Mixture setting on a hot day
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Mixture setting on a hot day
Hi guys, I have a question which has left me a little confused.
Q: On a hot day climbing IAS and mixture setting at take-off should be:
A-Faster IAS, Mixture Rich
B-Faster IAS, Mixture Lean
C,D - Slower IAS, Mixture Rich/Lean... Wrong
I said B because obviously you use a faster IAS due to improved cooling, and a lean mixture, because you have a lower density, hence needing to reduce mixture to precent flooding and restore the correct fuel air ratio.
Problem is the quoted answer is A and i cant quite work it out
Q: On a hot day climbing IAS and mixture setting at take-off should be:
A-Faster IAS, Mixture Rich
B-Faster IAS, Mixture Lean
C,D - Slower IAS, Mixture Rich/Lean... Wrong
I said B because obviously you use a faster IAS due to improved cooling, and a lean mixture, because you have a lower density, hence needing to reduce mixture to precent flooding and restore the correct fuel air ratio.
Problem is the quoted answer is A and i cant quite work it out
Halfbaked, you've never had the pleasure of flying an overloaded, poorly cowled piston A/C on a hot day, have you? It's great fun trying to clear the mountain without frying the engines. 
Student, I think the question and answers are poorly written; ie, how rich or lean are they talking about? I think they are assuming that the "standard" mixture setting is that for best power. Ergo, if the air flow isn't enough to keep the engine cool, then you can increase the mixture a bit and the extra fuel will provide some cooling. Your thought process re fuel required per air density is correct; even running a little rich for extra cooling, you will still not be anywhere near the fuel flow required for the same throttle setting on a cold day. If you set up your fuel system on a cold day, and then try to fly on a very hot day, your engine might not run at all with the mixture full rich, even at full throttle.
Student, I think the question and answers are poorly written; ie, how rich or lean are they talking about? I think they are assuming that the "standard" mixture setting is that for best power. Ergo, if the air flow isn't enough to keep the engine cool, then you can increase the mixture a bit and the extra fuel will provide some cooling. Your thought process re fuel required per air density is correct; even running a little rich for extra cooling, you will still not be anywhere near the fuel flow required for the same throttle setting on a cold day. If you set up your fuel system on a cold day, and then try to fly on a very hot day, your engine might not run at all with the mixture full rich, even at full throttle.
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Halfbaked, you've never had the pleasure of flying an overloaded, poorly cowled piston A/C on a hot day, have you? It's great fun trying to clear the mountain without frying the engines.
Years ago, I flew DC-6B aircraft from KLAS, in the summertime, average temp 45C.
Even though the engine cowls were of the pressurised NACA design, slightly faster climb speeds needed to be used, in order to keep cylinder and oil temperatures in check....not to mention the passenger cabin, with its assiciated ACM cooling (or lack thereof...
).
