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Great post, Al.
Just a comment on one of the "assumptions" we all start with when
discussing estimation of cooling system sizes for hot weather: How much hp
is being developed, i.e., how much heat is to be rejected?
- I wonder if everyone is "assuming" that they must calculate radiator
areas based on heat being generated/to be rejected while generating 100%
power, i.e., 160 or up to 180-205 for a n/a rotary, depending on what you
think you will be getting?
-- My point is this: At a 90 or 100 degree F day, at full
throttle, sea level, std day pressure (29.92), you won't be generating 100%
power - because, you are sucking in hot ambient air through the induction
system (lower density than standard) and, without looking up the equations
for normalizing measured performance to "std day", I guess we'll be
generating only 90% of the 160 or so hp.
--- So, when we are analyzing or talking about cooling in hot
weather, we need to state a percentage of "std day hp" that we estimate to
be generating in the stated "non std hot day" we are designing for. I'll
try to dig out that info, unless someone else beats me to it.
- We should be able to post a two column table with "Outside Air
Temp (OAT)" & "% of hp at full throttle".
-- And, while we are at it, we ought to be factoring in the
manifold pressure drop across the air filter:
---- My Cessna 182 or 172 with the oiled foam induction air
filter drops 2" - that's what I always see.
---- A friends Mooney with dry paper filter drops 1". That
is consistent with what others have observed on takeoff and posted.
So, these become "constants" to be applied at the outset of setting up
(calculating) our "heat to be rejected" value, which value is the starting
basis of all subsequent calcualtions of radiator size, pressure drops,
velocity changes, etc, etc, that the equations work on.
- In other words, if I have a 160hp engine, I should NOT design my
cooling system to reject 160hp worth of heat on a hot summer day climb out.
I should be using some lower value that was objectively arrived at as
discussed above.
David Carter
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