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OAT Location

Flybuddy2

Flybuddy2

Well Known Member
Currently have my Dynon OAT probe in my NACA vent and getting a higher temp reading than actual there. Looking to move it onto the left wing inspection plate just behind the pitot tube. I've got a stock Van's ss pitot tube. Even though the OAT is wider than the pitot will it suffer any inaccuracies mounted there due to the turbulent airflow from the stem of the pitot. If so, how far should I offset it?? thx Tom
 
You might try mounting it to the bottom wing fairing cover. That get's it out of the exhaust and/or cowl airflow. No need to go all the way out into the wing. If you do put it out there, I don't think the airflow around the pitot will bother it.
 
f1rocket said:
You might try mounting it to the bottom wing fairing cover. That get's it out of the exhaust and/or cowl airflow. No need to go all the way out into the wing. If you do put it out there, I don't think the airflow around the pitot will bother it.
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Is yours there?? if so, does it read accurately? Seems like it might get some limited exhaust flow there. Heck of a lot easier to mount it there though. Pierre did a super job on grommets and sealing and I'm having a tough time routing wires through the wing. Ground a small hole in the pvc tube that houses the strobe/nav/landing light wires and fishing 041 wire through there but it keeps hitting something before it makes it to the fuselage. Sounds like the bottom wing fairing is worth looking at.
 
Info

When i did my research on this, I was led to believe that mounting it on the back emp deck of the fues, under the emp fairings was as good a location as any. It is out of the weather, but also out of turbulent air, exhaust etc.

Made sense to me at the time. I dont think you want it in the "wind" do you?

At the back there should be no ill effects from cabin heat etc that would effect it. Further, for the skyview it now plugs into the back side of the adahrs so the wire run is very simple in that location.
 
Mounted ours front right of the lower fuselage - bottom skin by the fuel vent.

Outside where the exhaust can't touch it - easy to wire in, haven't flown yet but got that position from the forums ages ago.
 
That is where I have mine. It works in the air when there is a bunch of air movement around in there. On the ground, it will read high if the Sun is beaming on the airplane.

It is a little slower to respond to changes in that location but it works good enough.

I am not sure there is a perfect place for these things. Also, I don't think many if any of the affordable EFIS makers dynamically adjust for all the variables that can impact a OAT probe.....

RickWoodall said:
When i did my research on this, I was led to believe that mounting it on the back emp deck of the fues, under the emp fairings was as good a location as any. It is out of the weather, but also out of turbulent air, exhaust etc.

Made sense to me at the time. I dont think you want it in the "wind" do you?

At the back there should be no ill effects from cabin heat etc that would effect it. Further, for the skyview it now plugs into the back side of the adahrs so the wire run is very simple in that location.
Click to expand...
 
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In the cabin NACA duct, it gets air warmed by the engine that flows out of the inlets and down the side of the fuselage and into the cabin. That is why it indicates a higher temperature. On one airplane I tested it hardly dropped in going from 2000' to 10,000'. It is subject to stagnation heating if out in the airstream, which is 7.2F maximum at 200 mph. If you mount it in the tailcone just above and behind the elevator spar cutout you will get reasonably fast response and true OAT.
 
Under wing inspection panel - yes! But why behind the pitot???

The OAT probe should ideally be in the air stream in clean flow, away from the prop wash and away from structures that may otherwise affect the local air flow, out of direct sunlight, and away from any other sources of heat (engine, exhaust, cabin).

Underneath the wing is probably about as good as it gets. But why choose a spot right behind the pitot? There are plenty of other convenient spots (other under wing inspection panels) away from the pitot.
 
I had originally mounted mine in the NACA vent, only because I had recalled seeing others mounted there. Then I read the installation instructions (Duh).

"It is important that the OAT probe be mounted somewhere on the skin of the aircraft where it will not be affected by heat sources (sun, engine, aircraft interior, etc). It is acceptable to extend the length of the included wiring for the OAT. The ideal location would receive no heat from the aircraft engine or any other source in the aircraft body. While this may be impractical, it is a good idea to mount the probe as far away from heat sources as possible. On the RV series, common locations include the wingtip and under the horizontal stabilizer." (Dynon D-100 installation manual, Appendix B)

I remounted mine beneath the horizontal stab. I've had no troubles with it there.

Hope that helps.
 
Flybuddy2 said:
Is yours there?? if so, does it read accurately? Seems like it might get some limited exhaust flow there. Heck of a lot easier to mount it there though. Pierre did a super job on grommets and sealing and I'm having a tough time routing wires through the wing. Ground a small hole in the pvc tube that houses the strobe/nav/landing light wires and fishing 041 wire through there but it keeps hitting something before it makes it to the fuselage. Sounds like the bottom wing fairing is worth looking at.
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Yep, and it reads fine. I mounted mine forward of the spar to make the wiring easier. The oil pattern from the exhaust indicates that the exhaust flow doesn't (can't) get that far forward on the bottom of the wing.
 
Yeah, I don't know about anywhere along the fuselage. It seems to me that you're going to pick up heat from the engine just about any place down the fuselage, but it may be less noticable the further back you go.

Either way, I put my OAT in the NACA duct and constantly read higher than ambient. Anywhere from 5 degrees all the way up to 12. It depended a lot on whether it was hot outside, I was in cruise, the sun was on that side of my airplane, etc, etc.

So, I moved it to the middle inspection panel under the wing. Now it's spot on. I personally wouldn't even bother moving it from one location (NACA duct) to another (along or close to the fuselage) if there's potential for inaccurate measurements. If you want accuracy, move it out of the engine/exhaust stream and put it under the aircraft to keep it out of the sun. Unless you have a white paint job, the color will absorb SOME heat from the sun and throw off your temp (ask me how I know!).

;)
 
OAT location

I installed mine just ahead of the inspection plate in the left wing. I talked to the Dynon reps at Sloshkosh this year, and bought the probe.
They said that the bottom of the wing would offer best results, as there is no sun, no exaust heat, and less chance of it getting smacked.

So far, I have enjoyed the results........and they seem to be accurate.
Nice addition to a great EFIS unit!!!
 
mine is inside the wing root fairing on the just above the trailing edge, out of the exhaust, and the sunlight... I think it is fairly accurate but who knows.....

Doug Rozendaal
F1 EVO
 
B25Flyer said:
mine is inside the wing root fairing on the just above the trailing edge, out of the exhaust, and the sunlight... I think it is fairly accurate but who knows.....

Doug Rozendaal
F1 EVO
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If you really want to know if it's accurate and not picking up stagnation heating, which it is if it's in the freestream, then do this. Climb to some altitude, and fly the plane as slowly as comfortable, and see what the OAT reads after it has had a chance to stabilize. Now bring the plane up to cruise speed and again allow the OAT to stabilize and see if the OAT rises.The maximum rise is (TAS mph)^2 / 5520, or (TAS kt)^2 / 4167. Those who fly high speed jets understand the concept of stagnation rise and total temperature.
 
On the RV-8, we put it just inside the NACA inlet under the right wing that feeds the rear seat vent. It works great there.
 
Mine is also mounted under the H-Stab...seems pretty accurate.

However, I may just have to go out and do Paul's recommended slow-to-cruise check, to see how much, if any Stagnation/Ram Rise I'm getting.

In my GPS speed tests, I've noted that my Dynon TAS indicator reads 2-5 knots fast during high speed test runs (for speed mods). So here's a question for ya Paul:

If the max rise at 210 KTAS is about 10.58 degrees (if I did the math right), and in doing your test I see some or all of that 10 deg rise in my OAT, to me that means that some of my airspeed error may be due to T rise at the probe (higher T makes EFIS think I'm in less dense air, TAS reads high)...sound right so far? Can you relate the T rise to an increase in TAS indication (X knots TAS rise per X degrees stag/ram rise)?

Why do I ask? Just part of my mission to watch my TAS in races, so I watch out for Vne/flutter issues. I've considered tweaking my static port to see if I can narrow the delta, but 2-5 knots is pretty acceptable, and if some of it is due to Stagnation Rise, then static port futzing is probably not needed. I'll have to run the test and see if the rise is linear with speed, or not so. Now, just gotta find some thermo-static air over the desert to test in (good luck there, eh!)

Not sure how or if I'll use the resulting data...but its always fun to see Paul write formulae that I have little to no chance of comprehending...and I got an A in aerodynamics! :eek:

Thoughts?

Cheers,
Bob
 
Numerical compensation for ram air heating

The EFIS could, in theory, take the raw pitot / static and temp sensor measurement data and numerically compensate for the ram air effect when computing the OAT and TAS. Wouldn't be difficult, but I don't know if they actually do this, or if they just use the raw temp reading from the sensor. That'd be a good question for Dynon.
 
Flybuddy2 said:
Currently have my Dynon OAT probe in my NACA vent and getting a higher temp reading than actual there. Looking to move it onto the left wing inspection plate just behind the pitot tube. I've got a stock Van's ss pitot tube. Even though the OAT is wider than the pitot will it suffer any inaccuracies mounted there due to the turbulent airflow from the stem of the pitot. If so, how far should I offset it?? thx Tom
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Mine is located in the NACA duct and works now. It is commonly believed that engine heat effects it but I didn't believe that. I tested the sensor and found heat on the brass mounting side affected the reading. So if the cabin is warmer than outside air you get an average between the two. I insulated the inside mount with RTV and insulation material and now mine works. I have the same probe in the FAB measuring intake air just past the Carb heat flap. It had the same problem where the mount was heated by engine cooling air. I insulated it and it works now as well.
 
Well, Bob, the density changes about 1% for each 5.2F change in temp, and the IAS to TAS changes with the square-root of the density, so that would mean that the TAS would increase about 1% with a temperature increase of 10.4F. That's the same as the power change with temp, too! Typically, according to Ron Mowrer of RMI, the external probe recovers about 80% of the stagnation rise. And since the stagnation rise is proportional to V-squared, the temp will rise 2% for a 1% speed increase. The Reno guys going 400 mph have to deal with 27F rise. BTW, I'm really stoked. Tom Aberle called me yesterday and said his biplane Phantom qualified at 260.8 mph at Reno with the four-blade I designed. Of course for the thinner-fewer-longer bunch of propeller experts, that means that if he increased the diameter from 54" to 72" decreased the thickness from 15% to 6%, and dropped the number of blades from four to two he would probably have hit Mach 1!;)
 
Interesting Paul, thanks! So if I do have the full Ram Rise of 10 deg at 210 KTAS, then the TAS rise from that would be 2 knots, right?...that's close to what I measure the error to be with my GPS. Maybe a little T rise error, maybe a little static port error. Will have to play a little to see what I see.

I just talked with Tom's mechanic day before yesterday, and we were looking at your prop, and discussing his updraft cooling (air goes in the bottom of the cylinders, and comes out the top, and runs down the sides of the canopy). Really interesting! Then again, everything on that airplane is interesting! Your prop gets a LOT of attention here!

Good week to skulk around the Sport Class airplanes and look at speed mods!

Cheers,
Bob
 
Hey, Roberto, I'll be there tomorrow afternoon. Also take a look at Jeff Lo's four-blade prop on "Miss Gianna", and the prop tip LE mods I designed for Bryson's "Son of Galloping Goat" and Andre's "Zipper" biplanes.
 
Well, this thread got me thinking if mine are reading accurately since I had never tested them with the exception that they seemed to be reading reasonable. I have two and they are installed in the NACA vent on the plastic part very close to where the air will enter the duct work and both are within one degree of each other at most. Today I went for test, I got the temp on the ground which matched the AWOS and went for a flight. I tested them flying slow, fast and in between and all seemed to be right on the target varying by one degree at the most. This was also confirmed against another unit in a different plane.

Hope this is of any value to you.
 
Bavafa said:
Well, this thread got me thinking if mine are reading accurately since I had never tested them with the exception that they seemed to be reading reasonable. I have two and they are installed in the NACA vent on the plastic part very close to where the air will enter the duct work and both are within one degree of each other at most. Today I went for test, I got the temp on the ground which matched the AWOS and went for a flight. I tested them flying slow, fast and in between and all seemed to be right on the target varying by one degree at the most. This was also confirmed against another unit in a different plane.

Hope this is of any value to you.
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Yep, this is where I had mine, too. I put insulating foam on the back to make sure it wasn't picking up heat from inside the cockpit. I still had high OATs. I'm guessing that most of my problem was that the area is under a black stripe of paint, and it holds the heat very well. BUT, I had inaccurate temps even before I painted the airplane. If yours is white, then you probably won't have too much of a problem (still may get some heat from the engine, though).
 
lostpilot28 said:
Yep, this is where I had mine, too. I put insulating foam on the back to make sure it wasn't picking up heat from inside the cockpit. I still had high OATs. I'm guessing that most of my problem was that the area is under a black stripe of paint, and it holds the heat very well. BUT, I had inaccurate temps even before I painted the airplane. If yours is white, then you probably won't have too much of a problem (still may get some heat from the engine, though).
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Mine is light blue color but can not see who it could be affected by the engine heat? I will keep an eye on it but yesterday test proved accuracy of within 1 degree which is plenty accurate for me.
 
Bavafa said:
I tested them flying slow, fast and in between and all seemed to be right on the target varying by one degree at the most. This was also confirmed against another unit in a different plane.

Hope this is of any value to you.
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How do you verify the temp is reading accurate in flight? I see three ways.
- Compare against another flying plane that has been varified to read accurately. Maybe pretty accurate method but need to prove other aircraft OAT is accurate.
- Launch a weather ballon just before flight to get accurate temps. This is what the big companies do before flight test. This is probably problematic as we do not all have extra ballons in our hanager to launch.
- Use the winds aloft forcast. This is usually old data (and maybe even just predicted data) and do not know exactly where the data was taken from.

Those that claim their OAT is accurate, how did you prove that?
 
plehrke said:
How do you verify the temp is reading accurate in flight? I see three ways.
- Compare against another flying plane that has been varified to read accurately. Maybe pretty accurate method but need to prove other aircraft OAT is accurate.
- Launch a weather ballon just before flight to get accurate temps. This is what the big companies do before flight test. This is probably problematic as we do not all have extra ballons in our hanager to launch.
- Use the winds aloft forcast. This is usually old data (and maybe even just predicted data) and do not know exactly where the data was taken from.

Those that claim their OAT is accurate, how did you prove that?
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Funny, I just moved mine and first flight was today. Flew formation with a Mustang II and I did exactly as you suggested. I asked what his OAT read. But then, I thought, how do I know his is right? Anyhow I think the answer is that's it a lot easier to tell when they are NOT accurate. On the way back I checked local temp on ground, checked against that and did the usual adiabatic lapse rate to get a ballpark on where it should be at altitude.
 
The OATs mounted in the NACA duct are always 5-7 degrees hotter than mine....

I learned not to place it there, thanks to these forums.. :)

L.Adamson --- RV6A
 
Flybuddy2 said:
Funny, I just moved mine and first flight was today. Flew formation with a Mustang II and I did exactly as you suggested. I asked what his OAT read. But then, I thought, how do I know his is right? Anyhow I think the answer is that's it a lot easier to tell when they are NOT accurate. On the way back I checked local temp on ground, checked against that and did the usual adiabatic lapse rate to get a ballpark on where it should be at altitude.
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I just moved mine as well. After 400 hours of it on the RH side of the aircraft 1 ft behind cowl and the temp being off by 20-30 degrees, I have made the effort to get it right. I now have it under the RH horizontal tail. Seems pretty accurate as I only get 2 degree difference with throttle (or maybe speed change based on the discussion in this thread) but the numbers are 4-5 degrees above the winds aloft forcast temps.
This picture has 3 seperate plots with OAT being the same on each. Temp on the ground (engine off) reads right on. I am just thinking I need something better in the air to compare against (not the winds aloft forecast) before I go moving it or recalibrating the sensor.
RV-6A%252520performance%252520charts.jpg

Also notice the slight temperture inversion layer just before 15:50Z during the decent between 5100 ft and 4400 ft. Same thing on the climb (but off the LH side of the plot).
 
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plehrke said:
How do you verify the temp is reading accurate in flight?
Those that claim their OAT is accurate, how did you prove that?
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Exactly! What I did, and this is good enough for me, was to "calibrate" my OAT probe (I can tweak it up or down in the EFIS) compared to 2 separate thermometers (1 laser & 1 digital) while sitting on the ground. I did this at different temps, too. I can't tell you whether my 2 thermometers are accurate, but they DID read within 2 degrees of each other...they were both within a degree of the local AWOS reading, as well.

Sooo, what I did was split the difference between the 2 and "calibrate" my OAT probe according to what the 2 other sources showed.

As for the OAT being in the NACA duct - I have 2 OAT probes (1 for each EFIS) and they were each in the NACA duct they were closest to (and for the most part, they read the same temp). After removing the pilot's side probe and moving it under the wing, I saw very large differences between the NACA mounted probe and the under-wing probe. It varied between 5 and 12 degrees cooler under the wing!
 
By the way, in looking at TAS calculators online, a temp difference of 10 degrees F will only change your TAS by 2 mph. For the kind of flying I do, being within 1 or 2 degrees of the "real" temperature is close enough. I'm sure I'm not more than 2 degrees (F) off. But I figured I'd put the whole "accuracy" discussion into perspective. :)
 
lostpilot28 said:
As for the OAT being in the NACA duct - I have 2 OAT probes (1 for each EFIS) and they were each in the NACA duct they were closest to (and for the most part, they read the same temp). After removing the pilot's side probe and moving it under the wing, I saw very large differences between the NACA mounted probe and the under-wing probe. It varied between 5 and 12 degrees cooler under the wing!
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And which one do you believe is reading closer to reality now? Was there any subsequent test method to determine which one is accurate?
 
Bavafa said:
And which one do you believe is reading closer to reality now? Was there any subsequent test method to determine which one is accurate?
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I haven't tested my under the wing & out of the prop arc probe for accuracy, but placed it there, because of all the reports I had read over the years. I does show temps of 5 to 7 degrees lower than a friends NACA mounted probe. That difference falls in line with what I had read, too.

L.Adamson --- RV6A
 
I think sometimes that the temperature in the cabin affects the OAT when it is mounted in the NACA. Part of the probe is in the cabin. When I turn on the cabin heat the OAT would go up. I now have it mounted in the wing root fairing.
 
Norman CYYJ said:
I think sometimes that the temperature in the cabin affects the OAT when it is mounted in the NACA. Part of the probe is in the cabin. When I turn on the cabin heat the OAT would go up. I now have it mounted in the wing root fairing.
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Do you get accurate reading in the wing root fairing? Or where in the wing is the suggested place for the probe?

I am wondering does it have to be exposed to the air flowing over it, or could it be just inside the wing or wing root area?
 
Bavafa said:
And which one do you believe is reading closer to reality now? Was there any subsequent test method to determine which one is accurate?
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The under the wing probe is definitely more accurate. As I noted before, the NACA duct read anywhere between 5 and 12 degrees F warmer.

Norman CYYJ said:
I think sometimes that the temperature in the cabin affects the OAT when it is mounted in the NACA. Part of the probe is in the cabin. When I turn on the cabin heat the OAT would go up. I now have it mounted in the wing root fairing.
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Norman, I thought of this, too. About 8 months ago I tried insulating the backside of my probes with insulating foam. I didn't see any change at all. They both still read high.
 
lostpilot28 said:
The under the wing probe is definitely more accurate. As I noted before, the NACA duct read anywhere between 5 and 12 degrees F warmer.



Norman, I thought of this, too. About 8 months ago I tried insulating the backside of my probes with insulating foam. I didn't see any change at all. They both still read high.
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I insulated my probe and it just took longer to warm up to the cabin heat then read high.
 
Philip,

Your graph seems to show (to my untrained eye) a pretty stable set-up. Expected decrease with climb, a slight Ram/Stagnation rise (actually drop, 'cause you were slowing), and the rise on descent. It was sensitive enough to indicate the inversion, and it did not change with power settings.

Pretty good from those aspects...as far as how accurate the actual temps are, you started with it close to actual field OAT, so it sounds pretty good there too. However, temps from winds aloft may not be a good measure, as they (to my mind) cover a wider area, and may not represent your postiion's reality...as you know, winds aloft are never more than 180 degrees off :rolleyes:, so the temps may be a bit of a SWAG too. Not sure what would be a better tool...maybe another airplane, but who is the "control" sample?

As others have said, if we get purty close, we should be good to go!

Had an interesting talk with Paul Lipps and Jim (the gent with his prop and triangular wingtips on an RV) at Reno today. After looking at his props on the biplanes, we sat under my cowl for a while and talked cooling drag, wingtips, and lotsa neat stuff. I missed it in his earlier post here, but he pointed out that some of the NACA-placed OAT probe high temps are from hot air flowing back out of the spinner gap, and even out of the cowling half seam, and running down the side of the cowling and right into the NACA. Insulating the back of the probe in the NACA won't help there, and perhaps that's why some NACA-placed probes showed no changes with insulating. The NACA under the wing that an earlier poster mentioned as doing well would not be so affected (methinks).

By the way, Paul (and I know you'll see this) can pull those formulae he posts about right out of the air...we talked Coanda, Oswald, tip geometry, exhaust flow...man just a bunch of stuff...and he had the numbers right there! I really need to stay at a Holiday Inn Express more often! (I R A Pilot :D)

I'm also interested in the installations in the wing root and the tailcone. They sound like they work well...anyone have pix? Any issues with skin heating on a sunny flight, or does the airflow keep it pretty much at ambient?

Fun discussion!

Cheers,
Bob
 
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Bavafa said:
I am wondering does it have to be exposed to the air flowing over it, or could it be just inside the wing or wing root area?
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At least two reasons why in the open air flow is preferred:

1. Faster response to temperature changes. If the sensor is in an enclosed or semi-enclosed space, then it'll take some time before the stagnant air in that space reaches the temp of the outside air when it changes. [Yes, I know the weather doesn't change that quickly... but the air temp does change rather quickly for you as you climb or descend.] This effect will create lag error in your OAT reading. Depending on how enclosed the space is, i.e. how slowly the air inside it gets replaced, the lag could be on the order of minutes, which at RV climb / descent rates can translate to 10+ degrees of transient error.

2. More accurate steady state readings. An enclosed space will trap heat from nearby sources, or even not so near by. The air inside the enclosed space will be a little bit warmer, since it has a heat source and is partly insulated from the outside air by the enclosing structure. How much temp difference in the air inside the space vs. the outside air depends on many factors, but the effect is there.

You can avoid both these problems all together by installing the sensor in the free-flowing outside air.
 
Currently I have the most accurate OAT readings that I have had on any of my previous aircraft. I have the AFS system and initially I had the probe located on the inboard lower flange of the fuel tank. The probe was on the underside of the wing, in the airflow. This location always read high, too high for me to get accurate TAS numbers.
As an experiment I simply moved the probe up so that it did not extend into the airflow. I plugged the hole that was there and now the probe is totally inclosed in the airspace between the tank and the fuselage.
Based on TAS calculations and comparisons with known ground temperatures this location is quite accurate. I just checked my data from the AVC race and the temperatures dropped nicely as I climbed to 17,500 feet. If there was a lag it was quite minor and in no way would it effect the practical use of the instrument. Finally I have a probe location that is both reliable and accurate. This results in a simple installation with no extruding probe to snag clothing or create drag.
 
Tom, just a couple of thoughts:

1. The original location of your probe probably read high because it was too far inboard, placing it in the prop wash and in close proximity to the exhaust and cowling. A location further outboard would have likely worked well.

2. If you really are getting very accurate readings and no significant lag with the probe in its current location inside the wing root fairing, then that suggests that you're getting significant airflow through your wing root fairing. If that's the case, that means drag.
 
Moving the probe to the interior of the structure was not just a whim on my part. Paul Lipps had spoken with me regarding stagnation error on temperature probes exposed to the free airstream. He had suggested mounting the probe in the tail under the main fairing. I had done this on another aircraft with success but since my probe wiring was already done on this plane I simply moved it inside the wing.
Yes I have considered that there might be air movement in the wing and also that it will be causing drag. My farings are quite tight in this area and I believe that there can not be much air movement there but I will tape it off before the next race and see if anything happens. Even if there were no air movement at all the temperatures between this region and the actual surrounding air would equalize quickly enough for my needs. At 227 knots, my race speeds, the passing air mass is going to bring the aluminum parts quickly to the ambient temperature. If it takes a minute or two for the temperature to stabilize that is good enough. As my aircraft is approaching Vne in level flight I am quite interested in the accuracy of my TAS reading and this location is working well.
 
It would seem to me

It would seem to me that there is a fair amount of airflow in this area as evidenced by the the fact that we have all experienced the air flow under the seat pan from the push rod pass through hole. If this were'nt happening, there would be no need for fabric covers on this hole.

I suspect that that flow is always going to be there because of the low pressure area inside the wing. Yes it is drag but it is pretty much un-avoidable.
 
Tom Martin said:
Paul Lipps had spoken with me regarding stagnation error on temperature probes exposed to the free airstream. He had suggested mounting the probe in the tail under the main fairing.
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Tom,

A few years ago I did some test flights to measure this, and found about 3?C rise between 60 knots and 170 knots (sad to say I was unable to test at Rocket speeds...). My temperature probe is mounted to the plastic NACA vent that RV-8s have under the wing, exposed to airflow but not exhaust or engine heat.

The E6B function on my Garmin 396 asks for "total temperature", which I think means it automatically makes a correction for this temperature rise effect. If the AFS system is making this correction, then putting the sensor out of the airflow might give a more accurate temperature but maybe not an accurate true airspeed calculation?

temprise.jpg
 
He had suggested mounting the probe in the tail under the main fairing
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That's where I have mine, but it consistently reads far too high. I may move it to the exterior, under the HS.
 
I did a test for stagnation rise on a X-C this weekend by slowing, then accelerating in level flight. My probe is under the H-Stab on the right side. Here is what I saw:

10,500' / 195 KTAS / 152 KIAS / OAT = 52F
10,500' / 90 KTAS / 70 KIAS / OAT = 47F

OAT decreased 5 degrees over a 105 KTAS decrease in speed (80 KIAS decrease). Just to be sure I wasn't flying into colder air (and to get back to a reasonable cruise speed ;)), I accelerated back to 195 KTAS, and the OAT came right back up to 52F.

Obviously I'm getting stagnation rise, so what to do? I've seen posts saying tail fairing works well, and others saying it reads hot. I have my tail fairing taped for races (I leave the tape on all the time there, and it stays well). Not sure if that will have an impact...any more thoughts on aft (tail/fuse) mounting? Good, bad, indifferent?

Tom, your rectracted mount sounds interesting. How did you secure it? Any pics (of the inside, I know its covered on the outside :p). Think it could it be replicated in the under-stab part of the fuse?

Not a critical item for everyday use...but I would like to make it as accurate as possible for high and fast regimes!

Cheers,
Bob
 
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Why not add a fairing in front of the probe? Or, what if you covered the probe in a material, like vinyl or plastic, that doesn't transfer heat very well? Just ideas...probably bad ones, but that's all I come up with this early in the morning! :D
 
Just to re-itterate the numbers I gave earlier to calculate stagnation rise, you square TAS and divide by a constant. For kts, it's 4167.3, and for mph it's 5520.9. I was told that usually you will get about 80% of the rise from a probe in the airstream. My Garmin 196 asks for total air temperature, too, in order to get TAS on the E6B function. Since my two probes are out of the freestream, one in the tailcone and one on the wing rear spar ahead of the flap, they usually agree within 1C and give true OAT, so I have to play a game with the Garmin. What I usually do is put in all of the info except I leave IAS at zero and look at the density altitude. Then I keep adding to total temp until the dalt comes back to the original number, usually about 5F.

Your power will drop 1%/10.4F or 5.8C, so if you're trying to get a handle on your power, having good temps helps. 'Course, this is for you number-crunching racers out there who are trying to eke out the last little bit of mph.
For the graph numbers of 146 kt and 75 kt, the respective numbers are 5.1F and 1.4F, a drop of 3.8F. Using 80% recovery would give 3.0F drop, as against the 2F seen.
On my flight back from Carson City, I saw 7C near Sacramento and 11C near Paso Robles at 11,500'. Typically I will see forecast temps on my trips. Maybe around Ca. the temps are more predictable due to the influence of the winds coming from the Pacific Ocean.
Bob, that's one hot rocket!!!
 
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