lml@lancaironline.net Mailing List Archive

Gary,

Operating at WOT LOP does not mean operating the engine “hard”. And operating the engine at 2300 RPM and 30” (shorthand for 29.92) and LOP often means operating the engine in the 65 to 75% power range - - - JUST LIKE the traditional high power ROP down low operation of the engine. Only it is done on much less fuel, with cooler CHTS and lower peak internal cylinder pressures.

And the notion that if you are 24” x 2300 normally aspirated that you will also be at the same power when 24” x 2300 at altitude when adding a compressor is wide spread - - but rather incorrect. There are a whole host of problems with the assumption that those are equivalent power settings. They are not - - in large part due to the absence of an intercooler!!!

But there are other losses - - mostly back pressure. When operating the engine near WOT the back pressures losses are mostly gone and with the intercooler, the degradation due to higher compressor discharge temps are mostly gone - - and life gets very very good.

As someone once said, “It is not how hard you run the engine. Rather, it is how you run the engine hard!”

George

Ps I have not done a Vy climb (except for flight test work) in a decade, or more.

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Gary Casey
Sent: Tuesday, December 22, 2009 6:50 AM
To: lml@lancaironline.net
Subject: [LML] Re: io-550 vs. tsio-550 differences?

I agree with George, the difference of his and my position is based on the assumption of operating conditions, not the technology itself. I was assuming "turbonormalized" carried with it the assumption that the operator would use "traditional" power settings. Most naturally-aspirated engines will have a sea level WOT manifold pressure of maybe 29 inches, so operating at 30 is already ever-so-slightly higher. Most traditional methods shown in POH's and, as I have observed, used by many, if not most, pilots utilize a cruise of 75% of max power. For an engine rated at 2700 rpm at sea level, that equates to a setting of something like 2400rpm and 24 inches. I made the (incorrect?) assumption that "turbonormalized" means the pilot will continue to use approximately the same settings, but use the turbocharger to maintain those settings at altitude. That's probably the most conservative assumption - and one that matches my own past experience, limited as it may be. George assumes that essentially all operators will take full advantage of the capability of "modern" turbonormalizing systems, which are apparently dramatically improved compared to traditional approaches, and operate the engine at almost full power regardless of altitude. I say "almost" because LOP operation will result in a loss of maybe 5 or 10% compared to ROP operation that would presumably be used for takeoff. I suppose rpm could be reduced to drop the noise level, but the manifold pressure would presumably remain at some maximum. 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. 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? 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, 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. 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

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