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Growing a
Grumman - Part 8
2009-09-01
Believe it or
not the majority of the time since moving the aircraft to the airfield
in Oct 2008 has been consumed in trying to get the darned engine
to cool - and battling with problems with the wheels and brakes!
Once the decision
to use a belly-mounted radiator has been made then we needed to
figure how it should be mounted. To this end I engaged in quite
a study of similar installations, starting with the obvious one
- the famous P51 of WW2 - and I was quite amazed to find that apparently
some of the information on cooling of large reciprocating engines
from that era is still classified!
Nonetheless
I found a great deal of info - much of it relating to P51s used
in air racing. Also info on such radiators in various home-builts,
the most valuable of which was from a Zenith builder's website,
which in turn led me to another site of a guy building a Zenith
in the US - I have basically copied his set-up which had the benefit
of being simple to fabricate and obviously sturdy - don't want the
thing flying off the aircraft at the amazing speeds my Grumman should
be capable of !
According to
the literature the secret appears to be "divergence - convergence"
within the shape of the scoop - divergence leading the cold incoming
air to the front of the rad to slow and pressurise the air and convergence
to aid in extracting the heated air from the radiator fins. Almost
the opposite of what one might expect!
However the
structure of the underbelly box has none of that shape - it is a
straight sided box at present, with the air passing through the
rad mounted at a 30 deg angle nose-down to the front of the aircraft
and exiting via a cut-out in the lower panel - no airflow aids at
all but it works! Of course my intention is to build the divergence
into the inlet scoop shape once we get around to making this of
fibreglass - but really it looks as if this will be mostly for cosmetic
effect, although if this does improve cooling more than we have
so far achieved, then the motor may in fact run too cool - we will
see. Making a sleek inlet scoop and fairing in the rear of the box
will help drag however. Right now the drag must be horrendous from
the underslung "bucket"!
Before mounting
the rad in the box however some mods were needed - the top tank
of the rad with the filler cap was cut off, since the filler will
be part of the firewall-mounted header tank - lo and behold on inspecting
the innards of the rad it was obvious that the design was not the
best - there was no internal baffling to direct the water-flow correctly
so this was added to create a double-pass radiator which is supposedly
more efficient. Then a flat alloy plate was welded to the open top
of the rad. Also the inlet and outlet pipes had to be relocated
and this was done.
Once the alclad
box had been built and riveted to the belly, it was time to figure
out the pipes to run the coolant around the entire system - the
pipes I used are 1.5 in ID aluminium and add very little weight
or drag. They are supported by brackets to the belly of the aircraft.
All hoses are standard automotive hoses of decent quality using
ordinary hose-clamps. It would be great to change all of this for
hoses with AN fittings and some say this is the way to go with an
aircraft installation - I am not sure that this is true, given the
additional mods needed to the entire system, but I will agree that
we will double up all the clamps at least - and keep a careful eye
open for any signs of leakage. To that end I need to locate a water-pressure
gauge as this would be a good indicator of any coolant-loss which
would quickly show up as falling pressure in the cooling system.
Let the hunt for that item begin soon!
One problem
which reared it's head was air entrapment - exacerbated by the fact
that the rad is lower than the engine. Any air at all in the cooling
system means that no cooling can take place as circulation of the
coolant is prevented by the air bubble. A 6mm ID return pipe to
the header tank from the top of the thermostat housing, which is
the highest point in the water-reticulation system has solved that
problem along with careful, slow filling. I think another return
pipe from the highest point of the belly-mounted rad would also
help to make the system self-bleeding and foolproof.
The good news
- once the system is filled with water and all air has been evacuated,
the motor cools beautifully! The video below was taken during the
final static tests, where the motor ran for 40 minutes with no sign
of overheating at all!
Since that time
I have taken the aircraft out for taxi-runs - taxi out to the runway
threshold - power up to about 4000rpm - high speed run for about
600 metres - power down, slow down and repeat - showing no sign
of excessive coolant temp. Once we get the brakes to actually retard
the aircraft with any force we can try FULL power.
I think we have
the cooling monster licked!
Next we will
move on to getting the brakes to work properly.
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More pics -
stills taken from the video! Video below the photographs
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