OAT Differences

[Robert_M]

Hi gang!

I'm planning to install the D-180 into my RV-9. I would like to mount the OAT probe before I get too far into the construction of my fuselage. Does the Probe for the D-180 come with a ten foot long cable or is it longer or is there a cable? why is it nearly half the price of the other OAT probe?

Thanks,

Robert

 

[dynonsupport]

Robert,
The difference in the probes is that the EMS (and thus D180) have hardware for OAT already in them, so the probe only needs two wires and is $37. The EFIS doesn't have this hardware, so the probe has the hardware inside, has three wires, and is $65. You can use the more expensive probe with the EMS, but not vice-versa.

Both probes have 10' of wire hanging off them. These can be extended or cut by you if needed. We do not sell probes with longer or shorter wires.

 

[65xx]

Rober: I mounted my 2 wire OAT on the first inspectiion plate on the left wing and used two knife connectors to facilitate removal. Dan

 

[Robert_M]

Thanks Dynon Support, that's the exact information that I was looking for.

Dan! Hadn't thought about placing the probe there, I was fixed on placing it in the tail of the plane at the open area covered by fiberglass. Interesting idea.

Thanks again!!

 

[Ratman]

Hi Robert,

I was planning to install mine in the wingtip but was told that was a bad idea as it needs to be in the wind stream for the most accurate reading. Inaccurate OAT will give inaccurate density altitude as well as TAS. I ended up putting it under the horizontal with the remote compass in the fairing but on the wing on an inspection plate is a good idea as well.

John

Thanks Dynon Support, that's the exact information that I was looking for.

Dan!  Hadn't thought about placing the probe there, I was fixed on placing it in the tail of the plane at the open area covered by fiberglass.  Interesting idea.

Thanks again!!

 

[dynonsupport]

Sticking out of the inspection plate into the free air stream is totally different from being inside the wingtip.

 

[Robert_M]

Okay, so what's acceptable and what's not. I thought I understood that the probe should not be exposed to the wind stream but tucked away. The location should be in a place outside of the cabin where the probe can get as accurate a reading as possible without direct air flow over the probe. What exactly is meant by "in the wind stream?"

Thanks again

 

[dynonsupport]

Not sure where you got the idea they should not be in the airstream. They work fine out there and that's where the majority of them are. If you have a location that is tucked away but still outside the airplane, that should work fine as well as long as it's not behind the exhaust or other heat source. Being inside an insulated cavity like the wingtip will not be accurate.

 

[khorton]

Dynon - ram temperature rise is a small factor for some faster amateur-built aircraft.  For example, the OAT probe feeding the GRT engine monitor on my RV-8 seems to have a recovery factor somewhere in the range between 0.8 and 0.95 (more testing is needed to get a more accurate estimate), which means that at 180 kt TAS the indicated OAT is approximately 6 - 7 deg F higher than the actual OAT.

Is ram temperature rise still ignored with the current firmware?  If not, what probe recovery factor is assumed?

 

[dynonsupport]

Kevin,
No ram rise is assumed or factored in in the firmware.

We don't know the recovery factor of our probe.

The real problem with trying to deal with ram rise in any homebuilt is that the probe location does matter, and we have to support hundreds of different airframes. The local speed of the probe is not always the speed the pitot reads. The only real way to overcome this is with a TAT probe, but those are total overkill in this market.

Yes, if you are building a machine capable of Mach numbers above .3 in cruise, then maybe you should try and find a place that isn't as exposed. But then again, if the air has already slowed down, hasn't it also experienced some rise? The airframe is also warmer by some ram rise, so it contributes as well. How well insulated can you get the probe from the skin of the aircraft?

Maybe someday we'll let users put in a recovery factor of their own, but this issue hasn't been high on our list, and we don't imagine many people will go through the rigorous process of calculating recovery factor for their install.

Also, to get a 7 deg F rise with a .9 recovery, you have to be doing 195 MPH IAS at 10K feet. Very few RV's cruise at that speed. At 145 IAS at 10K feet, rise is less than 4 deg F (2 deg C).

 

[khorton]

Also, to get a 7 deg F rise with a .9 recovery, you have to be doing 195 MPH IAS at 10K feet. Very few RV's cruise at that speed. At 145 IAS at 10K feet, rise is less than 4 deg F (2 deg C).

How did you calculate the ram temperature rise?  There is some sort of error in your calculation.  

There are two mathematically equivalent ways to calculate ram temperature rise.  You can do it using mach number, or using TAS.

Using mach number, we need to work in absolute temperatures.

IOAT = OAT * (1 + 0.2 * K * M^2)
IOAT = indicated outside air temperature, express in degrees K or R
OAT = outside air temperature, in the same units as IOAT
K = recovery factor of the OAT probe
M = mach number.

If you know the calibrated airspeed and the altitude, you can calculate the mach number.  In the example you gave, 195 mph IAS at 10,000 ft is M0.308.  The standard temperature at 10,000 ft is 23.3 deg F, or 483 deg R.  With a recovery factor of 0.9, the IOAT would be:

483 * (1 + 0.2 * 0.9 * 0.308^2) = 491.2

Thus, there was 8.2 deg R (or deg F) ram temperature rise.

The other way to calculate ram temperature rise is to use true airspeed.  If we are using deg C and kt, the ram temperature rise =

K * (TAS^2)/7592

In the case above, 195 mph CAS at 10,000 ft is 226 mph TAS, or 196.5 kt.  The ram temperature rise in deg C =

0.9 * (196.5^2)/7592 = 4.6 deg C or 8.2 deg F.

At 145 mph IAS at 10,000, with a recovery factor of 0.9, the ram temperature rise would be 2.5 deg C, or 4.6 deg C.

The faster RVs are capable of over 180 kt at 75% power.  At 180 kt TAS, with a recovery factor of 0.9, the ram temperature rise would be 3.8 deg C or 6.9 deg F.

 

[Camps]

Hi Dynon

I'm having some troubles with my OAT probe.

The temperature reading is increasing with the altitude!

Do you have any clue?

Regards

Roberto Camps :-/

 

[dynonsupport]

Temperature inversion? ;-)

Actually, was this a one day thing, or all the time. Temperature inversions do happen from time to time.

Beyond that - is it right on the ground? And if you warm it on the ground, does it increase or decrease?

 

[KellyF]

I may have missed it on your webpage, but is there a picture pf the probe?  How far should it protrude into the airstream?  Would it work OK if it were mounted outside of the aircraft, but not directly in the airstream?  I'm thinking maybe between the rear spar and flaps on an RV-7.  I know I'm splitting hairs, just trying to eliminate as many drag producing items as I can while I build

 

[dynonsupport]

It looks like this.

You're asking questions that are tough to answer precisely. I guess I'll note that most OAT probes on manufactured aircraft often sit well proud of the skin. You're basically after a probe that is the same temperature of the air around it (*with the caveat that at higher airspeeds, as discussed elsewhere on the board, there will be a bit of ram rise on the probe that we do not take into account). This means avoiding enclosed spaces that might trap heat, and keeping out of the sun (which will heat the probe), and keeping it away from the exhaust stream. Beyond that, you'll need to do some experimenting.

 

[KellyF]

Fair enough, thanks! Looks like it's about 1" long, 1.4" in dia. I was thinking about placing it as close to the gap between the flap and aileron as possible, thinking there would be some air circulation around there to get a clean temp. reading from.

 

[airguy]

Another possibility - coming out of the fuselage just behind the step mount plate. If you place it in trail of the step, it's already in disturbed air and will not contribute significantly to parasitic drag - but will have more than sufficient airflow across it to register accurate free stream temperature.

Dynon - could you have come up with something a little more kludgy-looking perhaps? :-? How about a teardrop drag-reducing semi-airfoil shape for the OAT sensor?

 

[Camps]

Hi Dynon

My OAT probe is not working well. There are some kind of wrong readings, I don't know.

I tested putting ice in contact with the probe and I found 0 c ( zero degree celsius) OK

Local temperature inside the hangar is OK, correct.

Putting my hand in contact with the probe we can observe the temperature increasing.

When flying the temperature reading is always some degrees higher then the correct.

Is it possible that the exaust gas are heating the probe? It is located 1,2m behind my seat ( Remos G3 + Rotax 912 ULS)

Maybe something related with electrical?????

Any clue?

Regards

Roberto Camps

 

[dynonsupport]

If it reads right on the ground, but not in the air, it's probably a location problem. It can't be anywhere behind the exhaust. Out on the wing or on the side of the body back by the tail generally works well.

The exhaust is 1300 degrees F so it doesn't take much of it getting to the OAT to bump it up a bit.

I doubt it's electrical.

 

[Camps]

Hi Dynon

Bingo you were right.
The probe was installed in the belly of the plane about 3m behind the engine and in fact it was not a good place. The exaust gas were still reaching the probe.

I tested this during a flight putting the engine in idle and aplying rudder ( left/right). The temperature droped 10 C ! !

Thanks

Roberto Camps