the rich rich condition is, in a sense, on/off because as soon as it passes the "best power" mixture it is rich rich.
In terminology, yes. But like you later said (contradicting the previous) "Not by very much at first".
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...
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).
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?
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.
ither 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...
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...