NATO STO

Exploratory Team ET-225 sought to establish the current knowledge on Reynolds number effects for future combat aircraft aerodynamics, identify risks for next generation aircraft programmes and establish a plan and a team to address these risks under a research task group. Through workshops, questionnaires, background and literature search and computational fluid dynamics investigations, the following conclusions were drawn. The last significant effort to document Reynolds number effects for military aircraft was conducted in 1994, reported in AGARD-AG-323. The author brings together various wind tunnel and flight test evidence to give examples of Reynolds number effects. There are a number of examples that are relevant to current configurations of interest and significant effects are reported. For legacy combat aircraft, lateral/directional characteristics in particular have shown to be sensitive, which often limit manoeuvre performance. In the recommendations, the author strongly supports activities such as the one proposed and recommends exploiting the capabilities of the ‘new’ cryogenic facilities European Transonic Wind tunnel (ETW) and the NASA National Transonic Facility (NTF). Since AGARDograph 323, there has been relatively little new literature on the topic in either the public or NATO domains. In recent aircraft programmes such as F-35, Typhoon etc., anecdotal evidence suggests that significant aerodynamic discrepancies were encountered in flight tests and it is likely that they are attributable to Reynolds number scaling effects. The ongoing activity AVT-298 is investigating Reynolds number effects for a blended wing body configuration. The proposed activity will build on that knowledge and experience and extend the range of flow physics to cover complex fighter configurations that exploit non-linear vortex dominated aerodynamics. Design requirements for the next generation of combat aircraft mean the likelihood of encountering issues due to Reynolds number effects may be increased for the following reasons. Low observable (LO) shaping leads to new aerodynamic configurations, which may be more susceptible to Reynolds scaling effects (e.g. higher trailing edge sweep, tailless etc.). Internal stores carriage increases vehicle size, which reduces wind tunnel model scale, so the wind tunnel to flight scale effect is likely to be larger. Traditional aerodynamic fixes such as vortex generators, fences and notches are no longer acceptable due to LO constraints.