Top-Heavy Airborne Pyramids for Improved Stability

Fascinating NYU experiment reveals surprising data on fluid-dynamics for top-heavy airborne structures.  

From ScienceMag (video) via Wired Science

"Think that floating pyramids are more metaphysics than physics? Think again. Results just in from an experiment that levitated open-bottomed paper pyramids on gusts of air reveal a curious phenomenon: When it comes to drifting through the air, top-heavy designs are more stable than bottom-heavy ones. The finding may lead to robots that fly not like insects or birds but like jellyfish.

The researchers placed hollow paper pyramids inside the cylinder. The objects were about 1 to 5 centimeters high and were made of tissue paper or letter paper on carbon fiber supports, like tiny homemade kites. Physicist Bin Liu led the experiments, attaching a beadlike weight to a post running down the center of the pyramid and changing the height of the bead to give the object a different center of mass. Common sense says that the pyramid should be most stable when the bead is at the bottom of the post, like ballast in the hold of a ship. But when the team released the pyramids over the subwoofer, the opposite was true: the bottom-heavy pyramids were likely to flip over and fall, whereas the top-heavy ones remained upright and continued to hover (see first video), the group reports in an upcoming issue of Physical Review Letters.

Zhang's team suggests that flapping pyramid or cone robots could combine stability and maneuverability. They would quickly right themselves if they leaned further than 30° in any direction, but within 30°, they should move freely."

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Comment by Shannon Morrisey on February 10, 2012 at 6:18pm

I gave this a little more thought and have a very simple theory about what's happening here.

Building on Cliff-E's idea - but assuming the force's orientation to the cg is not the driving factor:
Gravity has little actual bearing on fluid dynamics and in this case had distracted me from a more obvious explanation.

While the lower center of gravity will force horizontal orientation it can also have adverse effect on a surface's natural tendency to align and balance itself by seesawing. There's probably some well established theory/Law/equation on this.

As the bottom-weighted pyramid begins to tilt and the high pressure side rises, the downward force from the bottom weight condenses those air molecules by pulling the elevated edge down jut enough to trap the high pressure until it reaches a 'boiling' point and then topples over.

The top-weighted configuration on the other hand has a subtle reactionary / pendulum^-like effect. When air pressure increases on one side, the weight adds just enough additional downward force to the lower (low pressure) side to increase that pressure and balance it off. Alternately / concurrently the top-weight may be 'torquing' the high edge to tip just enough to release the pressure without toppling, then levels off.

…or it could just be this subwoofer artificial thrust environment 'insulating' the top-weight in a vortex [Coanda/gyroscopic-like effect] which would be a huge letdown.

For anyone interested, I've been trying to adopt a fresh empirical perspective on things like this (and life in general) after having read this riveting article on how our notions of causality are wreaking havoc on scientific method and progress.


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