NELSON MKII WIND TUNNEL DETAILS

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THE SCIENCE OF MEASUREMENT

 

You would not believe the inane comments we get about small scale wind tunnels. It seems some folk miss the point entirely or are deliberately obtuse. A tunnel of this scale is a handy tool for gauging performance relative to other designs tried at the same scale. It is not a definitive tool. For that you'd need a full size tunnel. The point though is that here you have a tool in you own home/office/workshop where others do not. With such an instrument on your doorstep you can do so much more economically and faster than other conceptual designers. You will learn a lot just from making it. The steerage is invaluable compared to the time it takes to build the unit. You can add to the accuracy of measurements and analysis by going digital, linking a computer to the strain gauge tunnel outputs.



A key component of any wind tunnel is the fan assembly.  A fan should be able to generate high wind speeds, sufficient for the vehicles being tested. 

 

Take a look at the propeller and motor to the left.  The 1hp motor is mounted on eight springs to reduce vibration, rubber damped.  The whole fan module is then positioned on rubber mounts, which in turn channel vibration through a weighted plywood frame.  The net effect is to pass the vibration through materials with different natural frequencies, so acting as a vibration filter, much like coils and capacitors are used to filter out unwanted frequencies in Hi Fi speaker systems.  Lead weights are also employed to give mass to the mild steel base frame unit.

 

 

 

 

 

Spring mounted 1hp fan motor

 

 

Air temperature and wind speed were also monitored and smoke could be trailed over models to visualize airflow, to highlight problem areas.  This tunnel could fit comfortably on an office desk.  It measured just 60" x 40" x 15".  Being a closed circuit design it was not unbearably noisy in operation. 

 

The plexiglas chamber provided superb all round visibility, which is so important.

 

Strain gauges provided measurement of drag, lift and down-force.  A rotating table was incorporated so that vehicle models could be tested in side wind conditions.

 

Plexiglas test chamber viewed from underneath

 

 

 

 

It is necessary to have variable speed control of the main fan unit.  The control panel above shows temperature gauge, amp and volt meters for the main motor and hours run meter.  There are other switches for smoke trails, etc.

 

An ordinary domestic light dimmer switch was used to vary the voltage supplied to a mains transformer, which in turn supplied 12 volts to the fan motor.

 

Wind tunnel control panel

 

 

 

 

A wind tunnel is a complex piece of engineering.  The Wright Brothers were the first inventors to use such a tool to compile lift and drag tables for various wing shapes.  It is much more difficult to design a balance to measure lift and down-force on each wheel of a vehicle.  When working at such a small scale, accuracy of measurement is essential.  Fortunately, electronics come to the rescue, with compensated amplifiers providing multiplication of movement (lift/downforce) up to ten thousand.

 

 

 

 

Five element balance with oil damping units (slightly the worse for wear)

 

 

 

 

Five of the above RS strain gauge amplifiers gave separate channels of information to provide a range of measurements to include drag, lift and down-force for each wheel.

 

Each amplifier is fed information from a bridge of four foil resistors bonded to each element of the balance, two on each side.  There are five elements to the balance, consequently, you will need quite a few (20) strain gauges and some patience during the epoxy bonding and positioning.  The marking out must be exceptionally accurate for consistent results.

 

 

 

 

RS strain gauge amplifier

 

 

 

 

 

 

The components used in the making of this tunnel, were not exactly cheap if you are on a small budget, but more importantly, with suppliers such as Radio Spares (RS) they at least are widely available to amateur scientists and enthusiasts, such as myself.

 

Plan out any project and cost it well in advance, or spend will spiral.

 

 

 

Single laser cut 5mm strain gauge - foil resistor

 

 

 

 

 

Here is the close up wiring of one of the five steel elements against which bend fractionally stretched one side of the plate (increasing resistance) and compressed (lowered resistance) the other side of the plate. The sensitivity (voltage) multiplication effect of such a bridge is well known.

 

The completed assembly is coated in silicone for environmental protection.

 

The build of this wind tunnel took approximately 2 months from start to conclusion.

 

Strain gauges bonded to steel element - 

2 on each side to form a wheat sheaf bridge

 

 

 

Nelson's small scale, desktop wind tunnel, with the BE1, bluebird electric model in the chamber

 

 

MODEL MAKING STAGES

 

Steps 1.  Mark out your plywood and cut formers

Step 2.  Assemble plywood formers 

Step 3. Fill formers with foam and shape

Step 4. Coat foam plug with plaster

Step 5. Sand, add detail mouldings and prime

Step 6. Paint finished model

 

 

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The latest electric world land speed record car design, 350mph plus

This is the kind of result you can expect from your wind tunnel - a super slippery 350+ mph electric streamliner. This particular car is charged by solar power, making it probably the world's fastest solar powered electric land speed record car. She also has battery cartridge exchange built in, for instant EV refueling.

 

Bluebird batteries, electric motors and solar panels legend

 

 

 

European Commission star circle logo  NineSigma, Canada investing in innovation  Technology Strategy Board planet earth logo

 

 

 

 

 

LINKS

 

http://www.grc.nasa.gov/WWW/k-12/airplane/tunnel1.html

http://www.roboticsschool.ethz.ch/sfly-summerschool/programme/1.1-Leutenegger-Airplanes_Aerodynamics_Performance.pdf

http://www.landracing.com/forum/index.php?topic=862.235;wap2

http://www.geversaircraft.com/wt/wtsmall.htm
http://mae.uccs.edu/jroney/SpaceGrant/WTReport.pdf
http://www.grc.nasa.gov/WWW/K-12/WindTunnel/wandering_windtunnel.htm
http://www.ezcomm.com/Windtunnel/index.html
http://www.windtunnels.arc.nasa.gov/pics/9x7/9x7dimPlan.html
http://www.theplanpage.com/Months/2306/tunnel_files/tunnel.pdf
http://www.galcit.caltech.edu/awt/index.html       http://www.galcit.caltech.edu/awt/tenfoot/wirerig1.jpg
http://lisar.larc.nasa.gov/IMAGES/SMALL/EL-1999-00415.jpeg
http://www.nlr.nl/documents/flyers/f137-02.pdf
http://www.lboro.ac.uk/departments/tt/research/aerodynamics/wind-tunnel/tunnel.html
http://connect.larc.nasa.gov/connect_bak/pdf/measure.tools.guides.pdf

 

 

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