On 04/16/2010 12:54 AM, GRAHAM SINGLETON wrote:
> in theory only is a 2 blade more efficient, you get more vibration that
> can't be balanced out.
I agree about the vibration, although if this is a valid argument, why
not add even more blades? 7 blades gives even less vibration. ;-)
> More vibration gives more disruption to laminar
> flow so more drag.
Well, I can't follow this one. Can you explain why? The vibration of a
2-blade prop has nothing to do with the blade itself, but to the fact
that one blade is working harder than the other blade if the wind isn't
exactly axial. Each blade however doesn't "know" that there is another
blade. In fact, this same inbalance exists with a three blade prop, but
you just don't notice it because the "peaks" are no longer symmetrical
but 120 degrees out of phase. For the air flowing over a blade, it
doesn't matter though.
> This is also why 2 blade windmills are more likely to throw blades.
Not because of a less efficient airflow, but because vibrations can
weaken the attachment of the blades.
Luckily, airplanes have been flying for decennia with two blades, so
there is still enough knowledge to construct a blade that doesn't fall
apart.
Frans
|