Aerodynamics:
Laboratory and CFD Investigation:
Aerodynamic Characteristics of a Slender Wing-Body Configuration at High Incidence
Introduction:
The objective of this experiment is to investigate the aerodynamic characteristics of a slender wing-body combination over a large incidence angle range. In particular, the non-linear aerodynamic behaviour associated with the separated-vortex flow from the highly-swept, sharp leading-edge wing is to be studied. The results obtained in the experiment are to be compared with predictions using the vortex-lattice computational method. The predictive capability of this method when using the Leading-Edge Suction Analogy of Polhamus is to be investigated.
Experimental Arrangement:
The slender body has an overall length of 1.094m and a maximum diameter of 0.06m. The wing is of cropped-delta configuration, with a leading-edge sweep angle of 70o. No.1 Low-Speed Wind Tunnel is to be used for the experimental work. This wind tunnel has working section dimensions of 1.14m width and 0.82m height. Lift forces, drag forces and pitching moments are to be measured over an extended range of positive and negative incidence angles. The pivot point for pitching moment measurements cpivot is 0.14475m behind the root quarter-chord position for the wing. The geometric characteristics of the wing are given in the table below:
Leading-Edge Sweep Angle: L.E. 70o
1/4-Chord Sweep Angle 64.1022o
Root Chord 0.437m
Tip Chord 0.08m
Wing Span: S 0.26m
Zero-Lift Incidence Angle: aL=0 0o
Mean Aerodynamic Chord: cmac 0.2996m
Experimental Procedure:
With the wing-body model at zero incidence angle, and with the wind tunnel switched off, set to zero the voltage outputs from the load cells for lift, drag and pitching moment. Switch the wind tunnel on, dial up a speed setting of 700 and allow the wind tunnel flow conditions to stabilise. Pitch the model to an incidence angle of -14o. Record the temperature in the wind tunnel working section. Once flow conditions stabilise, read the voltage outputs for lift, drag and pitching moment and record the Betz manometer reading (in mbar). Repeat these measurements at incidence angle increments of 2o up to a maximum value of +28o. Record the temperature in the working section of the wind tunnel after the final set of measurements. Return the model to zero incidence angle and dial the wind tunnel speed down to zero. Record atmospheric pressure (in mmHg) using the barometer in the laboratory.
Data Analysis:
Convert all lift, drag and pitching-moment measurements in mV to N and Nm respectively using the force balance calibrations displayed near the wind tunnel working section.
Dynamic Pressure in Working Section
Force and Moment Coefficients
Wing Tunnel Wall Corrections
The wind-tunnel walls will influence the downstream development of the wing trailing vortex system. The measured incidence angles and drag coefficients need to be corrected, as follows:
S is the wing area and CR is the cross-sectional area of the wind tunnel working section.
Air Density and Viscosity
The average air density and viscosity during the experiment are required in order to determine the average free-stream velocity and Reynolds number of the experiment. Using the measured atmospheric pressure Patm (in mmHg) and the average temperature in the wind tunnel working section Ttunnel (in K):
Average Velocity and Reynolds Number
Polhamus Leading-Edge Suction Analogy
This accounts, in an empirical way, for the additional lift and drag generated as a result of the vortex-flow separation from the sharp leading edge of a swept wing. The lift and drag coefficients are given by:
where:
Note that these empirical equations are applied to the linear aerodynamics results predicted by the VORTLATM1 vortex-lattice method. The output file from VORTLATM1 contains both the linear and non-linear aerodynamic predictions.
Presentation and Discussion of Results
1. Plot experimental lift, induced-drag and pitching moment coefficients against incidence angle.
2. Run the vortex-lattice method VORLATM1 to calculate the linear aerodynamic characteristics for the wing and the non-linear aerodynamic characteristics using the Polhamus Leading-Edge Suction Analogy. Compare both sets of predictions with the experimental data for lift and induced-drag coefficients, plotting experimental data and the two predictions on the same graphs. Note that the panel-sensitivity investigation discussed in 4 below should be carried out first, to determine the required span-wise and chord-wise panel densities to give converged lift and drag predictions.
3. Discuss the ability of the vortex-lattice method to predict the aerodynamic characteristics of the slender wing-body configuration, particularly at high incidence angles.
4. Investigate and discuss the sensitivity of the vortex-lattice method predictions to the number of span-wise and chord-wise panels used in the calculations. Use a minimum of 2 chord-wise panels.
5. Discuss the limitations of the vortex-lattice method for predicting high-incidence aerodynamics of slender configurations involving vortex-separated flows.
6. Discuss how vortex-separated flows can be exploited in aircraft manoeuvring performance.
Dr L J Johnston:
TAXES: Topic sentence, Assertion statement, example, Explanation and Significance.