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In a message dated 4/1/2007 3:47:38 PM Eastern Standard Time, marv@lancair.net writes:
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.
OK, so far the required power has in creased by a factor of 1.015 vs 1.07 for the engine efficiency. Looks pretty good.
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.
Those engineers at SMA also thought a diesel engine made sense as an aircraft power plant. You seem to be arguing both sides of this point.
Ordinarily, the cooling drag number encompasses all of the cooling drag associated with the power plant installation. In other words all of the drag created between the cowl entrance and the cowl flaps. If you want to add the intercooler in separately, you can still increase the cooling drag an additional 36.7% before you reach the break even point with regard to engine efficiency. Or to look at it the usual way, using your numbers one could increase cooling drag by 46.7% before the efficiency was reduced by the engine change.
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?
There is no reason one couldn't increase the compressor capacity to handle cabin pressure the same as used on the TSIO 550 in the IVP. The additional weight and power loss become negligible with this approach.
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 ?
On the contrary. It looks like you reread some of the previous posts on this subject and found the answers.
The main negative performance issues with the diesel are loss of payload, increased sensitivity to altitude and slightly slower speed for a given power and gross weight.
Rob
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