Humidity

[Michael Baxter]

1st and intro: My name is Michael Baxter and I'm new to the list. I'm building
an EFI AMC 401 and you guys/gals will be hearing from me in the future on this
project :-). I'm headed out on a trip on Thur. so, I'll ask all my questions
after I return next Wed.

 2nd, Humidity: I'm a pilot and during flying career I've been taught that
humid air hampers performance compared to dry air under the otherwise same
atmospheric conditions. I was taught this in ground school when I was flying
piston engine airplanes all the way up to ground school for the B-727. The
engine performance charts in the manuals for all airplanes have some
correction factor for high humidity and it is always detrimental.

 What I've been taught goes like this: Air in our atmosphere is made-up of 78%
nitrogen, 21% oxygen and 1% other gases. That mixture stays pretty much the
same all the way to space. It's a common mis-conception that oxygen content
decreases with an increase in elevation. It's not true. The oxygen molecules
are just spread out more. As you go up, there is less and less of our
atmosphere on top of you pushing down. Less weight and hence less pressure
pushing the molecules together. The air is less dense and the molecules are
more spread-out.

 You have two sealed boxes/parcels of air each a cubic foot. One is totally
dry and one is just short of total saturation. Both are at the same temp. and
pressure. In other words, the density of the molecules in each box is the
same.

 All the molecules of water in the near saturated parcel are displacing
molecules of air. If you remove all of the water molecules from the saturated
box/parcel, you are left with less air molecules than you have in the
box/parcel with dry air inside. The air molecules in the previously saturated
box will spread-out and become less dense. Less dense air equals less engine
performance whether it's a internal combustion engine or a turbine engine with
7 stages of compression.

 Of course, all my piston engine aircraft experience has been with air cooled
engines. I always noticed that my engines ran cooler on humid days. Some of
this may be attributable to decreased power output because of less dense air
but, I believe the humid air cooled the engine better. If that is indeed the
case, I'm sure I gained some lost power back due to a cooler more dense
fuel/air mix. reaching the cylinders.

 -- Michael Baxter, 74172.1164 at Compuserve.com
 http://ourworld.compuserve.com/homepages/MBaxter
 From Reno, NV USA on 23-Sep-1997 at 23:51:17 PDT

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