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  #1  
Old 03-06-2018, 08:24 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 4,511
Default OAT Location: PIREP w/data

Well, There is a saying - "striving for perfection, oft we mar what is well"

That is an appropriate statement. My OAT probe is located in the wing root cavity aft of the spar. Now in Phase I, some of the first items to be verified are the ASI speeds and OAT accuracy. A K-type thermo couple was placed well outboard on a wing and flight data recorded. It was ok, but today just happened to plot some data on Savvy Analysis. It showed a very surprising result. The OAT is, indeed, affected by the exhaust temps. The low/high range below is 1.0 deg C I don't know the mechanism for the effect, but the data is clear that it does. Having advocated this location before, any such recommendation is retracted until the cause is known. Maybe, this result is specific to the aft-of-spar location, but that will be an edit to this post if data is taken there. For now, it is another issue to be evaluated on the squawk list.


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Phase II 8-3-18

Last edited by BillL : 03-06-2018 at 05:43 PM.
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  #2  
Old 03-06-2018, 09:09 AM
KatanaPilot KatanaPilot is offline
 
Join Date: Feb 2015
Location: Locust Grove, GA
Posts: 373
Default I've stayed at a Holiday Inn Express before...

Any chance it's related to engine RPM/alternator output/bus voltage - more than any effect from exhaust that is several feet away?
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DA20-A1 Katana "Princess Amelia"
RV-7A Phase 2
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  #3  
Old 03-06-2018, 09:31 AM
BillL BillL is offline
 
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Location: Central IL
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Quote:
Originally Posted by KatanaPilot View Post
Any chance it's related to engine RPM/alternator output/bus voltage - more than any effect from exhaust that is several feet away?
It does seem odd that the exhaust has that effect, I wonder if it is from cabin heat bleed. The voltage is dead stable at 14.5 volts. The TAS is a function of density and the effect of OAT [C] on that is what . . 274/273 ratio? It would not seem that TAS will be affected much and the other reading important to OAT is icing.

I am now wondering if 1 C is enough to have any practical concern. It is not affected by the sun as tested. My TAS vs GPS testing was so darn close. (0.12 kts error)
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  #4  
Old 03-06-2018, 10:12 AM
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rleffler rleffler is offline
 
Join Date: Jun 2006
Location: Delaware, OH (KDLZ)
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You made me go look at my last flight. Unfortunately, I only have data from my right wing OAT in Savvyanalysis.com. Both OATs are mounted just inboard from the first access panel in the wing on my RV-10.

While it was only a two degree rise, my OATs went up as the EGTs went down. For all intensive purposes, OAT was pretty flat lined through out the entire flight, regardless of EGT readings.

I'm happy to share data with anyone interested. I just don't have an easy method of sharing images at the moment.
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  #5  
Old 03-06-2018, 04:29 PM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 4,511
Default More confused

In trying to let the subconscious work on the how exactly the OAT is connected to EGT, the response time is nearly zero. Like it is somehow connected.

I am just puzzled. Note the upper graph has a time offset to the lower graph.
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Phase II 8-3-18
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  #6  
Old 03-07-2018, 09:04 AM
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Snowflake Snowflake is offline
 
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Location: Victoria, BC, Canada
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Quote:
Originally Posted by rleffler View Post
...For all intensive purposes...
Eggcorn alert... "intents and purposes"
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  #7  
Old 10-16-2018, 10:56 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 3,272
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Quote:
Originally Posted by BillL View Post
Well, There is a saying - "striving for perfection, oft we mar what is well"

That is an appropriate statement. My OAT probe is located in the wing root cavity aft of the spar. Now in Phase I, some of the first items to be verified are the ASI speeds and OAT accuracy. A K-type thermo couple was placed well outboard on a wing and flight data recorded. It was ok, but today just happened to plot some data on Savvy Analysis. It showed a very surprising result. The OAT is, indeed, affected by the exhaust temps. The low/high range below is 1.0 deg C I don't know the mechanism for the effect, but the data is clear that it does. Having advocated this location before, any such recommendation is retracted until the cause is known. Maybe, this result is specific to the aft-of-spar location, but that will be an edit to this post if data is taken there. For now, it is another issue to be evaluated on the squawk list.


Bill,

I am considering the wing root area for the OAT probe on my 10 build. I am curious what differences you saw between thermocouple and the OAT probe temps.

I am not too concerned about the EGT data you posted here. The relationship shown between EGT and OAT is literally immediate or directly related and therefore somewhat impluasible. Given that direct exhaust flow can't make it to the wing root at cruise speeds, it seems quite unlikely that a 5% increase in EGT could substantially increase the engine compartment air temp enough to create an immediate increase in temp from a nearby probe. I would suspect that if it is possible, it would take quite a bit more than a second or two for it to register.

I would consider something electrical in the box is causing the difference that is related to the K type thermocouples or possibly something else that was changing at the same time.

Larry
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Last edited by lr172 : 10-16-2018 at 11:01 PM.
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