correct as-is Engine bearings damage too sensitive?

[PL_Harpoon]

Overall I like the new changes to engine damage model, however I think it needs some tweaking.

Especially engine bearings.

 

Currently, if you dive at full RPM you're at a very high risk of damaging engine bearings.

I'm guessing it's because of overspeeding the engine and reducing the RPM to 2500 is enough to prevent this but here's why I still think it's a problem (and not accurate):

 

First, I've searched through both the original USAAF Pilot's Flight Operating Instructions and Pilot Training Manual and found no mention of keeping the RPM below max when diving. There is one sentence in the Training Manual about reducing Manifold Pressure to keep the engine from overboosting, but  in DCS the bearings get damaged regardless of MP.

Surely, if the risk of damaging the engine were so great there would be at least some mention of it.

 

 

Secondly, it's a common practice, when designing engines that when you know the RPM limit of your engine you give yourself some safety room when placing the limiters. It seems very unlikely that the engineers/designers would setup the max RPM at 2750 RPM if they knew it could break apart at 2800. Especially for a combat aircraft where you know things (like the RPM governor) will get damaged.

And yes, during dive and recovery the RPM needle barely moves past the 2750 limit (if at all).

 

Here are the links to the documents I mentioned (they're free if you create an account)

https://www.avialogs.com/aircraft-r/republic/item/5091-aaf-51-127-3-pilot-training-manual-for-the-p-47-thunderbolt

https://www.avialogs.com/aircraft-r/republic/item/5089-an-01-65bc-1a-pilots-flight-operating-instructions-for-p-47d-25-26-27-28-30-and-35-airplanes

P-47 engine main bearing damage.trk

Edited December 25, 2020 by PL_Harpoon
correcting RPM limit to 2750, adding a track file showing the problem

[grafspee]

wow, you are doing it wrong, your engine did not die because over speeding engine, you have your throttle retarded to 0  while you have high rpm, you cant do that. it will kill main bearing in matter of seconds.

next time leave at least 25' of MP or reduce engine rpm. But avoid at all costs cuting throttle to 0 while flying p-47

From  very beginning you cut throttle to 0

Edited December 25, 2020 by grafspee

[PL_Harpoon]

Ok, so it's not related to engine overspeed after all.

I haven't found it in any manuals I've read (in fact I had no idea about such limitations until now) so I guess it's something inherent to radial engines, right?

 

Well, you learn something new every day.

Thanks for clarification.

[grafspee]

yes all radial suffer from this, same as FW190 but in FW190 Engine rpm are binned with throttle so when you retard throttle engine rpm will go down as well.

[-0303-]

On 12/25/2020 at 3:16 PM, PL_Harpoon said:

(in fact I had no idea about such limitations until now) so I guess it's something inherent to radial engines, right?

It's a thing with Piper Cubs and Pawnees towing gliders. Full power going up, don't crazy cut throttle leaf flipping down. To rapid cooling develops engine cracks over time, driving up maintenance costs.

[grafspee]

12 minutes ago, -0303- said:

It's a thing with Piper Cubs and Pawnees towing gliders. Full power going up, don't crazy cut throttle leaf flipping down. To rapid cooling develops engine cracks over time, driving up maintenance costs.

Rapid cooling is not a big deal in that case, nether less rapid cooling is an issue too.

But radial engine has single main bearing which is not designed to with stand forces when engine is driven by prop airflow. V12 engines are naturally immune to this because the number of main bearings per cylinder is much higher.

Solution for p-47 is to latch prop handle with throttle this will secure engine from getting damage. So when you are doing some ground attacks and you need to cut throttle in dives connect throttle with prop, when you cut throttle engine rpm will drop and main bearing will be safe.

[Nealius]

If cutting throttle damages the bearings, how do we do dive bombing? Shouldn't the throttle be in idle during a diving run?

[Yo-Yo]

1 hour ago, Nealius said:

If cutting throttle damages the bearings, how do we do dive bombing? Shouldn't the throttle be in idle during a diving run?

No, just reduce MP and prop rpm if necessary. 2300/36 or 2400/46 will be fine.

[grafspee]

Not sure which version had this but you can latch throttle and prop lever, quite useful for bomb diving.

Then, when you cut throttle engine should be fine.

[Nealius]

Should prop be reduced when cutting the throttle on landing? After 50 minutes of flying around at 2550/42", I landed with 2550 and whatever throttle required to maintain safe pattern speed. Cut throttle just before the threshold like the manuals say. My engine died after landing, while still on the runway. Temps were all good, and I was able to restart the engine normally, but it was just weird. 

[grafspee]

On 1/10/2021 at 5:31 PM, Nealius said:

Should prop be reduced when cutting the throttle on landing? 

It does not matter, you set 2550rpm only because in case of landing reject, you have available up to 42" to perform go around.

When you retarding throttle in flight prop pitch will decrease to keep set rpm. But at some point when airspeed drops and throttle will be at low position you will notice that prop rpm starts decreasing, This mean that prop blade pitch is already at fine pitch position at this point prop lever does not change a thing is it at max or minimum prop rpm will not change as well as prop's blade pitch remain constant.

I always try to remind ppl who memorizes  coarse/fine pitch as full forward/back prop lever, this makes very hard to understand how constant speed prop works.

 

On 1/10/2021 at 5:31 PM, Nealius said:

Cut throttle just before the threshold like the manuals say. My engine died after landing, while still on the runway. Temps were all good, and I was able to restart the engine normally, but it was just weird. 

One thing which comes to my mind is mixture control. Are you sure that you had mixture lever at right position ? Auto Lean or Emergency Rich may not support idle engine rpm.

Edited January 11, 2021 by grafspee

[Nealius]

My mixture control should have been in Auto rich, but I'll double check if it happens again. I also landed with a belly tank with that tank selected, but it had plenty of gas left to get my engine re-started and taxi to parking, so unless the gas sloshes around on landing or something I don't think it was the belly tank that caused it.

[grafspee]

10 minutes ago, Nealius said:

My mixture control should have been in Auto rich, but I'll double check if it happens again. I also landed with a belly tank with that tank selected, but it had plenty of gas left to get my engine re-started and taxi to parking, so unless the gas sloshes around on landing or something I don't think it was the belly tank that caused it.

It could be that, iirc manual says select main tank before landing. It would require to carefully analyze fuel system in P-47, could be that fuel pumps will not drain fuel from drop tanks at idle speed.

If you can replicate this watch fuel pressure gauge, if pressure drops mean that fuel is not fed to fuel system.

Edited January 13, 2021 by grafspee

[Yo-Yo]

On 1/11/2021 at 7:20 PM, grafspee said:

It does not matter, you set 2550rpm only because in case of landing reject, you have available up to 42" to perform go around.

When you retarding throttle in flight prop pitch will decrease to keep set rpm. But at some point when airspeed drops and throttle will be at low position you will notice that prop rpm starts decreasing, This mean that prop blade pitch is already at fine pitch position at this point prop lever does not change a thing is it at max or minimum prop rpm will not change as well as prop's blade pitch remain constant.

I always try to remind ppl who memorizes  coarse/fine pitch as full forward/back prop lever, this makes very hard to understand how constant speed prop works.

 

One thing which comes to my mind is mixture control. Are you sure that you had mixture lever at right position ? Auto Lean or Emergency Rich may not support idle engine rpm.

 

Or engine had initial master bearing damage. As MP and speed  were relatively high the engine runs. No speed and idle MP - bye-bye.

 

[Yo-Yo]

No problem to land if the power is reduced gradually if you have excessive speed : 2700/52 - 2700/46 - 2500/46 - 2500/30 - 2400/30 - 2000/20  (rpm are actual, if the speed is quite low at final the rpm lever can be advanced to 2700 for possible go-around but the actual rpm will be lower).

[grafspee]

I think that our p-47 has option to interconnect throttle and rpm lever. I think it is good solution for landing as well as dive bombing

 

[Yo-Yo]

No, just throttle and boost.

 

[PL_Harpoon]

It definitely has.

I've done it a couple of times.

 

 

Also, all the original manuals I've seen  (including the "N 01-65BC-1A Pilot's Flight Operating Instructions for P-47D-25,-26,-27,-28 -30 and -35 airplanes" which is the most relevant for us) tell you to set the RPM to 2550 for landing, so using lower RPM settings cannot be mandatory unless you already have a damaged engine.

 

If you have a damaged engine the safest approach is to perform a power-off landing anyway (my opinion).

Edited January 13, 2021 by PL_Harpoon

[grafspee]

I was recalling that i played with this as well in recent past, but it may be dependent on jug version, we have 3 versions iirc.

Edited January 13, 2021 by grafspee

[Whisper]

41 minutes ago, grafspee said:

I was recalling that i played with this as well in recent past, but it may be dependent on jug version, we have 3 versions iirc.

 

I just tested the 3 versions, seeing your post, and the 3 have the interconnect between RPM lever and throttle

[Tiger-II]

Interesting they really are destroyed in seconds! I'll stop trying to break it now!

[GUMAR]

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

Edited January 31, 2021 by GUMAR

[grafspee]

Agree, 3000rpm 52" main bearing damaged after 8 second.

[AJaromir]

On 12/30/2020 at 10:41 PM, grafspee said:

when engine is driven by prop airflow

It's called windmilling.

[303_Kermit]

On 1/19/2021 at 8:19 PM, 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 · 70 downloads

 

@Yo-Yo Is it corrected?