P-47 and strange relationship between MP and RPM

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Jacques
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P-47 and strange relationship between MP and RPM

Post by Jacques »

If I understand correctly all I've read about this subject, with a constant speed propeller any movement of the throttle to increase or decrease MP will result in essentially no change (maybe some fluctuation before settling as the governor reacts?) in RPM. On the other hand, a change in RPM- say a decrease- will result in a change in MP. The decrease in RPM will, or should result in a slight increase in MP.
For the record, the P-47 on my computer is completely up to date. On a recent flight I wrote down these characteristics on a flight in which I did not use the turbo:

Takeoff at 42" MP and 2550 RPM. Once cleaned up and at 3000', I reduced to a fast cruise setting of 32" MP and then brought the RPM down to 2150 as suggested by the manual. As I brought the RPM down the MP dropped as well, ending up at a bit below 28". I raised the MP back up to my target of 32" and RPM stayed steady at 2150, as it should.

I noticed this a few weeks ago and thought it strange. I don't know if this was new behavior, but it seemed odd, and since some time and several updates had passed since I last flew the P-47, I couldn't really trust my memory. So, some more testing was in order.

Moving the MP back up to 42" requires the RPM being moved first. I accomplished this, moving the RPM up to the recommended 2550 for a climb setting. This time, the MP climbed as I moved the RPM up and stopped at 35". Once RPM was steady at 2550 I moved the MP the rest of the way up to 42", with no change from RPM, again as expected.

One more rep, this time back down to an economy cruise setting of 32" MP and an RPM of 1700. The throttle was set first at 32" and then the propeller slowly brought back to 1700. The MP drops into the basement, finally stopping at 23.8". So, I move the throttle forward and set it again at 32" with the RPM steady at 1700, again as expected.

This kind of drastic drop in Manifold Pressure when the propeller is moved back to a slower speed just doesn't seem right. And I'm not too sure about the RISE in Manifold pressure as the Propeller speed is increased.

Can anyone, first of all, confirm this behavior in their P-47? Second, if this behavior is correct, can they then explain why it works this way?

Thanks for the help,

JP

Just as an aside, this may have been discussed in another post, but I cannot find it anywhere in the P-47 thread, so maybe it was tacked on the end of an unrelated post?

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gulredrel
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Re: P-47 and strange relationship between MP and RPM

Post by gulredrel »

Don't know the other post, maybe in the P-40 forum.
Both engines have a supercharger directly attached.
So every change in rpm changes speed of the supercharger, this directly influences manifold pressure. lower rpm, lower pressure output of the blower and vice versa.
P-47 adds the turbocharger for higher altitudes. But down below or at ground level, boost comes from the supercharger only.

Hope, this helps.

Jens
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Jacques
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P-47 and strange relationship between MP and RPM

Post by Jacques »

Hi Jens,

Yes, I understand that there is a gear-driven supercharger on the P-47, but I don't understand how that explains why, when Manifold Pressure is reduced, and then RPM is reduced, the Manifold Pressure continues to fall as the RPM is reduced. This behavior means that you could then reduce either MP first or RPM first, so there is no need for any procedures for reducing or increasing power. At least with my P-47, a reduction in RPM only leads to a reduction in MP instead of an increase, however slight, as might be expected.

I believe the A2A B-17 preserves this characteristic of power changes, and I thought the P-47 did as well when I first started flying it, but it was a long time ago, and there were several updates in the interval, so I don't trust my recollection. I can't see why there would be a difference in an R-1820 and an R-2800 in how they behave....I mean they are both just air pumps of a different size.
I'm still puzzled.

JP

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gulredrel
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Re: P-47 and strange relationship between MP and RPM

Post by gulredrel »

I think, B-17 should do the same.
Maybe the location of the pressure sensor is different in the induction system or the regulator comes into play there, even if it is set to 0.

Maybe anyone else has some ideas?
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Jacques
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Re: P-47 and strange relationship between MP and RPM

Post by Jacques »

Hi Jens,

I did some tests with the B-17 yesterday and again this morning. I set the TBS at either 8, 5, or 0. Carb Filters On or Off. Aside from some settings (MP @ 29" and RPM variations between 2000 and 1400 with TBS at 0), increasing RPM resulted in MP fluctuating lower and decreasing RPM caused the MP to fluctuate higher. When MP was adjusted higher or lower, RPM stayed relatively flat...maybe a small fluctuation that then settled out to the original, or near original setting.

Which is really different from the way my P-47 behaves.

It is almost as if the two gauges are mirror images of each other....a reduction in RPM is mirrored by a reduction in MP, and vice versa. What I'm trying to say is...if you the RPM is reduced in 5 or 10 or 20 degrees in arc on the gauge, then the MP is also reduced in 5 or 10 or 20 degrees in arc, which to me is just completely counter intuitive! I hope I'm explaining this clearly enough.

Oh, I also did some flying with the Throttle and the Turbo control lever linked in the P-47 and the results were identical to what I've described with no Turbo being used.

I really just want to know if this is how the P-47 truly responds to changes in throttle settings and RPM!

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Jacques
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P-47 and strange relationship between MP and RPM

Post by Jacques »

I just found the forum where this was discussed earlier and, yes, it was the P -40 forum. Erik raised the question, Dudley confirmed that a lowering of RPM would result in a rise in MP. And, then it died a quiet death. So I guess there is no interest in exploring why the opposite is happening in some A2A Accu-sim aircraft.

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gulredrel
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Re: P-47 and strange relationship between MP and RPM

Post by gulredrel »

mmmhh... maybe one could reanimate the P-40 thread :wink:

Engine cylinders work as airpump against an almost closed throttleplate at idle. So lower than outside pressure.
In an engine without super/turbocharger, open throttle always means MP equals outside air pressure, regardless of engine rpm.
With charging, this becomes far more complex. More rpm, more pressure is delivered from the supercharger, so you'll restrict it with partially closed throttle. As MP gauge shows pressure differential campared to ambient pressure, one should note the difference between MP values below or above ambient pressure...
And maybe different supercharger systems behave differently at lower rpm, the charger is turning so slow and ineffective, that it will not deliver useful higher pressures?

Even if there's something wrong, don't know it A2A will change something with the non accu-sim core birds (B17, P-47, B377).
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Scott - A2A
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Re: P-47 and strange relationship between MP and RPM

Post by Scott - A2A »

This comes down to the supercharger efficiency. The P-47 is the one Warbird we never were able to flight test, so what we have in terms of supercharger efficiency is what we believe it is, based on the available data. Some superchargers are extremely inefficient at low and even mid range RPM, and come on strong in the higher RPM. Others are fairly linear.

We modeled the P-47 with the type of supercharger that over comes the opposite manifold pressure / rpm relationship of a normally aspirated engine.

I can't say with certainty, this is the way it is. What I do know is all of the performance numbers of the P-47 are very nicely in line with the actual flight tests from the military and manuals. The speeds based on manifold pressure and RPM. This tends to indicate the MP / RPM behavior is correct as if it wasn't, it would show in the flight tests.

Scott.
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Jacques
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P-47 and strange relationship between MP and RPM

Post by Jacques »

Thanks to you both. I didn't realize that the P-47 supercharger could overcome the normal relationship between RPM and MP. I was under the assumption that since it was driven directly by the engine then it was still subject to those characteristics. I assumed that a reduction in RPM would still always result in an increase in MP! So thanks, Jens, for trying to drive that point home to me repeatedly. A2A must have been somewhat (maybe very?) frustrated by not being able to conduct flight tests to verify results. In the past few weeks I've covered a lot of ground on the internet to find video or text to confirm how MP and RPM interact in the P-47... And my searches were fruitless, so I have a small idea of how difficult your task must have been to try and fine-tune behavior solely on the original test data.

I am always happy to fly your team's rendition of the P-47, so I didn't want to come across as whinging or dissatisfied, and I hope I was successful in that regard. I just needed someone to confirm that what I was seeing was correct, and then explain why. I'll take a closer look at the test data you refer to Scott ( in the A2A manual, I think). If I find any more interesting bits of information, I'll post them here.

JP

Edit-- Jens, do you have any good links describing/explaining different types of geared superchargers? Maybe "geared" isn't the correct term?

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Re: P-47 and strange relationship between MP and RPM

Post by CodyValkyrie »

It's extremely hard to find detailed information about planes to begin with. Now, make that a rare aircraft that flew nearly 70 years ago, and the rare few pilots that still have experience with them, or once flew in them, and you can imagine how hard this is. A2A does a good job of talking to maintainers and pilots, dissecting PIREPS, and multiple sources including manufacturer specifications for engines, etc. Tough work.
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Jacques
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Re: P-47 and strange relationship between MP and RPM

Post by Jacques »

Apologies if you thought the thread was hostile..it wasn't meant to be taken as such.

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Re: P-47 and strange relationship between MP and RPM

Post by Lewis - A2A »

No worries Jaques I don't think it was seen that was at all.

cheers,
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Jacques
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Re: P-47 and strange relationship between MP and RPM

Post by Jacques »

Thanks, Lewis! Yesterday, I spent some time at the local used bookstore and discovered on the shelves a beautiful, hardbound copy of Graham White's R-2800- such an odd coincidence. But, what a fantastic find! I think I've found my book of choice for the foreseeable future!

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Re: P-47 and strange relationship between MP and RPM

Post by n421nj »

Very interesting read
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