The A2A Simulations Community

With the talk of fuel economy I'm surprised no-one's mentioned winds-aloft, if we're stretching it - it's another factor to consider. Is it worth that climb to benefit from that faster moving body of air, or not.

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~ Make love to the sky, don't shag it.

Yes, winds aloft are important but also more of a navigation problem, I've stayed out of that because it does not have a bearing on engine performance at altitude.


Thrust is proportional to power (ignoring prop efficiency), so as long as you can maintain power you can maintain thrust. And yes, the supercharger makes all the difference. With a naturally aspirated plane your critical altitude (altitude where best power is made) is sea level or below since maximum manifold pressure is ambient pressure, adding a supercharger brings that critical altitude up to many thousands of feet. Turbocharged planes, as that article mentions, can keep their throttles open virtually all the time since boost under climb and cruise is maintained by adjusting wastegates on the exhaust (see the B-17 and B377) so throttle losses become irrelevant and both power (thrust) and fuel burn rate are pretty much flat until you redline the turbos at critical altitude and have to sacrifice manifold pressure beyond that to prevent overspeeding them. In that case you might as well fly at the critical altitude for cruise power, all other considerations aside, to get the highest TAS for your burn rate.

With superchargers your throttle is progressively opening until you hit critical altitude at any given manifold pressure (increasing efficiency), then closing after the gear change on a multi-speed supercharger (decreasing efficiency), then steadily opening again (increasing efficiency again) and this means your throttling losses are always changing with altitude and with it your power output (but not your fuel burn rate). And, as I mentioned earlier, your power losses to drive a mechanical supercharger are higher in high gear because the blower is required to do more work to maintain boost beyond the initial critical altitude (you have one critical altitude per gear of the supercharger). All together this means that with a specific manifold pressure and burn rate a turbo engine can operate at peak output across a broad range of altitudes whereas the supercharged engine hits peak output only at it's critical altitude(s). That means that your Spitfire's specific fuel consumption will be lowest at your critical altitude, or your lowest critical altitude had you a multi-speed blower. A higher true airspeed at any additional critical altitudes as well as increased thrust from the ejector pipes, since exhaust pressure is proportional to manifold pressure and thrust is proportional to the difference between exhaust and ambient pressure, may make up for the increased SFC but I don't have the data to make that call.

-Dave

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I agree with the last two paragraphs, but I think that the first one is a bit of simplification.

I'm not sure if ignoring efficiency is desired in case of propeller planes - it illustrates that just power doesn't always mean enough thrust. Imagine a plane with propeller with blade angle that is set to about 90 degrees. The engine is roaring, producing huge amounts of power, yet the thrust is barely anything. It's the jet engine that produces thrust, in the propeller plane engine produces power, which is then converted to thrust by a propeller. Even with propeller with efficiency=1, the thrust is proportional to power and inversely proportional to TAS - which means that the faster the plane is flying, the lower thrust is generated assuming constant power. The point I was trying to make was that climbing higher in a propeller, normally aspirated plane that is flown with IAS to achieve maximum range results in increasing TAS, which results in lower thrust. Higher fuel flow is required to maintain thrust, but thanks to increased TAS the ratio of fuel used per distance unit remains about the same irregardless of altitude. This is the most basic case that ignores some factors, but I believe it is a good beginning on a trip to the world of turbo- and superchargers

Propellers, I once heard them described in this way; "It can make the difference between your boat belting across the oggin, or being an expensive egg-whisk."

Translates to aircraft, of course - air being a fluid.

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~ Make love to the sky, don't shag it.

I'm enjoying the technical discusion although much of it is beyond me, I'm afraid.

The one thing I can tell you is that 20,000ft is the optimum cruising altitude for the early Spitfires. The "Pilot's Manual" notes (or similar, can't remember the exact wording) that the disance travelled at 200mph IAS at 20,000ft will yield the same RANGE as 200mph IAS at 2,000ft, but the TIME is , of course, markedly shorter at 20,000ft. To my knowledge this includes a high power climb to 20,000ft and the PRU certainly climbed at high power...and they, more than any other Spit operator, were concerned with fuel efficiency and range.

regards

Darryl

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...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Aha! High power climbs proven through practice, thanks for that one Darryl!

-Dave

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Your logic would be valid if the RICH RICH condition was ON/OFF. If, however, the mixture is a continuous curve as on the page 33 of Accusim manual, there's no clear line when you're RICH RICH and when you're RICH, and when your stoichiometric, and when your LEAN. From those curves it seems that the best economy can be obtained with 60% of power with AUTO-LEAN setting. How does that give us "2600rpm with 6 psi boost with AUTO-RICH"? Because to me, it gives the high end of cruise power setting (with AUTO-LEAN), not the climb power setting (with AUTO-RICH).

Thus, it would be most economic to climb with high power cruise setting... as long as the engine doesn't overheat from being run with lean mixture. The engineers have calculated the climb power figures... but the big question is: did they calculate the climb power settings to give you best economy... or to give you best sustainable (non-overheating) performance? Because usually during the wartime, they didn't give a flying duck about fuel economy. It didn't really matter. Especially not during Battle of Britain when the battle was over Britain and the Messerschmitts had less fuel in their tanks than the defenders did.

My hunch, which is also based on Accusim manual page 33, is different from yours. To me 2500rpm 5psi AUTO-LEAN would seem the closest to optimal (being rought 60% of 3000rpm 12psi)... but impossible due to engine durability concerns. Reducing power gives lower efficiency but considering the added consumption of switching from AUTO-LEAN to AUTO-RICH, I think a lower throttle (like 2400rpm 2psi) while maintaining AUTO-LEAN might win in specific fuel consumption. Not necessarily by a huge margin, though.


Actually, it's producing zero usable power. Zero thrust, zero power. Power is torque times angular speed. If the engine is running at a constant speed and not increasing it's rpm, and not producing thrust, it has zero net torque. With zero torque, no matter the rpm, it produces zero net horsepower. (It does produce a heck of a lot waste heat, though.)

Actually, we'd also have to assume zero air resistance for the prop in addition to assume the flat pitch.

I wouldn't expect that illustration to be representative of the actual mixture curve for the Merlin. Yes, it follows a curve something like that but that same picture is in every Accusim manual so it could be for any engine, we don't know. Furthermore, intercooling and carb air temp are big factors in determining when full load enrichment is needed and, as we all know, the Merlin has a very well developed intercooling system compared to the supercharged, non-turbo radials as well as no real need to maintain carb temps higher than ambient. And the rich rich condition is, in a sense, on/off because as soon as it passes the "best power" mixture it is rich rich. Not by very much at first but the important thing is that beyond "best power" the carb adds more fuel than necessary, thus fuel is wasted. Oh, and the percent power mentioned in the diagram is in reference solely to manifold pressure since RPM does not determine mixture requirements. So really it's more like percent load...

And regarding the engineer's figures, I'll agree with you, who can say whether fuel economy was or was not factored into max climb power? Either way, my opinion is that getting to altitude under max climb conditions will reward you in terms of total fuel burn in the long run, fewer minutes to cruise altitude, more minutes under cruise conditions, etc; it seems experimentation may be necessary there... Regardless, it's inaccurate to say that the engineers did not care about fuel economy at all. It's a primary factor in figuring range, something that was very important post-BoB and a crucial factor in the design of the P-51. Also, many of the factors that influence high power outputs (efficient intercooling, low CAT, etc) also improve fuel economy by virtue of improved thermal efficiency so one can run less RPM for a given power output or less boost or both compared to simpler engines.


I think the theory was that if the blades were fully feathered and the engine was at max RPM and manifold pressure that you would get zero thrust, which is correct. The engine is actually producing maximum horsepower under those conditions, it's just being absorbed by the turbulence in stalling the prop blades (meaning we are assuming normal wind resistance). I don't think net power really relates to thrust at all, really it just means power at the prop shaft after all losses are taken into account, since the propeller is a separate system its efficiency or lack thereof is irrelevant to net power output of the engine. So in simpler terms, net power is how much juice the engine can send to the prop, how much thrust you get is the prop's problem, not the engine's.

-Dave

PS I hope I'm not getting on your nerves with this back and forth banter, lemme know if I do!

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In terminology, yes. But like you later said (contradicting the previous) "Not by very much at first".


That means that if the graph was applicable to Merlin (it could be for some other engine as well) the best economy would be 60% of MP (or 60% of power if assuming rpm as constant) instead of 60% of power with rpm freely selectable. This also means that my calculation of best economy being around 2500rpm +5psi AUTO-LEAN can be fixed to just 60% of full MP. Full MP being +12psi and minimum MP (vacuum) being around -13psi. That's 25psi non-relative MP at full boost. 60% of it is 15 psi. Switching back to relative to normal pressure at sea level, we get +2 psi. So if I set it to around 2300...2400rpm +2psi AUTO-LEAN, it should be the most economic power setting for specific fuel consumption... which is quite a bit more bearable than +5psi with AUTO-LEAN would have been. 2400rpm +2psi is actually pretty good, as long as the temperatures stay low. Some extra attention toward monitoring temperature is required for climbing with lean mixture but it's doable.

EDIT: Correction: one atmosphere is 14.7 psi (not 13 psi). That means that absolute manifold pressure at +12psi relative to standard air pressure is 26.7 psi (not 25 psi). 60% of it is 16 psi. Substracting 14.7 psi from it to make it again relative to standard atmosphere, we get +1.3 psi AUTO-LEAN to be the most economical power and mixture setting. To optimize STF, that is. Not necessarily the most economic way to climb (and definitely not the most economic way to cruise as you end up flying uneconomically fast).


Even with PRU (Photographic Reconnaissance Unit) being the most concerned about fuel consumption because during BoB, they were the only ones taking it to the enemy territory, and were very much concerned about range and speed because their only method of survival was surprise and outrunning the enemy.

Still, I could imagine what PRU was also interested in getting the planes as fast as possible to high altitudes because in air combat, speed is energy and altitude can be converted to speed. A slow target is a dead target. A climbing (or landing) plane is as good as a sitting duck. Even if it was an unarmed PRU Spitfire.

Also I don't know the details of how PRU operated but Spitfires were used for hit-and-run type low-altitude reconnaissance as well. I'd assume it could be useful in some situations to start a climb again after taking the low altitude photos. If this occurs on enemy territory, the need to get up fast becomes quite a bit more important than climbing fast over Britain's soil.


I think it's definitely necessary. Making assumptions only from specific fuel consumption won't help us get the whole picture.

To me, the biggest assumption would be to assume that a halved power output would result in halved climb rate. In reality, it could be more... or less:
- if you reduce power, significance of air resistance starts to play a bigger role than energy needed for climbing... but on the other hand
- if you add power, you need more airspeed to keep the engine cool, and have to use a shallower, non-optimal climbing angle, and
- you have to account to how much terrain has been traveled horizontally while you climbed to your cruise altitude.

I think that absolute certainty would require tests done in standardized conditions between two points using a predetermined climb profile, a set cruise altitude, and identical cruise settings (as we're only interested on the difference in consumption of the climb) and measure fuel consumed. The second point could be in the air, over some landmark, so that we get rid of varying fuel consumption of descent, approach and landing, and don't have to specify them. In reality it might be difficult to conduct such a test with accuracy (because the fuel gauges are rather approximate) but in FSX, it's possible and you get rid of inaccuracy factors.

I don't expect climbing with fast cruise power in AUTO-LEAN to save much fuel. I merely do it because I don't feel pushing the engine to +6 psi for the entire climb. I use climb power only to burst through cloud layers or climb above mountainy terrain (and of course I do switch to AUTO-RICH for that short period of power-on-demand)... it's also gives a very different sense of power when after a lengthy cruise you just let her rip to avoid some specific cloud you'd rather not fly into, compared to climbing with steady +6 psi from take-off to target altitude because the brain has a habit of filtering out constant noises.

Ok, there I went ranting again...

Again, assuming 60% load is the point of best power mixture for this engine. That said, I just found some real interesting test data on the Merlin XX, close enough for our purposes, and it indicates an air fuel ratio at or close to 13.7:1 (7% fuel air ratio) from manifold pressures as high as 50" Hg (20 psi boost) down to 20" (-5 psi boost) in rich mixture and low gear. That's actually leaner than the 12.5:1 AFR (8% FAR) best power setting in our diagram. That shows that the Merlin never actually enters a rich rich condition, ever, during their testing. No lean mixture data is available.

http://www.enginehistory.org/members/articles/ACEnginePerfAnalysisR-R.shtml
(table is at bottom of page)

-Dave

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"It's not the maximum allowed climb power but it's not stupid to throttle back and level off for a while for the engine to cool down from the take-off run." Tally ho..this works for me, as im not chasing 109's anymore. I just cruise along with a cool engine and no steam streams.

Well that's fair, I just get the impression that a lot of pilots here use power sparingly for fear of overheating but with a little practice it's no sweat to push her hard (haha). I usually cruise high and since the air gets pretty cold above 10,000 ft I don't have to worry about overheating if I can punch through that barrier quickly.

-Dave

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The original RAF manuals were extremely conservative about heat. The venting system was designed specifically to AVOID damage to the engine, and thus is only a preventative safety measure and not necessarily indicative that actual damage is occurring. The manuals often like to talk about 100C being the limit, but you can safely and easily go above that. It's when I'm around 120 that I start to get a bit worried, as that happens to be the temperature that the vent pops open. You can fly her quite a bit above that temperature before damage occurs or becomes accelerated, and the proof is that the RAF to my knowledge changed the manuals based upon how pilots were actually flying the aircraft. The early Spits still need to be babied, but I think too many people get stuck on the heating issue. Plenty of Spitfires then and even know have come in with steam pouring from the vent. It happens.

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How dangerous is high coolant temperature, or what temperature is dangerous to the engine... I don't think there's a simplified answer to that question.

You can for example let the engine overheat after landing and at 120 deg C the vent would start hissing steam to reduce pressure (and to cool the engine by evaporation cooling - sacrificing coolant for added cooling in a situation when no airflow is through the radiator). I don't really see a big problem here. The evaporation probably occurs at or near the vent, where the pressure is reduced, there's where pressurized liquid becomes steam.

However, if you get above 120 deg C during war emergency power or otherwise high power setting, I could see the same temperature being more harmful:
The vent would prevent pressure build-up in the system because if it was to increase without limit, it would burst catastrophically at one point. Howeven, when the vent opens, the pressure stops increasing within the system while the coolant temperature may still be rising. This could lead to a situation where the pressure inside the cooling system is very high but still insufficient to prevent the coolant from boiling near the cylinders (instead of near the vent). Bubbles forming, separating and collapsing at explosive speeds might cause damage to surfaces of coolant channels or head gasket (damage might be similar to cavitation damage on water turbines and props). Also, with bubbles forming on the hottest surfaces inside the engine, they act as insulation and causes metal that's supposed to be cooled to heat more rapidly... while some other parts of the engine are still adequately cooled because insulative bubbles only form on hottest parts. This means increase of temperature differences within the engine.

There's probably even more stuff that I left out why overheating with low power is much less harmful than overheating with full power. If there was no vent, then even slow, gradual overheating could become hazardous (and give no prior warning of failing) but as it so graciously starts to vent steam visibly and audibly. Most likely no harm done, if engine is not pushed hard in this condition. The evaporative cooling of vent opening might also slow down further warming up (if modeled).

Very in depth, yes you are right about boiling on the cylinder liners (also could happen in the heads). The pressure, however is kept pretty constant in the system with the vent closed (and probably even when open) since that is how it's designed. The way a pressurized cooling system works (on gas engines, diesels, nuclear reactors, etc) is that some water is allowed to boil in the header tank and the resulting steam pressure prevents the rest of it from boiling while the overall pressure is regulated by a valve like the emergency valve, thus you can run it up to 120º C instead of only 100º C. More importantly for aircraft, boiling point is proportional to ambient pressure, early liquid cooled aircraft started boiling well below 100º C since they were vented to ambient pressure all the time.

As for steam pockets around the cylinders, yes it's a big danger factor when overheating, mostly because of the vapor's insulating properties compared to water and the fact that the pockets are insulating the hottest parts of the cylinder or head. I've never heard of cavitation-type damage under these conditions and I actually doubt that it would occur since the bubbles are not under any pressure to immediately collapse as they are during cavitation, they can simply float to the top of the coolant jackets just like when you boil water on the stove.

A recurring thought of mine regarding overheating, though, is the possibility for detonation. As I've stated in another thread somewhere, high coolant temps (100º C or more) are actually good for thermal efficiency and thus fuel economy during cruise since it limits heat transfer out of the cylinders so more heat can be turned into power at the crank, however at high power you want low coolant temps (75º-85º C) to keep peak cylinder temps during the compression stroke down below the pre-ignition/detonation point (those temps are from automotive race engine tuning books, by the way). Now reading these manuals it seems the Merlin and even the P-40's Allison have pretty high limits for that sort of phenomenon since they allow war emergency power up to 120º C (125º C in the Allison) so I'm curious exactly what ambient temp, coolant temp and boost combination will cause knock, if any.

-Dave

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