propeller pitch in the governing power range. At low power, the governor no longer governs, so then the propeller control can move the blade angle to fine pitch. The P-51 Pilot Operating Instructions do state that on approach (so power reducing, flaps being extended) the propeller is to be set to 2700 RPM, which is the maximum continuous RPM, so increase RPM, decrease pitch.
For the information which you are selectively interpreting from the report, while overlooking some simply realities of piloting constant speed propeller planes, I think you're leading yourself off the track somewhat.
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The pilot wrote that he "cut the throttle"? What did he mean? He cut the throttle effect, which effect would reduce power, so power was increased? Or, he cut back the throttle control, which has the effect of throttling the engine, and reducing power? The language is skewed in two places in the report, why focus on one language error, and not the other?
All I can say to this is you have got to be joking...
In reading the original report, if that were being reported to me by a competent pilot, who was probably still justifiably excited reporting such an encounter, I would interpret what the pilot meant to say to be that he moved the propeller pitch control to the "increase" position - which is a lesser blade angle. Interpreting that the pilot rapidly reduced power, and extended some flaps, there is no plausible reason to then coarsen propeller pitch. I bet that if you asked the pilot did he mean to say that he "increased the propeller RPM?, he's say: "yeah... that's what I meant". If power is being reduced in maneuvering, or anticipation of landing, you want the prop to be in fine pitch. If not, if/when the power is increased, perhaps quickly, and to a high power setting, a constant speed propeller would govern to a lower RPM, and cause damaging overboost of the engine.
For the P-51, the propeller control is labelled: "RPM" "Increase" (forward), "decrease" (rearward). This is because that control controls the propeller governor, so the pilot selects RPM, not pitch - the governor controls propeller pitch in the governing power range. At low power, the governor no longer governs, so then the propeller control can move the blade angle to fine pitch. The P-51 Pilot Operating Instructions do state that on approach (so power reducing, flaps being extended) the propeller is to be set to 2700 RPM, which is the maximum continuous RPM, so increase RPM, decrease pitch.
For the information which you are selectively interpreting from the report, while overlooking some simply realities of piloting constant speed propeller planes, I think you're leading yourself off the track somewhat.
For the P-51, the propeller control is labelled: "RPM" "Increase" (forward), "decrease" (rearward). This is because that control controls the propeller governor, so the pilot selects RPM, not pitch - the governor controls propeller pitch in the governing power range. At low power, the governor no longer governs, so then the propeller control can move the blade angle to fine pitch. The P-51 Pilot Operating Instructions do state that on approach (so power reducing, flaps being extended) the propeller is to be set to 2700 RPM, which is the maximum continuous RPM, so increase RPM, decrease pitch.
For the information which you are selectively interpreting from the report, while overlooking some simply realities of piloting constant speed propeller planes, I think you're leading yourself off the track somewhat.
I'm afraid your argument that increasing the throttle could mean less power is reavealing as to YOUR bias...
You also say, "there is no plausible reason to then coarsen propeller pitch" but you don't mention that explaining this oddity is precisely the entire point of my post. I ask you one more time: Forget your -biaised- interpretation of increased pitch, and find me a reason other than what I came up with to explain why he might have found it better to coarsen the pitch...
There was asymmetrically accelerated air inside that prop, that's why the coarser pitch... This is the simplest explanation that makes the most sense, if we accept the basic terms used.
And yes, that and a 30 lbs/square foot aircraft being vastly out-turned (in all instances that I know of), in low speed sustained turns, by a 45 lbs/square foot aircraft, does mean there is something fundamentally wrong with flight physics on these particular types. If you can't accept that, then accept you can't discuss this with an open mind, as the throttle issue seems to indicate...
Gaston
The fact is, as I pointed out before, everywhere a pitch increase is mentionned by real pilots it means an increase in the blade pitch angle. Your argument is dependent on interpreting a forward movement of the control, as designed in the cockpit, as "increase", which is not how the term is used, and in fact makes no sense, since here an "increase" would be refering to lesser speed use, which is exactly why it is never used that way...
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other you are well out of it DAR.
.....is mentionned by real pilots it means....
You mention that you are trying to get us to accept a specific statement. Please keep in mind that the accepted flight physics have been like this for many more years than I've been playing with aeroplanes, because everyone involved, from humble pilots all the way to some very eminent researchers, have been able to prove that a specific model and the actual behaviour of the aircraft match. I get the idea from your posts that your assumption only fits when we are willing to accept some pretty abstract constructs that don't match with what we, as pilots, instructors, lecturers, have come to know and understand over the course of our careers. Please don't take this the wrong way, but from a basic research standpoint it could mean that your theory needs some more work.
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So you could have a pilot state that he increased the drag of the prop, by moving the lever forward.
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Yes, and I sometimes do this just a little in my flying boat on very short final onto the water. I had it demonstrated to me aggressively in a Bellanca Viking decades back, however, the rapid drag rise I experienced, and transient RPM changes, seemed abusive to me. However, it's considered a poor technique for those engines with gear reduction, as the gears are not supposed to drive the engine. Some WW2 fighters had geared engines, though I agree that in combat, the pilot might disregard good technique in battle.
The model is not just not predictive, it is the OPPOSITE of observable reality in the crucible of actual combat experience... Test pilots of the era appearing, on the other hand, to be at odds with what the machines are trying to tell them.
The assumption that the model is correct in the face of the ENTIRE, but less precisely technical, historical record seems fragile to me...
Last edited by Gaston444; 7th Apr 2020 at 17:42.
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Happily, I have no combat experience, so I'm unqualified to comment on combat flying techniques. My experience with many engine and propeller changes on many small civil types has shown me that airplane handling may be effected by the propeller, if the propeller is managed/mismanaged during maneuvering. Also, when installing an engine of a different mass, or at a different location (I've done both), the handling of the plane could be affected. If ballast is needed in the tail, more so. So, certainly, there could be expected to be a difference in handling between airplanes changed from inline to radial engines.
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Happily, I have no combat experience, so I'm unqualified to comment on combat flying techniques. My experience with many engine and propeller changes on many small civil types has shown me that airplane handling may be effected by the propeller, if the propeller is managed/mismanaged during maneuvering. Also, when installing an engine of a different mass, or at a different location (I've done both), the handling of the plane could be affected. If ballast is needed in the tail, more so. So, certainly, there could be expected to be a difference in handling between airplanes changed from inline to radial engines.
There is nothing in any model known that explains an improvement in slow speed sustained turn handling for heavier radials of similar power. The obvious explanation is internal leverages are affecting the wingloading, but there is no model for that either.
Not only is there no model for these particular rare and esoteric issues, but even a question as basic as: Does more power improve the maximum available sustained speed rate of turn? This basic question is still answered today as yes, more power increase the sustainable speed rate of turn, yet not one single wwII combat account I have found supports this in 25 years of searching for it. The implications of this alone are enormous; it means, for these specific configuration/weight/power, there is an interaction going on between prop load and wing load, an absolutely foreign concept to current flight physics.
It seems to me the prop effects that are well known are all easy to detect minor roll and yaw effects. Since there are huge vertical to fuselage forces in play that are cancelling each other out, gravity/momentum on one side and lift on the other, there is plenty of room inside those two immense forces to hide massive smaller forces that also cancel each other out, but alter the assumed outcomes of the two larger vertical forces.
If you assume the smaller forces hiding within the bigger forces are not there, they are never going to come out and tell you of their existence (other than by indirect means like competing turn rates)
As I said, precise vertical prop load variations in turns, or wing bending measurements in turns, for these old types, would instantly reveal these forces, but if you never do these tests on these particular types, how would you ever know 20 000 lbs of forces are cancelling each other out? the airframe will perhaps groan a little more than you expect, but it won’t really complain in an obvious way if you don’t dogfight everyday... Yes I do think something that gigantic could easily be hiding in plain sight of decades of air show flying, since that is all they do.
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tail ballast was changed when altering radically from inline to radial, which was rare.
Yes I do think something that gigantic could easily be hiding in plain sight of decades of air show flying, since that is all they do.
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If gigantic forces associated with propeller vs wing loading forces were hiding, or cancelling each other out in flight, surely there would be correspondingly different handling and performance during power off maneuvering, as one of these sources of force was dramatically reduced. Though power off maneuvering usually involves trading off altitude, I have otherwise found it to be within the handling expectations for the type. To offer an example that propeller powered airplanes with a decent power to weight ratio can be very nicely flown in maneuvering power on, or power off, I would refer you to Bob Hoover's demonstrations in the Shrike. He seemed to suffer nearly no handling defect power off through some very impressive maneuvering and turns.....
WwII pilots never show any interest in using emergency power in horizontal turns, which is contrary to theory for competitive turns.
Since high power and a low wing seem critical, the only type I have seen you mention that roughly matches WWII fighters is the trainer, A Harvard or Texan or something similar.
Can the Harvard sustain faster turn rates at full power than it can at reduced power? I do mean SUSTAINED speed on fully horizontal turns.
Let’s say the minimum turn time for that Harvard, at sustained speed and 70% power, is 18 seconds per 360, indefinitely. If my theory was correct, you would be incapable of matching this sustained speed turn rate with 100% power.
Current assumption is that 100% power will sustain horizontal 360s FASTER at a constant speed.
I predict 100 % power will be at least one or two seconds slower per 360... Hard to notice as significant in ordinary maneuvers, but hugely obvious in combat...
It really is that simple.
It could be that a requirement is a hard 5 g turn entry, followed by an ability to sustain 80 degree banks and 3.3 gs at a constant horizontal speed. The effect may not scale anywhere downward from such relatively high values for a prop type of over 6000 lbs... It could be that a “tumble” of air must be set up by a hard entry at high bank.
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the only type I have seen you mention that roughly matches WWII fighters is the trainer, A Harvard or Texan or something similar.
Can the Harvard sustain faster turn rates at full power than it can at reduced power? I do mean SUSTAINED speed on fully horizontal turns
I agree that there are likely technique differences between combat flying a WW2 fighter, and today's civil airplanes, so the typical experience of civil pilots may not represent the nuance factors of WW2 fighter flying. Hopefully you can find a warbird resource for your research....
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I will try.
As I mentioned, there are two avenues by which sensors could detect the forces I am talking about: one would be wing flex sensors during horizontal turns: these exist but requires wing disassembly to put x layout “metal tape sensors” (?) directly on the spars, with electricity running through the whole thing.
The other would be, I presume, a thin pressure sensitive “patch” on the back face of the propeller blades, with wire or cordless recorder, and a light beam hitting the pressure patch to record the location orientation of any pressure variation within 1% accuracy of maximum load, and this fast enough to locate within one foot of the disc the variation within one rotation.
I do not know if such a patch device exists or even can exist, at reasonable cost at least, but if anyone can enlighten me I would certainly be able to invest a serious amount to investigate this: It seems to me like it would be a cheaper alternative to stick a patch on a prop than disassembling a warbird’s wing...: It would also be closer to what I think is the source of the phenomenon.
All I could find online about prop pressure sensors concerned marine prop research, so any help to find a maker relevant to aircraft propellers sensors would be greatly appreciated. Finding a warbird for such a simple test would actually not be that difficult, given the mild nature of the test, and I even have a local flying museum where several fighters are part of the collection.
As I mentioned, there are two avenues by which sensors could detect the forces I am talking about: one would be wing flex sensors during horizontal turns: these exist but requires wing disassembly to put x layout “metal tape sensors” (?) directly on the spars, with electricity running through the whole thing.
The other would be, I presume, a thin pressure sensitive “patch” on the back face of the propeller blades, with wire or cordless recorder, and a light beam hitting the pressure patch to record the location orientation of any pressure variation within 1% accuracy of maximum load, and this fast enough to locate within one foot of the disc the variation within one rotation.
I do not know if such a patch device exists or even can exist, at reasonable cost at least, but if anyone can enlighten me I would certainly be able to invest a serious amount to investigate this: It seems to me like it would be a cheaper alternative to stick a patch on a prop than disassembling a warbird’s wing...: It would also be closer to what I think is the source of the phenomenon.
All I could find online about prop pressure sensors concerned marine prop research, so any help to find a maker relevant to aircraft propellers sensors would be greatly appreciated. Finding a warbird for such a simple test would actually not be that difficult, given the mild nature of the test, and I even have a local flying museum where several fighters are part of the collection.
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MT Propeller in Germany make propellers for Focke Wolfe 190's, and they have the technology to attach strain gauges to the propeller faces to measure strain in flight, I've seen there set up of this during a factory tour a few years ago. MT know what their time is worth, and it's worth a lot, so don't approach them with a small budget, but they are fully capable.
A strain gauge can also be attached to a spar, but knowing where, and how to resolve the observed data is more complex, and, as you say, requires a co operative warbird owner. Warbirds are high cost, as MT knows their market.
A strain gauge can also be attached to a spar, but knowing where, and how to resolve the observed data is more complex, and, as you say, requires a co operative warbird owner. Warbirds are high cost, as MT knows their market.
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MT Propeller in Germany make propellers for Focke Wolfe 190's, and they have the technology to attach strain gauges to the propeller faces to measure strain in flight, I've seen there set up of this during a factory tour a few years ago. MT know what their time is worth, and it's worth a lot, so don't approach them with a small budget, but they are fully capable.
A strain gauge can also be attached to a spar, but knowing where, and how to resolve the observed data is more complex, and, as you say, requires a co operative warbird owner. Warbirds are high cost, as MT knows their market.
A strain gauge can also be attached to a spar, but knowing where, and how to resolve the observed data is more complex, and, as you say, requires a co operative warbird owner. Warbirds are high cost, as MT knows their market.
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The only remaining question is if the device can discriminate the within-disc location of the pressure variations, and with sufficiently fine percentages,
If your preferred language is English, there will be no problem. The technical staff I worked with there speak better English than most people in North America - they have not learned to be sloppy speakers yet. But, remember, don't present yourself at MT with a limited budget, they already have lots of work to do, so they aren't looking for budget minded projects...
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Assure that the strain gauge(s) on each propeller blade can be distinguished 1/2/(3) by the data recorder, then add one more channel with a rotational position sensor, and you'll know where each blade was to within a few degrees when the load was measured. This is all well within MT's capability and understanding. I have worked with them on several propeller certification projects, which resulted in my issuing STC approval for their installation.
If your preferred language is English, there will be no problem. The technical staff I worked with there speak better English than most people in North America - they have not learned to be sloppy speakers yet. But, remember, don't present yourself at MT with a limited budget, they already have lots of work to do, so they aren't looking for budget minded projects...
If your preferred language is English, there will be no problem. The technical staff I worked with there speak better English than most people in North America - they have not learned to be sloppy speakers yet. But, remember, don't present yourself at MT with a limited budget, they already have lots of work to do, so they aren't looking for budget minded projects...
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Would 10 thousand Euros be considered too small for them?
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Great! It is a good starting point for knowing where to look. I will be investigating this and other options later this year, after my big move... Thank you very much for all the specialized info you provided. I seems like pprn is a good place for specifics.🙂