I found this post about wingsails in the cruiser thread. But I'm posting my/this reply in the downwind sail topic instead, since it seems to fit better there.
>>Also a Greenbird wing averages about twice the lift of a sail
To me, no trim tab makes sense, because a trim tab would only help acceleration. At top speed (Greenbird's target), the low angle of the apparent wind, and the fast wind speed, means less lift, to prevent a capsize. The trim tab would only be weight and drag at top speed.
A sail doesn't necessarily have a bad lift coefficient. It might even be better than a two element wingsail, depending on the shape that can be made with the sail.
About L/D-ratios. A perfectly trimmed sail has a good L/D. But the zone is very narrow. If trimmed a little wrong, the sail starts to drag a lot. The wingsail doesn't drag very much unless it's far from optimum trim. That's the big benefit with the wingsail, imo. In an true sailing environment, with a gradient and fluctuating wind, changing boat direction/pitch/heel, it's impossible to keep all of the sail, from top to bottom, in trim, all the time. (That's why competetive sailing is hard, imo.) With a wingsail, the penalty in drag, should be a lot less.
Bjorn
On Mon, Nov 2, 2015 at 4:11 AM, taladorwood@yahoo.com.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
Because the Greenbird style wing doesn't use a traveler to control the angle of attack, it uses the stabilizer and trim tab, it can't sail directly downwind no matter what area it has, because the lift is perpendicular to the wings sides.
There is a ~60˚ dead zone directly down wind, much like there is a ~60˚ upwind zone that the boat can't sail into or from. If the boat is very efficient however, those zones can be greatly reduced and/or eliminated because of the apparent wind.The big advantage of the Greenbird wing apart from its automatic trimming is its lack of drag. If there are no stays and if the boat is aerodynamically shaped, the total drag on the boat is reduced by half at displacement hull speeds and a lot more than that at higher speeds. Also a Greenbird wing averages about twice the lift of a sail, with much less drag, so half the surface area of a sail, will produce about the same lift as a sail. The Greenbird wing should weigh and cost a lot less than a comparable wingsail, sail, mast, lines, winches, hardware, etc. and be easier to build.The speed limiting factor is always drag, drag always increases faster than lift, (induced drag anyway).Talador
__._,_.___
Posted by: =?UTF-8?B?QmrDtnJu?= <bjornmail@gmail.com>
| Reply via web post | • | Reply to sender | • | Reply to group | • | Start a new topic | • | Messages in this topic (13) |
.