Density varies with:
Water vapour is important because it is invisible, and affects the humidity ratio of the air. Adding water vapour reduces air density because its molecular weight is lower. So, as humidity increases, the air density reduces. This causes the thrust and therefore the output power or performance to decrease at all phases of flight.
An increase in humidity reduces the burn rate and increases the combustion duration. This increase in combustion duration has the same effect as retarding the time of ignition. As a result of this the thermal efficiency is reduced. The chance of the engine misfiring increases as humidity increases.
1. A turbine engine because of the increased humidity will spin faster to draw more air in the combustion section. Thus, the engine will run hotter, but turbine engines are designed to cope with these temperarures.
Thus, the effect of humidity on a turbine is not great, especially as only 20% of the air going through it is used to generate power, but it should still be taken into account, particularly in the wake of a heavy shower.
2. On a piston engine, the apparent change in the time of ignition is uncompensated and, therefore, has a more harmful effect in performance.
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