Posted for "George Braly" <gwbraly@gami.com>:

Rob,

I'm still not trying to be difficult. Just realistic.

Lets assume that the engine cooling drag on a 200 knot airplane is in
the neighborhood of 15% of total airframe drag.

And lets suppose that the heat rejection issues increase the total
airframe drag by 1.5%.

Then we need to ask, how much additional Brake horsepower will it take
at the engine to overcome that 1.5% increase in total airframe drag?

Since the cruise BSFC numbers appear to be around 0.365 (diesel )
and 0.39 (LOP turbocharged piston engine) - - - then the increased Bhp
must not exceed the ratio of 0.39/0.365 = 1.07 or you have gone
backwards in terms of NM/lb of fuel consumed.

And then - - to be realistic one does have to consider that
"intercooler" that you want to ignore. As I said, in the case of the
SMA 230 Hp engine - - - it is huge. So you may want to ignore it - -
and claim it is unnecessary - - but then the best engineers at SMA sort
of thought it was essential.

And then - - if you want a pressurized diesel - - one has to plan on
the weight and complexity of an add on gear / belt driven compressor.
That system and its associated complexity and power losses extracts
another 5 to 10 Hp or more from the engine. How heavy is that system?

And the engine ?

DRY it is claimed to be around 620 lbs.

So... wet? How much coolant ? 25 to 30 lbs minimum?

Oil ? 12 lbs?

And the weight of the gear

Driven cabin pressure compressor ? 15 lbs?

Without even getting started on the details one is up to 675 lbs.
At sea level, that is a pwr/weight of about 350 / 675 = .52 Hp/lb

That compares to a wet TSIO-550 of about 585. Or about .60 Hp/lb.
So there is at least a 90 lb weight penalty - - 15% off of the
power/weight ratio.

And at 25000 feet - - there is a loss of a lot of horsepower.

The 25000 feet pwr/wt of the TSIO-550 is around .45 at high power
cruise.

For the diesel it will probably be around .33. That is a huge step
backwards in real world cruise performance for the aircraft.

Am I missing something here ?

Regards, George