investigating Curtiss Electric Prop Angle Limit Switches Modelled Incorrectly

[kablamoman]

I wanted to do a bit of research with regards to the Curtiss Electric prop as modelled in the DCS P-47 modules in order to ascertain what is going on with the blade angle limits, and whether or not the behavior of the automatic governing vs. manual RPM adjustment was correct as we see it in the sim.

As modelled currently, the governing range of the blades in the automatic mode reach their limit at 48 degrees coarse. Yet, by going into manual mode, you can further coarsen the blades to ~88 degrees -- what would be considered feathered. But this does not make sense for a single engine installation.

I suspected that maybe the devs had erroneously modelled propeller limits for a multi-engine installation of the Curtiss electric prop and set out to understand how the limit switches (analogous to mechanical high and low pitch stops in a hydraulically actuated prop) function and how they should be configured in a P-47.

For this I started out referencing the P-47 series Erection and Maintenance Instructions handbook (AN 01-65BC-2) 10 August 1944, revised 15 December 1947:

 

As you can see, the depiction of the limit switches (mounted on the aft portion of the power unit, which also contains the electric motor) specifies three limit switches: Low angle, high angle, and feather angle. It is important to understand that all Curtiss electric prop installations have these switches as the props can be fitted to many types of aircraft. Indeed, some installations would be for multi-engine aircraft (eg. the B-26), where a separate feather limit would be needed.

If one were to look at the wiring diagram for the control mechanism, and also if you read the user manuals for the Curtiss electric props, you can see and read that while the manual increase RPM switch uses the same circuit as the automatic governor, the manual decrease RPM switch makes use of a separate control circuit, one that doesn't pass through the high limit switch, but instead the feather limit switch (follow the yellow line):

Indeed, in the generic Curtiss prop user manuals, it mentions that pilots can manually decrease the angle all the way to feather limits (albeit, at a slower rate than using the dedicated feather control on a multi-engine installation) if they use the manual RPM decrease control. One must remember that these manuals (you can check one out here for yourself) are made to encompass general usage of Curtiss props across several types of installations, and so are a bit ambiguous in specifying whether this feature works for a single-engine installation.

If this was all there was to the story, the DCS modelling is correct as is.

However, we also have this description of the limit mechanism:

 

It mentions that the switches are tripped by cam segments mounted onto the ring gear. It turns out that how those cam segments are mounted, determines when exactly the switches will be tripped. How should they be mounted in our P-47? Which Curtiss electric prop models are installed on our P-47s? Here's what the Erection and Maintenance Handbook says:

As you can see, we're already getting some hints about prop pitch ranges, but seeing as how we've got specific models listed and they all seem to be variations of the C542S-A series, let's take a look at that manual:

There's a ton of information in here about installation, disassembly and description of components, but we're interested specifically in those cams:

Here's the diagram, parts labeled 7-11 are the cams that determine when exactly our limit switches will trip.

Notice at the bottom, we've got a note about blade angles?? Hey, we may be getting pretty close here. Let's have a look:

Here, I've cross referenced all the specific model numbers listed in the P-47 Erection and Maintenance handbook, and you can see that for each of them, the High and Feather limits are exactly the same -- this means that a manual decrease of RPM should result in the same blade pitch governing range as the automatic mode.

But why is our high/feather limit different than what we're seeing in game? To shed some light on this, here's a technical order from June of 1945:

 

As you can see, at a certain point, blade angles were increased (the procedure involves moving those cams by 5 notches towards a higher coarse limit). There is still a discrepancy between this technical order and the curtiss prop manual, but I am assuming that is because the curtiss manual is 5 years more recent (1950). The one thing this does seem to prove, though, is that in all instances the curtiss electric prop installation on the P-47 had the same coarse pitch limits, whether governing in automatic mode, or set manually with the decrease RPM switch in fixed pitch mode.

Also worth noting is that the increase limits in the technical order were done to help prevent engine overspeed in high-speed dives.

Lastly, would like to reiterate the fact that, as others have mentioned multiple times on this forum, that the allowable engine overspeed limit in the P-47 pilot manuals is listed as 3060 RPM, probably owing to the fact that you WILL exceed the RPM redline in a dive at high speeds with any power applied:

 

I believe getting the proper blade angle limits implemented would be a great first step on the road to really tightening up the P-47 module.

Thanks for following along and I hope the devs really consider taking a second look at this.

Edited October 28, 2023 by kablamoman

[Hobel]

Even the dcs manual gives a higher value.

56

 

Nice post.

[kablamoman]

Here are the equivalent -59 numbers:

 

Edited November 6, 2023 by kablamoman

[Art-J]

On 11/6/2023 at 6:04 AM, SRF_Robert said:

PS: The P-47 is incredibly well-engineered for DCS in comparison to real life. Even with NACA test data and R-2800 engine limit data, this module behaves exactly how the reports say down to the buffeting of a specific control surface in a specific flight regime. 

 

Well, let's just say it does some things well and some things... possibly not so well. Infamous adverse compressibility effects in a dive are, for example, an elephant in the room, because in practical terms they're non-existent in DCS Thunderbolt. Emphasis on "in practical terms", because even though a very mild Mach tuck and mild elevator stiffness are indeed simulated, easy and effective pitch control is still maintained at every power, dive speed and high altitude combo you can test in mission editor. So, contrary to flight manual notes, you can just forget about compressibility and about assigning controls to dive recover flaps, as you will never need to use them anyway.

About the engine RPM, please note that even the 2880 limit for 30 seconds posted in your manual is not possible in DCS, let alone higher one from other revisions. Testing in v 2.9 shows the redline of 2750 is when damage simulation starts every time, with time-to-failure depending on how long and how far we are above that number. For example: staying between 2750 and 2800 kills master bearing in about 45 seconds. Staying between 2800 and 2850 kills the bearing in about 18 seconds. Measured with stopwatch and data from post-mission briefings.  Going to 2900+ range will kill the bearing below 12 seconds. That's with power-on, power off would be even faster for the forces reasons explained in your manual.

Let's get back on topic, though. The current engine RPM limits are here to stay for now, as was officially confirmed in the other thread on the subject, with suggestion for revision at a later date forwarded to the devs by Nineline. Propeller pitch limits, however, might indeed help figuring out what the most plausible simulated engine behaviour in a dive we should expect. Kablamoman presented some valuable prop reference data here.

[kablamoman]

3 hours ago, SRF_Robert said:

The second point is showing windmilling. Most people in DCS are not trained 47 pilots  and end up diving at max rpm and blow up before they even hit compressibility.

Seems there may be a bit of confusion. The 2880 and 2980 limits you posted vs. the 3060 limit are simply due to them being figures for different engine models.

Max diving speed (30 seconds):

R-2800 S1A4-G: 2880 RPM

R-2800 2SB-G: 2980 RPM

R-2800-59: 3060 RPM

The first two engines seem to be export variants (an A series and B series engine, respectively), while the -59 is the B series engine used in our DCS P-47. The figure has nothing to do with the prop.

The notes in the procedure about “some cruising value” are suggesting a lower RPM and manifold for entry (unless MIL is being used tactically, ie. fighting) to give the governor more of a head start on blade angles. Indeed, the 47 manual recommends a 2600 RPM entry.

Once the blades are at their coarse limit, however, as they will be eventually in a high-speed dive, it does not matter what the pilot has selected for their RPM — the blades will have reached their coarse limit and the engine will start to overspeed with increased airspeed.

Where the material you shared is really helpful is seeing that it recommends at least 12 - 15 inches of manifold (and even higher if able, according to the chart) even if the engine is already overspeeding!

It’s saying that to help prevent reverse loading on the master rod bearings you should keep as much manifold applied as practicable, likely overspeeding the engine even further, up to the 3060 RPM value (not to exceed 30 seconds). This procedure is absolutely impossible with the current engine modelling, and highlights a deficiency many have noticed in the sim.

Increasing the allowable high angle of the blades to 53° as per the technical order posted would likely bypass the issue entirely. However, if they choose to retain the current 48° angle governing limit, these overspeed failure modes should probably be properly addressed.

In either scenario, I do not believe the separate “feather limit” of 88° (or anything beyond the auto-governing limit) one can manually achieve by using the manual decrease RPM switch is accurate, and it should probably be fixed.

 

 

Edited November 7, 2023 by kablamoman

[kablamoman]

Just to provide some missing context from my previous reply (as the other user removed the post with the dive procedures from the Pratt manual) here's the relevant bit again:

 

The above was extracted from a -8/-10 version of the manual (which has the same overspeed limits as the -59), and the wording matches that from the generic A/B series manual that was previously posted.

[grafspee]

32 minutes ago, kablamoman said:

Just to provide some missing context from my previous reply (as the other user removed the post with the dive procedures from the Pratt manual) here's the relevant bit again:

 

The above was extracted from a -8/-10 version of the manual (which has the same overspeed limits as the -59), and the wording matches that from the generic A/B series manual that was previously posted.

This imho should close discussion.  Last time i checked in DCS even if i was holding 50inch and rpm went up to 2900 engine died. DCS P-47 is far away in that regard from RL.

[Cool-Hand]

1 hour ago, kablamoman said:

Just to provide some missing context from my previous reply (as the other user removed the post with the dive procedures from the Pratt manual) here's the relevant bit again:

 

The above was extracted from a -8/-10 version of the manual (which has the same overspeed limits as the -59), and the wording matches that from the generic A/B series manual that was previously posted.

Pretty cut and dry that the P-47 needs a tweak or two.

[NineLine]

Will take a look, thanks.