correct as-is Engine bearings damage too sensitive?

[NineLine]

On 1/19/2021 at 11:19 AM, GUMAR said:

Topic starter is damn right!

 

Made a simple test.

Manually set 2800-2900 RPM. MP well above 40". 50 seconds --> Master bearing damage. While "maximum permissible diving RPM" is 3060 from P-47s manual.

Conclusion.

Bearing damage occurs whenever you exceed 2800 RPM regardless of manifold pressure.

 

P.S. As a background, I did numerous dive tests before to find operational limits for P-47.  

P-47_bearing_dmg.trk 39.53 kB · 75 downloads

 

I am not sure what to say but you manually adjust your RPM to exceed the limits, why would you do this and why would you expect it to be ok?

You put the RPM above the redline then when it became damaged you kept it above the redline, and the engine got worse. In what scenario would you need to do this?

Now, bearing damage is bad, but it is also not a blown engine just yet. If you correct and fly within the limits you can still survive and even fight. Put it back on auto, don't run the engine at its max and make plans to go home, but if you had to you could still fight, at least in my experience as someone who tends to be hard on his aircraft.

Some clips from the manual as well on diving and such.

[kablamoman]

How about the fact that you can't exceed 400mph indicated without the RPM going over its redline and killing the engine when the prop is governing in automatic mode?

There is simply something wrong with the way the engine and/or the prop is modeled:

1. The prop having a coarser "feather" limit in manual mode on a single-engine installation is suspect. The Curtiss electric prop hub has a separate "Feather" limit switch, but in single-engine installations the pitch angle for this should match that of the "Coarse" limit switch (the equivalent of a high-pitch stop).

2. As modeled in the sim right now, If you want to take advantage of the P-47's diving capabilities and fly within 100 mph of the redline airspeed, you have to switch the prop to manual mode and coarsen beyond the high limit to keep the engine from over speeding due to the windmilling prop.

3. Any kind of windmilling kills the engine way too fast. The standard practice of not allowing the prop to drive the engine with radials like the R-2800 was something they discovered post-war with airline operations trying to extend their TBOs with aircraft like the DC-6, and even then, it wasn't something that caused immediate and catastrophic engine failure. They certainly didn't care too much about it in their jugs during the war when time between overhaul was a secondary consideration to fighting in their aircraft. Can anybody cite specific mention of fragile main engine bearings or specific procedures to follow in pilot manuals to avoid the problem? There is certainly no mention of flipping the prop control to manual when diving.

4. Folks have already pointed out that 3050 RPM was noted in performance charts as the max permissible RPM overspeed in a dive.

5. Slightly different aspect of the module, but compressibility seems completely absent.

 

Edited September 22, 2023 by kablamoman

[Art-J]

@NineLine That's the point, the limit itself is the problem. To add more specific reference to Kabloman's point No. 4, please check:

a) 01-65BC-1 manual for B-through-G Thunderbolts. For -21 version of the engine (which belongs to -B series as the one in DCS), engine chart on page 32 states max permissible diving RPM as 3050;

b) AN 01-065BC-1A manual for D-25 through D-35 Thunderbolts. For generic -B series of the engine, engine chart on page 36 states max permissible diving RPM as 3060;

So, even if we skip the Curtiss prop pitch stops issue for now, as I can't provide relevant documentation (yet), there already seems to be a flaw in DCS, with bearing damage simulation kicking in exactly at 2750 RPM figure. Following the real manuals and getting close to 3000 in a dive is not possible, as the simulation kills the engine before 2900, after 10-15 seconds depending on dive angle. Very short over-3000 RPM "spikes" are indeed possible, but only during incorrect stall & spin recovery, and usually don't kill the engine if the governor loads the prop fast enough to drop RPM below 2700. That's a different case then.

To sum it up, the manuals clearly show the takeoff (gauge) redline wasn't the same as ultimate engine "failure redline" so to speak, while in DCS it is. Granted, 3050-3060 surely couldn't be maintained for long. How long was "safe"? These manuals don't say. N Thunderbolt manual says "up to 30 seconds", and even at 3120 RPM, but that's for -C series engine which was a different kettle of fish compared to "ours". It's still some reference, though.

Edited September 22, 2023 by Art-J

[Art-J]

48 minutes ago, kablamoman said:

5. Slightly different aspect of the module, but compressibility seems completely absent.

That's the one which always cause head sctratching although I think some of it is kind-of present actually. Mind you, it's supposed to be more noticeable above 20-25000 ft and it doesn't take long to get to lower, denser altitudes in a dive. However, in my numerous test mission attempts, air-starting at 35000, I've noticed that when initially trimmed for a dive, the plane does tend to pitch down a little by itself, so a bit of Mach-tuck simulation seems to be there?

Now, in a grand scheme of things it doesn't matter at all, because control loss doesn't seem to be simulated indeed and we always have full elevator authority, making recovery flaps implementation redundant. I've only read one guy's post here on the forums, claiming he had experienced control loss in a dive, but without replay track I call it a bluff  Will believe it when I see it.

[Bremspropeller]

1 hour ago, kablamoman said:

3. Any kind of windmilling kills the engine way too fast. The standard practice of not allowing the prop to drive the engine with radials like the R-2800 was something they discovered post-war with airline operations trying to extend their TBOs with aircraft like the DC-6, and even then, it wasn't something that caused immediate and catastrophic engine failure. They certainly didn't care too much about it in their jugs during the war when time between overhaul was a secondary consideration to fighting in their aircraft. Can anybody cite specific mention of fragile main engine bearings or specific procedures to follow in pilot manuals to avoid the problem? There is certainly no mention of flipping the prop control to manual when diving.

Not only do the manuals not suggest it was a thing. There's also a complete lack of anecdotal evidence of what would have been a serious operational constraint.

I don't buy the glass bearing story.

[kablamoman]

Just for fun, here's some random Curtiss electric jug images I could find where the prop angles were legible. You may notice that the ones with the 48.5 high angle limit like ours in-game have the same feather limit if it's also specified. One wonders how these machines would have been able to dive beyond 400 mph without the engine self-destructing:

Edited September 22, 2023 by kablamoman

[NineLine]

What you guys are failing to acknowledge is that in most if not all manuals you must pull back on the throttle in a dive. Most tracks I am seeing are with guys either full throttle or for some weird reason using manual prop control. 

If I go into a dive as most manuals suggest, I have no engine issues. 

And trust me, this bearing issue was worse and was tuned, but I am not seeing where if you run the engine like the manual states (and max permissible RPM is not the recommended RPM), I am not seeing an issue.

Also not seeing instant damage at 2750, you need to look at all aspects of the flight. Now Yo-Yo has already commented and it seems correct.

The only thing I will personally concede is that running the engine at about 3000 RPM the damaged bearings happens a little fast, but then I don't spend much if any time at that RPM as there is no need to.

 

[Reflected]

18 minutes ago, NineLine said:

What you guys are failing to acknowledge is that in most if not all manuals you must pull back on the throttle in a dive. Most tracks I am seeing are with guys either full throttle or for some weird reason using manual prop control. 

If I go into a dive as most manuals suggest, I have no engine issues. 

And trust me, this bearing issue was worse and was tuned, but I am not seeing where if you run the engine like the manual states (and max permissible RPM is not the recommended RPM), I am not seeing an issue.

Also not seeing instant damage at 2750, you need to look at all aspects of the flight. Now Yo-Yo has already commented and it seems correct.

The only thing I will personally concede is that running the engine at about 3000 RPM the damaged bearings happens a little fast, but then I don't spend much if any time at that RPM as there is no need to.

 

Throttles pulled back to approx 1/3  to keep below the red line in MP. Still damaged. 

 

p47bearingdive.trk p47bearingdivewep.trk

[NineLine]

The dev has commented on this correct as is, I even showed you a track how to dive properly. This issue is closed for now. Thanks.

Edit: I asked for the time to damage to be reviewed even during what would be considered abuse.