Just what my CR-3 says it should be, in the standard atmosphere, so the ideal gas law seems to be valid to use for the lapse rate. Using your figures to calculate the ratio of T at altitude over T at SLS,Īt altitude, that'd mean R*T/V = 6877/7385, or. Let's forget those pesky non-ideal properties of the atmosphere for a bit and play with the ideal gas law.ĭensity (n) and pressure go hand in hand, unless the temperature changes. Quick-and-dirty and possibly poorly thought through thought experiment below - do not use for operating nuclear power plants, and ignore unless willing to risk subjecting yourself to gross errors - corrections welcome. I'm not going to venture a guess either way, but I'm willing to guess that the temperature difference won't be enough to matter. Now, is the atmosphere lapse rate with pressure higher or lower than what you get as you compress the air in the induction system? That determines whether the manifold air is warmer or colder than it was at a lower altitude, for the same MAP. ![]() As you raise the pressure in the induction system, the temperature will rise again. the OAT is lower as the pressure is lower. I'm looking for a detailed technical answer as to what and why, if anyone has some info on this that would be fantastic!īut. I noticed the cruise performance charts on a certain piston airplane also showed that the manifold pressure was generally lower, with an increase in altitude, to maintain the same % power. The lower pressure ratio, compared to the density ratio, I believe might indicated that you require a lower manifold pressure at altitude to create the same power. But, I do know that static pressure and atmospheric density don't decrease the same with an increase in altitude so I'm assuming there would be a slight difference. ![]() I would assume that as long as you are moving the same amount (density) of air into the cylinders, at roughly the same temp, then you should require the same manifold setting. ![]() I am wondering what happens to the required manifold pressure (and why), as you increase in altitude, if you want to maintain the same power? So, if at sea level, with 65% power set at 23"/2400RPM, and 20GPH - what do I need to set the manifold to if I wanted to keep 65% power (and 20GPH) with 2400RPM at 8,000'? Assume standard atmospheric conditions. This is not the standard question about how altitude affects manifold pressure.
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