What position in the airframe do you recommend to instal de OAT sensor for your engine monitor?

[email protected] wrote:What position in the airframe do you recommend to instal de OAT sensor for your engine monitor?

Most I've seen are up under the wing, maybe a foot and a half or so aft of the leading edge, with the sensor mounted in the wing root fairing. You want it far enough under the wing so that it's shaded by the wing from the sun.

MTV

Thanks MTV.

Has anyone put that sensor inside a wing leading edge cabin air inlet? Results?
(Don´t even know if it would fit in)

[email protected] wrote:Thanks MTV.

Has anyone put that sensor inside a wing leading edge cabin air inlet? Results?
(Don´t even know if it would fit in)

I doubt it would fit. And, it would be less accurate on the ground, I suspect.
And probably much harder to mount if it did.
MTV

You want to put it outside of the prop slip stream and in ambient air. In the cabin vent may make it less accurate. Its really easy to just put it in an inspection panel which is how I've done all mine. Easy to access for maintenance as well.

I installed my OAT sensor in the inspection panel just outboard of the pitot tube.

A1Skinner wrote:You want to put it outside of the prop slip stream and in ambient air. In the cabin vent may make it less accurate. Its really easy to just put it in an inspection panel which is how I've done all mine. Easy to access for maintenance as well.

Why outside the prop arc?

MTV

mtv wrote:

A1Skinner wrote:You want to put it outside of the prop slip stream and in ambient air. In the cabin vent may make it less accurate. Its really easy to just put it in an inspection panel which is how I've done all mine. Easy to access for maintenance as well.

Why outside the prop arc?

MTV

That way it is outside of the prop wash when sitting on the ground. I know EI specifies this in their install manuals, not sure about JPI. But it makes sense.

Thx A1 and Barra.

Unfortunately one sensor was put yesterday close to the pitot tube.
One more has to be installed and think I will use the air duct as four persons on FB said it worked well there.

As MTV said, it will not be accurate on the ground but I belive once airflow starts it all will gradualy go accurate and there wont be other sticks on the outside surface.

Here is my Garmin OAT probe for the GAD 109 ( not sure the right number) Anyway it's the air data source for my G-5s




The probe is on an inspection panel and works great. It is just outside the prop arc.

MW

A1Skinner wrote:

mtv wrote:

A1Skinner wrote:You want to put it outside of the prop slip stream and in ambient air. In the cabin vent may make it less accurate. Its really easy to just put it in an inspection panel which is how I've done all mine. Easy to access for maintenance as well.

Why outside the prop arc?

MTV

That way it is outside of the prop wash when sitting on the ground. I know EI specifies this in their install manuals, not sure about JPI. But it makes sense.

I don’t think JPI has this in their install manual. I’m still trying to get my head around why inside or outside the prop arc would make any difference. You say it makes sense.....that’s my question: why does it make sense? Curious, not arguing. Trying to grasp any physics, other than maybe prop wash throwing stuff at it??

Curious in Bozeman.

MTV

mtv wrote:

A1Skinner wrote:

mtv wrote:

A1Skinner wrote:You want to put it outside of the prop slip stream and in ambient air. In the cabin vent may make it less accurate. Its really easy to just put it in an inspection panel which is how I've done all mine. Easy to access for maintenance as well.

Why outside the prop arc?

MTV

That way it is outside of the prop wash when sitting on the ground. I know EI specifies this in their install manuals, not sure about JPI. But it makes sense.

I don’t think JPI has this in their install manual. I’m still trying to get my head around why inside or outside the prop arc would make any difference. You say it makes sense.....that’s my question: why does it make sense? Curious, not arguing. Trying to grasp any physics, other than maybe prop wash throwing stuff at it??

Curious in Bozeman.

MTV

They want it in clean airflow. There's a chance that the prop wash can blow warm engine heat over the probe causing an incorrect reading. Their instructions don't specify outside of the prop wash, but they do specify clean airflow. FWIW, they do say a cabin air vent scoop is acceptable. Here is a link to JPI: https://www.jpinstruments.com/shop/oat-probe/

Called JPI.

What they want is for the sensor not to be in danger of receiving any hot air from the engine normal exits or leaks.
Asking if there was something directly related to the prop wash they said no, that its really the above.
Paper with the sensor says to put it ideally two feet out from wing root.

Thx for the inputs Gents.

Despite what JPI says, I would think they want it outside the arc because what the sensor is actually reading is total air temperature (TAT), not outside air temp. TAT is a function of OAT plus a rise due to the air molecules impacting the probe from forward motion. At normal GA speeds, the difference between OAT and TAT is negligible, but perhaps the speed of the prop wash gets out of the negligible range for the probe.

Thanks for the explanations, gang! I can certainly understand avoiding potential engine heat.

Now trying to get my head around spiraling airstream effects...

MTV

Right wing.
Cabin air inlet duct.
Temp sensor instalation.

fredy wrote:Despite what JPI says, I would think they want it outside the arc because what the sensor is actually reading is total air temperature (TAT), not outside air temp. TAT is a function of OAT plus a rise due to the air molecules impacting the probe from forward motion. At normal GA speeds, the difference between OAT and TAT is negligible, but perhaps the speed of the prop wash gets out of the negligible range for the probe.

If I remember correctly some Pipers or Beechs had their sensors on the windshields or very close to it, right behind the prop wash.
Would also think ram rise is negligable in this cases. It would be interesting to see the numbers.

Those were direct reading gages. With todays remote sensors one can be more creative.

I've seen some of those, but the ones I've seen have holes in the "probe" that is out in the free stream and the actual temp sensing part is inside, thus being somewhat shielded from the the airspeed. It's also possible that due to being direct reading, as you mentioned, this location was simply the best compromise given the technology available at the time. All of the above is speculation on my part, based on some science (probably just enough to be dangerous).

Allways very interesting.

Actually the JPI 900 came with a shielded sensor. Did not take a detailed look at it but only got to see one small hole in it,
We will leave the ram rise computation for another day.

Thx for the inputs!

Putting OAT sensors inside air inlets worked very well.
Less danger of damage when cleaning the aircraft and no holes to the outside of the airframe.

On the ground as expected, they read higher but once you have airflow all OK.
If one wants a fairly aqurate density altitude readout on the ground one should consider not putting the sensor in the duct and find a shady place outside.

One of my sensors had already been installed under the wing, say a little behind and to the side of the aft part of the pitot tube.
Still reads higher to some extend until airborne.

Think I have more OAT sensors on the 180 than on the Airbus now.