Jerry Hey wrote:
Paul, I have been designing and dreaming about my cooling system
for six months and still don't have it designed. I have defined large
areas of ignorance that I will never bridge without the help of the
engineers on this list. The goal is to avoid increasing cooling drag
over what an air cooled aircraft engine requires and to have the
engine run at the proper temp regardless of outside conditions.
Sometimes I feel these two goals are mutually exclusive.
Additionally, hanging ugly, draggy looking, scoops out in the air
stream is not really an option. I want my Tailwind to be SLICK,
period. I don't care how much work is involved or, within reason,
how much it costs. Effort is not the issue, ingenuity is.
Instead of scoops ducting air into the fuselage, I could build a more
roomy cowl to put the rad up front. The result is the same, however.
MORE DRAG. And the beautiful lines of the Tailwind would be
destroyed.
I realize that I am just venting the frustration that most people
feel when trying to hang a auto engine. To be honest. most of the
"solutions" I have seen are hard to accept. Because of cooling drag,
the aircraft end up going slower.
Would it be possible to deliver sufficient air to a fuselage
mounted radiator using NASA ducts on the belly of the fuselage? In
the Tailwind, internal structures limit the length of the 7º ramp of
the NASA duct to about 20 inches which would provide about 10 sq.
inches of inlet per duct. I could put four of these side by side
across the bottom of the fuselage for a very clean looking
installation. But that is only 40 sq in. of intake.
I recall you writing that the inlet size should be at least 25% of
the size of the rad. This seems impossibly large to me. I am not
sure if this refers to the volume of the core or just its surface but
it is much more than required for an air cooled engine either way. I
just walked down to measure the intake openings on my friends 182,
which has, I think, a 470 cu in Lyc. They are about 80 sq. in.
total. To cool an equivalent auto engine using say a 1000 cu in RAD,
would the inlet have to be 250 sq in? I must have this wrong. Does
a water cooled engine require three times as must air as an air
cooled engine?
A streamlined belly scoop could be made for the Tailwind. It could
be 20"/30" wide easily, but the airplane's looks would be destroyed
if the thing has to hang down 4 or 5 inches. What if I combined NASA
ducts with a two inch deep scoop. In all cases except taxiing, we
can assume a minimum of 125 mph relative air steam. That must be
worth something.
Thanks for listening, Jerry
Not to worry Jerry. I think we finally have a handle on why so many people
are having trouble with cooling. It has to do with a pressure build up
inside the cowl that Chuck discovered. More messages on this subject today
as I have done a bit of research. More on your message too when I get some
of this other stuff out of the way.
Paul Lamar
Bill Freeman wrote:
Jerry,
Long EZs are cooling O320s (one I know has been souped and dynoed to
176 hp) with a belly mounted NACA duct. Mine is about 4" deep and about
12" wide (from memory, I can measure it if you want) at the outlet. These
belly
NACA ducts work pretty well because they are in a positive pressure field when
climbing. I have a moveable floor in mine, set to about 3" deep since I only
need to
cool 108 hp. Seems like a 12-14" wide and 4-5" deep NACA duct with
a good diffuser would be a good starting point for a rear fuselage radiator
system. An upright radiator with a bell type diffuser dumping out into the rear
fuselage with a pair of exits under the horizontal stab seems worth a
try. Make the ducts out of two plies of BID glass over a simple styrene foam
mold covered with duct tape and melt out the foam with gasoline when done.
Mount the rad to the fuselage tubes, sealed to the diffuser, which is sealed
to the
NACA duct.
Might be good to check out what the pressure inside the rear fuselage is at
cruise
in a std Tailwind. If you have a friend with one flying, ask him to put a
tube into
the aft fuselage, run it up to the cabin and connect to either a water
manometer
connected to a static port. This is a critical piece of info. If the
pressure is high, you'll need to make an outlet duct and run it to a lower
pressure location on the aft fuselage. This would be best done with a
pressure survey on a flying Tailwind, looking at the
exterior pressures at various locations. I would expect the top surface to
be low
pressure, but that is a pure guess. You need an outlet location that is low
at both cruise and climb.
What do you think, Paul??
Bill Freeman
Sounds like a good program to me Bill. I wholeheartedly agree.
Nothing like real world, full scale data.
Paul Lamar
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