Adding an OAT indicator to my M20F

[leesh]

My new (to me) 1967 M20F doesn't have an outside air temperature indicator. Perhaps they didn't think it was necessary down in Florida, but here in Massachusetts I consider it an essential instrument. I'm wondering if there's any real advantage of going with a digital in-panel gauge rather than the analog probe that goes through the windshield? Considering that my pockets aren't lined with gold, and I'm already having to replace the VOR/CDI head that is connected to the Garmin 430, I want to keep my cost down, but if there is a compelling reason to consider some sort of fancier unit, I'd consider the investment.

Also, is an A&P required to install the simple 'through the windshield' analog type gauge? I assume it is with the fancier ones.

Thanks,

Alisha

[N601RX]

Davtron makes some small digital dash mount models that are a little over $100.  They also have some combo units that have clocks, timers, voltage, ect. 

www.aircraftspruce.com

 

[Hank]

Why stick something the size of your finger out the windshield?

Since replacing the windshield requires an A&P, and replacing side windows does not, I can suppose that modifying the windshield would require an A&P also, to say nothing of running wiring and hooking up to the electrical system.

My C-model has an in-panel digital readout, with the probe in the right-sdie air scoop, nice and easy to run the wires to. Other people put them way out on the bottom of one wing or the other. Personally, I think the air scoop is a quick, simple location, and it picks up very little engine heat.

[carusoam]

Leesh,

Having a reliable temperature indicator is a flight safety "requirement" for flying in the NE this time of year......  Requirement, meaning if you are experiencing icing (freezing rain), you would really want to know if you have escaped those conditions or are they getting worse....?  It is also an important tool for density altitude calculations, hot summer, short runway, full load, will I get off the ground or should I wait till later calculation?

I think the rest may be just convenience.

A digital read-out is the best way.  You want to glance at the gauge and know what it is reading.  You don't want to have to stare at it and take a statistical sampling and average the input to get a result.

The windshield mounted mechanical device is lowest cost, but vibration and placement makes it incredibly difficult to read accurately.  If you decide to go this way, an A&P with experience and tools for drilling in acryllic windows is the lowest cost way to go.  A simple mistake drilling the hole in the windshield could cost a fortune.  It is incredibly easy to make a mistake while drilling acrylic.........

I always use my thermometer to determine whether or not I need pre-heat to start the engine.  My M20C would start pretty reliably above 28 dF. 

A study of other links will provide information on where to locate the thermocouple, out of the sun and away from exhaust streams = under a wing and on an access panel.

Best regards,

-a-

[Vref]

Quote: leesh

My new (to me) 1967 M20F doesn't have an outside air temperature indicator. Perhaps they didn't think it was necessary down in Florida, but here in Massachusetts I consider it an essential instrument. I'm wondering if there's any real advantage of going with a digital in-panel gauge rather than the analog probe that goes through the windshield? Considering that my pockets aren't lined with gold, and I'm already having to replace the VOR/CDI head that is connected to the Garmin 430, I want to keep my cost down, but if there is a compelling reason to consider some sort of fancier unit, I'd consider the investment.

Also, is an A&P required to install the simple 'through the windshield' analog type gauge? I assume it is with the fancier ones.

Thanks,

Alisha

[Magnum]

Quote: Vref

Alisha, I had the mechanical "scott" OAT probe sticking out of the window. The left window cracked because somebody pushed against it...So I was forced to install a new side window... I am having an edm800 installed with OAT probe. IMHO the best place is under the wing and go digital davtron etc....

I was concidering also the naca inlet however as you will read in some post if you do a search this will not give you the exact temp due to air compressibility wich casues temperature variations, less air flow etc... The aerodynamic drag caused by the probe is insignificant and as stated already by some oter members OAT is a very very important parameter ...

 

[Immelman]

Alisha,

I just removed the old "meat thermometer" scott probe from my airplane as I was installing a new windshield this year and just hated having the protuberance. I had installed an engine monitor a few years ago so I purchased an OAT probe and added it to the monitor, and mouned it out underneath the wing.

One thing to be careful of is where to mount it for an accurate reading. I did some experimentation with a device that has two temperature probes from our electronics lab at work and did some measurements with one probe in the cabin air scoop on the side of the fuselage, and the other taped out under the wing. The probe mounted in the cabin air scoop (on the opposite side of the exhaust) was consistently 5 degrees fahrenheit warmer than the one under the wing while in flight (equal readings on the ground with the engine off), which tells me warm air from the cowl is influencing it. I imagine that the original 'top of the windshield' position will yield good data since that's where the factory put the original OAT probes.

If you decide to go with the old style OAT that goes through the windshield and want to save a few bucks, you can have mine for shipping cost.

 

[Hank]

Jesse,

Do you have any data from the cabin air scoop ON the exhaust side? That's where my probe is located . . . Not that I expect the exhaust to contribute significant additional heat, except maybe when I'm using cabin heat since that hose is much, much closer.

[leesh]

Ok, so it sounds like the windshield probe isn't the way to go. I did wonder about the potential risk of damaging the windshield also. Now the question is what instrument to buy -- whether to go with a basic Davtron that shows just the OAT, or a fancier unit that adds things like density altitute, etc. Of course, being a computer geek, I found myself then looking at something really fancy like a new EGT/CHT system that includes OAT to replace my old classic gauges.

Too many choices, and too many ways to spend every bit of money I can throw at it. I agree with Anthony, that it really is important to have an OAT indicator (especially here in the New England area).

Alisha

[Immelman]

Quote: Hank

Jesse,

Do you have any data from the cabin air scoop ON the exhaust side? That's where my probe is located . . . Not that I expect the exhaust to contribute significant additional heat, except maybe when I'm using cabin heat since that hose is much, much closer.

[MARZ]

Quote: Hank

Jesse,

Do you have any data from the cabin air scoop ON the exhaust side? That's where my probe is located . . . Not that I expect the exhaust to contribute significant additional heat, except maybe when I'm using cabin heat since that hose is much, much closer.

[KSMooniac]

The scoop-mounted locations DO read warmer than the optimal under-wing locations as Jesse confirmed with his measurements.  The engine/cowl is of course a big heat source and air flowing over it will be warmed before it hits the probe.  If this were a *good* location, you can bet the factory would have put their probes there in later model aircraft, but they didn't!  Personally, I'd really like to know if is 31 dF or 33 dF and not just "somewhere" around 32 dF!

My '77 J had the original probe located under the right wing in the center of an inspection panel near the main gear.  This is away from the engine, exhaust stream, and the sun, so it is a great location.  When it died, I opted for a Davtron M655 multi-function device (OAT, pressure and density altitude, voltage) and installed the probe in the left wing opposite the original location.  We had to do a 337/field approval but it sailed through.  I installed it myself and prepared the paperwork under my IA's supervision.  Someday I plan to add an OAT probe to my EDM-700 and install that probe in the original location on the right wing.

Alisha, I highly recommend the Davtron unit.  I got a deal on it NIB via ebay for $150-200 or so, which was quite lucky.  If you don't already have an engine monitor, then I would strongly urge you to consider adding one of those and just add an OAT probe to that installation.  While the unit costs of something like my EDM-700 are relatively modest, installation can run upwards of 15-20 hours to run the probes and wires.  They are the best bang-for-the-buck instrument and I'll never own a plane without one.  If you can swing an engine monitor, I'd go that route...if not, then the Davtron units are great.  If you're a little bit handy, you can do some of the easy/tedious work to remove the interior panel and route the harness for the probe to the wing and minimize the installation costs.  It is an important measurement, and you should really put the probe in the proper location!

[Magnum]

Quote: KSMooniac

The scoop-mounted locations DO read warmer than the optimal under-wing locations as Jesse confirmed with his measurements.  The engine/cowl is of course a big heat source and air flowing over it will be warmed before it hits the probe.  If this were a *good* location, you can bet the factory would have put their probes there in later model aircraft, but they didn't!  Personally, I'd really like to know if is 31 dF or 33 dF and not just "somewhere" around 32 dF!

My '77 J had the original probe located under the right wing in the center of an inspection panel near the main gear.  This is away from the engine, exhaust stream, and the sun, so it is a great location.  When it died, I opted for a Davtron M655 multi-function device (OAT, pressure and density altitude, voltage) and installed the probe in the left wing opposite the original location.  We had to do a 337/field approval but it sailed through.  I installed it myself and prepared the paperwork under my IA's supervision.  Someday I plan to add an OAT probe to my EDM-700 and install that probe in the original location on the right wing.

Alisha, I highly recommend the Davtron unit.  I got a deal on it NIB via ebay for $150-200 or so, which was quite lucky.  If you don't already have an engine monitor, then I would strongly urge you to consider adding one of those and just add an OAT probe to that installation.  While the unit costs of something like my EDM-700 are relatively modest, installation can run upwards of 15-20 hours to run the probes and wires.  They are the best bang-for-the-buck instrument and I'll never own a plane without one.  If you can swing an engine monitor, I'd go that route...if not, then the Davtron units are great.  If you're a little bit handy, you can do some of the easy/tedious work to remove the interior panel and route the harness for the probe to the wing and minimize the installation costs.  It is an important measurement, and you should really put the probe in the proper location!

[takair]

Alisha,

 

Any chance you have OAT burried in one of the menus of a piece of equipment you already have?  Not knowing what you have, it's hard to say.  For example, a Garmin 430 will take OAT from an air data module and can display it on one of various sub-menus.  Many engine analyzers have OAT burried on a page of sub-menu and I think there are even some clocks that do it.  Usually when the meat thermometer comes out, it is because of a superior replacement.  If you describe some of your avionics, maybe somebody here can point out where to look. 

[leesh]

Quote: takair

Alisha,

 

Any chance you have OAT burried in one of the menus of a piece of equipment you already have?  Not knowing what you have, it's hard to say.  For example, a Garmin 430 will take OAT from an air data module and can display it on one of various sub-menus.  Many engine analyzers have OAT burried on a page of sub-menu and I think there are even some clocks that do it.  Usually when the meat thermometer comes out, it is because of a superior replacement.  If you describe some of your avionics, maybe somebody here can point out where to look. 

[danb35]

Quote: leesh

Can an OAT probe be directly to a 430?

[N601RX]

The 300 series is temp only, the 655 will display temp, pressure altitude from the encoder, density altitude, and voltage.

http://www.davtron.com/overview.php#600

[captainglen]

There are numerous advantages to a Davtron digital unit. The first of which is flexibility in where you mount the probe since it can be as much as 10' from the indicator. An obvious advantage is accuracy in calculating performance especially density altitude for takeoff performance. Another advantage is visibility they are illuminated and can be mounted in the instrument panel; just try reading an old style Aerotherm OAT gauge at night. Flexibility in that the Davtron instruments are multifunction, the simple ones also function as a Voltmeter/voltage alert while the more complex (M655-2) units display voltage,temprature, pressure altitude and calculate density altitude.

[captainglen]

Just purchased a Davtron M655 for all the advantages I earlier pointed out. Wired a "T" connector harness which allows the Davtorn to power up from the transponder without actually modifying the transponder harness, You simply unplug the transponder from the encoder, plug the davtron into the encoder and plug the transponder into a connector in the davtron harness. I am still mulling over where to install the probe for best accuracy to avoid effects from sunlight and onboard heat sources. I really do not want to drill a hole in the bottom of a wing! which is the advised install position (any advice wound be appreciated) I discovered another advantage the instrument has in that calculating TAS allows you to skip a tedious step on the E-6B, you simply enter the density altitude and read TAS on the outer scale over IAS on the inner scale. You avoid the eye strain of trying to set the temprature over the pressure altitude while trying to fly the airplane at the same time. Digital E-6Bs are OK but with this method the old one is faster and easier! BTW spent $100 on the Davtron but had to spend $60 for a new probe.

[carusoam]

Check where the modern Mooneys put there's....

Under the wing in an inspection panel.

Away from the engine...

Avoid the fuel tanks...

There is probably a few threads on TC location...

Best regards,

-a-