On 01/10/2010 19:25, ThinAirDesigns wrote:
@Ronald Raygun
The exactly-at-windspeed scenario is a bit of a crazy special case ...

small snip

Therefore the presumption that the wheels must generate 1hp
and the prop must therefore provide 10lb of thrust is a bit silly,

You did notice that it was *you* who brought up the "crazy special
case" of exactly at wind speed. :-)

Actually I brought it up a few days ago... but I'm not sure anyone
noticed...


It is your own special case that has brought into play the seemingly
odd (but true) case of the 100% efficient propeller which can produce
10lbs of force on 0hp.

The way the term "efficiency" is applied in any given situation can be
a bit arbitrary. As I previously mentioned, propellers and fans use
the term very differently because they have very different
applications and thus very different definitions of "work". What is
most important is that the the term and it's relevent formula be used
consistently throughout any given. We can't use one definition at 2x
wind speed and another at exactly wind speed. I have been consistent
in my application.

It just so happens that the standard propeller efficiency formula
turns up looking a bit odd (but still valid) in static conditions.


Ron,

imagine a case where in order to generate that 10lbs of thrust at zero
relative windspeed the prop needs half a horsepower. Real losses, quite
achievable - in fact I think TAD did better in real life.

The cart has that 10lbs of thrust, and it's going downwind at exactly
windspeed, and that 10lbs through the wheels at that speed makes a whole
HP. You have half a HP left over to accelerate with!

Andy