Anthony P. Brown

Flight Measurements of Wake Vortex Circulation in Crossflow Shear

NRC Aerospace has conducted flight measurements of the wake vortex flowfields of heavy jet transports. Reduction of B767-300 and B747-400 wake vortex flowfield flight data has been conducted. Contour path integrals have been used for the identification of net vortex circulation of the wake vortex trailing pair domain within the crossplane flightpath contours and of the combined wake vortex trailing pair and jet wake (upper) domains of the wake, within larger flightpath contours. Thereafter, the derived circulations of the domains, which include the inherent circulation of the non-frozen background crossflow windshear, have been examined as functions of mean vertical crossflow windshear magnitude, or crossflow vorticity γC in the absence of any significant background vertical wind. The derived circulation was approximately zero for γC≈0. Dependence upon crossflow vorticity was observed to be much greater than the variation of circulation with wake length. For the B767-300, derived circulation of the trailing vortex pair, including inherent background shear circulation, was about 400m 2 s -1 at a crossplane vorticity magnitude of 0.035s -1 . For the combined wake vortex and jet wake domain, circulation was approximately 1000m 2 s -1 at │γC│=0.020s -1 , again including the background shear circulation. Data-set standard deviations were approximately 100m 2 s -1 . As an endeavour to remove the inherent circulation of the background crossflow shear, the mean crossflow wind profiles were regressed to estimate the shear strength, and the circulation calculated for each contour path integral, for the assumed frozen shear. This was subtracted from the wake vortex and wake vortex plus jet wake circulation, to estimate the net circulation of the vortex flowfields. The so-estimated net circulations were approximately zero at γC≈0, rising a mean 150m 2 s -1 for γC≈0.036, with no effective difference between

Derivation of Wake Vortex Circulation from Flight Data using Path Integrals

In 2003/2004 NRC Aerospace conducted a wake vortex flight measurement proof of concept programme, aimed at the demonstration of the cost-effective data gathering of wake vortex flowfield characteristics from jet transports in commercial service. Survey flying consisted of helical flight paths around condensationdelineated wake vortex flowfields. Instrumentation consisted of inertial and air data systems capable of the high-fidelity derivation of wind perturbations for the identification of the flowfield characteristics of trailing pair vortex circulation, separation, axis orientation, descent path and perturbation state. For such identifications, the technique of trial and error kinematic fitting of the derived wind perturbations in the time domain was used. In regions of highly-perturbed vortex states, with attendant highly dynamic flight attitude changes and possibly frequent but poorly-defined vortex core passages, a time domain kinematic fitting process lacked accuracy. Rather, moving-crossplane open path and contour path integral methodologies are proposed and applied. Whilst the association between contour path integrals of induced velocity fields and the enclosed field total circulation is well understood (an application of Stokes’ Theorem), the relationship between open path integrals of induced velocity and the induced-velocity field motive circulation, and the application thereof to the derivation of wake vortex total circulation, are perhaps less used. Examples from the NRC wake flight measurement programme are provided. The usage of open path integrals provides insight into planning optimal wake vortex flight measurement profiles, in order to maximise the effective data return per task flight hour.

Aircraft Encounters with Advection -driven Horizontal Vortices within the Atmospheric Boundary Layer

In the early Northern spring of 2005, the Flight Research Laboratory of the National Research Council undertook radiometry flight research in support of snow hydrology verification and validation ‘North o f 60’, to the north of Yellowknife, North we st Territories. During one flight, in a s trong south easterly airflow, stream s of large -scale coherent vortices , of substantial strength, w ere encountered within the atmospheric boundary layer. The vortices were near -horizontal, with cold -air core axial flows, opposite to the direction of the advection wind. The core air temperatures were 5 ° to 10 °C colder than the surrounding air. The flightpath consisted of a number of parallel grid legs. The vortices were encountered during several grid legs. A plot of the encounters provided evidence that the two largest vortices were wavy and quasi -stationary, during the period of the encounters. Two line vortex identification processes were used to estimate the core rad ii sizes and vortex strengths from each vortex core penetration. Overall , the identification yielded core radii estimates between 260 and 520 metres, and line vortex strengths between 2,700 and 11,000 m 2 s -1 . Within the vortex cores , along the line of fli ght, many smaller vortices were encountered . The strength and core radii of the core vortices were estimated to be 1½ orders of magnitude lower than those of the primary vortex. The strength and core radii of the horizontal core vortices and the primary vortices were logarithmically related by a best -fit empirical power -law of �=3.11 rC 1.246 . The effects of the vortex encounters upon flightpath stability were sudden deceleration and loss of climb performance , followed by turbulent loading from the vortice s within the cores , thence a similarly -sudden acceleration and performance gain towards the end of the encounter .

Flight Measurement of Enroute Configuration Wake Vortex Characteristics: A Feasibility Study

*The Flight Research Laboratory has undertaken a feasibility study into obtaining inflight measurements of wake vortex flowfields generated by jet transport aeroplanes in enroute configuration. The objectives of the study included the establishment of air traffic procedures for the usage of jet transports in commercial service as wake generators, the usage of condensation wake conditions for delineation of wake vortex regions, and the usage of FRL-developed air data and inertial data instrumentation for the accurate identification of wake vortex state parameters from the relatively small wind perturbations induced by the trailing vortices, whilst sampling around the perimeter of the delineated condensation wake domain. Using the NRC Falcon 20 as the wake sampling aeroplane, the study was conducted in two phases. The first phase studied wakes of length 1-6nm. The second phase, presently reported, studied wakes from 5 to approximately 30 nm in length. The proof-of-concept of the techniques was satisfactorily demonstrated. Condensation wake domains enabled the visual identification of the characteristic upper and lower vortex structures and instability states, and wind perturbations enabled the identification of some vortex characteristics. Results are presented, including examples of vortex strength dissipation, wake descent rates, and the details of a number of flightpath disturbances induced by close encounters with the trailing vortices.

In-situ Measurements of Enroute Configuration Wake Vortex Scalar Field Characteristics

The scalar fields of vortex-pair-induced pressure, temperature and velocity magnitude from the NRC wake vortex inflight measurement proof-of-concept programme are presented for an inbound B767-300 wake of Γ≅350m 2 s -1 (wake age ½ to 1 minute), an outbound B747-400 wake of Γ≅600m 2 s -1 (wake age 1 to 1½ minutes), and the vortex core characteristics of an inbound A310 wake of Γ≅430m 2 s -1 . Correspondence between induced temperature and pressure fields indicated that the fields were representative of near-adiabatic compression or expansion. Between the two wakes the scalar fields exhibited similarities and differences in characteristics, possibly attributable to differences in wake ages/lengths. Similarities were the existence of generalised areas of temperature and pressure rises above the vortex pairs and temperature and pressure reductions in the immediate vicinity of vortex core edges, and between the vortex pair. Differences were notably the lack of distinction of vortex core delineation, in the case of the B747-400 wake and different asymmetries in the scalar fields. Possibly the former was attributable to the greater age, length and state (near to vortex-linking) of the B747-400 wake. However, the differences in asymmetry were probably attributable to background atmospheric flowfield differences. In particular, the B767-300 wake existed in a strong crosswind shear from the right, within which the vortex pair appeared to roll, with the downwind vortex displaced and stretched upwards. From a vortex core transect of the left vortex of the A310 at a wake length of 6nm (wake age 0.77 min.), internal core-flow features of significance have been identified.

Observations of Wake Vortex Instability Flowfields and Aircraft Response to Encounters

* from 0.5 to 3. The second phase, studied wakes from 5 to approximately 30 nm in length, or t * from 1 to 6. Sampling was conducted at 32 Hz. More recently, the NRC CT- 133 aeroplane, with 600Hz sampling air and inertial data systems, has conducted an additional data gathering flight against enroute jet transports. Through the course of all flights, a variety of condensation wake conditions prevailed, with condensation intensity and longevity varying between the upper engine exhaust jet wake and the lower trailing pair vortex wake domains. The sampling aeroplane conducted helical flightpaths around, rather than through, the condensation flow regimes. However, following the onset of significant multi-wavelength long- period instability (at enroute speeds, generally beyond 6 nm wake lengths), a number of entrainments of the Falcon 20 into wake vortex cores occurred. The effects of wake vortex core entrainments upon aerodynamic loading, flightpath stability and systems operation have been of particular interest. In loading, such entrainments were multi-axis, with severe (defined as >0.3g deviation) aZ and a Y and severe (defined as >90°s -2 for roll and >30°s -2 for yaw) roll/yaw angular accelerations. The angular acceleration loadings resulted in gross flightpath deviations and upsets. Such encounters with a B747-400 wake are studied in detail, in terms of their flowfield characteristics and the effects upon the Falcon. Cross-plane re-construction is used, in order to correlate structural characteristics and the changes in structure, with increasing wake age. In the case of the B747-400 entrainments, one occurred at the point of vortex linking, and one occurred with the wake vortex in a probable ring state. Although the loading was multi-axis in the former case, in the latter case the loading affected aZ only. The aircraft loading and flightpath upsets of an NRC CT-133 survey of an A310 wake vortex, which included deliberate vortex core traverses, are also discussed. Initial comparisons between the Falcon and CT-133 core penetrations suggest that the parameters of be/rC and bf/be might be significant factors for the magnitudes of loading and flightpath upset experienced during core passage.

Dynamic Stall Flight Data, Effects of Pitch Rate, Surface Roughness and Ground Effect

Internally-funded NRC dynamic stalling flight research has been conducted, using the NRC CT-133 research jet. Initial data analysis and results are presented for the parameter matrix of varying surface roughness, stall-entry pitch rates and the presence of ground effect. For the first set of such flight manoeuvres, inertial data, air data and video data of surface flow visualization, using low-density wing tufts, were recorded. The inertial and air data were used for the derivation of incidence angle, α, and integrated force coefficients, notably CL . The video records have been used to define the spanwise / chordwise position and nature of the onset of wing upper surface flow separation lines. Stall onset has been defined as the onset of a pitching moment break, as observed in the occurrence of an uncommanded pitch rate change. Considering surface roughness in the form of the application of sandpaper to the upper and lower wing surfaces, extending +8% c from the leading edge, the data for the unroughened wing showed that stall onset angle, αSO, increased with stall-entry pitch rate, qSE. However, for the roughened wing, αSO and αCLmzx generally reduced with increasing qSE, such that the combined effect of roughness and pitch rate resulted in reductions in αSO which were in exceedance of the Brumby curve prediction by up to 130% at intermediate roughness (#120 and-#80 grit). Ground effect (GE) on the #40 grit roughened wing, at low qSE, resulted in a further decrement in αSO of approximately 2°.

Aviation Emissions Index Derivation Methodologies from Flight Data, including Black Carbon and Aerosols

The NRC has undertaken aviation emissions flight research. Principally aimed at Heavy and Super Jet Transports, the project has applied the NRC T-33 to measure condensation nuclei (CN), black carbon (BC), volatile organic compounds (VOC) and oxides of nitrogen (NOy) from aircraft flying enroute at high altitude. Most recently, the T-33 and flight profiles, developed under the research, have been applied to the comparative measurement of jet biofuel emissions. During the course of the project, techniques have been developed, for the derivation of estimated Emission Indices (EI) of pollutant species, measured on the ground (during taxy, departure or arrival runway operations) or inflight. In particular, if the jet engine exhaust jet streams were very young, air temperature measurements were used to correlate to empirically modeled turbulent jet mixing fluid dynamics, thereby deriving the applicable dilution factors (DF) to apply to the measured concentrations. For exhaust plumes which were not very young (i.e. not in a state of turbulent mixing, but rather in a state of turbulent or quiescent diffusion), flight data has involved techniques for the full cross-sectional measurement of wakes, including jet wake and wake vortex regimes. Not only has this captured the nature of wake structures under very strong static and dynamic influences, but permitted the measurement of aerosol emission index numbers directly.

Correlation of AIRS Inflight Icing Lift/Drag Effects with Supercooled Droplet Spectra Atmospheric Conditions

During the course of AIRS 1.5 and AIRS II inflight icing flight research operations, the NRC Convair conducted aerodynamic characterization manoeuvres, following and during icing accretion in a wide range of environmental conditions of altitude, air temperature, LWC and droplet spectra. The paper describe the correlation of the effects of inflight icing accretion on the lift and drag of the NRC Convair, a turbo-propellor powered transport aeroplane, with full droplet spectral parameters, for which the small (FSSP) and large (2D) portions of droplet spectra have been combined. The changes to profile drag, CDo, lift-dependent drag, k, and lift-curve slope, a, were analysed initially for correlation with Ta, LWC and FSSP MVD. With the airframe IPS ON, ΔCDo increases did not correlate with SLW exposure. Maximum Δk and Δa degradations correlated with each other, and with air temperature. With airframe IPS OFF, maximum Δa degradations correlated with exposure to SLW, whereas maximum ΔCDo and Δk correlated with exposure to small droplet conditions. Following 2D data archiving by Environment Canada, a detailed natural icing with atmospheric conditions analysis was conducted, in particular for correlation with LWC and the full-spectra droplet spectral parameters of 50VD (or MVD, when defined as median volumetric diameter), 80VD, 95VD and spectral width, approximated as maximum droplet diameter. When combined with LWC, the parameters of 95VD and maximum droplet diameter provided the better correlation of CDo, k and a icing-induced effects, essentially limiting canyons in the contour cross-plot of effects. The analysis also highlighted the need to include the full droplet size distribution for the determination of the most detrimental icing accretion environmental regimes.

Wake Vortex Flight Measurements and Tactical Aircraft WVE Design & Flight Testing Requirements

The NRC has undertaken re-instrumentation of the NRC CT-133 ruggedised research aeroplane, for high altitude atmospheric research. In the process, a new air data system has been designed, and optimized for high data acquisition rates, in particular matching the capability of the aircraft’s FRL inertial reference and navigation system, to derive and acquire wind vector data at 600 Hz. The platform has been used to conduct transport aircraft wake vortex (WV) surveys, and acquire instability wake vortex flowfield data, most recently in comparative straight flight and turning flight manoeuvres, using a Falcon 20 wake generator, to 3 nm distance. Vortex core traverse analysis has highlighted a greater frequency of small-scale vortex segments with high (>150% exceedance of the primary vortex) tangential velocities in turning flight; also, a greater bandwidth in short-wave instability wavelengths, and higher amplitude in long-wave instability, also multi-wavelength, but centred at 3 to 5 generator wingspans, whether in straight or turning flight. Such observations and general considerations of possible tactical jet aircraft wake vortex flowfields suggests the need to establish from flight data, wake vortex flowfield environmental design requirements, in order that contemporary and new tactical aircraft types may be developed with robust flight control systems, able to handle the [frequency amplitude] environments experienced during wake vortex encounters.