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The title of this section says it all. There are two ways we
can fly and climb faster, either develop more power or reduce
the airframe drag. Today's air-cooled engines require a substantial
amount of air pressure in order to
move enough volume of air over and through a large and complex
engine structure. This creates a lot of drag and that drag
adds to the overall parasitic drag of the airframe. This type
of drag is called cooling drag. In addition it is very difficult
to get the airflow over the engine to pass evenly by each of
the cylinders. The result is uneven cooling of the cylinders
that frequently results in cylinder to cylinder temperatures
varying as much as 50F from each other. This substantial variation
of cylinder temperatures makes fuel efficiency a much more difficult
problem since the hottest cylinder will typically limit how lean
the other cooler cylinders can be run. Using an air-cooled engine
means that there are few options on how to pickup cooling air
and where the heated waste air can be dumped.
A water cooled engine offers several benefits to reduce cooling
drag. The first benefit is a sleek, tight cowl since little
space is required to be maintained between the engine and the
cowling. We can now completely control the cooling airflow requirements
in a separate radiator design that is not constrained by the
engine location, envelope and surface area. Such a radiator can
be designed to provide substantially more thermal conductivity/efficiency
than would ever be possible with direct air cooling of cylinders.
A radiator can be located in more physically suitable locations
on the airframe. Air pickup can also be located in more aerodynamically
optimum locations and internal ducting offers the opportunity
to create a far more efficient air stream to the radiator. Slowing
the incoming cooling air and allowing it to expand towards the
radiator surface allows much more efficient cooling. More heat
is dissipated to a smaller air volume over a compact and lightweight
radiator surface.
Cooling air pressure = Drag
We all want to minimize the drag on our aircraft to achieve
maximum performance and their are a myriad of modifications on
the market to help reduce drag. However, the one major source
of drag that nobody can really do much about is cooling drag.
Generally, cooling air pressure drop is expressed in terms of
Inches of Water ("H2O)
- a much more sensitive scale than the inches of mercury ("Hg)
scale we use to express atmospheric pressure. The typical
4 or 6 cylinder air cooled aircraft engine typically requires
a minimum of 6-8"H2O
of air pressure in order to force sufficient air through the
engine to keep it cool. The typical GA aircraft engine installation
generates anywhere from 8-12"H2O of
pressure - well more than needed in level flight.
In short - high cooling air pressure translates directly to
cooling drag. Wind tunnel testing has shown that as much as 10%
of the mechanical horsepower generated by an air cooled aircraft
engine is lost to cooling drag - just to keep the engine cool!
This is a huge waste
of energy. What makes this worse is that in addition to the parasitic
cooling drag that is the result of the cooling air pressure drop
across the engine, there is also additional airframe drag that
is created by the turbulence in front of the engine cooling inlets
that are typically located near the top of the cowling. This
turbulence creates airflow separation over the airframe - especially
over the top and sides of the cowling and airframe which are
low pressure areas and that adds even more parasitic drag over
the entire length of the airframe.
Lower cooling drag =
better performance =
better fuel economy =
better range.
It isn't so much about the volume of air
required to cool the engine but the pressure needed to force
the airstream through the heat exchanging surface. A radiator
can be designed to present far less drag on the passing air flow
than does a complex structure such as an air cooled engine with
baffles. Reducing the air pressure needed to allow air to flow
through the radiator translates directly into an overall reduction
in cooling drag.
Better still - ducted radiator air inlets can be located
in more aerodynamically efficient locations. On our 1971 PA28-180,
we located the intake cooling air inlet on the lower part of
the cowling. The single air inlet provides cooling for both the
radiator and oil cooler as well as engine induction air. The
flow separated and slanted inlet design we used is derived from
the P51. The design minimizes airflow disturbance around the
inlet and any spillover from the inlet is directed underneath
the airframe into the high pressure region that contributes little
to the overall parasitic drag of the airframe especially when
considering that it is mixing with the already turbulent exit
air from the cowling just a few feet further back.
The result of this approach is that a far smaller air inlet
is required and that the airstream carries off
far more waste heat. A smaller volume of air with less pressure
is handling a higher heat load. It also means that the cooling
air inlet can be located and shaped in such a manner as to take
advantage of the pressure dome that will be created at the inlet.
This pressure dome
can be blended into the surrounding airframe structure to produce
a smooth aerodynamic contour. The overall result is substantially
reduced cooling
and parasitic drag.
We have found that very little research has gone into radiator
design for light GA type aircraft. At Liquid Cooled Air Power,
we have spent a substantial amount
of time and effort not only designing our CoolJugs system but
also the airframe installations as well. We have learned how
to design small, very sturdy, lightweight radiators for use in
the tight confines of light GA aircraft that provide amazingly
high thermal efficiencies with minimal cooling drag. On our 1971
PA28-180 our radiator installation creates about 3"H2O of
air pressure drop at 167 mph - about 1/3 that of the air-cooled
installation on the same aircraft. The resulting increase in
top speed performance has been an astounding 10% (from 154 mph
to 169 mph) and climb rate improvement of nearly 60% (from 900
fpm to >1500 fpm). No, our airspeed indicator isn't bent.
We verified everything on multiple test flights using a precision
calibrated airspeed indicator and cross referenced readings
with winds aloft and GPS based ground speed.
Just to
make certain we ran a race head-to-head with a 1984 Piper PA28R-200
Arrow IV! Our
1971, 180HP, fixed gear, hershey bar winged, PA28-180 out
climbed and flew faster than a 200HP, retractable gear, semi
tapered winged, 1984 PA28R-200 Arrow IV! Let us assure
you that based on precision engine dynamometer testing we verified
that the water cooled engine in our 1971 PA28-180 was producing
the same 180HP
as the original air-cooled installation! If you don't believe
that this level of performance increase is really possible
merely by converting to a water cooled engine installation
you can watch the video of
the head-to-head air race on our website.
The much more efficient radiator also means that far less air
pressure is required for cooling on the ground - eliminating
the need for supplemental boost fans in both of our installations.
In fact we have been able to idle the PA28-180 Cherokee indefinitely
with as much as a 15 knot tailwind on the ground - something
that would overheat the typical air-cooled installation! We can
run at full takeoff power, leaned to best power mixture (about
25F rich of peak) at sea level on the ground on a 95F day for
well over 10 minutes with only a 10F rise in coolant temperature!
In our LongEZ installation - a pusher configuration with major
ground cooling challenges in its original air cooled configuration
we were able to perform the same ground run tests at the Mojave
airport in California on days when the outside air temperature
was >120F!
Simply stated - there were no ground cooling problems at all
and we did not need a supplemental electric cooling fan for the
radiator.
As for flying - both the Cherokee and LongEZ were able to climb
out at maximum effort (Best Angle, minimum airspeed) on the hottest
days all the way up to 10,000 feet without any cooling problems!
Imagine how hot the cylinders would get on your current, air
cooled engine on a Best Angle climb from sea level to 10,000
feet holding Vx when
the outside air temperature is 120F!
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