A new Approach to Aerodynamic Performance of Aircraft under Turbulent Atmospheric Conditions

In the present study the influence of atmospheric turbulence on aircraft performance has been investigated and a relation between flight physics and meteorology has been established. Special attention was paid on aircraft with natural laminar flow airfoils because they exhibit an additional possibility of performance loss due to increased drag caused by a premature laminar-turbulent transition. A theoretical analysis was performed and the aerodynamic problem was extracted from the performance problem. A new wing glove for the G109b measurement aircraft as well as measurement equipment capable of detecting unsteady aerodynamic effects were developed. In-flight measurements for the validation of performance loss theories were carried out resulting in a new approach to aircraft performance under turbulent atmospheric conditions. Generally, a loss of flight performance can be the result of decreased lift, increased drag or a combination of both. The aerodynamic state and therefore the possible influencing mechanisms of atmospheric turbulence vary with the flight condition. Based on aircraft performance considerations, three principal flight conditions were determined for an in-depth study of the aerodynamic state related to these flight conditions. The flight conditions are slow flight, best glide and cruise flight. A new wing glove with favorable characteristics for aerodynamic in-flight experiments, retaining the flying qualities of the aircraft besides the asymmetric configuration, has been designed. A survey of the base flow on the glove in non-turbulent conditions by means of flight tests, wind tunnel tests and numerical simulations was conducted prior to the investigations under turbulent conditions. The results were essential as baseline data and showed that the numerous design requirements for the new wing glove were fulfilled. The flight test results show that the assumption of steady inflow conditions is incorrect for flight in atmospheric turbulence. An elevated level of micro-scale turbulence in the atmosphere is related to increased angle of attack variations. Therefore, unsteady changes in the airfoil pressure distribution are prevalent when an elevated turbulence level is encountered. Turbulence levels of 0.5% and more, which lead to different transition scenarios according to the results from flat plate experiments in transition research, do not occur in only light atmospheric turbulence, which is in turn the prerequisite for almost steady pressure distributions. The unsteady lift variations related to the angle of attack variations due to gusts are well predicted by unsteady thin-airfoil theory. A quasi-stationary approach does not cover the entire unsteady lift effects but in the case of laminar airfoils it predicts when the laminar drag bucket is left and airfoil drag increases. Especially in slow flight very close to the upper limit of the laminar drag bucket, angle of attack variations lead to increased airfoil drag. Flying at a lower angle of attack simply solves this problem. Only a slight increase in velocity is required to lower that angle of attack sufficiently.