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George,
I have compressor maps for lots of aircraft centrifugal compressors - - and
only one of them gets to 80% at any point on the map.
It is unlikely that the fixed rpm ratio version of that unit would operate
at
that optimum efficiency for more than a small portion of its operating life.
You've got me there. I don't have any maps for "aircraft compressors". I do however have many for general applications and it is not uncommon for them to peak in the upper 80s to 90% efficiency. I only assume that aircraft would use equipment with similar efficiency.
Why wouldn't an aircraft compressor operate at or near its optimum efficiency nearly all of its life? Airplanes are routinely flown at roughly the same altitudes and power settings for the majority of there lives.
>> 4) To get the air temperature back down to tolerable (detonation
margins) ranges requires an intercooler All sorts of engines run without
intercoolers, without damage. Detonation is a function of temperature AND
pressure in the cylinder at, or near, full compression, not the temperature in
the intake. Besides, if you are flying up high, where an add on system makes
sence, the air starts out colder so it isn't all that much warmer than it
would have been at ground level.
The principle reason for adding an intercooler is the same as for adding a
supercharger, to make the intake air denser and increase the mass flow through
the engine. <<
Well, after reading that, I am sort of beginning to wonder that NACA
never
knew what it was doing.
NACA was part of the government. Enough said
The primary reason in an aircraft to use an intercooler is not to gain more
charge density. It is a LOT easier to do that with the compressor.
By your own words, a compressor saps power from the engine. An intercooler ups the load on the compressor slightly, but the result is more efficient than jacking up the compressor ratio.
The primary reason is to provide adequate detonation margins at higher power
settings.
If you think that one of the primary component causes of detonation is NOT
the temperature of the induction air - - - then I would suggest that you
take another look at the hard data.
I never said it wasn't one component. But there is a lot more to it than just that. What is important is the combination of factors in the combustion chamber. If all you want to do is get away from detonation, cut the compression ratio by a quarter point. If you want to make more power, ad an intercooler.
I have been teaching this precise subject matter on Saturday mornings every
other month for the last 34 months.
I studied the subject at the graduate level for nearly that long.
In addition to teaching the subject, I have measured the results, directly,
on the test stand on a TIO-540J2BD engine - - with and without an
intercooler.
George, I spent 5 years playing around with race cars and yet I know full well that there is absolutely no way I could ever catch up to the hands on experience you already have with piston engines. I respect that. By and large, I have agreed with the vast majority of your statements on this forum over the past few years and even picked up a detail or two that I didn't know before. Thanks.
However, when you make the blanket statement that a mechanically driven supercharger will result in a slower airplane, we have to part company. Neither thermodynamics nor history support such a stand. On the contrary, both show that well matched components will result in improved performance. That is not to say that it couldn't be done even better with a turbine driven supercharger or some other system.
Rob
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