Sunday, November 21, 2010

DDWFTTW

14 comments:

Frankie said...

...I'll have croutons with this soup, thanks.

ChrisP said...

Passed some RS200s racing today and their spinnakers got them going pretty fast, but not faster than the wind....

Litoralis said...

Oh, so that's why it works.

JP said...

Ok, useful looking graph: now explain what it means and through in a couple of equations for good measure.

JP said...

Doh!

"throw" not "through"

Tillerman said...

Either you get it or you don't. I'm afraid that I can't add any more explanation to this chart today because I have taken a vow of silence as part of my new religion of Wordless Weekends.

Oops. I broke my vow. Now I will have to appease the gods by sacrificing two pizzas (no anchovies, please), a bottle of Gran Patron, and a wash cloth.

ChrisP said...

I realised how it worked (at least on an intuitive level) when someone said 'the prop isn't driving the wheels - the wheels are driving the prop'.
So...if you imagine the vehicle going at 30mpg with a following wind of 10mph, the wheels think they are doing 30mph, but the prop only has to provide the power to do 20mph. It is the difference that provides the power.

ChrisP said...

All references to mpg should be interpreted as mph. Mpg is a different argument....

O Docker said...

I have no idea what this graph shows, but I think if you have any mutual funds, you should sell them now.

Then, drink the Gran Patron.

Tillerman said...

Damn. I already sacrificed the Gran Patron.

I still have the wash cloth, though.

Baydog said...

Wash cloth. Did someone say bed?

Joe said...

Zzzzzzz

BeachComber said...

I was thinking about this in the shower when it came to me. First here's the technical background. Props have a certain ideal efficiency that cannot be exceeded (and it is always less than unity) because they're operating in a stream of moving air. If the air they were working in was perfectly still, the ideal efficiency would be one, but the air can't be still because props have to accelerate air to create thrust.

In the case of a helicopter hovering in ground effect (IGE), the ground slows down the air to an extent, so hovering (IGE) is more efficient than out of ground effect (OGE).

Now, if you imagine the machine in still air, a prop driving a vehicle whose wheels drive the prop, it wouldn't work. It would be a perpetual motion machine, undone by the fact that efficiencies are always less than one. Now if you add the tailwind component to the air the prop is working in, it becomes viable.

Having said all that, Chris P's explanation is much more succinct just as valid.

Pat said...

Just strap on a couple of JATOs.

Or sail a board-up Laser down a log flume chute.

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