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Thank you again to you, Sergeant and Darryl!

Do not be sorry Darryl, my problems are not from you, but rather from my weakness with english.
I must admit I need help from an online translator and dictionnary to understand you and also write my answers ...
I read each of your answers dozens of times before my pc, or in the subway ...
I realize that theoretical knowledge base is lacking.
So thanks for taking the time to explain to me like you do.

Darryl, first of all I think I understand the concept of optimum climb, similar to the car.
I can actually uses more fuel with high power, but as we'll rise faster at cruise altitude, there's gain.
I think I understand also that all parameters can not be locked because with the altitude, the cards changes.
Most often the boost (or if necessary the speed) should be adjusted to keep constant the other parameters, rpm and rate of climb.
Finally, a high cruising speed is better.
An aircraft operates efficiently at high altitude at high speed.

This is consistent with your words, Seargent.
Cruise in altitude allows proper cooling.
Low rpm and high boost can generate more heat, and slower coolant flow, resulting in improved temperature maintenance.
That said, I often harder to adjust my needle cooling at "nine o'clock position", because it is rather closer to ten hours than of seven...

Actually, I had forgot that I can reach the required boost without opening full the throttle, as at take off.
But we are still not far from the maximum opening with a MK1, with +6.5 psi at low altitude, right?
Because without the extra boost (small red push lever) that I never use, the Merlin of Mark1 hardly reaches more than +7psi is that correct?

That said, for me it there's a major misunderstanding in your example, Sergeant.
I read you, I reread you, and I finally find that I am not competent.
You say +6 psi of boost throttle not completely open like as take off, delivers less power than +6 psi of boost throttle fully open?
In your last answer, you try to explain myself, but I can't understand, that it causes a deficiency of power as we can not fully open the throttle for the boost required!
But yet the power is proportional to the intake pressure at constant rpm, no?

This difference come from just a poor filling of the cylinder when the throttle is not fully open?
Or it there's something else I'm missing?
Why with the same boost, it does better with the throttle fully open rather than an incomplete opening?
Why do we consume less at equal boost (and rpm), with the throttle fully open?
It is incomprehensible to me!

Still, once admitted this conclusion that full throttle opening is better for the boost cruising, I understand why the Spit feels ok at 20,000 feet. It is at this altitude we obtain boost for cruising about +3 or +4 psi, with throttle fully open.

Thank you for your patience ...
Cordially

May I chime in and politely ask you to expand this subject or point me to sources explaining relation between load, efficiency and desired coolant temperature?

Call me Dave

Anyhow, max power in the Mk XII Merlin is at the aforementioned +12 PSI, nearly double the 6.5 you've mentioned, and that is where the cutout engages. If you disengage the cutout (hit the red tab) you can overboost beyond that and damage the engine, so yes, you can get a lot more boost than it seems. The issue here is that your gauge cannot reliably tell you how much boost you are running when you get past the edge of the dial, which is especially easy to do in the Mk I Spitfire. Furthermore, boost delivery is non linear, the closer you get to the end of the throttle gate the more boost you get for every inch you move it. This is why when you're getting up high it only takes small tweaks on the lever to give you another couple PSI. The reason behind this is that the throttle is in front of the supercharger. As such, opening the throttle increases the blower's effective pressure ratio (the number it multiplies ambient pressure by), so by linearly increasing pressure at the blower inlet by smoothly increasing throttle we are actually non-linearly increasing manifold pressure.

Which ties into the next issue. Putting the throttle plate ahead of the supercharger allows it to throttle both the engine and the supercharger and, in doing so, reduce the power draw of the blower for non-full throttle conditions (i.e. most of the time). Now, power at the crank CAN be seen solely as a factor of RPM and manifold pressure, however that is the power generated before parasitic losses are taken into account, like the aforementioned supercharger. So, at 6 PSI with the throttle open we are generating X amount of power at the crank minus the full power draw of the supercharger at that RPM, as well as the draw from the generator, magnetos, air compressor, gearing friction losses, etc. At 6 PSI with the throttle more closed the power at the crank (X) is the same as with a wide open throttle but, because a closed throttle creates an obstruction that the engine has to work against to overcome, the power at the prop (thrust) is reduced.

So, in either case you are burning the same amount of fuel to produce the same power initially (at the crank), however closing the throttle creates a parasitic loss between the crank and the prop. This loss is invisible to the pilot, however, since it does not show on any engine gauge he has (not sure if it can be seen with torque meters). As I mentioned above, though, closing the throttle also reduces the draw of the supercharger, so while we create a new parasitic loss by closing the throttle we are also reducing another one. As such, I am not certain just how much effective power (thrust) is lost due to throttling on this engine.

Long story short, all of this stuff actually happens and actually changes your fuel economy (less thrust for a given fuel burn means lower fuel economy) but as the pilot you cannot see any of this happen and there's usually little you can do about it. The only thing you need to keep in mind is that when setting for cruise power you should find an RPM that allows you to keep the throttle as open as possible to maintain your desired manifold pressure.

All of the above is why later marks of the engine have two-speed superchargers, by having a low gear and a high gear you can maximize your throttle during climb and cruise across a large range of altitudes. If the engineers just used the high gear to get you up to 35,000+ feet you would be severely throttled at anywhere below 20,000 feet or so and probably noticing a lack of low altitude performance despite RPM and boost being constant (our Spitfire has the equivalent of the low gear on the later engines).

A side note on efficiency is that your compression ratio (around 6:1) is set for avoiding detonation at full boost. At lower boost (idle, cruise) your ideal compression ratio is actually much higher, but since there are no production engines with variable compression it's just another part-throttle loss (lower thermal efficiency) that is out of the pilot's hands.

More reading here, if you're interested: http://en.wikipedia.org/wiki/Engine_efficiency

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Still digging for official sources on that one, I've been going from memory. The short version is that at high power a low coolant temp allows more heat to bleed out of the cylinder to keep peak temperatures down and, through that, prevent detonation. This constitutes a loss of thermal efficiency but the loss of heat there is actually beneficial, overall, since it keeps the engine in a safe operating condition. It's a lot like full load enrichment in that regard... No, it isn't efficient to put in more fuel than the engine needs but the loss of efficiency is acceptable if it keeps your engine from blowing up when at max power.

At cruise power, though, detonation is no longer an issue because we are no longer pushing peak boost. Because of that, the heat lost to the coolant and its related loss of thermal efficiency is now the only factor in play (remember, heat in the chamber is what makes power). So by raising coolant temps you are raising the cylinder wall temps and preventing heat from so readily escaping the chamber, heat that then turns to power at the crank. Regardless of coolant temp you are burning the same amount of fuel, might as well get as much power as you can from it!

For more info check out this page (beware, it's math/physics heavy): http://en.wikipedia.org/wiki/Thermal_efficiency

-Dave

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Thanks for the answer! Yesterday I read that Wikipedia article (and a bit about Audi car engines ), but couldn't be certain if my "mental model" of the process was right (as I wasn't sure if "thought" relations between efficiency and temperature of basic heat engine approximate piston engine closely enough for that mental exercise) - I didn't take CHT and risk of detonation into account, instead I wondered about coolant properties (such as viscosity versus temperature relation). Thanks for clarification, don't hesitate to share if you find more on this subject!
One more benefit of keeping higher coolant temperatures while cruising is the ability to close radiator (as cruise speeds are higher than climb, the parasitic drag caused by open radiator would be higher), but it's of course Spitfire specific.

Spitfire specific indeed. Scott mentioned somewhere that the most aerodynamic position for the Mustang's radiator is about 1/3 open. Interesting given that the radiator is fully shut under most cruising conditions... Either way, the Meredith effect radiator is a fascinating concept. One would think that the more closed the shutter is the more thrust one would generate in addition to the reduced form drag.

I've often wondered if putting the radiator on the right wing was a means of adding indirect rudder trim for all phases of flight. Open shutter simulates right trim for takeoff and landing, somewhat open shutter simulates a little less right trim for climb, closed shutter more or less negates any trimming effect for cruise.

Update: Here's some real world data that should be applicable to the Spitfire on this topic, it's an article on P-51H flight testing: http://www.wwiiaircraftperformance.org/mustang/p-51h-64161.html

Check out the speed vs altitude charts near the bottom, the engine hits peak horsepower at its two critical altitudes, power is lower below the critical altitude due to throttling losses and the high blower peak output is less than low blower peak output because of the extra power needed to drive the blower faster.

-Dave

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Dave, thank you for these many explanations!

Okay for the boost cut out, I'll have to try it for curiosity to reach this incredible +12 psi.
Although I try to take care about my engine over time, with A2A, it can be easily repaired if accidentally damaged it, why not try?
In this regard, under what circumstances pilots were they required to use this extra power?

For the large chapter of the throttle, I understand the need to focus at the full opening.
The emplacement of throttle just before the compressor allow to regulate the load on it.

I better understood several things:
The compressor also uses power, as a lot of accessories.
I think I read that it could provide an additional of about 400 horsepower and it consume about 150, so a net gain.
So the more you sollicit it, the more it consumes.
Throttle not full opened decreases thermic efficiency and creates resistance, that's acquired.

But, but... I still have a problem:
"At 6 PSI with the throttle closed the more power at the crank (X) is the Same as with a wide open throttle purpose, because a closed throttle creates obstruction that year the engine has to work to overcome against, the power at the prop (thrust) is Reduced. "

There's one thing I still do not understand Dave, I'm sorry!
The engine can't "see" the throttle, he sees "only" +6 psi intake exiting the compressor?
That the butterfly is half opened with high static pressure, or fully open with a low pressure, if the compressor delivers +6 psi in both cases, how the engine can see a difference?
The loss of power between the crank and the propeller is not the same at constant intake pressure?
What is this mystery?

Finally it does not matter, I must remember that it is better to focus the throttle open (in the correct limit of boost when at low altitude).

Another question if you allow: Do the low dynamic pressure which should help better exhaust gas burned, increases the efficiency of the engine at high altitude also?

Your answers make me see much of very interesting documentation, and I discover how all of Merlin is a marvel!
I'm sorry to insist on certain points, and bounce off others, but the more I documented with your help, the more I want to learn...

Have a good day , cordially

Haha, not a problem, let me see what I can do... The boost cutout override is like War Emergency Power (WEP) in American planes and kinda like the afterburner on modern jets. As such, it was only used in combat when the conditions were either to push your engine too hard or get shot out of the sky. A little extra wear and tear on the engine is a small price to pay for coming home alive.

As for getting two different power outputs at the same boost pressure, check that table on the link I posted for the proof, the first table shows power and speed at various altitudes at WEP of 67" Hg (about 18 PSI boost), and the second table is at military power of 61" Hg (about 15 PSI boost). Looking at the numbers for 61" Hg, we can see that at 5000 ft the engine makes 1420 hp yet at the same manifold pressure at the critical altitude of 16300 ft we see that it makes 1498 hp, a 78 horsepower difference. The only thing that changed between the two altitudes was the throttle opening.

EDIT: I forgot there are two sets of two tables each, the above numbers reference the CLIMB PERFORMANCE charts since ram air boost is less of a factor at max climb rate speed.

Now as for why this happens (at our aforementioned boost of 6 PSI), all of the components of the engine have to be viewed as a single system all together, and vacuum needs to be taken into account. At wide open throttle air flows freely straight through the carb into the supercharger, gets compressed, then gets blown into the cylinders (due to high altitude this comes out to our 6 PSI of boost). With the throttle closed air flows freely until it hits that throttle plate then gets severely restricted, like damming up a river. Because of that restriction there is now a vacuum immediately behind the throttle plate and before the supercharger, which the pilot cannot see (due to low altitude we still get 6 PSI boost). So basically, the engine as a whole "sees" the same 6 PSI of boost in the manifold in both cases but the throttled engine also "sees" a vacuum ahead of the supercharger on top of that. It takes power to create a vacuum and that power has to come from somewhere.

Long story short, in the high altitude situation we are only multiplying air pressure by "X" amount in the supercharger to get "Y" amount of boost whereas in the low altitude situation we are first dividing air pressure by "Z" amount first, then multiplying by "X" to get our boost of "Y." X and Z both take power to do so if you avoid one step you have fewer losses.

Oh, and regarding this:

Are you talking about thrust from the ejector exhausts at high altitude? If so, then yes, I think high altitude (low ambient pressure) probably would increase thrust from the pipes and perhaps increase total engine efficiency in the process (correct me if I'm wrong people).

-Dave

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Don't forget that the "external conditions" also changed - lower pressure at higher altitude is the obvious one, but different air temperature and density also slightly affect the engine performance. It is also possible that the airplane is a bit lighter due to fuel burned, and the drag may differ due to radiators position. And the gravity is tiny bit lower up there... don't worry, I'm just kidding

If we consider the exhaust as a rocket engine, then according to the equation below you are right - lower ambient pressure results in higher net thrust.
http://en.wikipedia.org/wiki/Rocket_engine#Net_thrust

So now I wonder if the increased exhaust thrust makes up for the loss of power in high gear blower, you're making less horsepower at the prop but more thrust at the pipes...

-Dave

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Ahhhh, thank you Dave, I think this time it's good.

Sorry for the tables that show the Mustang losses throttling, I had not seen them when I posted ... It is not far from the hundred horses, a supplement which is that of my Ducati , compared to a thousand horses is certainly a little invisible.

I recognize that this time, your examples have convinced me.
The concept of depression just before the supercharger, which consumes power is not so too difficult to grasp for me.
The other concept of multiplication of losses whenever air is successively compressed or relaxed is also easy to understand.

Finally, I wanted to talk as you suppose, about the ambient pressure at high altitude, my term "dynamic pressure" was completely inappropriate.
Belfegor, thank you also for your comments and link just as instructive on the subject than Dave.

It's good to learn a little more each day.
I return now to learn the basics of this fabulous machine ...

Sincerely,

I've seen some sources claim that Spitfire's prototype ejector exhausts could generate about 70 pounds of thrust, but the description is rather vague. It's an interesting topic and it also appeared at A2A forums:
http://www.a2asimulations.com/forum/vie ... =10&t=3862

Cheers, thanks for interesting topic to discuss !

Yeah, I've never seen any specifics on it either. In that other thread it's mentioned that you gain the equivalent of 125 hp or so from the pipes at 20,000 ft (I'm assuming this is on an early model, single speed engine like ours). Another interesting topic is how radiator drag changes with altitude and power. Looking at it like a jet engine (which was the whole point of the Meredith effect radiator) you want both high heat added to the air and high velocity at the duct outlet. Higher power output provides higher heat and a closed duct provides the highest exhaust velocity so presumably max power at critical altitude with the radiator shut should give you minimum drag (I don't think any of these radiators actually produced net thrust).

Definitely. It's rare I get to debate the finer points of engine physics.

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