Experimental Setup Report Samples

REPORT: WIND TUNNEL MEASUREMENT ON AN AEROPLANE MODEL

Report: Wind Tunnel Measurement On an Aero-plane Model
Abstract
The experiment makes a great use of a wind tunnel to determine a few features of an aeroplane model. The results are used to determine the trimmed lift coefficient at the two tail settings with the CG assumed to be in a realistic position and the corresponding static margins and neutral points. The aircraft-less-tail has an aerodynamic centre position which was to be determined through calculations of the data achieved from the experiment. In finding out the lift curve slope of the aircraft, with or without the tail, a (1+F), the lift curve slope, a1 is also to be determined. The aeroplane model also experienced variations of the downwash at the tail with incidence and induced drag factors, which were also calculated and established from the results of the experiment.

Introduction

In aerodynamics, wind tunnels are more often used to test aero-plane models to ascertain the proposed engine and aircraft components. At this stage, the aero-plane model is put at the test section in a tunnel, while air is flown towards the model. During this a number of instrumentation are involved to determine the different forces acting on the model. Specifically, there are four major types of wind tunnel tests. Some instance of model testing, the forces along with the moments are determined (measured) directly. The model is usually mounted on a force balance in the tunnel. The output as a result of the balance is quantity that has a special relationship with the forces and moments prevailing on the aero-plane model.
The balances can be used to derive and measure drag and lift forces on the model. Most important is that the balance should be calibrated against established (known) force values either before, in the midst, or the test. Fluid force measurements put into consideration data post-test or reduction processing for March number or Reynolds number and what they bring about on models while testing. Another vital factors to keep in mind is always to note the reference values or rather, the bench mark measurements that are used for data reduction.
Therefore, the experiment probes and aims at measuring of lift, pitch, and drag acting on the model of an aeroplane. Three configurations out of the seven possible incidences of the practices are done in a wind tunnel, while the data obtained from both the configurations are used to derive quantities that relate to its static stability, mainly the longitudinal ones.A twin-jet, executive aeroplane is represented by the model.

At first, the equipment was first switched on before loading the main tunnel control program. While the model datum is kept horizontally, the incident indicator was pegged at zero. The console was left switched on throughout the experiment. The experiment continued and conducted in three phases: without the tailplane; with the tail plane at +1; and with tailplane at -3.
The barometric pressure and the tunnel air temperature were recorded while controlling any variations in either temperature or pressure. The incidence was set to zero after checking if the tunnel doors were closed. The balance was then zeroed and the take zeroes command button was then clicked. Wind-on measurements were taken for every tail setting at incidences of -20 with increments of 20 up to 100. Each test was to have an air speed of about 37m/s. Speed setting that was with small errors were acceptable as such errors would slightly alter the Reynolds number while maintaining the dimensionless coefficients.
The model has a tail plane that can move in both ways without necessarily having a separate elevator. It is to ensure that the pilot control column movements alter the tail setting angle ȠT while the normal a2n disappears from expressions relating to the tail lift or overall pitching moment. A single pin attaches the tail plane. The pin is in a series of holes used to fix the angle of tail setting. Movements range from the most forward hole at +20 to -50 at intervals of 10. The angles are not taken relative to the zero-lift line, but in relation to the fuselage datum line. A 3-component mechanical balance which can be operated remotely is used to mount the model. The arrangement of the balance is in the positive directions as seen from the operator’s position. The incidence is in the clockwise direction with degrees as units. The lift is in the upward direction, while drag in the downstream direction with both having 1bf as the units. Finally, moments are in anti-clockwise direction with 1bf-ft as the units.
The model has a mounting at the inverted position to retain a clean surface to the wing, and avoid the wake from the supporting struts impinging upon the tail. The positive directions of all of the quantities listed above should be in the correct senses. One can achieve this by correcting the measured qualities from the point of view of the model. The locks on the weigh-beams are always engaged when adjustments are made to the model and while concluding the experiment. Upon pressing the tunnel stop button, the balance is automatically locked. An operator of the balance has to be conversant with the natural frequencies of the balance, hence he requires a little practise before serious use. There is a necessity of estimating the mean point of balance in situations whereby the conditions are unsteady.
The tunnel was started and the wind speed adjusted as the balance was operated. The force values were read and the results used to get some calculations previously described. The fluctuation of the force reading was noted and the reading accuracy estimated. Later, the incidence was set to the next value and the process repeated. The tunnel, stopping button was pressed after making the measurements over the incidence range with tailplane off and the tailplane set at ȠT =+1. The previous procedure was then repeated. The tunnel was stopped after taking the measurements over the same incidence range with ȠT =+1 and the tailplane fitted at ȠT =+3. The measurement procedure was repeated for various incidence from 00 to 1.

Results

The static pressure difference of the airstream at entry and exit from the tunnel contraction was the wind speed. Tunnel contraction was usually the section immediately upstream of the test section. The reference pressure in this tunnel at the speeds used in the experiment also was exactly equal to the dynamic head and displayed in mm of water on a Betz manometer. The manometer provides an accurate guide to the tunnel speed than a petite-tube mounted in the test section, in areas where the presence of a model might alter the local static pressure. The tunnel used was closed-return type with closed-section and driven by a 30 kW motor. Its maximum speed was approximately 50 m/s while it’s working at atmospheric pressure. The tunnel was ran at 37 m/s for the sake of the current experiment.
Significant features of the flow about the model were observed by means of wool tufts. These features include the onset of the wing stall and its spread across the wing, the location of the wing wake in relation to the tail and the sail of the tail. The incidence was then increased beyond the maximum of 120 as used in the previous measurements in order to measure the stall. It was done in gentle steps up to about 180. The incident was finally returned to 00 before switching off the equipment.

The longitudinal static stability experiment characteristics of the model are as listed below.

The actual T.E. and the T.E. of the MAC coincides in both cases because the train edges of both the tailplane and the wing are unwept. The datum forms the leading edge of the wing MAC with the quarter-chord point of the tail MAC being 0.438 aft of datum. The attachment of the main balance struts to the wings are 0.0586 aft of datum.

Discussion

The graph of CMp against CL is as shown below.
For a give value of CL, the values of CMp at the two tail settings ȠT1and ȠT2 are CMp1 and CMp2 respectively. Thus,∂Dm∂ȠT=CMp2-CMp1ȠT2-ȠT1, gives a mean value for CL to be 0.5. The correction δCM is added to all the tail-on values of CMp.
A graph of CL against true incidence α for the three configurations have the slopes at each tail-on curve at the lift coefficient being CMG=0 at the tail settings.
The plot of CD against CL2 for the tree configurations were straight lines. The coefficient of zero lift drag CDo would be the intercept on the CD axis. The gradient of the graph would be k/ (πAR). The “free-flight” values would differ from the obtained values since the model is generally not in trim making the tail lift differ from the trimmed value.

Conclusion

The experiment achieved its aim of determining the pitch, lift and drag acting on the model aeroplane. The three configurations helped determine the exact features of the aeroplane at different conditions as quantities were derived in relation to the system longitudinal static stability. The experiment makes a great use of a wind tunnel to determine a few features of an aeroplane model. The results are used to determine the trimmed lift coefficient at the two tail settings with the CG assumed to be in a realistic position and the corresponding static margins and neutral points. The aircraft-less-tail has an aerodynamic centre position which was to be determined through calculations of the data achieved from the experiment. In finding out the lift curve slope of the aircraft, with or without the tail, a (1+F), the lift curve slope, a1 is also to be determined. The aeroplane model also experienced variations of the downwash at the tail with incidence and induced drag factors which were also calculated and established from the results of the experiment.

References

National Aeronautical And Space Administration, 2014. NASA. [Online] Available at: http://www.grc.nasa.gov/WWW/k-12/airplane/tuntest.html[Accessed 13 March 2015].
Doane, Stargel R. (2001). A Wind Tunnel Technique For The Identification Of Ship Airwake/Rotor Downwash Coupling. Print.
Peters, Rhonda R. et al. (2010). 'Static VAR Compensation Of A Fixed Speed Stall Control Wind Turbine During Start-Up'. Electric Power Systems Research 80.4: 400-405. Web.