Effects of Non-Standard Atmospheric Temperature on Barometric Altitude Measurements and Calculated Aerodynamic Coefficients

摘要

In parachute testing, it is common practice to take non-standard air temperatures into account when calculating air density. Less common - and oftentimes ignored - is the effect of non-standard air temperatures on pressure-based altitude measurements. Failing to take the effect of non-standard air temperatures into account will result in errors in altitude measurements with respect to true altitude. While errors in altitude measurements themselves may not be significant, the resulting errors in vertical velocity calculated from altitude and time data may be. Velocity errors also magnify errors in the calculated parachute drag coefficient and may result in misleading predictions about the canopy and performance. Various scenarios where non-standard air temperatures have or have not been taken into account in pressure-based altitude measurements and determination of air density have been investigated, and resulting velocity and drag coefficient errors have been quantified. The greatest error in calculated drag coefficient occurs during the apparently common scenario where air density is corrected for non-standard air temperatures, but pressure-based altitude measurements are not. Somewhat surprisingly, the error resulting from ignoring the effect on non-standard air temperatures entirely (no correction for non-standard temperature in pressure altitude measurements, and basing air density on pressure altitude rather than density altitude) is approximately half the error when accounting for non-standard air temperatures in determining air density, but not correcting pressure-based altitude measurements for non-standard air temperatures. An additional consideration is atmospheric temperatures and temperature lapse rates are usually quite different from those defined for the standard atmosphere, and it can be difficult to obtain accurate air temperature data during testing. So, there can be considerable uncertainty in the air temperatures used to correct pressure altitude measurements and calculate air density. Depending upon test conditions and objectives, availability and accuracy of air temperature data, a case can be made for ignoring air temperature entirely during testing utilizing pressure-based altitude instrumentation, and basing all drag coefficient calculations on a pressure altitude and an air density that corresponds to the standard temperature air density at that pressure altitude.