Thoughts about air scoops.

[Tomanek, Wojtek]

Adam
 
It really does not matter where the air exit is, as long as it is adequate.
If the engine is completely cowled and the pipe is covered by the belly pan,
there is usually more than enough area between the engine compartment and
the pipe tunnel for the cooling air to exit (I am assuming that the belly
pan has plenty of openings for pipe cooling and exit of air that entered
through cheeks or scoops - this also cools the pipe.  What is important is
that the airflow is directed/forced to flow as close as possible to the item
being cooled (fins for cooling the engine head and to the engine case for
cooling the bearings).   Just adding an air exit may not improve cooling if
the air is not forced to flow immediately adjacent to the engine case.  In
planes with large engine bay compartments the air may be slipping adjacent
to the engine case and not doing very effective cooling job, or heat
transfer, - we want a high convection heat transfer coefficient which
increases with air velocity.  Just like with water, air will take the path
of least resistance in the middle of the available flow area, or in our case
NOT next to the engine case.   You may significantly improve cooling just by
adding cooling channels, (race cars are doing that for engine cooling,
engine air intake, and break cooling) ducts, or barriers to prevent free
airflow far away from the engine case and forcing it to flow as close as
possible to the engine.  Finally, it is probably more important to slightly
overpressure the cowled engine bay with incoming air than to create a slight
vacuum since the heat transfer characteristics are better with higher
density air than lower density air.  
 
Finally, it is good that you are concerned with cooling - it is very very
critical.  When I installed a scoop (forcing more air over the engine case)
on my Omen (based on recommendation form George Asters - the Omen designer)
the engine increased max rpms by 250-300 and the power increase was
noticeable in the air, especially on hot days.  In my case the engine bay
compartment is very tight and additional ducting was not required.  But do
not assume that air entering the chin scoop will flow around the entire
engine case - most likely just around the fins.   
 
I think one of the Canadian plane makers markers sales inverted fiberglass
scoops to be used as air exits (or just use cut to fit small wheel pant or
large plastic spoon).  Also, the new Temptation is actually designed to have
air intake and air exhaust through the cheeks.  
 
Directed/controlled air flow is probably more important than the ratio of
air entry to exit areas.
 
Hope this helps.
 
Wojtek
 
 
-----Original Message-----
From: Adam Quennoz [mailto:aquennoz at ipa.net]
Sent: Tuesday, February 04, 2003 8:34 PM
To: Pattern List
Subject: Thoughts about air scoops.
 
 
It seems the trend these days are air scoops.  Before a scoop is effective,
the area of the air exit must equal or exceed that of the air intake thus
allowing a positve air flow to occur.  With all the scoops people are
putting on the nose of the aircraft, there must be a pretty good sized hole
somewhere for the air to escape.  There are cheek scoops, chin scoops and
now with the concern of cooling the crank cases, there are scoops above the
engine compartment.  I call them "nose scoops".  
 
Has anyone thought of making EXITS on the top of the nose above the engine,
and/or exits on the cheeks?  Basically put the scoops on backwards.  The air
flowing over the backward facing scoop would actually create lower pressure
inside the engine compartment allowing a larger volume of air to enter the
chin scoop.  The air entering the chin scoop can flow all around the engine
as it makes way to the exits of the engine compartment, and what's left can
flow down the pipe tunnel.  
 
I'd like to hear what others have to say about this idea.  I'm thinking of
trying it on my pattern plane.
 
Regards,
Adam Quennoz
AMA 383866
IMAC 2439
NSRCA 3546
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