Re: Attention erg fans!
- From: Tinus <martijn.weterings@xxxxxx>
- Date: Wed, 24 Aug 2011 19:41:55 +0200
On 08/23/2011 10:19 PM, Carl Douglas wrote:
On 23/08/2011 19:51, Tinus wrote:On 08/23/2011 03:05 PM, Carl Douglas wrote:On 22/08/2011 15:55, Tinus wrote:On 08/21/2011 11:14 PM, Alistair wrote:Human lady-powered helicopter:
Plus, I'm pretty sure that's Mike Sullivan helping out in the back of
the video at the bottom of the article.
What an enormously large rotor blades.
On the principle that there is no spare energy to waste, you need to
maximise air column diameter in order to minimise the energy content of
the air flow (proportional to mv^2, where m is mass flow of air & v is
the induced downwards velocity) for the required thrust (proportional to
mv). This is not a Black Hawk helicopter with energy to burn but has
more in common with a moth.
You get large spans & low foil surface loadings in all low energy, low
speed aircraft (man-powered, solar-powered & gliders).
Although indoors is the safest place to fly such a thing, I wonder about
the influence of ceiling/roof & surroundings on the air flows & the
resulting performance - a point I raised recently in another topic.
Yes you often see large spans in low energy aircraft. But, for this type
of helicopter the main structure needs to be as big as the rotors. This
is not true for mono-helicopters, air planes or gliders where the
aircraft can be made of almost purely wing.
You need to maximise the total foil surface, and not necessarily the
column diameter. If they'd put multiple foils on the same rotor then
they could reduce the rotor diameter. The rotor would hardly require
more material but the chopper main structure can be made a lot smaller.
It is a common misconception that more blades gives more lift where, in
reality, more blades just generate unnecessarily more drag. That's why
modern wind turbines have, at most, 3 long slender blades, & why we
don't see many biplanes & triplanes these days.
As I said, you need to move the entire cylindrical air column. As it
happens, a couple of blades slicing spirally through that column may be
hard to improve on for actual air movement (all the air moves, not just
that which is above the blade), while adding more blades than absolutely
necessary to do this job will add more drag (against the rotation of the
blade) for rather small increase in total air displacement (& may also
increase rotation in that air column).
Nothing in fluid dynamics is very intuitive, but it is fairly logical!
That is if you are indeed moving the entire air column.
In can imagine it is not the case for this low rotor frequency. Because the human needs to get the rotor into motion the operating speed is limited. The scale of the distance travelled by the air before the next blade passes trough is much larger. The situation is much different from the high speed machinery. For low speed systems you see much more blades.
It is much more likely that these blades are accelerating the air close to the blade at a higher velocities than you may think from your mv relation. The entire column is moving on with impulse mv but only after a lot friction and turbulence in the air.
This human driven helicopter rotor should be modelled according to a ventilator instead of the typical machine driven helicopter rotor.
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