Re: How does the body of a whirlygig affect the time it takes to fall?

What Fun!  And a very cool way to do some actual scientific research!

First off, a helicopter is different from a gyrocopter, which is different 
from your whilygig.  The difference is in how the air is striking the 
blades.

The engine in a helicopter is turning the blades through the air. The 
blades work like narrow wings generating lift.  This is why a helicopter 
can hover in one place.  If the engine quits, the pilot reduces the cyclic, 
flattening the blades so that they keep spinning due to momentum.  This 
provides enough lift to let the helo drop slowly so a “soft” landing.  This 
is auto-rotation.

The air acting on your whirligig is coming from directly underneath.  
Notice that the two blades are pitched upwards at the ends, and are 
staggered from each other. When you let it go, the air it drops through 
hits the blade and is deflected towards the tip.  This creates what is 
called a force-couple, which causes the whirligig to start to rotate around 
its vertical axis. The faster it drops, the harder the air strikes the 
blades, and the faster it spins.

This, to me, is not the interesting part.  A science experiment changes one 
aspect of a system and observes what happens differently.  I made a 
whirlygig similar to your from your sketch.  Then I made another, with 
wings exactly the same size – only using one strip of paper, so the wings 
were directly opposite each other. This one wouldn’t spin.  I dropped both 
of them together – and the spinning one consistently fell more slowly than 
the same sized but not-spinning version! 
 
So, what I think is happening is this:  Air is moving faster over the wings 
of the spinning whirlygig. This is generating a very small amount of lift 
due to Bernoulli’s Principle, which essentially says that air moving over a 
surface creates a slight vacuum on the top and higher pressure underneath.  
It is what keeps airplanes flying.  This extra lift is slowing the decent 
of the spinning version.  

But don’t take my word for it.  You have a wonderful field for 
experimentation here.  Make whirlygigs with long, thin blades, short, fat 
blades, curved blades, trim tabs on the tips and edges, single-strip 
versions with the blades tilted slightly so they spin too.  Add more 
weight, lengthen the stem, use heavier paper.

 Change only one aspect of any whirlygig at a time, compare the drop times 
to a “constant” whirlygig so that every one you make ic compared to a 
single model, and record your results.  Use a stopwatch if you can get 
someone to help you and record drop times.  Also, any interesting things 
that happen during the drops (“the whirly gig flew up to the ceiling, then 
out the window”—kidding)

All these results may not advance science at all, but you will have done 
some real science—if only for yourself.  Now excuse me, I have to go find 
some paper, weights and a tall ladder.