Note: the exhaust area requirement is greater than the inlet
combination of oil and coolant due to the now considerable hotter air
temperature. Rule of thumb: Coolant air inlet opening for 200 HP
coolant cooled engine ~60 sq. in., oil inlet opening ~30 sq. in. equals a
total opening of 90 sq. in. A good place to start with exhaust opening
is 1.4 times the inlets or 126 sq. in. Close the exit area down with
cowl flaps to as little as 80% of the inlet combination at cruise
conditions!
...cut
There was an excellent
article about that in Kitplanes of Feb.2004, about Brian Schmidtbauer's
Mustang II. Though Lyc powered it still goes 250 mph, just about par with his
friends RV-4, same engine (Dave Anders of CAFE foundation fame). Of course
Dave is part guilty for that Speed demon, as he gave some ideas to Brian about
cooling.
I do understand that an
air-cooled installation is a different animal, but by the end of the day ( or
at the cowl exit...for that matter) everything is air cooled. The interesting
thing is that both run a intake/exit ratio of 78%, the intake being bigger -
just about confirming the above statement!
Both seem to use exhaust
augmentation for the exit.
Their main study material was
"NACA reports back to the 1920's and Hoerner's book about Fluid Dynamic Drag"
( by the way that book is still available, but at around a 100 bucks rather
pricey, but worth it - says hearsay. Out of my range for
now....)
I mentioned that on the other
list, but I was dismissed as " you can't compare aircooled engines with a
rotary".
I don't think there is a
great difference on the amout of cooling necessary, as the efficiency of both
engines are fairly close. So some heat goes out the exhaust and the rest has
to be cooled. For sure you need different ducting, but the amount of heat
energy should be about the same and you want to get rid of it with the least
drag, either way.
Schmidtbauer mentiones the "
rule of thumb" - exit about 150% of inlet. By fine tuning the ductwork he got
rid of up to 30% of the total drag, just by eliminating most of the cooling
drag.
Anders beat John Harmon in
his Harmon Rocket, by over 1000 points (CAFE system, Harmon 1316,45 - Anders
2381,24)
Harmon around 300 hp, Anders
200hp (6mph faster than Harmon - 250+ mph for the RV-4)
So much to "thumb-rules", and
" not doing ones homework, because I don't like formulas,
etc."
I believe until there is a
solid FWF- instalation for every popular airframe, you will HAVE to do your
homework, or give away efficiency in a big way!!
There are so many guys on
this list that know their ways around formulas, being on the list to share
info and trying to help on every corner, you don't even have to dig in that
hard, just ask someone who enjoys formulas!
Back
lurking,
Thomas J.
PS: I have 3 pages of that
article scanned, if anyone wants/needs them I can forward it. 200-300Kb
each....