OAT best place installation

[Owl-Eagle]

Hello all the communty

I have a Zenair 601 XL fitted with a D100 and a EDC-D10A.
I would like to know: where is the best place to install the OAT probe on this plane ?.
Tanks  in advance for any good answer.

Francis.
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[dynonsupport]

We can't speak for the 601 in particular, but generically, you want it away from sources of heat such as engine exhaust and sunlight. The area on the lower rear fuselage, underneath the horizontal stab, tends to work pretty well. So would the underside of a wing.

 

[PilotKris]

Don't forget about "ram temperature rise".

Member Kevin Horton (khorton) can tell you far more about it than I.

 

[dynonsupport]

On the 601 you can basically safely ignore it. Up above 200 knots, which is a rare customer for us, you need to start thinking about those effects.

 

[khorton]

Ram temperature rise is a speed squared function.  The difference between actual OAT and indicated OAT = K * TAS^2/7592, with TAS in kt and temperatures in deg C.  K is the recovery factor of the temperature probe, which is supposedly around 0.8 for typical probes. The approximate value of the recovery factor can be determined via flight test.

If we assume our probe has a recovery factor of 0.8, the ram temperature rise at various TAS would be:

TAS ram temp rise
(kt) (deg C)
100 1
150 2.4
200 4.2
250 6.6

It is up to you to decide whether ram temperature rise is worth worrying about for your aircraft.  Certainly at CH-601 speeds there are probably other errors in the OAT indication system that are larger than the ram temperature rise.

 

[PilotKris]

Hey Kevin,

Wouldn't ram temperature rise also depend on probe placement?


PilotKris

 

[dynonsupport]

Yes. That ram rise assumes the local air hitting the probe is going as fast as the plane. A probe in a NACA duct, behind the prop, or up in a gear faring will be totally dependent on local airspeed. The reality is that ram rise is totally dependent on probe shape, location, and installation, so there is no way to generically correct for this with a formula.

 

[khorton]

Hey Kevin,

Wouldn't ram temperature rise also depend on probe placement?

Yes, it certainly could. The recovery factor would vary with the mounting location, as well as the probe type. The more the OAT probe is buried in the boundary layer, the lower the recovery factor will be. Prop wash could also complicate things.

You can get a rough idea of the recovery factor by stabilizing at max speed in level flight, letting the indicated OAT stabilize, then recording altitude, IAS and indicated OAT. Then slow to the lowest safe speed at the same altitude and record the data again after the indicated OAT has stabilized. Calculate the TAS for those two conditions, then plot indicated OAT vs TAS^2/7592 (with OAT in deg C, and TAS in kt). The slope of that line is the recovery factor. Repeat on several flights to be sure you have a repeatable result.

 

[Owl-Eagle]

: Tks a lot for your informations,

I will try 2 possilities for my 601, inside the naca on the right side behind the firewall, or at the intrado of my right wing close to wing locker. :
I will let you know.
Francis.

 

[meljordan]

I think you will find the NACA unacceptable. Two factors work against that location; the heat from the engine escapes from the cowl and will raise the local temp at least several degrees and the sunlight falling on the probe in certain attitudes will also cause an elevated reading. A location under the wing or empanage, away from engine and exhaust air should be your goal.

Regards,
Mel Jordan

 

[khorton]

I think you will find the NACA unacceptable.  Two factors work against that location; the heat from the engine escapes from the cowl and will raise the local temp at least several degrees and the sunlight falling on the probe in certain attitudes will also cause an elevated reading.   A location under the wing or empanage, away from engine and exhaust air should be your goal.

Also, for NACA scoops on the forward fuselage, typically the inboard end of the OAT probe would be exposed to cabin air. If you are using cabin heat, this would likely affect the temperature sensed by the OAT probe.