lml@lancaironline.net Mailing List Archive

>> Incidentally, I see no substantial difference in the compression ratio of "modern" engines compared to "traditional" (NA or TN)engines - 8.5 to 9.0 seems to have been the norm for many years. <<

Lycoming does not make an engine with a compressor that has more than 7.3:1 compression ratio. So there is a fundamental difference in the CR between the current generation TN engines and the older baseline TCM and Lycoming engines with 7.5:1 and 7.3:1 compression ratios.

>>And what about that takeoff from Centennial at 110F? Yup, the non-aftercooled engine will suffer in that condition. But how often is one likely to take off from a "hot AND high" airport? <<

How often? I do that several times a year while stopping in Colorado or Utah in the summer time in the middle of the day on a cross country.

>> I've never seen Centennial at 110, but that doesn't mean it can't happen there or somewhere else. Maybe Flagstaff or Durango.

George also suggests that "modern" pilots will operate "modern" TN systems in the maximum performance mode all the time (30 inches manifold pressure). For the specified 4-hour flight this would include a climb at 1,000 fpm up to 18,000 to be followed by a descent at 500 fpm all the way to the ground. Really? For most reasonably heavy aircraft this would entail climbing at Vy …. <<

Hmmm… We climb the TN Bonanzas at Vy + 20 knots and are normally able to make 1000 fpm unless it is unusually hot. Again, it is important to note that operating at 30” MAP does not mean operating at 100% power. On the lean side, the %power is only a function of fuel flow - - not manifold pressure.

>>, but I find myself, when pointed where I want to go, climbing at a speed of at least 20 knots faster than Vy, resulting in a lower climb rate. <<

>> After all, I want to get where I'm going, not just get high. and then I find myself starting the descent early and to save fuel descend at less than 500 fpm and I spend some time maneuvering at low altitude before landing. And, unless the aircraft is equipped with an electronically-controlled oxygen system, Many fuel stops will also result in a visit from the (expensive) oxygen cart. <<

Because of the not very expensive pulse O2 delivery system, I do not even remember the last time I had to top up with O2 when I was away from my hangar and was able to do that out of the home base bottles in the hanger. I can fly to the west coast and back with several people and still have O2. In the summer time, I don’t like slow descents because it leaves you bouncing around in hot air. IMC in the winter - - it leaves you in potential icing when IMC.

>>And George maintains that pretty much all operation will be at 18,000, so I guess that includes westbound against headwinds. But maybe I'm as old-fashioned as I look :-).

Gary, not TN, but always LOP, thanks to George<<

18,000 all the time? Not at all. One of the beautiful aspects of the turbocharged piston engine operating LOP is that it runs very efficiently at both low and high altitudes. So you get to take advantage of the difference. It is not uncommon for me to have a tail wind going both ways on an out & back trip, just by picking two different altitudes.

Gary - - you should make arrangements to take a good cross country trip in a turbo normalized aircraft flown by a knowledgeable pilot. It could become habit forming!!!

Regards, George

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