Thanks Ty.........

Not sure it will solve my problem, but I need all the help I can get.

Jim

Bill,
In my experience, You do need to adjust the needle. I fly at home, 3000' altitude, typically 80 degree weather during summer, and very low humidity. I fly contests typically at about 200 ' ASL, 60 to 70 degree weather, and 50% humidity. As I have learned to understand my power system, and refiine it. I do find myself making changes to the fuel load I put in, and to the needle setting. There is more or less slugs of oxegyn in the air the engine is breathing, so it needs different levels of fuel to burn and perform the same. This is probably the simplest breakdown of what I see possible. Others can give the technical numbers if they want, I was just trying to keep it simple,,

I think it has to do with the water added to the air, the total volume of oxygen available to the engine per stroke goes down since it is fed through a restrictor, the venturi.  The same volume of air fuel mix goes into the engine, but there's water in there that doesn't burn, and with less o2, less power.

We racers see a drop in performance in Muncie compared to the West Coast.  In a lot of the classes where we either have venturi diam specs or "standard fuel", we have to richen the needle and lower the prop pitch to keep our engines from overheating.  Result, slower speeds and fewer laps/tank.  Those used to flying in the high humidity probably don't see any difference.

Still can't get you to calculate anything, eh? The more computers and fancy software we get, the less we cipher. 

I wondered about this awhile back and took four extreme conditions:

59 degrees F, 12% relative humidity
59 degrees F, 95% relative humidity
100 degrees F, 12% relative humidity
100 degrees F, 95% relative humidity

lb. of water/lb. of air for these cases:
.0105
.0015
.005
.04

Looking at my notes, it appears that I figured the mole fraction of water for each case and figured the density change and oxygen fraction change for constant temperature and pressure.  As a rough approximation and without going to any real effort, it looks like the oxygen change from Phoenix (12% relative humidity) to Houston (95% relative humidity) at 100 degrees about the same as a 1,700-ft. altitude change at standard atmospheric conditions.  The change from Eastern Washington (59 degrees F, 12% relative humidity) to Western Washington (59 degrees F, 95% relative humidity) would be about the same as a 500-ft. altitude change.

I think the change you saw in Paducah was due to temperature.  I betcha that the fraction of water in the air was constant, although I'm too lazy to look it up, let alone learn how to pronounce "psychrometric".  The density fraction of the low and high temperatures there = (459 + 76)/(459 + 87) = .98, which is about the same as 800 ft. altitude change at standard conditions, i.e. interpolating the standard atmosphere in my Pratt & Whitney Handbook, which is as much effort as I'm willing to put out.

I hope that one of you guys who worry about stuff like this will do the calculating and make a chart so I'll know what to do going between Seattle and Muncie.  I still haven't got my stuff working right since getting home.  I'm going to give the props another twist and try that tomorrow. 

Clouds are up high not because they are lighter.  They are up there because that is the condensation level, the point where the temperature drops to the dew point of the air.

But water vapor is less dense than air.  The bad news for full scale is hot, humid, and high.  Leads to needing a LOT more runway.