Vince Orton wrote:
The main problem is that liquid cooling is more difficult (less
efficient) than air cooling - mostly due to lower delta-T (temperature
difference).

<rant mode="on" style="slightly absurd">
All cooling is air cooling.  If you're going to insist on calling a 13B
"liquid cooled", then to be fair you should call a Lyco/Conti "aluminum
cooled".  All the liquid does is transport it from engine to radiator.
<rant mode="off">
(I feel better now.)

Which brings me to the point ... what are the relative merits of these
systems?  Here's some questions I have:

- I think good ducting design goes without question.

Except that determining what constitutes "good design" is unfortunately
a matter of interpretation. There's no one golden design that Joe
Builder can copy, and a distressing amount of junk science to mislead
Joe Shadetree Designer.

- Cowl flaps are reasonably easy to fabricate, but what can/do they

add

to cooling capacity?  10%?  50%?  100%?

Cowl flaps do not add to cooling capacity.  They serve as a way to
restrict cooling capacity.  "Why would one want to restrict the cooling
capacity?" I hear you ask.  Because the cooling capacity is sized for
unfavorable conditions that *do* happen, albeit for substantially small
portions of the running time of the engine.  The majority of the time,
the engine's cooling requirements are less, sometimes *a lot* less.
During that time, it is convenient to reduce the capacity of the cooling
system with cowl flaps, because doing so also reduces the drag produced
by the cooling system.

- Exhaust airflow augmentation is almost free to implement.  How
effective is it?

Almost free except for all of the engineering it would take to research,
design, and implement.  And if you get it wrong the first (second,
third) time, the cost is usually not cheap (your time doing it, and your
time fixing or eliminating it).

The *limited* research seems to indicate that it can have a modest
improvement in low airflow, high RPM situations such as takeoff roll and
Vx climbs.

- Liquid spray on radiators may be the cheapest way to add takeoff
cooling capacity, and is VERY easy to implement.  Also, any boil over
lines should be routed to the radiator face to get the "last use" out

of

any lost coolant.  Are there any drawbacks?

Drawbacks: Possibility of scale buildup on the radiator.  Yet another
subsystem that can fail.  Yet another component with a consumable
element that the pilot can forget to refill.

As for routing boil over lines to the radiator face: there's little
point.  By the time the cooling system generates sufficient steam to
cause boil over, the engine (internally) has already entered the
"overall boiling" phase.  The cooling system rapidly loses effectiveness
when this happens, even if the radiator were to be dunked in liquid
nitrogen (i.e. darned near infinite delta T for all practical purposes).
Spend your effort preventing overall boiling if you want to save the
engine.

- What are the drawbacks to alternate coolants?  Are they worth the
trade-offs?

Trade offs: cost and effectiveness.  Water has a remarkable capacity to
transfer heat.  I am not aware of *any* additive or commercial
replacement that improves on water's capacity.  What they can do is
reduce the unwanted side effects of water (corrosion primarily) and
raise the boiling temp of the solution so that overall boiling is
postponed and delta T is increased.  If the remainder of the cooling
system can take advantage of the higher delta T, then additives or
complete replacement fluids can work well.

- I understand that electric fan(s) are too weak to cool a 200hp

engine

without severe current draw, but are they useless as a "cooling system
augmenter"?

I think you are confusing electric water pumps with electric fans.
Electric water pumps are completely impractical for continuous duty; the
HP (equivalent to wattage) requirements are too high. Electric fans have
good promise as cooling system supplements. The biggest problem is that
at cruise the airspeed can be much, much faster than the stock
automotive radiator fans are designed to withstand freewheeling.

- I don't like the behind-the-engine radiator installation idea, but

can

it be made workable with some of these other solutions?  Using forced
air from a fan, etc.?  (Some installations may have restricted space
under the engine.)

Then build a pusher and the radiator will be in front of the engine.
:-)
But seriously, "can it be made to work?"  almost certainly.  Do you want
to undertake the engineering challenge?  Maybe.  Ignore the myriad of
tiny details; a cooling system basically needs to draw air from a high
pressure area, slow it down, pass it through a radiator of sufficient
capacity, speed it up, and eject the warmed air to an area of lesser
pressure.  Cooling system design flaws almost invariably trace back to
failure to heed one of those 5 simple rules.

- Why are engine driven fans not discussed?  We have pulleys, so the
fans could be located below, and in-line with the accessories.  Keep

in

mind the fan(s) could be smaller, or could be a "squirrel cage", like
home FAUs (Forced Air Units) use.  Ducting could control air paths to
pull through the radiator.  Ideas?  Problems?

The only serious problem I see is physical room to put the fan and
ducting. Engine cowls are tight spaces as it is.  Plus would one want to
spend HP spinning the fan at cruise?  If not you'll need an electric
clutch (one more thing to fail).

- Could the compressor section of a large turbocharger be used to pull
air from the backside of a radiator, and then dump it overboard (or

air

blast hot spots)?  Is this "out there"?  I think this has merit since
increasing power causes an increase in cooling airflow.  Heck, I

suppose

the turbocharger could still maintain an induction compressor section,
too.

Yes, that's slightly nuts.  Compressors are designed (what an
exaggeration... it's friggin' black magic!) to take a modest amount of
air and increase the static pressure (i.e. reduce the volume).  For a
cooling system supplement one would want to take a rather larger amount
of air and simply expel it (same volume).  And you wouldn't want to
compress warmed (post radiator) air for use in the intake as that would
drive the intake charge temperature up (not good!).

Russell Kent

 
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