I now have 100 or so hours in my Extra 400 with the turbocharged Conti TSIOL 550. I have decided that the cooling system needed investigation and acquired an 8-channel temperature data logger. The data from the first test flight confirmed my suspicion that the cooling system was suboptimal. But the most striking fact was not an anomaly, rather a shocking finding about how much thermal stress these engines have to endure.
One of the probes was on the inlet side of the engine intake intercooler, measuring the temperature of the air exiting the turbo, before being cooled down. I’ll call this “TOT” for the rest of this post (as in Turbo Outlet Temperature).
I was expecting this to be hot, but not that hot!
On this flight I deliberately set full climb power (37.5 inches, 2500 RPM) per POH and leaned to 125 liters per hour which gave a TIT of around 1500 F and good coolant and oil temps.
I kept VS constant at 1000 fpm all the way to FL 200 and this gave 115 to 120 KIAS. So the only change to cooling airflow was air density and temp.
OAT decreased from +12C at SL to -23 C at FL200 – quite near ISA.
QNH was 1022 at take off.
The data shows TOT to increase in a perfectly linear fashion during the climb from
roughly 50 C (40 C above ambient) at SL to a staggering 110 C (132 C above ambient) at FL200.
After level off I set 75% power / 206 KTAS. TOT was 95 C (118 C above ambient) . Economy cruise at 55% power / 185 KTAS dropped TOT to 75 C( 98 C above ambient)
Sadly the probe I had installed on the “cool” side of the intercooler was inop so I do not know how much of this heat was being eliminated by the intercooler.
I will post this data as soon as another “useful” flight is done. My present guess is that 70C would be the best case, which is still very very hot.
I initially thought that something was wrong with my engine, but, after researching it, it turns out that this is a normal situation for turbocharged engines operating at altitude. Part of the heat comes form the mechanics of compression, but another part is transmitted from the turbo’s hot side as an increasingly large amount of exhaust gases are used to spin the turbine in order to compress the air to 37.5 inches. Keep in mind ambient pressure at FL 200 is around 10 inches!
Now I understand why the POH requires us to reduce MP above FL200 on these engines.
The thermal strain is already huge at FL200 and I don’t even dare think of what happens on a hot day where both inlet and cooling air temps are way higher.
I think that we should have dedicated procedures for operating these engines in above-ISA conditions.
Just stating a different MP is not enough. A completely different (and realistic) climb profile should be defined and I am also thinking of adding to my panel a gauge showing air temperature at the inlet manifold.
Flyingfish wrote:
leaned to _125_*Bold* liters per hour
Is that a typo?
Why would it? A big-bore turbocharged 6 cylinder engine easily drinks 40 gallons when all levers are full forward. So, 125 litres would equate to a slightly reduced fuel flow…
I love my Turbo-Rotax. It sips 34l/h unleaded Autofuel while outputting 115hp full power. Fair enough – it wouldn’t do much in an Extra 400 
Brilliant work, Flyingfish.
It will be really interesting to find out the air temperature after the intercooler.
But if this is +70C or anywhere remotely near it will help understanding why turbo engines don’t achieve the great MPG which they are supposed to do (due to the TAS gain) at say FL250. For example the TB21 (turbo-normalised) has about 20% less range (in practice; no idea what the POH says) than the TB20.
These instrumented flight tests can yield results which nobody believes. For example this one (done by someone else, with a 4-channel PT100-USB logger) explained why you get fuel servo icing at an OAT of -15C, when the airframe is probably totally clean. In fact the whole inlet duct ices up, including the alternate air door which then won’t open! It drew some sneering comments for years afterwards but fortunately experimental data always trumps the theory
Have you tried running LOP? On the IO550B TN I also fly this makes all the difference in the world between efficient flying and wasting fuel and cylinders; obviously a liquid cooled engine is different but should be easier because you don’t have the baffles to affect each cylinder differently. We run the IO550B TN at about 17gph, LOP at about 85% up to about FL200 routinely. When high, the RPM needs to be increased to 2500 to keep things smooth.
boscomantico wrote:
125 litres would equate to a slightly reduced fuel flow…
Rather more than I am used to in the Turbo Arrow IV
My SR22T burns 40 gal/hr in a full rich climb.
Flyingfish wrote:
roughly 50 C (40 C above ambient) at SL to a staggering 110 C (132 C above ambient) at FL200
Flyingfish wrote:
I think that we should have dedicated procedures for operating these engines in above-ISA conditions.Have a look at the procedures to operate an intercooled Turbo Arrow. What you are looking for is to match the actual air density to the POH value at ISA.
I don’t know your level of technical knowledge but turbocharged engines need some special handling and care. I would recommend John Deakin’s Pelican Perch articles: Those Fire Breathing Turbos. John was here in June and is very accessible and glad to help.
Here is the link: Fire Breathing Turbos
