Question regarding engine performance and engine failures

[Hammer1-1]

Ive noticed over the years that the mustang doesnt really like to fly in the green - manifold pressure around 30 psi, engine rpm around 2700 or lower, basically the gauges all in the green. Every time I try to fly in the green, my engine will seize up while flying in straight level flight. Yet I have best reliability when flying at 3000 rpm, 61psi, basically just controlling the engine through the throttle exclusively. Why is this? I cant troubleshoot the engine because everything is in the green - temps, oil pressure, fuel pressure, manifold pressure, temperature...

[grafspee]

Can you provide the track?

[Hammer1-1]

Ill see what I can do, but Im just talking in general about this. Its not just me that have this question. It just seems to me the engine is more reliable at maximum performance than it is in its optimal performance.

[=475FG= Dawger]

Poor technique is the most likely culprit.

 

There are strict procedures drilled into every pilot brain when flying reciprocating engines aircraft. Prop forward then throttle forward when increasing power, throttle back then prop back when reducing power.

 

Violate this rule once when flying the DCS P-51 and the engine will be damaged and eventually fail for apparently inexplicable reasons.

 

Maximum manifold pressure allowed goes down with RPM. What are the numbers? Whatever the programmers put in.

 

The chart for the P-51 gives numbers necessary to achieve range or endurance. These are not necessarily engine limits but they may have been interpreted as limits.

 

In any case, 46 inches is the maximum allowable MP at 2700 RPM (Maximum Continuous). Exceed 46 at 2700 RPM and you damage the engine. Reduce RPM below 2700 when at 46 and you damage the engine.

 

The lower the RPM, the lower the allowable MP to prevent damage and it only takes one excursion in the DCS P-51D to cause damage and eventual failure.

[grafspee]

Ill see what I can do, but Im just talking in general about this. Its not just me that have this question. It just seems to me the engine is more reliable at maximum performance than it is in its optimal performance.

 

Because reasons of engine fail are plenty.

Why engine seems to be more reliable at or near max power settings then at cruise power settings?

There are some principals about engine management.

I will say that flying at 2400 rpm is pretty much safe from my point of view, because risk of overboosting is very low even when you descend or accelerate and your boost goes over 36" you are still safe, because auto boost control unit kick in at about 42" which is still safe for engine, in in case when you cruising at very low rpm like 2000, 42" boost will be very dangerous for engine. There is another thing, when high supercharger gear is engaged minimal rpm rise from 1800 to at least 2000 rpm.

I remember guy complaining about engine fail during climb in spitfire, he included track of it, he was climbing at 2500rpm and high blower kicked in increasing boost instantly to +18lbs after couple of seconds engine died.

Yes flying at 3000rpm will be safer it will impact engine life time, but you will avoid catastrophic engine fail because you cant overboost at this rpm :)

I read couple of P-51s spitfires manuals and pilot's notes, my conclusion is that what you call optimal performance which refers to cruise power settings aren't so healthy for engine as most ppl think.

in pretty much all manuals they say that pilot must use at least 80% power setting after long low power cruise in order to clean engine before landing, high level of sludge inside combustion chamber is contributor to detonations as well.

[=475FG= Dawger]

In DCS, there really isn't any reason to get below 2850 RPM, ever. The maps are too small.

[Hammer1-1]

Well the ONLY time I go under 3000 rpm is when I am on approach to land. The checklist says 2700RPM on final, flaps down gear down. Ill tinker around to make a track file soon, but Ive never been able to reliably fly around in the green so to speak.

[grafspee]

In DCS, there really isn't any reason to get below 2850 RPM, ever. The maps are too small.

 

Exactly and engine life time doesn't matter either.

[grafspee]

Well the ONLY time I go under 3000 rpm is when I am on approach to land. The checklist says 2700RPM on final, flaps down gear down. Ill tinker around to make a track file soon, but Ive never been able to reliably fly around in the green so to speak.

 

How do you change power setting, when attempting to change to cruise power ?

I will test it, i will try 30min flight at 30" 2400rpm from cold start.

My temps after 50nm cruise, i will say temps are not in the green. ambient temp is 13C on this server

i made 120nm cruise first 60nm at 12k ft second 60nm at 20k alt, engine was fine all the time.

[Hammer1-1]

I drop my throttle back to the green band, then I pull the engine RPM back to 2700. Theres no real reason (for me anyways) to fly maximum throttle and back off using the engine RPM lever - thats a waste of gas.

[grafspee]

I drop my throttle back to the green band, then I pull the engine RPM back to 2700. Theres no real reason (for me anyways) to fly maximum throttle and back off using the engine RPM lever - thats a waste of gas.

 

Quite opposite, running engine at low rpm and high boost will increase fuel efficiency, but you cant do that.

[Hammer1-1]

How? You lose speed when you drop your engine RPM but you are consuming more fuel since the throttle is wide open. Doesnt the prop pitch angle govern engine RPM by increasing the torque at the hub?

[grafspee]

How? You lose speed when you drop your engine RPM but you are consuming more fuel since the throttle is wide open. Doesnt the prop pitch angle govern engine RPM by increasing the torque at the hub?

 

Both MP and RPM determine fuel flow, at higher rpm engine makes more cycles in period of time then at lower rpm, higher MP will make engine burn more but at the same time lower rpm will decrease burn rate. Prop governor governs engine rpm thats it.

Whole idea of variable engine rpm is about economy.

Engine will burn much less fuel at 30" and 2400rpm then at 30" at 3000rpm with minimal cost of speed. That is why you cruise at low rpm and highest boost as possible to achieve top economy.

But flying in DCS at cruise power settings is just cosmetic thing, maps are too small for anything below continuous power.