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Anyone put OAT probe in the Tail Area?

lr172

Well Known Member
I put my OAT probe on the rear H stab spar, under the emp cover. It gets a good dose of ambient air in flight (negative pressure under the emp cover) and am confident that it is getting a good sampling of ambient air without a drag penalty. I recently came to realize that the G3X modifies the OAT probe reading to account for heat induced from air stream friction. My location is not in the free stream, so it is likely reading a few degrees cooler than actual. Someone posted some data that implied that this friction accounts for about 2-3 degrees at cruise speeds.

I do not want to re-wire the probe, so my only alternatives are to leave it where it is at or put it on the side of the fuselage under the H stab. My 6 has it located there, but I always wondered if I was getting some exhaust heat affect in that location. I fly IFR, so am more comfortable with a slightly low reading over one that is slightly high. I suppose that I could splice in another 6 feet of wire and put the probe under the H stab out near the tip, but am not sure that I would be able to mount the probe there.

Has anyone put there probe in this location with good results? I suspect most of us really don't know how accurately our probe is measuring that actual temps.

Appreciate any guidance here.

Larry
 
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I put mine on the starboard side, underneath the HS.
I did calibrate the probe to the EFIS.
Mine was very accurate.
 
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I saw substantial heat rise due to the engine with the probe mounted in the tail on the Rocket. I tried multiple locations and never could get away from the heat until I put the probe out in the wing.
 
OAT location

I flew my first flight a couple of weeks ago where I was somewhat concerned about in-flight icing, and after closely reviewing icing levels and actual conditions, I feel that my probe, being mounted under the right HS, reads a few degrees hot.
 
Both OAT probes for my Dynon system (dual AD-AHRS) are inside the emp fairing pointing aft. Working great for 3+ years.
 
Mine are lined up in line with the airflow, seperated by about 3 inches, high under the left horizontal stab. Work fine there.
 
What i heard

There was another thread recently where the G3X expert chimed and said the probe should be in the air stream, not under a fairing. i believe in this case he was talking about in the wing root fairing. I did not understand all the science, but the Garmin probe apparently uses the free stream temperature to calculate compressability effects.

So mine is going in the wing bottom skin, adjacent to an access plate.
 
So, for those of you placing your probes along the fuselage, you will have a higher temp from the engine heat. Your TAS will read higher, too. :)

Vic
 
I've got dual ADAHRS, so an OAT probe on both sides of the fuselage under the HS.
IMG_8747_heic-M.jpg

OAT is always reasonable and accurate.
 
Science and art

So the science says in free-stream air, but the art (the pilot / builder's PIREPs) say it doesn't matter. Maybe I will put mine in a convenient place and follow the art
 
I placed my temp sensor in the triangular space between the flap brace and the rear spar near the outboard end of the flap. It's shielded from radiant heat from the ramp and gets airflow from the flap/aileron junction.
Image pilfered from Bob C.

Cheers, David
RV-6A KBTF
 

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I saw substantial heat rise due to the engine with the probe mounted in the tail on the Rocket. I tried multiple locations and never could get away from the heat until I put the probe out in the wing.

Me too. But sadly not a Rocket. :eek:

I took two thermocouples and located them is possible locations then flew (many times) and plotted the data. I was surprised at the difference in the two, and wandering of the inboard one.

If it was advice to offer, it would be "outside the prop sweep". That maybe too conservative, but out under the wing (like near an inspection plate) was very stable.
After all - why pay $400 for a super accurate RTD and then allow it to measure dirty air?

Hey Larry, hop down to C75 and we can tape some TC's where you would like to compare them.
 
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Me too. But sadly not a Rocket. :eek:

I took two thermocouples and located them is possible locations then flew (many times) and plotted the data. I was surprised at the difference in the two, and wandering of the inboard one.

If it was advice to offer, it would be "outside the prop sweep". That maybe too conservative, but out under the wing (like near an inspection plate) was very stable.
After all - why pay $400 for a super accurate RTD and then allow it to measure dirty air?

Hey Larry, hop down to C75 and we can tape some TC's where you would like to compare them.

Thanks for the offer. Always looking for a reason to fly and I plan to take you up on the offer, as I would like to get as accurate data as I can. I often fly directly over Peoria on my way to Columbia, MO.

Larry
 
It might be worth considering engine size when polling the community. Just because locating in the empenage seems to work on a 180 HP RV-8 does not mean its going to perform as well when you add substantially more heat from a 540.

On the Rocket I tried the inspection panel under the stab, rear facing on the fin spar, and under the HS empenage fairings. All three worked great engine off in the hangar, and all three showed a 5 -10 degree rise when in flight. Only when I moved the probe to the LH wing bellcrank panel did I get rock steady readings on the ground and in flight.

540's throw a LOT of heat allong the fuselage - I cant even touch the floor at the rear stick location with my bare hands due to heat - there is no way to expect a temp probe located 10 feet further back is going to be immune.
 
I am considering splicing in some cable and putting the sensor on the bottom of the H stab just inboard of the tip. That should put me about 6' away from the fuse.

Do you guys think this is far enough away from the heat? I just don't like the idea of taking off all of the side panels and trying to figure out which cable is the OAT. Not many nice days left here in the midwest.

Larry
 
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Doesn't answer the OP question, but I put mine just in front of the pitot tube (Van's) under the wing. Figured I'll already have drag in that location. Definitely free air. And opens the option of 3D printing a streamline for it and the lower part of the pitot tube.

Finn
 
A quick calculation (with estimated diameter/length and other assumptions) shows this sensor in the slip stream might take .1hp at 200mph at sea level...
 
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A quick calculation (with estimated diameter/length and other assumptions) shows this sensor in the slip stream might take .01hp at 200mph at sea level...

OK, Alex, I'll bite.

My Dynon OAT probe is 1" long, 5/16" diameter and tip radiused 5/32" or so.

Now, a simple 1.75" long 1/8" diameter rod transponder antenna apparently has so much drag that it's worthwhile to upgrade to a shark fin transponder antenna.

I'm having real trouble wrapping my mind around that.

Finn
 
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OK, Alex, I'll bite.

My Dynon OAT probe is 1" long, 5/16" diameter and tip radiused 5/32" or so.

Now, a simple 1.75" long 1/8" diameter rod transponder apparently has so much drag that it's worthwhile to upgrade to a shark fin transponder antenna.

I'm having real trouble wrapping my mind around that.

Finn

Finn, those dimensions are almost exactly what I used. I also assumed the entire length was in the full velocity slipstream. I used 1.0 as a coefficient of drag (which might be incorrect, I did not do any Reynold's number estimates), and 1.2 kg/m^3 for air. It results in a drag force of about .2 lbs, which, at 200mph is about .1 hp.

Perhaps someone with recent practice in study and application of fluid dynamics will chime in.
 
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Shark fins are much faster

Shark fins are much faster. Especially when one breaks off the transponder antenna cleaning the plane. Not going very fast, not going anywhere with a broken transponder antenna.
 
Shark fins are much faster. Especially when one breaks off the transponder antenna cleaning the plane. Not going very fast, not going anywhere with a broken transponder antenna.

"Believe it or not, a typical TED antenna reportedly has about 4 times the drag of a typical blade antenna. That data is taken from a soaring website that compared a RAMI rod & ball antenna (0.41 lbs drag) to a RAMI blade antenna (0.09 lbs drag) (both measured at 250 mph)."

https://vansairforce.com/community/showthread.php?t=70773

If a 1.75" long 1/8" diameter antenna has 0.41 lbs drag, how can a 1" long 5/16" diameter OAT probe only have a drag force of about .02 lbs??

Finn
 
"Believe it or not, a typical TED antenna reportedly has about 4 times the drag of a typical blade antenna. That data is taken from a soaring website that compared a RAMI rod & ball antenna (0.41 lbs drag) to a RAMI blade antenna (0.09 lbs drag) (both measured at 250 mph)."

https://vansairforce.com/community/showthread.php?t=70773

If a 1.75" long 1/8" diameter antenna has 0.41 lbs drag, how can a 1" long 5/16" diameter OAT probe only have a drag force of about .02 lbs??

Finn

The Garmin is much worse than the Dynon. For some reason, in addition to the 1" probe/rod, garmin put a 5/8" high, fat nut on the probe side of the hole. Not sure what they were thinking with that move. Probably double the drag of the dynon style or more.

I was always told that a Ted antenna is worth one full Knot. No idea if that is true.
 
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"Believe it or not, a typical TED antenna reportedly has about 4 times the drag of a typical blade antenna. That data is taken from a soaring website that compared a RAMI rod & ball antenna (0.41 lbs drag) to a RAMI blade antenna (0.09 lbs drag) (both measured at 250 mph)."

https://vansairforce.com/community/showthread.php?t=70773

If a 1.75" long 1/8" diameter antenna has 0.41 lbs drag, how can a 1" long 5/16" diameter OAT probe only have a drag force of about .02 lbs??

Finn

It might not - I said it was an estimate. Also, one was estimated at 200mph, the other at 250, and drag is basically proportional to the velocity squared.

I'd like to see the data on the .41lbs. That would take about .27 hp at 250mph. Speed is proportional to hp^3, and take as an example some plane that needs 250hp to go 250mph. If it only had 249.73 hp instead, the speed would be about 249.18mph.
 
Alex, just guessing that you may have missed a unit conversion somewhere. For 5/16" diameter, 1" length, Cd = 1, 200 mph, and standard density I get a force of about 1 newton or .22 pounds, power of about .12 hp. As you note, this is just assuming the entire cylinder sees 200 mph, with no skin or end effects. Looking at 1/8" diameter, 1.75" length, and 250 mph I get about .24 pounds, so not quite 2/3 of the drag apparently measured by others.
 
Alex, just guessing that you may have missed a unit conversion somewhere. For 5/16" diameter, 1" length, Cd = 1, 200 mph, and standard density I get a force of about 1 newton or .22 pounds, power of about .12 hp. As you note, this is just assuming the entire cylinder sees 200 mph, with no skin or end effects. Looking at 1/8" diameter, 1.75" length, and 250 mph I get about .24 pounds, so not quite 2/3 of the drag apparently measured by others.

Dave, good catch. Not a unit error, but I had an extra zero in the length (.0025m, .025m is correct for my example). With that correction, I get right at 1N for my hypothetical dimensions.

Thanks -
 
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