Aerodynamic testing at the Purbeck School Science Fair

tl:dr – Make LEGO models and test them in our Scale Wind Tunnel, test and improve!

At the recent Purbeck School Community Science Fair, I was fortunate enough to be running a Scale Wind Tunnel manufactured by Clive Evans at Scale Engineering.

Website

Quote in wired magazine about making your own wind tunnel. 

These are truly magnificent pieces of engineering, and we were very lucky to be able to use one for the day.

Our version had been outfitted with a Lego studded test bed, which allows for the rapid testing and prototyping of Lego models.   As you can see from the picture above, the tunnel is a finely-engineered beast.

 

On the left of the tunnel is a laminar air intake, which forces the air through a honeycombed aluminium structure to reduce the turbulence.  Once through this, the diameter of the tunnel reduces sharply, increasing the airspeed through the test bed.

The LEGO bed is attached to two load sensors as can be seen below.  One senses movement in the vertical direction, which indicates a Lift force in Newtons, and the other senses the Drag generated by the model in Newtons.  The machine outputs the readings to two digital 7-segment LED displays, and are accurate to 0.01 of a Newton.   The air exits via the exhaust side, and is driven by a mains-powered air conditioning fan.  This is adjustable up to 18m/s at top speed, so the tunnel can really push the limits of most lego models.

It is very hard to directly visualise the airflows without some seriously dense smoke (which would set off alarms, asthma), but we have cotton thread on a wand that can be used to directly visualise the airflow over a model by tracing it around the outside of the model.

After the initial setup in the school library, I opened up my Big Box o Lego (TM), featuring many classic LEGO pieces from the golden era of 1980s space lego, and infused with Dexter’s more contemporary LEGO star wars pieces.  We even took my younger son’s duplo airplane as seen above.

The basic procedure was outlined:

(1) Build a model, get it tested in the wind tunnel

(2) Record the Drag and Lift Values from the tunnel

(3) Refine your design, retest.

(4) Repeat.

We had an incredible variety of models made, from the austere and efficient to the beautifully sculpted and adjustable masterpieces.  Over 45 different models were made and refined over the course of the day, involving multiple rounds of testing.

The best models of the day tended to have the best Lift to Drag ratio, the best being 1.09N Lift,  0.38N Drag. We discussed how you could make them more efficient, including using flat pieces to reduce drag around the studs, removing unnecessary fandangles and gizmos.

The crowd that gathered was equal parts boys and girls, as well as equal part child and adult.  LEGO is a great leveller, and many families had to pull away their participants in order to see the amazing things on offer elsewhere in the Science Fair.

This close-up shows the control unit, with readouts as well as the test bed.  Models to be tested are also lined up on the bottom.  That's me twiddling knobs.
This close-up shows the control unit, with readouts as well as the test bed. Models to be tested are also lined up on the bottom. That’s me twiddling knobs.
This was a particularly sassy model, with adjustable angle of attack for the wings.  The builder decided that inverting the wings would reduce drag.  He was right!
This was a particularly sassy model, with adjustable angle of attack for the wings. The builder decided that inverting the wings would reduce drag. He was right!
I was constantly surprised at the amazing variety of designs that people were able to make out of my limited pool of LEGO.
I was constantly surprised at the amazing variety of designs that people were able to make out of my limited pool of LEGO.
Two different solutions to aerospace design: reduce the surface friction, or go for maximum lift
Two different solutions to aerospace design: reduce the surface friction, or go for maximum lift
The fan running at 70% power.  This was, of course, off limits to members of the public in the rare case of a 'testing to destruction' event.
The fan running at 70% power. This was, of course, off limits to members of the public in the rare case of a ‘testing to destruction’ event.
This gives an idea of the beautiful engineering that went into the creating of this incredible wind tunnel.
This gives an idea of the beautiful engineering that went into the creating of this incredible wind tunnel. Thanks to Clive Evans for the loan of the wind tunnel.
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Building the Phenoptix mearm from scratch:Sourcing parts and cutting your own

Having already bought and assembled a MeArm kit from Makersify, I decided to use the open hardwire files from thingiverse to laser cut all the parts from 3mm Acrylic.

Thingiverse files.  If your DT department in school is as awesome as mine, they’ll show you how to use the cutter so you can plan and cut your own designs.

 

download the DXF file from thingiverse and start cutting
Laser cut parts from the DT department, hot off the cutting bed. JCB yellow is a nice touch

I had some trouble with the .dxf files running in 2D design, so I printed the design out on paper first and then did all of the measurements, comparing it to the real mearm that I built in order to make sure the scaling was perfect.  (If not, the servos won’t fit in the holes, and the bolts won’t self-tap)

I also cut a mountboard version for the students to see, and get to measure with vernier callipers to check the dimensions.  (It turned out it had scaled wrong vertically, and the test-print would never have assembled as all the circular holes were ellipses!)

 

IMG_1678
Parts arrived in the post, enough for 4 robots, but we’ve only enough acrylic for one at a time. 

In order to build the robot, you’ll need a bunch of M3 bolts, and some Turnigy 9g micro servos.  I picked up a load of servos cheap on ebay , at £11.99 for 4, which seems like a good deal.   (I would get a spare set, in case you burn the servos out during a build!)

I used namrick.co.uk to get small batches of the screws, as they were by far the best deal I could find in the UK, cheaper by about £25 than all other dealers.

IMG_1680

 

Once the students had done the inventory, they began to follow the assembly instructions, starting with the baseplate.  Here you can see that the longer M3 bolts are used like standoffs in the assembly.  You can also see the mounting holes to bolt on an arduino for controlling the servos.

 

We’ll keep you posted on the build as it happens, and then how to control it with scratch on the raspberry pi.