do you think that we could construct something like this
to reduce OAT error?
I am not familiar with this probe, but I believe that it attempts to correct for further non-linear effects associated with compressibility as the Mach numbers rise to levels substantially above where we operate. Think jet speeds, Mach 0.7 and above. I do not think such a probe will help at our more modest probes because the problem they attempt to address is negligibly small at our speeds.
However, I could be wrong. I leave it to others to research Rosemont probes.
As I noted elsewhere, at our lower speeds, compression heating at stagnation points and frictional heating elsewhere (and mixtures of both near stagnation points) makes the OAT error occur everywhere on the airframe. The Piper curves published earlier give you a good estimate of the OAT error. Going in the wheel wells only adds confusion if there is a fuel tank near by with a big thermal sink in the form of avgas that will be very slowly heating and cooling. Additionally, the OAT response for a probe in the wheel well will be really slooooooooowwwwwwww.
The thing to do is accept the error, and remove it. Use the Piper charts to estimate the error in OAT. To get an accurate TAS, you have to also correct for compressibility. The total airspeed error is about 2/3’s from the termperature error, and about 1/3 from compressibility (varying depending on speed, true OAT, phase of the moon, and other secondary effects.)
You can compute the air speed correction with a fancy Jeppesen whiz wheel developed to compensate for all these effects in jets, or you can use the simplified chart attached. It shows the total airspeed error (compressibility error in the pitot tube and frictional heating error from boundary layer friction) assuming you have a conventional steam gage air speed indicator and are reading an “uncorrected” OAT as indicated by your OAT gage. This assumes you are not using any fancy black boxes that make these corrections for your.
The procedure is simple. Calculate an “E6B TAS” using an E6B or that ring correction on the outside of your steam gage airspeed indicator. Then go to the chart on the horizontal axis with this number. Go vertical to one of the curves that best approximates your OAT. Then go horizontally to the left to the vertical axis to get the correction factor. Subtract this from your “E6B airspeed” to get a very good estimate of your “True TAS.” Caution: any static port errors will remain, and they can be considerable. But at least you can now accurately compare your computed “True TAS” against a four way GPS calculation of “True TAS” and make an estimate of your static port error. Then you can modify your static port, and do it again. And again. And again. When your static port error is less than 1-2 knots at cruise, you will at last have a fairly accurate number. (Whew.)
Fred
-----Original Message-----
From: marv@lancair.net [mailto:marv@lancair.net]
Sent: Wednesday, 5 August 2009 4:27 AM
To: lml@lancaironline.net
Subject: Re: [LML] Ice with OAT 36*F (LIVP): Ram Recovery on OAT
Posted for "Bill" <n5zq@verizon.net>:
Ice with OAT 36*F (LIVP): Ram Recovery on OATHey Fred,
Airliners have an OAT probe called a "Rosemont probe". The idea is to give a
more accurate OAT reading. It's an enclosure around the probe with a bunch of
little holes in it. Since only a percentage of the air gets through the holes,
the local velocity inside the "cage" is much less. I guess that the theory is
that the cage absorbs most of the ram and friction rise leaving the air inside
the cage at a low relative velocity and at a temp somewhere near true OAT.
There might be much more to this device that I don't know about (I never
really gave them much thought other than to make sure that it was "there") but
if this is all it is, do you think that we could construct
something like this
to reduce OAT error? I'd be interested in your thoughts on this.
Bill Harrelson
N5ZQ 320 1,650 hrs
N6ZQ IV under construction
[Discussion about this very thing long ago led many folks to install their OAT probes in the gear wells.... the gear doors aren't air-tight and the environment inside the gear well is the same as outside (except for rain & stuff) for all intents and purposes. Just another data point. <M> ]