>>I certainly agree with George in his comments in that a
well-designed intercooler will offer significant performance benefits at
altitudes like 18,000. When I said "marginal benefit" I was
referring to benefit in a more global sense. It all depends on how much
of the operation is conducted at 18,000 feet. From my experience I would
guess it would be very, very rare for someone to spend 50% of of the total
hours at or near 18,000. A 4-hour flight will spend maybe 30 minutes in
climb and 3 minutes in descent from 18,000 for 50% spent at 18,000. A
serious cross-country flier might be there for, what, 20%? And then how
much of that time is spent at 30 inches manifold pressure, burning close to 30
GPH? <<
Ah.. I
think I see the problem. How much time is spent at 30 “ MAP ? In the fleet
of well over 1,500 turbonormalized airplanes that TAT supports - - - the
amount of time in cruise that is spent at 30” MAP is very close to 100% of
climb and cruise. Essentially everything but descent and landing. But it is
not spend at 30 gph. It is spent at about 16.5 to 17.5 gph. Essentially,
WOTLOPSOP.
And, the
climbs are more typically 15 to 18 minutes, not 30 minutes. And the portion
of a 4 hour flight that will be spent at altitude at 30” WOTLOP will be around
3 hours. And a very large percentage of those owners fly those aircraft at
altitude a large percentage of the time.
In
short, the current generation turbonormalized aircraft are very very
different from the low compression engine operations at reduced manifold
pressures that you are referring to and which I spent thousands of hours
pursuing back in the 1960s, 70s and 80s .
>> At lower manifold pressures the higher inlet
temperatures are not as detrimental as they are at full power, where they limit
the power output. In other words, 25 inches with a 150F inlet might be
equivalent in power - and nearly equivalent in efficiency - to 24 inches at
100F inlet. <<
I can
tell you that it has been more than 13 years since I deliberately operated a
turbocharged engine at 24 or 25” in cruise, or at anything less than 29.x “
MAP in cruise. That is a terrible waste of a good engine and turbocharger to
do that!
>> Another thing to be aware of - the inlet air
temperature does not exactly match the temperature of the inlet charge as it is
trapped in the cylinder, which is, after all, what counts. Tests I have
seen show that the charge is heated about halfway to the cylinder temperature.
If the cylinder temp is 300F and the incoming charge is 100 the resulting
charge temperature is about 200. Increase the inlet temp from 100 to 200
and the trapped charge will be at 250, only 50 higher. While no one
is truly an average operator, my point was that chasing the optimum performance
at 18,000 feet may not be worth the cost and development expense. Depends
on your goals. All flights involve operation below 10,000 feet while a
limited number involve operation above 18,000. <<
If you
are operating the airplane WOTLOP at 8000 feet, on a hot summer day, a good intercooler
will drop the induction air temperature around 80d F. That may mean the
induction air temp is down around 80dF rather than up around 160dF. That
provides a huge margin in protection from detonation - - even when operating at
lower altitudes.
>> When flying my TR182 on a lot of cross-countries I
flew between 12 and 15 a lot, but flew above 18,000 only a couple of times in 5
years. Like in the drug commercials, these comments are directed only at
turbonormalized nonpressurized oxygen-equipped aircraft operated in
non-commercial environments. Oh, yeah - and by "average"
resource-limited owners :-) <<
>>I'm not sure I agree with George when he says that "Intercoolers have large benefits - - - even at sea
level." In this case the turbo is producing negligible pressure, so
the temperature rise is dependent only on the heat picked up going through the
turbo and the small compression heating when the turbo has to overcome its own
flow restriction. This is also true when cruising at less than 10,000
feet at reduced manifold pressure. Is the added weight, cost and
aerodynamic drag more of a detriment than the cooling is a benefit?
Gary <<
Consider this example: Non intercooled. Denver/Centennial.
August. 110dF on the runway. Full power takeoff. 30” MAP.
Ask yourself what the induction air temperature is with a
typical compressor operating off its design point at about 60-65% efficiency ?
Then ask if you would like to have an intercooler, or not ?
Then try the same thing at Durango.
Regards, George