Jan and I42 each hold one edge of a two-edged sword
- water vapor in the fuel/air mix displaces some oxygen and fuel, which reduces burnable fuel and thus power. However
- water vapor in the mix acts as a coolant, both to the charge itself, and to the cylinders. That has the same effect as a cooler day (denser mixture charge). It also evaporates to steam, increasing the cylinder pressure on the power stroke, or adding "reaction mass" to a jet's exhaust.
(The actual "mix" injected is usually about 50/50 water and alcohol, with a trace of oil. The alcohol burns, and acts as an antifreeze; the oil limits water corrosion (rust) inside the engine.)
Which effect predominates depends on the mode of operation: idle, low power, cruise, TOGA, "full military power." And how dense the water load is (ambient humidity being far less than the amounts injected intentionally).
In a civvy GA engine, I venture the cooling and lower power would make the engine "purr" more sweetly, even though technically it is producing less power. Definitely, if the engine is a bit mis-tuned, and closer to detonation conditions (spark timing, overall temp, or running on totally unleaded auto gas (STC)), water vapor/humidity will act as an "anti-knock" agent.
EDIT - in re post #4: Ahhh! Humid air is less dense than dry air (Molar weight of water lower than molar weight of "dry" O2/N2/trace elements mixture). All other things being equal (temp, altitude). Lower density = lower drag.
Last edited by pattern_is_full; 14th Jun 2016 at 22:18.