Posted: Mar 06, 2011 6:46 am
reksio wrote:Onyx8 wrote:
In the video quite a deal is made when the ribbons fluttering from the mast start being blown to the rear of the craft,
You mean this at about 3:00 ?
[youtube]http://www.youtube.com/watch?v=EEuAqq8FINw[/youtube]Onyx8 wrote:it also would seem that it would mean that the wind is now acting to slow the craft down.
The apparent wind at the chassis is creating drag that acts to slow the craft. The apparent wind at the rotating airfoils is creating lift that acts to accelerate the craft.Onyx8 wrote:
Or has the pitch on the propellor changed to now provide forward force to the cart even though the wind is now from the front?
No, on this early prototype the blade pitch was fixed. You don't need to change the pitch, to accelerate past windspeed. Just like you don't need to change the sail setting on an ice boat, to accelerate past downwind VMG = windspeed on a downwind tack.Onyx8 wrote:
Variable pitch propellors are non-trivial things, you notice if you happen to build one.
They later implemented variable pitch to optimize acceleration. But it works without it, just very slowly at the start as you see in the video.
Well, I am still struggling. When the ribbons are hanging straight down, the craft is traveling at the same speed as the air at the propellor. That means that there is no air moving past the propellor, ergo no force on the propellor. How then with no force being applied to the propellor by the wind can the propellor provide force to the wheels to drive the craft faster? The only thing keeping the propellor turning at that point is inertia.
