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Regarding the cooling section of the spreadsheet.
Calculations do compensate for the effects of less air
density at altitude which for the same airspeed results in less air mass
cooling flow at altitude compared to sea level. However, changes in Air
Temperature at altitude are not automatically compensated for - you need to
ensure that your engine intake air temperature is adjusted for altitude - if you
are that into it.
The cooling section also includes a factor that adjusts
cooling flow rate based on engine rpm - higher engine rpm = higher coolant flow
rate.
Concept:
Basically the cooling section uses air mass flow and
cooling mass flow to calculate heat transfer. Air mass flow primarily
dependent on:
1. Airflow velocity in duct for climb
configuration = 0.3*airspeed
2. Air Mass flow which factors air density, frontal area
of heat exchangers and the velocity of the air in the duct
3. Air density compensation for altitude and
temperature
4. Specific heat of air = 0.25 cp
Cooling mass flow is dependent on:
1. A 0.7 cp compared to pure water of 1.0 Cp -
this compensates for the typical anitfreeze dilution of the specific heat of
water.
2. Mass flow of coolant dependent primarily on
engine/pump rpm
There are some other factors which get a bit esoteric, but
play a lesser role. So the results are a bit conservative - for example if
you don't use antifreeze then theoretically you would pick up a 30% improvement
in cooling capacity due to the higher specific heat value for pure
water.
So these factors are used to calculate how many BTU/min
the cooling system can reject for the selected operational values. Then
this is compared to the BTU of waste heat the engine needs to reject based on
the power it is producing. The radiator and oil cooler can compensate to
some degree for a deficiency in capacity in each. So if the oil cooler for
example is showing overheating by 1% and the radiator shows a + 10% excess
cooling capacity - it will reject some of the heat not being rejected by the
oil. So as long as the overall capacity is adequate you are probably
OK.
Again, this is just a 1st order SWAG at cooling and its
has only been compared to tractor installations. It fairly accurately
predicts my cooling capacity at Sea level and altitude.
So if your cooling results were adequate for the power
setting/temperature at sea level and all you do in the cooling spreadsheet is
change the altitude, your cooling may no longer be adequate due to the less air
mass flow at altitude due to the lessen air density. But, since you will
probably be traveling at a higher airspeed which would increase your air mass
flow, the two factors in real life would tend to offset each other to some
degree. If you also increase your airspeed (as would probably happen at
cruise) along with the altitude increase then you would likely find you cooling
is back to adequate.
But, as stated before the cooling section is on less solid
ground due to the fact that installation variables play such a large role in
cooling and they are not taken into consideration. But, you can play with
it to see the effects of changing several variables on cooling.
Ed
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