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James W. Telford

Abstract

Many directly measured aircraft performance details related to the unstable behavior of the Desert Research Institute's (DRI) research aircraft after ice accumulation, which led directly to its crash were recorded on its final flight. The data system, with the fully gimballed inertial platform, remained fully operational during the flight, including the final spiraling dive, with negative (upside down) accelerators. The observations show a reduced lift effect involving transition to what seems to be partial stall on the inboard wing. This effect induced, at onset, a reduction of the lift coefficient at a higher angle of attack and at a greater airspeed than was consistent with flight measurements before and after. When normal conditions were temporarily reestablished, lift returned. This anomalous behavior appears to have produced an equivalent to control reversal in pitch, in which forward pressure on the control column could have induced increased lift and a nose up response. This seems to have led to an extreme nose up climb, followed by stall and a deep negative angle of incidence spiral dive, from which recovery could not be effected. The evolution of such instability does not appear to be widely understood and seems to be an apt topic for further investigation. The possibility of its occurrence seems to be a point requiring a cautionary note.

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James W. Telford

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James W. Telford

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James W. Telford

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James W. Telford

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James W. Telford

Abstract

The theory of isolated turbulent plumes given earlier is used to study a field of plumes. Each plume is immersed in the turbulent downdraft which comprises the return flow. The field of flow is specified by three parameters: the heat flux into the atmosphere at the surface, the depth of the convecting layer, and the intensity of turbulence at the surface (where turbulence is steadily generated by the wind) and where a plume element which leaves the surface returns there in a downdraft after a period of the order of 103 sec. The change in air properties during this period is of the essence of the problem. Since the process is driven by the changing density resulting from heating, the equations describing the field must be time-dependent in this essential respect. Wind is neglected, and the horizontal pressure gradient assumed to he the same at all heights.

The derived plume properties-size, temperature excess, upward velocity and turbulent intensity-are in agreement with observation. The formulation predicts a maximum possible depth for convection in the form of a field of plumes, depending on the magnitude of the heat flux and surface turbulence. As a result, it is suggested that the theory of a field of plumes could lead to a prediction of the onset of a different form of convection, such as on a larger scale, resulting from instabilities in the convecting layer as a whole.

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James W. Telford

Abstract

Crucial observations describing the mechanisms active in the convective planetary boundary layer are reviewed. Attention is drawn to the observed transport of heat and moisture upward against vertical stratifications stable to cloud-free convection. Occasional saturated parcels carry air of a different composition through regions well below cloud base. These parcels do not form sustained or easily visible cloud and are observed to penetrate even where no sustained cloud forms higher up. This mechanism is likely to be of major importance. The possibility of erosion into the warm overlying air at the top of the convective layer due to a negative heat flux is discussed. One must conclude from the observations discussed here that it does not actually occur. Despite claims to the contrary, the air above appears to enter the convecting layer only when the convection has effectively reached its temperature. When stratus cloud is present, mixtures with the overlying unsaturated air often have the same density as the cloudy air below. Both these phenomena illustrate the inadequacy of conceptual thinking based on averaged horizontal uniformity in the fluid, without recognizing that convective transport is a process where isolated thermal elements in the fluid carry practically all of the quantities such as heat and moisture from one level to another. Gradient turbulent diffusion usually has no role to play and is not a satisfactory concept in describing the planetary boundary layer.

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James W. Telford

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James W. Telford

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