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12-21-2016, 12:17 PM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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Quote:
Originally Posted by snopercod
This morning I did a single point calibration of my OAT sensor/display. I dipped the probe in a bath of crushed ice and water, and the Davtron indicated 32F/0C. My Fluke read 32.1F. At ambient the Davtron read 43F and the Fluke read 43.1F. So the 5C reading was probably accurate. BTW, my OAT probe is located under the wing, well away from the cowling exit air. I wish I had known about Skew-T charts last year 
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Please keep in mind that often times location of the probe matters and can change the temp reading during flight while a static reading can be accurate.
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12-21-2016, 02:10 PM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,104
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compressibility?
Quote:
Originally Posted by Bavafa
Please keep in mind that often times location of the probe matters and can change the temp reading during flight while a static reading can be accurate.
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Are you referring to "compressibility"? Most of my life I flew Cessnas and Pipers and at those speeds, compensating for mach number really wasn't necessary.  Even in my Lancair, I never even considered adjusting indicated OAT for mach number... until you brought it up. Using the formulas found at Ed Williams Aviation Formulary - if I did the math correctly - I was flying along at mach .25 on that flight !!! (Yes, my chest is definitely puffed out now  ) Because of my awesome speed  , the 5C temperature reading on my Davtron needed to be adjusted down by 3.5C! So the OAT really was close to freezing. I learned something here. Thanks!
Mach numbers, true vs calibrated airspeeds etc.
Mach Number (M) = TAS/CS
CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.
TAS is true airspeed in knots. Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately:
IAT=OAT+K*TAS^2/7592
The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots. Also:
OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15
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12-21-2016, 02:33 PM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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Quote:
Originally Posted by snopercod
Are you referring to "compressibility"? Most of my life I flew Cessnas and Pipers and at those speeds, compensating for mach number really wasn't necessary.  Even in my Lancair, I never even considered adjusting indicated OAT for mach number... until you brought it up. Using the formulas found at Ed Williams Aviation Formulary - if I did the math correctly - I was flying along at mach .25 on that flight !!! (Yes, my chest is definitely puffed out now  ) Because of my awesome speed  , the 5C temperature reading on my Davtron needed to be adjusted down by 3.5C! So the OAT really was close to freezing. I learned something here. Thanks!
Mach numbers, true vs calibrated airspeeds etc.
Mach Number (M) = TAS/CS
CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.
TAS is true airspeed in knots. Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately:
IAT=OAT+K*TAS^2/7592
The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots. Also:
OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15
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What you said. Also the physical location of the probe in relation to the passing air. Let's exaggerate and pretend the probe is installed right at the egress air passage of the cowl, I would imagine the hot air existing the cowl will have an effect on the accuracy. In RVs, many people report false reading when the probe is installed in the NACA vent.
In regards to the shade in the pix, my initial reaction was that was going to be ice but the clear/dry air rejects that idea. Also, three times that I had minor ice on my plane, it started at the wing root area but don't know enough to say if that is common or not.
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12-27-2016, 09:24 AM
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Join Date: Feb 2016
Location: CA
Posts: 10
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[quote=snopercod;1135867]the 5C temperature reading on my Davtron needed to be adjusted down by 3.5C! So the OAT really was close to freezing.
Still wondering where the moisture is coming from. Dry air doesn't usually cause icing.
I'm still going with the reflection theory. I know it isn't possible but it's too bad you don't have a better picture. If you were picking up ice in that air I would be interested in knowing why.
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12-27-2016, 01:08 PM
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Join Date: Jan 2016
Location: Southwest
Posts: 1,119
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Ice
Maybe what little moisture there is is turning to ice in the airstream casuing shadowing. But there is not enough moisture to cause the ice to stick. I bet you are making a small ice cloud with the particles so small they are following the airstream instead of impacting the wing. JMHO
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12-27-2016, 01:44 PM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,104
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must have been just a little moisture
Quote:
Originally Posted by runt
Still wondering where the moisture is coming from. Dry air doesn't usually cause icing.
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That's exactly what surprised me at the time. In retrospect, there must have been some moisture in the air as evidenced by the clouds in the background. I was flying at just about the altitude of the distant clouds, too, which tells me that the temp at my altitude must have been close to the dew point. Whether what I was seeing was ice or water, I guess I'll never know; All I can say is that the shiny gray stuff looked like it was actually coating the wing so I'm guessing it was an extremely thin layer of ice which - as someone posted earlier - immediately sublimated.
My Lancair isn't very stable; if I take my hand off the stick in cruise, I have only about 10 seconds before it wants to roll off in a spiral. I feel like I was lucky to get the photo at all, considering I was flying with my left hand and holding the iPad with my right while trying to aim the camera and tap the "button" with my thumb to snap the photo. I sure wish I had an autopilot for future situations like this  The iPad tagged the photo as taken near Mountain Home, ID, at a place called Grand View. I've been going through my cockpit videos trying to locate that segment, but haven't found it yet. If I find the video, it should show whether my airspeed dropped off at the time which would be one more piece of the puzzle.
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12-30-2016, 07:17 PM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,104
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Found the video
After going through hours and hours of shaky cockpit video, I finally found where I removed the iPad from its mount and took the photo of the mystery substance on the leading edge. For a while there, I thought I had lost my mind. First, the video is shaky because the mount that comes with the Garmin Virb is very flimsy. Since then, I've added a diagonal brace to steady the video. Second, the video was taken roughly 20 NM West of Twin Falls VOR, also known as "the middle of nowhere". I've synced the GPS data with the video as best I could, but there are no landmarks out there to allow me to do a good job of that. I didn't notice any airspeed dropoff as the "ice" built up, so maybe it wasn't ice. I had roughly a 15 KT headwind at the time, heading for Medford, OR. About an hour prior, there were thin puffy clouds at 12,500'.
Taking photo with iPad
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12-30-2016, 07:36 PM
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Join Date: Mar 2014
Location: San Marcos, CA
Posts: 415
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Clear air
I never liked it during my Power add-on training, and ground school that I was always taught that ice could not form unless visible moisture was present.
I was flying my glider near Mono Lake (California) near 18k and picked up enough ice to easily be seen. I was flying with a friend, and he iced up too. We continued to fly, but dropped in altitude until we became ice free again.
Cloudbase that day was about 22k, and their were cumulus clouds within 5-10 miles, but none where we were.
We had been up high for awhile, and my guess is that our wings were colder than the air we were in, so that might explain what occurred, but that's just a guess.
I would love to know why they teach it isn't possible, when it does in fact occur, albeit fairly rarely.
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12-31-2016, 09:11 AM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,104
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Me too
Quote:
Originally Posted by cdeerinck
I was always taught that ice could not form unless visible moisture was present.
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Me too, and also that you only need carb heat if visible moisture was present and we all know that's not necessarily true. Many years ago when I got my PPL, my instructor advised me that "This is only a ticket to learn" and boy was he right. I learned a lot over the years but flying an Experimental aircraft these last two years has really upped my learning curve. My guess is we were taught about "visible moisture" because the truth would have been much too complicated to explain. Most instruction is designed to teach the student how to fly the airplane, not how to become a meteorologist; We all pretty much have to learn that stuff on our own as we take longer and longer trips. At this late date I've been trying to teach myself Skew-T Charts, but there is so much information in those that I don't think I'll ever understand them completely. Thankfully, there are some simplified graphics that depict temperature and relative humidity at various altitudes. Those, I can understand. Here's a sample "RAP Chart". (I think RAP stands for Rapid Refresh.) Well, that which doesn't kill us...
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12-31-2016, 12:25 PM
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Join Date: Mar 2014
Location: Alabama
Posts: 127
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Here is an example on video
When the air moves over the wing and causes lift, pressure is lower, when pressure is decreased, temperature decreases. If the dew point is close, moisture will condensate.
https://youtu.be/-A0fN7PijgY
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