Speaking of Daniel Bernoulli, these last few posts have reminded me of back when I was about twelve years old, and very into helicopters. For a while, Igor Sikorsky was my hero, and I didn’t think it was fair that kids weren’t allowed to have helicopter licenses. Unlike airplanes, helicopters can hover in one place, and to my twelve year old mind, that was just about the most magical thing ever.
I learned everything I could about all things helicopter, from coaxial rotors to gyrocopters to swashplates — swashplates are amazingly clever.
The only thing that really bothered me about helicopters was the downwash. Those huge spinning rotors create a serious downward wind force. If you’ve ever seen people trying to get onto a helicopter while its rotors are spinning, you have an idea how large this force is.
So I set about trying to design something that could hover in the air but would produce no downwash. And it was, in fact, Daniel Bernoulli’s principle that inspired me to think of such a crazy thing.
More tomorrow.
One reason for the downdraft is that the upward force of the air on the rotors creates an equal and opposite force from the rotors on the air – the air moves so the force becomes a continuous draft to keep the vehicle aloft. We only don’t get the same effect from aeroplanes because they are higher and do not hover.
The rotors don’t so much create a vacuum above, rather a pressure difference between the air above and the air below; hence Bernoullli’s point about the aerofoil shape.
If you could create a circular craft with rotors that moved around the outside of it, then the same length of rotor would sweep a larger area, so given the same craft weight it would require a smaller pressure difference to stay aloft and thus have slower rotors; your downdraft is simply distributed over a wider area.
Phil: In regard to your second paragraph, the only sensible definition for a “partial vacuum” is “less pressure than somewhere else”. So I think we’re saying exactly the same thing in different words.
I agree completely with your larger point — indeed, the downdraft is distributed over a wider area (and therefore does not cause a problem for a person standing directly beneath).