Yes,
it tells you the pressure above the cooling fins of the engine (almost full
dynamic pressure). Yes I am making the assumption that the pressure
below the engine (other side of the cooling fins) is the same as the backside
of the radiator. Is that not reasonable? The cooling
exit is the same in both examples. Note that the pressure
above the engine was almost full dynamic pressure WITHOUT the exit being
blocked. Therefore my conclusion that the pressure differential is NOT
lower on the aircooled engine.
Tracy
I guess my point
was that there isn’t enough information to draw the conclusion. But if
the exits are the same, and the pressure in front of the radiator is the same
as that above the cooling fins; then the pressure drops should be about the
same. But of course it varies from one installation to another, rad
thickness, fin density, etc.
In any case; I
agree with your premise – I have seen no data that definitively supports the
conclusion of lower pressure drop for cooling the air cooled engine.
Should one also infer then, that the flow rate required is also about the same
– that the higher fin temp does not give a higher delta T to the cooling
air?
Al
A :
Yes, I do believe that the higher fin temps do not result in higher delta T to
cooling air (supported by some exit air temp readings on Lyc.
installations) The reason is that there are fewer 'collisions' between
each air molecule and fins than with fins in a rad (especially a thick
one). The average air velocity is higher in the Lyc than a rad. If
you slowed the air in the Lyc to the same speed as in a rad, it would not cool
the engine for two reasons. Not enough turbulence (required for good
heat transfer) and not enough CFM even if there were enough
turbulence.
You
have probably noticed the fins on the really high performance radial engine
heads from WWII era. They have an incredible fin density (similar
to a radiator). In the case of the cylinder barrel fin assemblies (a
separate part that is clamped to the cylinders) they look VERY similar to
radiator fins, including turbulators between the fins which accomplish the
same thing as the louvers in radiator fins. These air cooled
engines probably DID have higher delta T temps than our liquid cooled
roraries. If you just scaled up a Lycoming O - 360 cylinder assembly and
tried to get the HP/CI that the super charged radials did, they would
burn up in very short order.
Tracy