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
Hi Paul, I'm looking forward to your response. Following up on the
inlet measurements I made yesterday, today I learned that the typical
intakes for a 0-320 or 0-360 RV cowl are 3" x 7". That is a total of
42 sq in of inlet. Now this is a number that I can attain easily. I
feel like I am back in business. A close friend had cooling
nightmares on his One Design until he really opened up the the back
of the cowl. All of his problems went away immediatly. According
to him Lycoming states that 5.8" of water pressure drop between inlet
and outlet of cooling air is minimum. A very useful bit of
information, I think. Jerry
Yes I have heard of that Lycoming requirement and it is a good way
to be sure you have enough cooling air. Do you recall what air speed
that was specified for?
Until we have some of this pressure data
we cannot say for sure what the cowl opening or the rad size must be.
So far I am going mostly on reports from the field and racing car experience.
The field data we have is apparently flawed based on what Chuck
measured with his cowl. Nobody that I know about has sealed
the big hole behind the spinner except Piper.
I suspect the openings will get smaller as we get a handle on this cowl
pressure problem.
I am sorry there is no free lunch on the belly scoop.
I am not sure a combination type scoop as
you mentioned would work. The only real way to check the effectiveness
of the scoop is measure the pressure and relate it to airspeed
or dynamic pressure. Do you have a fast pickup truck you can mock
a scoop and rad duct up on a piece of eight foot long plywood up side down flush
with the cab roof in the back of the truck and drive 100 MPH or so?
However with a rad that large in order to take full advantage
of it you will need a large hole in any event.
Paul Lamar
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