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Engine RPM fluctuation


_Spad_

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Hi

Last time I flew the P-47 I experienced RPM fluctuations right from startup for the entirety of my flight.

 

I set my MP to give 900rpm on warm up, and noticed that the rpm would drop to around 6-700rpm before rising again to 900rpm? Even during cruise I was experiencing the same 2-300rpm fluctuation. Any ideas?

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11 hours ago, grafspee said:

Did you secure priming pump ?

Yes. I went through my normal startup procedure.

 

For what it's worth, I have only experienced this issue in the P-47-30bl1. Not sure if that would make a difference or not.

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Only things I can think of, other than the primer that grafspee mentioned, are a possible HOTAS issue or you may have turned the fuel pump rheostat to something other than the START & ALTITUDE setting (fully counter clockwise)

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  • 4 months later...

Last night it happened to me twice: once for taking off in a hurry without warming up the engine enough, the other after (almost) frying the engine on a lengthy dogfight. How are your temperatures (engine, carb, oil, etc?).

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On 2/3/2021 at 6:49 AM, _Spad_ said:

Hi

Last time I flew the P-47 I experienced RPM fluctuations right from startup for the entirety of my flight.

 

I set my MP to give 900rpm on warm up, and noticed that the rpm would drop to around 6-700rpm before rising again to 900rpm? Even during cruise I was experiencing the same 2-300rpm fluctuation. Any ideas?

Did you lock the primer pump handle and are you aware that they have corrected the animation for it?

 

 

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  • 5 months later...

This is in a P-47-40 and I've experienced this "desyncronized" Propeller effect several times. Most times I did the standard start up procedure also did the prop & magnetos last check before entering runway, but in this particular case I closed  the cowling flaps and switched on the oil diluter on ground, just to accelerate the warming up. I only took off once all needles were on blue, I switched off the oil diluter before entering the runway but I kept the cowling flaps totally closed all time. Also I'd tried to disconnect and reset the breaker of the auto prop pitch inflight, test the switch in manual, also tried to test the prop lever manually, but same issue in all flight phases. The A/C got an strong tendency for drifting right in the desync- sync, also I noticed the P-47 more floaty than I normally feel it in speeds below 200 KIAS.  

In the video you could notice the slide indicator and the magnetic compass moving every time RPMs decrease- Increase.

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1 hour ago, OLD CROW said:

This is in a P-47-40 and I've experienced this "desyncronized" Propeller effect several times. Most times I did the standard start up procedure also did the prop & magnetos last check before entering runway, but in this particular case I closed  the cowling flaps and switched on the oil diluter on ground, just to accelerate the warming up. I only took off once all needles were on blue, I switched off the oil diluter before entering the runway but I kept the cowling flaps totally closed all time. Also I'd tried to disconnect and reset the breaker of the auto prop pitch inflight, test the switch in manual, also tried to test the prop lever manually, but same issue in all flight phases. The A/C got an strong tendency for drifting right in the desync- sync, also I noticed the P-47 more floaty than I normally feel it in speeds below 200 KIAS.  

In the video you could notice the slide indicator and the magnetic compass moving every time RPMs decrease- Increase.

1) You haven't locked the primer handle (try to memorize that it should be turned right) - that's the reason for fluctuation;

2) Oil dilution has not been modelled yet, so don't bother with the switch - it's non-functional.

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3 hours ago, Art-J said:

1) You haven't locked the primer handle (try to memorize that it should be turned right) - that's the reason for fluctuation;

ok then.... appreciate. Do you have any explanation for that logics? I mean why a fuel primer handle position generates RPM's fluctuation? It really blows my mind and it would be awesome to understand the technical reason. Thanks anyway... with explanation or not.

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7 minutes ago, OLD CROW said:

ok then.... appreciate. Do you have any explanation for that logics? I mean why a fuel primer handle position generates RPM's fluctuation? It really blows my mind and it would be awesome to understand the technical reason. Thanks anyway... with explanation or not.

 

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On 11/26/2021 at 4:40 PM, OLD CROW said:

ok then.... appreciate. Do you have any explanation for that logics? I mean why a fuel primer handle position generates RPM's fluctuation? It really blows my mind and it would be awesome to understand the technical reason. Thanks anyway... with explanation or not.

The plunger type hand primers were the most common primers in use before electric primers. They typically squirted fuel in the intake of a couple cylinders (not all cylinders) to give the extra gas needed for a cold start. For most radials, it was usually the upper cylinders where the extra fuel was delivered. They operate similarly to a bicycle air pump where the upstroke will pull in fuel, and the downstroke delivers the fuel to the engine. The pump has check valves inside it to ensure the fuel flow went the proper direction.

The primer pump in the P-47 pulled fuel directly from the main tank strainer assembly, and then to the upper cylinders. The reason why the engine will run rough if one leaves the primer handle out is that there is now another pathway for fuel to flow into the engine.
 

For an engine to run smooth, every cylinder must get a similar amount of fuel and air as the rest of the cylinders (and ignited properly at the correct time) so that the combustion forces are fairly equal. Radial engines do a pretty good job at this as the intake manifold is pretty symmetrical, meaning that the fuel/air mix from the carburetor and supercharger travel about the same distance to get to every cylinder. At idle, there is some suction force in the intake, enough to pull some fuel through the check valves of an open and unlocked primer pump. And since the upper cylinders get the fuel from the primer pump, their fuel mix is now different than the other cylinders, making them run different than the others, and hence why it runs funny. Locking the primer pump closed off this path for fuel, ensuring that the carburetor was the only device delivering fuel to the engine for normal operation.

As aircraft electrical systems developed, the hand primer was replaced with a switch that controlled a solenoid valve which was fed from the fuel pump. That setup used the same method as the hand pump which only delivered fuel to a handful of cylinders. This later evolved to a single nozzle that sprayed fuel right into the supercharger so that all cylinders got extra fuel for cold start. Our DCS P-51 Mustang features this style of primer.


Edited by Diesel_Thunder
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7 hours ago, Diesel_Thunder said:

The plunger type hand primers were the most common primers in use before electric primers. They typically squirted fuel in the intake of a couple cylinders (not all cylinders) to give the extra gas needed for a cold start. For most radials, it was usually the upper cylinders where the extra fuel was delivered. They operate similarly to a bicycle air pump where the upstroke will pull in fuel, and the downstroke delivers the fuel to the engine. The pump has check valves inside it to ensure the fuel flow went the proper direction.

The primer pump in the P-47 pulled fuel directly from the main tank strainer assembly, and then to the upper cylinders. The reason why the engine will run rough if one leaves the primer handle out is that there is now another pathway for fuel to flow into the engine.
 

For an engine to run smooth, every cylinder must get a similar amount of fuel and air as the rest of the cylinders so that the combustion forces are fairly equal. Radial engines do a pretty good job at this as the intake manifold is pretty symmetrical, meaning that the fuel/air mix from the carburetor and supercharger travel about the same distance to get to every cylinder. At idle, there is some suction force in the intake, enough to pull some fuel through the check valves of an open and unlocked primer pump. And since the upper cylinders get the fuel from the primer pump, their fuel mix is now different than the other cylinders, making them run different than the others, and hence why it runs funny. Locking the primer pump closed off this path for fuel, ensuring that the carburetor was the only device delivering fuel to the engine for normal operation.

As aircraft electrical systems developed, the hand primer was replaced with a switch that controlled a solenoid valve which was fed from the fuel pump. That setup used the same method as the hand pump which only delivered fuel to a handful of cylinders. This later evolved to a nozzle that sprayed fuel right into the supercharger so that all cylinders got extra fuel for cold start. Our DCS P-51 Mustang features this style of primer.

Great tech explanation Sir. What really is still blowing my mind is the direct link between that anomally in the fuel injection system and the RPM's fluctuation in an constant speed propeller controlled by an electrical auto system. In my vid you can see an small fluctuation, about 1 or 2 P.S.I's in the manifold needle while in the RPM's tachometer you can visually observ the greater fluctuations, but anyway... all these tech explanations and the reminder of locking up the primer handle after pumpings have made me enough happy. Thanks to everybody.

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This does not explain power fluctuation at your video, as you can see that manifold pressure is 40", this mean that primer line can not distribute fuel in to the engine, but fluctuations are still present.

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I have a feeling that may be a DCS thing in the primer coding (if primer unlocked - then engine roughness/instability).

As grafspee pointed out, you were at 40 inches of MP. That should have forced the check valves in the primer pump closed, as the intake manifold is under pressure and not suction like at low power/idle.

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