How can an vehicle move faster than the wind that is powering it?

ThinAirDesign

I'm not sure what you need me to explain. My post was not in response to yours -- I hadn't even seen yours when writing mine. I was writing my explanation for schroder.

Same or different -- can't say as I wasn't referencing your comments.

JB

ThinAirDesign

I absolutely love the "still air relative to the moon" bit. If I'm picking a frame of reference for sailing -- that's the one I'm using every single time. And hey, if the wind is still, relative to the moon, we'll only need a one foot mast and a hankerchief -- saves a lot of room down below.

This "physics" forum is a real kick.

JB

spork

Someone made a comment about how diagrams are so "second degree university" - but there hardly seems to be 3 people here that understand the most basic high school physics concept of inertial reference frames. Discussing the prop-cart going downwind faster than the wind is beyond hopeless.

Heck, we've presented every possible type of evidence of what ice-boats are capable of - and all that has resulted in is silence.

Trond

I wonder why.....let me know when you are ready to discuss the cart

rcgldr

[QUOTE=Jeff Reid;1923648]Assuming the formula stated for a device with a Beta of 14 degrees, I made a graph of Vmg/Vt versus heading offset from true down wind. It peaks at about 2.56 at 38 degrees. http://jeffareid.net/misc/dwvhdg.gif[quote]I missed the obvious flaw that when the heading is 0 degrees offset from the wind direction, Vmg/Vt is 1.0, where it would have to be less than 1.0 because of drag. The Beta factor needs to vary based on heading.

Does anyone have an actual table of heading versus Vmg? Another issue is lift versus drag as it applies to sails. When heading downwind, aerodyamicd and ground lift are zero while aerodyanmic drag = ground drag. Also unlike wings, sails aren't designed to divert the air flow by 90 degrees. I need to do a rethink on this.

spork

I'll let you know.

I presume you mean measured heading vs. VMG? I don't have that. Certainly given the measured L/D of the sail and skates I can give you that data.

I'm not sure what you mean. Can you give me more detail?

I don't understand what you mean here either. I don't think I've ever seen a wing that diverts airflow by 90 degrees. If you compare a windsurf sail to a hang glider wing, they're practically indistinguishable. And before you all tell me how ludicrous that statement is, I've owned plenty of hang gliders and windsurf rigs over that last couple of decades.