Check the archives
Some long well populated threads on the topic. all the best
If you mount your OAT sensor so that it is being hit by the freestream, you will get errors from stagnation temperature rise, which is about (TAS,mph)squared/5521. That can be as high as 7.5 mph at 200 mph TAS. Each +10 F will give about +1% TAS error at 5000'. My recent prop testing with RVs shows there is a lot of room for improvement in placement of both the OAT sensor and the static port.
You make a great point there is instrument error and its almost impossible to measure perfect static temp or static pressure. The good news is for most of our purpose total temp (TAT) reading is good enough. We have ways to work with it. It only becomes an issue if you specifically need Static Air Temp (SAT) for weather reports and to determine icing conditions.
Where did you get your temp "Ram Rise" formula? If I am using it correctly @ 200 mph:
(200 mph)^2 / 5521 = 7.24F (must be F right?). That's sounds high even for degrees-F. I think its more like 2-3(C) or 3-5(F) rise. I don't have a formula off the top of my head for stagnation air temp rise, its kind of complicated formula I recall. It also depends on the "recovery factor". You never get 100% rise or stagnation. The type of "thermometer" you are using also affects the reading.
TAT = SAT + Ram Rise or Stagnation Rise; TAT is always higher than SAT; the temp increase is the due to the energy of bringing the moving air to rest and compressing it (on the OAT probe).
Many TAS "calculator"s expect you to supply TAT (total air temp) or indicated temp anyway, not SAT (static air temp). The TAS calculator takes care of the small "ram rise" temp error. The formula for TAS using TAT (kts) using TAT (total temp) & Mach is:
TAS (kts) = 39*Mach*sqrt(TAT)
Ref
For example the Dynon EFIS gives TAS - It already knows indicated or total air temp (TAT), altitude, airspeed (dynamic pressure) and static pressure. From this is can calculate (estimate) TAS fairly accurately provided the data is accurate. SAT is not needed but is accounted for.
Even if your temp is off 5F its only makes a less than 1 mph difference.
Really TAT/SAT is an issue for jets, but you make a good point, there can be some error due to temp. On jets they have fancy Rosemont probes or TAT probes, that are very accurate, since the recovery factor is known. From that the air data computer calculates SAT, which may not be directly displayed, except on a secondary FMC (flt management computer) page. The temp that is displayed prominently all the time on most large aircraft EFIS is TAT not SAT. TAT is what'ss used just like us. If ATC wants to know the air temp from a jet at their altitude, the crew should give SAT (but might require a few buttons get punched to read it). For RV's, SAT & TAT are close enough. Ball park assume SAT is a centigrade or two less than indicated by our simple OAT system. However most RV's read high OAT mostly from engine heat.
PROBE LOCATION:
Putting the OAT probes inside the planes structure might be and issue, since structure can heat-up & actually increase temp readings, if not well ventilated to ambient air. You could route some outside air to it and vent that air for continuous but slow flow, that would work (if heat is not picked up in the duct). I like the idea of not having the probe impinged directly by the free air; I suppose you could put it behind the pitot mast standoff and reduce the recovery factor or temp rise, but it's easy to just put it out in the breeze. Accept you're reading TAT not SAT. Like I say the electonic E6B's have ways to deal with that. The real source of temp error is the heat from the engine. The further away the OAT probe is from the engine the better.
I agree 100% that the static source can be a significant source of error.
