Hi -

I'm working on a balloon-driven helicopter toy for my daughter. You can buy these, but they waste a lot of air on an irritating whistle and I thought making our own would be more fun. The basic design is the same as commercially available ones, with a twist (sic): I add a second balloon on top for additional air and therefore prolonged flying time - hopefully...

So I am trying to ascertain the flow rate and velocity of air expelled from a deflating balloon through a tube. Applying Poisseuille's Law and using the calculator at

http://hyperphysics.phy-astr.gsu.edu...ase/ppois.html I get unexpected results...

I enter the following parameters:

pressure difference: 50mmHg (I found a YouTube video where a chemistry teacher measured pressure in an inflated balloon at +-810mmHg in a lab. Difference to sea-level ambient pressure of 760mmHg - I'm at the coast - is therefore 50mmHg. Several sources online estimate this as 0.5 to 1.5 psi and 50mmHg is about 1 psi - seems to fit)

tube radius: 0.2cm (the air is expelled through a 4mm diameter drinking straw)

tube length: 20cm

air viscosity: 0.000181 Poise (online sources)

I then solve for volume flow rate (by clicking Volume Flow Rate in red above as site directs) and get the ridiculous result of 11567.43 cm3/sec - that's 11.5 litres of air per second, through a drinking straw?! I have measured the flow rate by inflating the balloon to a guesstimate 2.5 litres of air and then timing how long it takes to deflate (around 13 sec). That is more or less 200cm3/sec - orders of magnitude out. Since I am fairly sure of the other parameters, I enter the measured flow rate and solve for pressure difference and get 0.864mmHg (0.017 psi) - equally ridiculous. I also tried setting the measured flow to 200cm3/sec and 50mmHg pressure and solving for viscosity - and get 0.01 Poise - very close to the viscosity of water!? What am I doing wrong?

Is there some principle that says that the static pressure in an inflated, sealed balloon is not the same as when the balloon is allowed in the process of deflating? Or, how relevant is the tube length really? Air has such a low viscosity, I can't imagine halving the length of the straw will double the flow rate as the calculator suggests. Is Poisseuille's Law maybe not applicable to compressible fluids? Can't find anything to that effect online.

I'm well confused! Please help...

Doogli (down in South Africa)