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Estimating Updraft Velocity from an Airplane Response

Donald H. LenschowNational Center for Atmospheric Research, Boulder, Colorado 80303

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Abstract

Airplanes have been used to estimate the magnitude and shape of thunderstorm updrafts by assuming that the airplane follows the updraft when the thrust, mass and pitch angle are held constant. This assumption is shown to be satisfactory, using simplified airplane equations of motion, for a Beechcraft Queen Air and a North American T-28, if the updraft is large with respect to the contribution of the drift of the pitch angle reference to the airplane vertical velocity. For thunderstorms, where updrafts >8 m s−1 with a diameter of ∼3 km are expected, the airplanes should follow the updraft closely enough that a “smooth” updraft profile can be distinguished from a “top hat” profile. The contribution to the vertical airplane velocity from horizontal wind variations is less than 20% of the horizontal wind variation if the pitch angle is held constant. If, instead, the airspeed is held constant, the contribution to the vertical airplane velocity would be as much as 100% of the horizontal wind variation.

A Queen Air, instrumented with a complete air motion sensing system, was flown through the updraft over an isolated mountain peak, which was similar in size and shape to a thunderstorm updraft, to check the analysis. The results verified the desirability of flying at constant pitch angle.

Abstract

Airplanes have been used to estimate the magnitude and shape of thunderstorm updrafts by assuming that the airplane follows the updraft when the thrust, mass and pitch angle are held constant. This assumption is shown to be satisfactory, using simplified airplane equations of motion, for a Beechcraft Queen Air and a North American T-28, if the updraft is large with respect to the contribution of the drift of the pitch angle reference to the airplane vertical velocity. For thunderstorms, where updrafts >8 m s−1 with a diameter of ∼3 km are expected, the airplanes should follow the updraft closely enough that a “smooth” updraft profile can be distinguished from a “top hat” profile. The contribution to the vertical airplane velocity from horizontal wind variations is less than 20% of the horizontal wind variation if the pitch angle is held constant. If, instead, the airspeed is held constant, the contribution to the vertical airplane velocity would be as much as 100% of the horizontal wind variation.

A Queen Air, instrumented with a complete air motion sensing system, was flown through the updraft over an isolated mountain peak, which was similar in size and shape to a thunderstorm updraft, to check the analysis. The results verified the desirability of flying at constant pitch angle.

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