Re: [RCSE] High Altitude Glider/off subject

[RCBrustEE]

In a message dated 1/18/2005 6:14:59 AM Eastern Standard Time, [EMAIL PROTECTED] writes: I am not doubting the good eyes, I have see Johnny B. tow too very high altitudes, but a Pegasus at 4000' agl is invisible. I will gladly stand corrected if someone can prove or verify these kinds of altitude claims. Maybe we should put a pic, a Sky melody, an Eagle Tree system, and a Casio watch in one plane and see what we get for grins. I will loan the Sky melody/sky panel. I calibrated mine last year. It is accurate to 3-10 feet in 1000' altitude gain. A barometric altimeter will not be accurate to within 3-10 feet in 1000.  This is a 0.3% to 1.0% error.  The error in an electronic barometric altimeter comes from 3 sources; the ability of the altimeter to correctly sense the atmospheric pressure, the accuracy of the atmospheric model that is used to convert pressure to altitude, and the deviation of the atmosphere from the standards set in the model, primarily temperature.   The first source of error, the accuracy of the altimeter's pressure sensing, is what can be calibrated.  Even so, it is difficult to get the unit to hold an accuracy of less than 0.5% over a period of time.   The second source of error, the atmospheric model, is based on an ideal version of the atmosphere where the temperature at sea level is 15C, the pressure is 29.92, and the temperature decreases with altitude at a rate of about 1.98C per 1000 feet.  Any deviation from these ideal conditions will introduce an error into the altitude reading.  Even under these ideal conditions, the model is still a model, it is not exact.   The third source of error is probably the greatest one, and that is a deviation in temperature from the ideal atmospheric model.  Full scale pilots know that when it's cold outside, a barometric altimeter will read high.  Conversely, if it's hot out, an altimeter will read low.  Fortunately, over the temperature range that we typically fly our models at, the effect is not that great, but you can see errors of over 5% if flying near freezing or 100F and not compensating for temperature.   We've done a lot of testing and have found that typically, after temperature compensation, our altimeters will read within about 2% of actual altitude.  The majority of the error we see comes from non-ideal atmospheric conditions, not the altimeters themselves.   Thanks, Randy Brust Soaring Circuits