Originally Posted by
Smythe
Ummm,, what?!?! You need High temps to have high specific humidity??
The standard is BASED on 80% to 34% rel humidity. Looking at the calculations, the variable in the calc for humidity is multiplied by .0621, and the humidity ranges between .0008 and .007. Therefore .0621 * .0008 to .007 is meaningless as an additive to the performance calculation. That is why is is negligible, and not added to any performance software, even at any altitude.
Temperature governs, plain and simple by a vast margin.
YES!!! You need high temperatures to have high specific humidity. Basic physics. Look at the chart Megan posted - specific humidity is all that matters since it reduces the air density - and
isn't accounted for by temperature. At ~90+% relative humidity and ~100 deg F, you've reduced the air density relative to dry air by ~1% - which is ~1% thrust at N1 (as noted, effect on EPR is significantly less).
You don't believe 1% thrust is meaningful?
PS Thanks megan