Engine management trouble

[twistking]

I'm new to the P-47 and following the official manual i can cold start and fly a sortie without the engine dying on me, which is a decent starting point, i guess.
However i sometime have the issue that while cruising the RPM drastically changes for a very short moment, unsettling the whole aircraft and generally giving me the impression that something is not right. First i thought, the prop govenor might be broken, but it could also be just a sudden reduction in engine power and the governor overcompensating, making the rpm needle jump around.

I'm new to the aircraft and this problem only occurs when i cold start, so it might have to do with warm-up, or some engine settings i forgot. I'll probably figure that out, however i would still like to know what happens with the engine that makes those RPM jumps. The engine will still run and reducing RPM also helps with the issue. I just wonder what might be going on there. On hot started aircraft i don't have this issue...

Thanks.

[razo+r]

Primer in and locked?

[twistking]

19 minutes ago, razo+r said:

Primer in and locked?

i think so, but will double check. locking is just one-clicking it, so that it rotates a little bit, correct?

[peachmonkey]

1 hour ago, twistking said:

i think so, but will double check. locking is just one-clicking it, so that it rotates a little bit, correct?

yes, it needs to rotate to the right (clockwise) to be closed.

[twistking]

at first i was suspecting that it had to do with improper warm-up (simply because proper warm-up seems to be more important on the p-47 then on other warbirds), but now that you mentioned fuel system, this would also make a lot of sense. i think i locked the primer everytime, but will also investigate if mixture setting was not correct. maybe i was running too lean.
there is no fuel booster pump or similar, that one would need to turn on, correct? if so, how does the fuel get to the engine though?

Edited February 1, 2022 by twistking

[unlikely_spider]

Shot in the dark, but do you happen to have water injection bound to something and you're hitting it inadvertently?

Also take a look at the turbocharger rpm when this happens to see if it is involved. It can have a 'multiplying' effect on engine rpm.

Edited February 1, 2022 by unlikely_spider

[twistking]

36 minutes ago, unlikely_spider said:

Shot in the dark, but do you happen to have water injection bound to something and you're hitting it inadvertently?

Also take a look at the turbocharger rpm when this happens to see if it is involved. It can have a 'multiplying' effect on engine rpm.

 

will check for both.

thanks to input from all of you so far. will do some flying now and check for everythign you mentioned

[peachmonkey]

51 minutes ago, twistking said:

there is no fuel booster pump or similar, that one would need to turn on, correct? if so, how does the fuel get to the engine though?

there is, but it works a little bit differently from other warbirds

1) the main fuel pump is turned on when you select the fuel tank

2) there's a booster fuel pump, which is controlled by the rheostat on the lower left panel (# 3 on the second picture)

None of them introduce the RPM fluctuations, I've tested it when I discovered the open primer handle rpm issue..

 

[Lixma 06]

There's a few threads on this - the turbo will suddenly surge (don't know why) drastically increasing the manifold pressure, and causing terminal damage if not quickly noticed and brought under control.

I've just had an awful couple of flights online with mysterious* high-speed vibration and engine failure while boosted.

*I'm pretty sure I followed the book on everything, but still....

[twistking]

1 hour ago, Lixma 06 said:

There's a few threads on this - the turbo will suddenly surge (don't know why) drastically increasing the manifold pressure, and causing terminal damage if not quickly noticed and brought under control.

I've just had an awful couple of flights online with mysterious* high-speed vibration and engine failure while boosted.

*I'm pretty sure I followed the book on everything, but still....

i think my RPM spikes are sth. different. i also once got the turbo surge you described, but i think it was different then my original issue. my issue felt more like engine loosing power. if i remember correctly the rpm spike was negative. did not safe a track unfortunately.

i did a flight again, double checked everything and did not get that behaviour. i had mixture on auto rich for cruise. will try again with auto lean to see if that has something to do with it.

[grafspee]

9 hours ago, Lixma 06 said:

There's a few threads on this - the turbo will suddenly surge (don't know why) drastically increasing the manifold pressure, and causing terminal damage if not quickly noticed and brought under control.

I've just had an awful couple of flights online with mysterious* high-speed vibration and engine failure while boosted.

*I'm pretty sure I followed the book on everything, but still....

When turbo kick it creates chain reaction, since turbo is powered by exhaust gases, sudden MP increase affects exhaust gases generation ratio and this speeds up turbo even more bringing even more MP, controling MP via boost lever is very difficulty due to big lag when turbo is spooling up. At higher alt this problem fades away since you have to use turbo all the time.

high-speed vibrations is indication of too high MP or too high carb air temp or both. for example 64" and 2700 is WEP rating but this not mean that you can use it, watch carb air temp if it hits 50C you are maxed out.

Same with military power 52" 2700rpm in hot day you may not be able to boost that high in case of P-47 RPM,MP and carb air temp are instruments needed to keep engine good.

Edited February 2, 2022 by grafspee

[Lixma 06]

4 hours ago, grafspee said:

high-speed vibrations is indication of too high MP or too high carb air temp or both. for example 64" and 2700 is WEP rating but this not mean that you can use it, watch carb air temp if it hits 50C you are maxed out.

I think this is the trouble I was having - during a fight I'd normally just throw everything to max (i.e. 72") instead of holding it at 64".

You can get away with max throttle/boost/everything on the deck, but above 100m everything starts getting complicated - god knows why.

I'm not convinced the turbo should behave like it does. Lag is one thing, but I see no reason for the turbo to spin up uncontrollably when you're flying straight and level with no throttle changes.

[grafspee]

42 minutes ago, Lixma 06 said:

 

I'm not convinced the turbo should behave like it does. Lag is one thing, but I see no reason for the turbo to spin up uncontrollably when you're flying straight and level with no throttle changes.

It is a reason you climb up and external pressure drops and turbo kick in. At deck turbo won't kick in right now if it is accurate i dont know. But it is how it is. Not a single p47 use 52 for take off

Edited February 2, 2022 by grafspee

[twistking]

On 2/1/2022 at 5:44 PM, razo+r said:

Primer in and locked?

I think your suspicion was right. i tested again and deliberately did not lock the primer: this gave me the issues i was describing.
case closed. thanks to all your helpful advice.

only question i still have is, why the unlocked primer would give those weird engine trouble? fuel delivery seems to be fluctuating, resulting in reduction and spikes in engine power... how come?

[grafspee]

30 minutes ago, twistking said:

I think your suspicion was right. i tested again and deliberately did not lock the primer: this gave me the issues i was describing.
case closed. thanks to all your helpful advice.

only question i still have is, why the unlocked primer would give those weird engine trouble? fuel delivery seems to be fluctuating, resulting in reduction and spikes in engine power... how come?

Fuel is sucked via primer line this makes this fluctuation, but this should be gone at MP grater then ambient pressure

Edited February 2, 2022 by grafspee