Turbocharged aircraft engines: facts and consequences

vic wrote:

And very small modern turbocharged car engines will probably see loads of 70 percent plus power for a lot of time, else you would not get anywhere in acceptable time. And that paired with lots of cold starts, very varying revs, from minutes in idle up to 6000-7000 rpm and a lifetime of several 1000 hours minimum.

I doubt that. I have a car with a “small” turbocharged diesel engine (≈100 hp) and I very rarely exceed 2500 rpm — and then only for a few seconds.

Exactly – and thanks for making the point, Peter and Vic.

Most internal combustion engines consume about 0.4lbs of hydrocarbon fuel per horsepower hour. So if a small car delivers 45mpg at 70 mph, that equates to about 1.6 imperial gallons per hour. For a gasoline car that means 12 pounds per hour or 30 horsepower. Hardly 70% – more like 30% of rated output. And trucks average between 0.38 and 0.45 pounds per horsepower hour – so are very similar. It is unlikely that even a formula 1 race car averages anything close to 70% of rated output on average. You simply cannot compare the duty cycles of aircraft and automotive engines. Flog a car round a test track at 150 mph for a few days and you will see what happens to its engine’s reliability.

Peter wrote:

or example the TB21 (turbo-normalised) has about 20% less range (in practice; no idea what the POH says) than the TB20.

This does not hold true for anything but very short trips where the turbo advantage are waisted on a climb or a lower cruise altitude.

To compare the POH of TB20 vs TB21 at same power settings at their typical cruise levels:

TB20: FL105@65% = 153 KTAS / 47,9 Ltr/H
TB21: FL210@65% = 166 KTAS /47 Ltr/H

Or for the approx same level and same power setting:

TB21: FL110@65% = 155 KTAS /47 Ltr/H

Now if you can get your TB20 to FL190 or so you will see great range and slow speed of course but the power output has dropped to 25% or so, so not real world example to compare.

if you can get your TB20 to FL190 or so you will see great range and slow speed of course but the power output has dropped to 25%

Hmmm I don’t think so

I agree it is slow flight (TAS of about 140kt) but also the fuel flow is 8-9 USG/hr. No free lunch in physics no matter how you shake it

Also very few people want to sit there on (quite critically needed) oxygen at FL210 as a routine matter, unless wx is a factor. I know – I’ve been there

I think the TB21 issue, reported by many who do long flights in them, is that the compression ratio is lower so you are chucking some engine efficiency out of the window already, and the turbo doesn’t do enough.

I am merely arguing the narrow point of whether a turbo gives you more MPG. I think the answer is “it depends” and it isn’t obvious. Obviously you can climb a lot faster at altitude, and achieve a higher TAS at altitude.

Some previous threads here and here.

Yes I didn’t check the exact power output for a NA at FL 190, and yes 25 % is too low, on the other hand the example you posted states leaned to best power and full RPM. In any case its a low power output for a NA up that high that compromises TAS too much IMHO + it will take you ages to get that high. The POH for the TB20 states nothing higher than 12500ft, likely because its outside the normal mission profile for that aircraft.

I dont say that the turbo is the better options in all conditions, but it certainly is better and more efficient under some conditions (up high and long flights). So I think its not fair just to say its gives 20% worse range without adding some conditions (low cruise altitude, low powersetting, short trip ect). That really was my point here.

I dont know what the TB21 owners you refer to are measuring against when they say some effeciency is lost? Is it reports on socata.org? I use the POH as source of information and I find the numbers are pretty close. I also do only long range flight in my TB 21 (between 3,5 and 5,5 hours). The turbo will give you 38 MP @ FL170 and 27,4 MP @ FL250. Much more than I would ever allow the engine to cruise with anyway.

As for the oxygen, it depends. I dont want to take my family to FL220 all wearing masks. But I usually fly myself at FL180 or with a single passenger and a Mountain High pulse nose system using the aircraft oxygen tank. It really isnt that big of a deal and I consider it a price well worth paying to get high. I occasionally do use a mask to get up higher than that but then only to catch a great tailwind or top some higher weather. But yes a pressurized cabin surely would be much nicer.

The collar is supposed to have a wire locked screw in it