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Moisture Effects

Wesael

Well Known Member
On a trip not long ago I took some screenshots showing the the TAS speed change while flying through high moisture air (clouds) and dry air (clear).

The first one is after about 30min of flight level in smooth IMC conditions.

screenshot-20121223-163138-168.jpg


This one is about 20 min latter after flying in clear air and drying the airframe off.

screenshot-20121223-165403-429.jpg


There are some subtle differences in, DA, MAP, CHT, EGT, FF, etc. but from the time the first shot was taken and the second shot there was no change in the power, prop or mixture settings.

Note the True Air Speed difference.:eek:
 
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seems that the clouds act like brakes

Your indicated airspeed is reduced in the clouds. It seems that the clouds act like brakes however I don't recall this happening in a Cessna at 110 kts. OK, I'll bite, why is this?
 
True airspeed

The true airspeed calculation requires an OAT input, which I don't see in your screenshot. TAS will vary with OAT.
 
OAT is displayed in the lower right hand corner.

I noticed several suttle changes in several data points besides the OAT, wind direction and velocity, altitude, fuel burn rate, etc. are these changes the result of atmosphic changes or something induced by performance or pilot changes?
 
IAS and TAS increased 8.5%. GS increased 11.5%. You flew straight and level in a slight downdraft and entered calm air or slight updraft and a tailwind. With the same power setting you will go faster. Rain will also slow you down.
 
Look at the pitch bars.

I think it's just a case of rising or falling air mass.
 
I guess I'm missing something, but it looks to me like the nose is below the horizon in the second picture. Doesn't take much of a "dive" to increase airspeed.
 
The nose is pointing down more however there is no indication of negative vertical speed? As speed increases the nose lowers, is this the case here? Not sure at these speeds if there would be a huge pitch change though.
 
Unrelated to this topic but man all that stuff on the left makes my head hurt. You'd think that some of the EFIS manufacturers could use UI-design inspired by an iPhone.
 
...there is no indication of negative vertical speed?

Not always. For example, I was flying a couple of days ago in some strong mountain waves on the east coast (and a quartering headwind gale :(). There were multiple occations where the autopilot pitched up and slowed the craft to around 120KIAS just to keep the VSI on zero. As you fellows know, this speed should have equated to better than 1000 FPM climb.
 
The VSI on the top shows an ever so small climb and the VSI on the bottom shows a very small descent. Looks like a moving airmass to me.
Why are both tanks full at 24+ in both pictures?
 
I don't see much of a difference from an airspeed perspective. If you take your trusty E6B and use IAS and Dens Alt to convert to TAS you get a number in the ballpark of the TAS shown on each of the screens. In this case the TAS calculated and the TAS shown are different by 4 to 5 knots which is probably position error or some other errors (e.g. OAT) built up in the calculations of the EFIS.
 
I think there are some exceptions (advection fog?), but a meteorologist I know made a flat declaration this year at a weather spotter meeting that all clouds are formed by rising air. Even if it is just "almost" always the case, that is reason enough for higher speed in clouds.
 
Except, it was slower in the clouds. If the OAT was colder I would have guessed ice. But I'll go with the clear air updraft theory for now. Certainly is a difference in nose attitude.
 
I think what has happened here is that you have identified one aspect of being in a cloud (high moisture content) and assumed it is the significant cause of the performance difference.

As others have noted, the velocity vector is on the horizon and the vsi is reading zero. There is a difference in attitude, with the lower nose attitude associated with higher airspeed. This would indicate that the aircraft is moving through an air mass that is rising. Both wave action and convection can be associated with cloud formation and both of these effects would be consistent with the performance difference noted.
 
Some good discussion here! Great to hear your ideas.


I think like some have said there is some variation in attitude due to atmospheric conditions that is changing TAS but will take some more testing to confirm.


however, I have noticed a drop in air speed almost every time I have been IMC while this particular flight was (probably) influenced somewhat by the atmospheric conditions, I still am seeing a drop in airspeed when in the clouds accross the board.


Does anyone else that flys IMC out there see this?

Another thing that happens when the air is saturated with water is there is work lost in the engine due to it having to evaporate the moisture. I have no idea how much intake air humidity has to do with HP output.
 
Unrelated to this topic but man all that stuff on the left makes my head hurt. You'd think that some of the EFIS manufacturers could use UI-design inspired by an iPhone.

Sorry Bob, That is due to me trying to cram all that I like to monitor into the 20% engine monitor section of the screen.

The Skyview engine monitor section is fully costomizable to what ever you like with may widgets and size/arrangement to your liking. my bad :rolleyes:
 
Another thing that happens when the air is saturated with water is there is work lost in the engine due to it having to evaporate the moisture. I have no idea how much intake air humidity has to do with HP output.

Ahhhhh....power loss. Hadn't thought of that. Good point. Water vapor changes inlet air density, fuel/air ratio, and thermal efficiency.

The equations for each are found in Taylor's "Internal Combustion.."
 
Ahhhhh....power loss. Hadn't thought of that. Good point. Water vapor changes inlet air density, fuel/air ratio, and thermal efficiency.

The equations for each are found in Taylor's "Internal Combustion.."

The tables/equations for "air density" that engine tuners use should do it....

Here's a calculator:

https://wahiduddin.net/calc/calc_hp_dp.htm

From memory, and playing with the calculator, it's a pretty weak relationship b/w humidity/dew point and "air density" or power output. Of course a cloud is >100% RH so there are suspended droplets, which would cause way more havoc than vapor.
 
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