What maneuvers will kill a P-51 engine?

[Boomer_G-Loc]

Done a search, haven't found....

Recently started doing a bunch of aerobatics in the P-51, and I've found that, whenever I do any ballistic maneuvers, like hammerheads, tail slides, or nose-overs, I find my engine breaks very soon after, like within a minute or so ---> the engine overheats, power dies, and then it stops.

I'm not pushing it during these maneuvers, I'm using lower manifold pressures, sometimes at idle, the prop speed is set around 28-29 hundred, and the maneuver is actually pretty gentle all the way through, yet every time, the immediate result is a broken engine. The engine is always running cool during the flight, right up to failure, then, overheat, power loss, and prop stops during a level recovery cruise. I'm not overspeeding the aircraft either, I rarely go over 350-400 indicated.

Are there other maneuvers I need to avoid to avoid breaking my engine?

thanks;

[razo+r]

Basically you have to avoid low speeds with high power settings. Auto rads are slow, opening them manually early enough might give you more room to play with. And maybe with the new cooling model, things will improve even further. 

[Boomer_G-Loc]

No high power settings, 20-30 inches at max when slowing down, but mostly below 15" to almost idle, temp needles are always solidly in the green, right up to the failure, and then that occures way after the maneuvers during basically a level cruise flight to set up speed. I'll use about 40" during a speed building dive, but then I'm already going fast because of the dive starting out.

The last two incidences I purposefully used power off dives to recover from the maneuvers and then just glid at best range glide speed to wait for the inevitable engine failure, and there it came....

I have used higher power during cuban eights & Immelmans before without problems in dogfighting, it's only during these ballistic dump overs that I'm having the problems.

I'm thinking oil? during the low G part, but that's just a second or two. But then I see other planes doing the same thing w/out incident.

Seems like a poor weakness for a fighterplane to have..... But then again, I have flown the plane inverted until fuel starvation quit the engine, but once righted, it came back w/out problems. So I'm at a loss here.

 

[grafspee]

@Boomer_G-Loc40" at what rpm ??

If you do tail slides you have to cut throttle to 0 before plane start sliding down, when P-51 slides down cooling is not existent and even idle will overheat engine if you keep any power above idle engine will cook in that situation, if you use 40" max you can set engine rpm to 2500-2600 you don't need higher rpm at all at this MP, this will help with cooling as well.

I was doing that kind aerobatics my self recently, i was using standard climb power rating 46/2700, in vertical climb when my speed was passing 150 indicated i was cutting throttle already still hitting 120C coolant at peak of this climb where speed was 0, any thing above then idle, temp would go beyond 120C.

When dcs p-51 engine seizure, over boost or over heat is involved or combat damage, fuel starvation do not involve engine seizure.

Edited May 25, 2022 by grafspee

[Jafferson]

For aerobatics use Radiators/Manually full open, in auto u can fry the in engine in a blink of an eye. Reduce Power below 200mph untill u gain speed. Radiator Control depends on ambient temperature, -10 or +30 can have a huge impact.

[M1Combat]

Yeah open the rads manually and reduce RPM when slow.  As soon as you get some speed back set them back to manual.  It's not only the MP that causes the heat.  RPM causes friction.

The Mustang just likes to stay fast.

 

[grafspee]

On 6/7/2022 at 6:57 PM, M1Combat said:

Yeah open the rads manually and reduce RPM when slow.  As soon as you get some speed back set them back to manual.  It's not only the MP that causes the heat.  RPM causes friction.

The Mustang just likes to stay fast.

 

Friction has very small contribution in heat generation, RPM and MP are engine power main power indicators. MP = how much fuel is combusted in single engine cycle, RPM = how much time engine need to complete 1 cycle. Both when increased, make engine burn more fuel and this mean more heat as well.

Any way it is good idea to drop rpm in aerobatics flying, 2400rpm and MP up to 40inch should generate a lot less heat and power should be enough for fun stuff. 

Edited June 10, 2022 by grafspee

[Ramstein]

flying upside down can often starve some engines of oil and blow them... not all plane and figthers were meant to fly upside down for extended periods... i.e. more than a few minutes

Edited June 10, 2022 by Ramstein

[M1Combat]

2 hours ago, grafspee said:

Friction has very small contribution in heat generation, RPM and MP are engine power main power indicators. MP = how much fuel is combusted in single engine cycle, RPM = how much time engine need to complete 1 cycle. Both when increased, make engine burn more fuel and this mean more heat as well.

Any way it is good idea to drop rpm in aerobatics flying, 2400rpm and MP up to 40inch should generate a lot less heat and power should be enough for fun stuff. 

 

I didn't mean like air friction or something :)...

What I meant was piston to cylinder wall friction...  so RPM :).  And that creates bucket loads of heat.

[grafspee]

32 minutes ago, M1Combat said:

I didn't mean like air friction or something :)...

What I meant was piston to cylinder wall friction...  so RPM :).  And that creates bucket loads of heat.

I know what you mean, but lower rpm provides less heat mainly because engine burn less fuel, piston actually make very gentle contact with cylinder liners, you are thinking about piston rings, but those are lubricated by engine oil. One of the main job for piston rings very important is heat transfer from piston to cylinder liners. This is why you don't want stress brand new engines, piston rings takes time to make full contact with cylinder liners. I woudl say that heat from piston rings friction is insignificant to heat generated by fuel combustion, ofc it adds up to pile.

Edited June 10, 2022 by grafspee

[Doughguy]

So you guys lower rpm settings eg in combat (as in aerobatics)?

still learning a bit so excuse the dumb question. gotta get used to the seperate handles in the pony.

[Ala13_ManOWar]

In combat usually not rpm, only manifold in vertical manoeuvres or maybe very low speed ones so you don't kill the engine. In aerobatics that's different, you set your rpm (not necessarily 3K, 27 is fine, or whatever you like) and play with your energy to fulfil your desired manoeuvres.

[Theodore42]

I would think you should keep RPMs at max and vary the manifold pressure. It is far more likely that the engine is overheating because the RPMs are set TOO LOW.

If you fly at 2700 RPMs then you're going to need to never let the Manifold Pressure exceed 46 inches (that's the maximum continuous setting).

If you really want to vary the RPMs then you're going to have to study all those flight operation instruction charts in the back of the manual. They'll give you safe RPM/MP ratios.

If you go against the manual and set RPMs too low for your Manifold Pressure then the Mustang will auto adjust the prop speed by turning the prop blades flat against the air until they make enough friction to slow the engine down to the set RPMs. Doing this to the engine produces a lot of excess mechanical stress and friction (heat). This is called something but I'm not an aeronautical engineer so I forget.

If you have 3000 RPMs but low MP, then the prop is spinning fast but the blades aren't turned into the air creating as much friction (or power). So really very little stress being placed on the engine and it's designed to operate at 3000 RPMs anyway, so it isn't a big deal. But when you have max MP + RPMs LESS than what the engine is designed for, that's gonna break stuff.

Probably the nature of the problem is going too slow for too long. Maybe you could design the airshow to have high speed maneuvers between the slow moving ones.

[grafspee]

On 6/11/2022 at 9:04 AM, Doughguy said:

So you guys lower rpm settings eg in combat (as in aerobatics)?

still learning a bit so excuse the dumb question. gotta get used to the seperate handles in the pony.

Combat is different, you want max power so you don't go below 3000rpm. Combat flying do not involve tail slides or very slow speed aerobatics it will make you easy target. If speed is kept high you won't have troubles with engine even when operated at military power for long periods.

[grafspee]

7 hours ago, Theodore42 said:

I would think you should keep RPMs at max and vary the manifold pressure. It is far more likely that the engine is overheating because the RPMs are set TOO LOW.

If you fly at 2700 RPMs then you're going to need to never let the Manifold Pressure exceed 46 inches (that's the maximum continuous setting).

First thing at lower rpm engine produce less heat not more. At lower rpm you can overboost engine so for 2700 keep eye on MP gauge is required, at 2700rpm you can get up to 50" but my sweet spot for aerobatics is 2600 and 45" MP.

Edited June 13, 2022 by grafspee

[Theodore42]

19 hours ago, grafspee said:

First thing at lower rpm engine produce less heat not more. At lower rpm you can overboost engine so for 2700 keep eye on MP gauge is required, at 2700rpm you can get up to 50" but my sweet spot for aerobatics is 2600 and 45" MP.

Hm well the Mustang was designed with 3000 RPM being the maximum and this had been an established limit for warbirds for many years. A prop rotating much more than 3000 RPMs starts to disrupt the air too much and there come to be diminishing returns. If only there was a way to compress the air before it gets to the prop, then the prop could spin as fast as metallurgy will allow...

The manual only gives guidance on RPMs and MP when they are LESS THAN 2700 and 46in. Above that and it says 3000 RPMs and 61 inches; and 3000 RPMs and WEP (68 or 72 inches or whatever). There isn't a reason to go above the maximum continuous setting except for combat and I'm sure the warbird was designed with these settings in mind. In other words, I don't think you should use less than 3000 RPMs when greater than 46 inches MP. If you do, keep in mind what the engine is doing is blasting 61 inches of pressure inundated with atomized fuel into the carburetor and it is the prop being pitched against the air that is causing the RPMs to slow down. That just sounds like a thing that would make an engine run hotter than if you just let it go at 3000 RPMs like it was designed. Of course any one of us could jump into the sim and check to see what speeds and heats you get at weird RPM/MP ratios.

I found a P-51 airshow done by Bob Hoover and then I promptly imitated it. All his maneuvers looked to be done at 300+ mph. He is usually diving down to the field right before the maneuver so I suspect he is doing repositioning maneuvers at lower engine settings to cool the engine off in-between maneuvers OR he is just flying at maximum continuous and gaining speed by diving to the showline. There is a cameraman in the jump seat behind him with sound. I'm not sure how accurate the sound is or if it is properly synced but it sounds like he's adjusting the throttle throughout some of the maneuvers.

I did the airshow at MAX RPM and FT the whole time. Hoover didn't do a hammerhead, but I threw one in and that's when I blew the engine. I found that all I had to do was drop the MP to 30 inches when the speed gets below 170 mph and I keep it there until the temperatures get out of the red. (Throttle down half way up and throttle back up half way down is what that looks like.)

I'm for sure going to practice this airshow but here is my attempt without blowing the engine: (Oh, don't laugh at my 16 point hesitation roll! I should have tried the 8 point one first, LOL!)

Bob Hoover's airshow:

 

[grafspee]

@Theodore42 running engine at 3000rpm all the time is very stressful. Manual gives typical power settings 2700/46 is unlimited continuous but this is not absolute limit for 2700 rpm. Not a single P51 is performing aerobatics at 3000 rpm today. I do aerobatics at lower power because it is enough. The way you are thinking is weird, you are saying that higher rpm require less power ? Engine at 2600rpm and 45" is developing about 1000hp and it makes a lot less heat in this process. At 3000 it makes a bit more power at 45" but there is no reason to stay at 3000 for this boost. I wish we could pitch up just after take off like hoover in dcs at max power, but coolant temp would shoot sky high if done so

I watched your flying and i noticed that you used mil power for take off and if you watch coolant temp you will see that temp was above redline.

Prop blade act like wing, difference is that lift is called thrust. You get lift/thrust from blade velocity and aoa. Both generates drag. At 3000 rpm indeed the pitch/aoa of blade is lower but speed is higher this increase drag.

Edited June 14, 2022 by grafspee

[Theodore42]

11 hours ago, grafspee said:

I watched your flying and i noticed that you used mil power for take off and if you watch coolant temp you will see that temp was above redline.

I used military power the whole time except when going around the top of the hammerhead. I wouldn't do that if I were doing a real airshow because that is insane, and it also feels like to get value out of military power you have to be unmaneuvering above 300mph (as weird as that sounds). When I post a more polished attempt at an airshow I'll have figured out a way to manage the engine in a reasonable way for people to imitate and not blow their engine. But of course I would never fly a virtual airshow and not bust out WEP. Even if all I get out of it is to say "WAR EMERGENCY POWER!"

But the point of my post is to reply to the question of "What maneuvers will kill a P-51 engine?"

And the answer I demonstrated is that you can do all the standard P-51 maneuvers at military power and not blow your engine, AND you can do the hammerhead (that I've never seen an actual P-51 do) as long as you decrease your MPs for a few seconds as you're going over the top.

My argument on 3000 RPM is kind of on the philosophy of the engineering of a warbird, and specifically the Mustang.

From this chart:

Maximum cruise is for operating with the most amount of time between having to rebuild the engine. All the green lines in the Mustang are for maximum cruise. It is normal to be above the green even in maximum continuous because being above the green represents the decrease in the amount of time between engine rebuilds, not that you're going to blow your engine. In war the warbird must get to the fight, and the Mustang is known for having extreme range for a fighter. This engineering trait is represented by this chart:

(Those different columns you see are for operating the engine to fly greater ranges). The engine operates from 2700 RPMs and 46 inches at max continuous all the way down to 1500 RPMs at 30 inches. Engineers designed the engine with these needs in mind and the engine settings in the manual here represent that design.

Engineers ALSO designed the Mustang to fight a WAR, so there is the setting of 3000 RPM and 61 inches. There is also the EXTRA CRAZY setting of War EMERGENCY Power, which I imagine the engineers had to spend a lot of time considering how to make the engine work with those settings.

So warriors need their aircraft to get them to the fight (1500-2700 RPMs) and then they have to FIGHT the fight (3000 RPMs). Why would engineers spend any time thinking about RPMs between 2700 and 3000 and MP between 46 inches and full throttle?

My argument is that the Mustang is engineered for fighting at 3000 RPMs and Full Throttle AND that the engineers had experience making engines of war that operated at 3000 RPMs. I don't think the main limitation was from blowing an engine but instead from the fact that if everyone in your squadron is using military power and WEP all the time then you are going to be spending a lot of time rebuilding engines rather than fighting the war. This is also probably why modern pilots fly their Mustangs at or below maximum continuous; they don't have a crew of mechanics rebuilding their engine between airshows.

The main limitation of the engine in my mind is speed, not RPMs. As long as you're going fast you're "fine." (although obviously there are many caveats)

[grafspee]

Engine is designed to operate at any available rpm set by governor. Military power is designed power for combat.

I was answering same question, and imho it is less likely to kill engine or stress it too much when doing aerobatics at reduced power then at military power.

[Obic]

What maneuvers will kill a P-51 engine?

Inverted  flight = Oil starvation Engine goes kaput. 

Side slipping = Impeding Airflow trough the radiator/Intercooler = Your Merlin that's running at 29 pound of boost, heat will start to build.
You have to remember once a component ''Cook'' it takes a lot of airflow to dissipate the heat.

Candle climb = Airflow is inexistent trough the radiator/Intercooler = Your merlin that's running 29 pound of boost with cook it's self within seconds.
Hanging the plane off the propeller will Multiply the speed at witch all the above failure will go.

The P-51 cooling system exist to operate at speed in excess of 250mph @ 20 000+ Ft. probably at -20c exterior temp
That's where it shines, 150mph high Angle of attack turn fight on the deck at +20c with the engine gated for 5 minutes not so much