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Doug,
There are basically three types of drag that you
wind up dealing with when planning a cooling installation.
1.) Pressure drag, due to the local pressure acting
on the surface of a the aircraft or duct
2.) Viscous drag, due to the scrubbing and mixing
of the air
3.) Momentum drag, due to any mass of air taken on
board (think of trying to scoop water out of lake from a speeding boat with a
bucket)
Your bleed air idea will more than likely cause all
of these types of drag, and it is dubious that it will help with cooling any. If
you want to achieve this the best way is to use the high energy present in the
exhaust gas jet to accomplish the same thing.
The bleed air ducts found on aircraft are to
accomplish the exact opposite of what you are trying to do. They ingest and
remove the low energy boundary layer, so that the main duct only sees high
energy airflow. You are ingesting the valuable stuff and then trying to use it
to accelerate the low energy stuff. What you really need to do is figure out how
to accelerate the low energy stuff without disturbing all that nice high energy
flow. I would copy Dennis' installation as much as possible. He seems to be
cooling well with the Renesis and the same cowl. He did mention to me that he
saw a large improvement when he separated the combustion air inlet from the
radiator inlet. He placed the combustion air inlet in the space between the
spinner and the radiator inlet. That is a really good place for
it. It will help keep the boundary layer from spilling around
the radiator inlet, and it should have really good pressure recovery due to
external diffusion.
I would also not recommend the location for cooling
outlet on top of the cowl where you have it drawn. Melmoth II has the
outlet near the front of cowl on top, just behind the prop. This is a low
pressure area, as you move aft, the pressure rises and at the base of the canopy
it is quite high. I can think of a lot of reasons not to use a top outlet in our
case. What if you have an oil/coolant leak? Wouldn't you prefer it splatter the
bottom of the airplane rather than the canopy? What if you have an exhaust leak
and the fumes spill up over the top and get sucked into the cockpit? I would
stick with the bottom exit and a cowl flap, or go for the side of the fuselage
with cowl flaps. A weighted door that is closed in flight on top of the cowl is
not a bad idea to let heat out when parked.
Planning a cooling installation is a big bunch of
compromises. I have my radiator behind my seat, the inlet under the wing, and
the outlets just aft of the thickest point on the side of the fuselage. That is
about as optimum as it gets. However, I have made several compromises to reach
that point. There are long runs of coolant that go through the center console.
Not something that makes me very happy. There has been at least one fatal crash
of a P51 due to coolant lines rupturing and filling the cockpit with steam. It
is also heavy. Coolant weighs a lot. I still have to have cooling air in the
cowl for the engine, exhaust etc so there is still going to be some drag due to
that. Then there is the fact that the inlet is placed farther back on the
fuselage and the boundary layer is thicker. A good deal of what could be baggage
area is eaten up with radiator and duct work. There is no perfect solution.
The winner has the best combination of compromises for a given
mission.
Monty
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