Jenny C. Holt

Development and application of optical fibre strain and pressure sensors for in-flight measurements

Fibre optic based sensors are becoming increasingly viable as replacements for traditional flight test sensors. Here we present laboratory, wind tunnel and flight test results of fibre Bragg gratings (FBG) used to measure surface strain and an extrinsic fibre Fabry–Perot interferometric (EFFPI) sensor used to measure unsteady pressure. The calibrated full scale resolution and bandwidth of the FBG and EFFPI sensors were shown to be 0.29% at 2.5 kHz up to 600 μe and 0.15% at up to 10 kHz respectively up to 400 Pa. The wind tunnel tests, completed on a 30% scale model, allowed the EFFPI sensor to be developed before incorporation with the FBG system into a Bulldog aerobatic light aircraft. The aircraft was modified and certified based on Certification Standards 23 (CS-23) and flight tested with steady and dynamic manoeuvres. Aerobatic dynamic manoeuvres were performed in flight including a spin over a g-range −1g to +4g and demonstrated both the FBG and the EFFPI instruments to have sufficient resolution to analyse the wing strain and fuselage unsteady pressure characteristics. The steady manoeuvres from the EFFPI sensor matched the wind tunnel data to within experimental error while comparisons of the flight test and wind tunnel EFFPI results with a Kulite pressure sensor showed significant discrepancies between the two sets of data, greater than experimental error. This issue is discussed further in the paper.

A wind tunnel investigation into the effects of roof curvature on the aerodynamic drag experienced by a light goods vehicle

Roof curvature is used to increase ground vehicle camber and enhance rear–body boat–tailing to reduce aerodynamic drag. Little aerodynamic data is published for light goods vehicles (LGVs) which account for a significant proportion of annual UK licensed vehicle miles. This paper details scale wind tunnel measurements at Re = 1.6 × 106 of a generic LGV utilising interchangeable roof panels to investigate the effects of curved roof profile on aerodynamic drag at simulated crosswinds between −6° and 16°. Optimum magnitudes of roof profile depth and axial location are suggested and the limited dataset indicates that increasing roof curvature is effective in reducing drag over a large yaw range, compared to a flat roof profile. This is primarily due to increased base pressure, possibly from enhanced mixing of longitudinal vortices shed from the rear–body upper side edges and increased turbulent mixing in the near–wake due to the increased effective boat–tail angle.

Investigation of the aerodynamic characteristics of a lifting body in ground proximity

The use of cambered hull shapes in the next generation of lighter-than-air vehicles to enhance aerodynamic performance, together with optimized take-off manoeuvre profiles, will require a more detailed understanding of ground proximity effects for such aircraft. A series of sub-scale wind tunnel tests at Re = 1.4 x 10 6 on a 6:1 prolate spheroid are used to identify potential changes in aerodynamic lift, drag and pitching moment coefficients that are likely to be experienced on the vehicle hull in isolation when in close ground proximity. The experimental data is supported by a preliminary assessment of surface pressure changes using a high order panel method (PANAIR) and RANS CFD simulations to assess the flow structure. The effect of ground proximity, most evident when non-dimensional ground clearance (h/c) < 0.3, is to reduce lift coefficient, increase drag coefficient and increase the body pitching moment coefficient.

Dynamic Wind Tunnel Simulation of Aircraft Wake Vortex Trajectory in Ground Proximity

The trajectory of the wake vortex generated by a transport aircraft wing during take–off and landing can influence the dispersion of the engine exhaust plume and therefore impact on local air quality in the vicinity of airports. A sub-scale atmospheric boundary layer wind tunnel (ABLWT) simulation, used to predict exhaust dispersion, would therefore need to include the effect of the wing wake. A preliminary series of wind tunnel measurements are presented which examine the effectiveness of both a quasi-static and moving-model simulation of a trailing vortex pair at nominally 1/200 th scale, within an ABLWT. A simple flat plate delta planform wing, at incidence, is shown to be an effective vortex wake generator at this scale and Reynolds number (nominally 0.3 x 10 6 ). The trajectory of the vortices within the wake in the range 2<(z/b)<26 are shown to be consistent with that reported for aircraft flight test; ground proximity is seen to influence both the lateral and vertical displacement of the vortex cores. Using this limited dataset, differences in vortex trajectory between a quasi-static and dynamic simulation only appear significant when an aircraft climb (+6 degrees to the horizontal) or descent ( -3 degrees) profile is simulated.

Vortex Induced Aerodynamic Forces on a Flat Plate in Ground Proximity

Vehicle underbody longitudinal vortices can have a significant effect on the aerodynamic loads experienced by a body in close ground proximity. A series of wind tunnel tests at a nominal Reynolds number of 2.26 x 10 6 ,were carried out to investigate both (i) the influence of a moving ground plane simulation compared to fixed ground and (ii) the effect of relative location and strength of underbody longitudinal vortices on a simple flat plate, at zero incidence, fitted with vane vortex generators.

The impact of inlet flow conditions on the aerodynamic performance of an NACA submerged intake for ground vehicle applications

Results are presented following a series of experimental measurements on a submerged NACA-type intake oriented between −30° and +30° yaw to the free stream in an atmospheric boundary layer wind tunnel at a unit Reynolds number of nominally 1 × 106. The intake was subjected to a range of upstream wall boundary layer conditions, and the mass flow into the intake (as measured by an orifice plate) was monitored to assess the aerodynamic performance. The mass flow data are supported by qualitative flow visualisation within the duct, using a smoke filament illuminated in a laser light sheet in order to gain insight into the flow physics. The intake performance, expressed in terms of a non-dimensional flow momentum coefficient, is seen to degrade both with increasing intake orientation to the free stream (changes of nominally 40% are seen for the angle range tested) and with increasing upstream boundary layer displacement thickness (changes of nominally 30% are seen for the range tested). These data are presented as a graphical carpet plot; it is intended that this is used as a guide to performance prediction in non-aeronautical applications where there are often significant changes in both the local flow direction and the boundary layer thickness. Flow visualisation studies show that the degradation in the intake performance with increasing yaw angle can be attributed to a progressive change in the vortex-pair structure within the intake as the local flow angle is increased. Both an increase in the lateral separation and an increase in the size of the respective vortex cores are considered to act so as to reduce the magnitude of the induced inflow into the intake.