Hmmm. I guess I'm confused. Isn't the TAS derived using the OAT? IOW, an EFIS needs an OAT probe to calculate the TAS, not the other way around. ICE-T and all that.
Joe,
Yes, it can be derived that way, but more generally... From the input variables static pressure, pitot pressure, and indicated air temp (what the "OAT" probe actually sees, which includes ram air heating) you can solve jointly for IAS, TAS, and the true ambient OAT. Tedious to do by hand, so they teach pilots only about the simpler relationship of starting with presumed true OAT and IAS and solving for TAS. But the more general solution is fairly easy to do in software, i.e. an air data computer or EFIS, so there's no longer a reason to limit ourselves to the former.
As for icing conditions...no thanks, not exactly "recreational" for me.
I'm with you there. The idea is to stay out of icing, and the OAT is a key indication to help us do exactly that.
And if 2 or 3 degrees makes or breaks it, I've got bigger problems.
The ram air heating effect in the RV speed range (let's say 180 ktas) can account for up to about 4 deg Celsius (7 deg Fahrenheit) of error. But again, that's actually not a problem because the EFIS can compensate it out numerically as long as the probe sees a known airspeed, i.e. the same airspeed as the airplane.
However, other measurement errors caused by locating the probe near other sources of heat, or in direct sunlight, etc. can cause even greater error, are not deterministic, and cannot be numerically compensated out. And likewise for lag error caused by placing the probe in stagnant air. The lag error could easily exceed 10 deg F with a lag on the order of minutes (consider RV climb/descent rates and atmospheric lapse rates).
I guess my point is, for our purposes--at the prevalent speeds and altitudes--other than the exhaust flow (and the propwash/exhaust mix), I don't think it matters one bit if the probe is on the bottom of the wing, in the sidewall NACA scoop, or inside a faring. I've seen flying RVs with all of the above. I'm sure the resultant error is minor.
You're certainly right that lots of folks put the OAT probes in all kinds of places and get away with it. It's not the sort of thing that's likely to get you killed unless you're really cutting your margins too narrow in the first place. However, I would contend that a better question to ask is this: Why would one knowingly and willingly choose a suboptimal installation and accept sloppy instrument readings if it's just as easy to do it right and get accurate readings? That's my take on it anyway.
