Growing a Grumman - Part 8
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.--------------------------------------------------------------------------------------------------------------------------------
More pics! Click the image for larger (1024x768) view - I have provided the large size images as I know you guys want to see DETAILS!