Johannes replied to me: > > Consider this ASCII art: > > > > / Sail > > / > > Wind -> O Center of mass > > / > > / > > > > Would this contragrav craft move in the direction of the wind, > > or at a slight angle? > > > > Also, would it start to rotate around its axis? > > > > With time, the craft would probably accelerate to the speed > > of the wind, and it would be at rest relative to the air > > around it, if the speed and direction of the wind is > > constant. What happens when there are gusts? > > > > I don't know about baloon sails, but they could also have been to maximise > the amount of wind you catch, when it blows in the right direction, and to > be removed at other times.
The area of the sails would be much smaller than the area of the gas bag, but the shape is different -- the gas bag is convex, a sail is concave. The air resistance from a concave shape is much higher than the resistance from a convex shape, does the same apply to sails? But my nagging problem is if the balloon travels at the speed of wind sooner or later, at rest relative to the surrounding medium. Then it could not maneuver any more. > Some types of sail, like the ones modern sailing yachts use, are not > intended to be used to catch the wind and get pushed, but they give a > shape similiar to an airplane wing, and that gives you the movement. This force is generated by air moving across the sail. The three requirements for that are: * Wind moving past the vehicle. See above. * The ability to set the angle of the sail relative to the directon of the wind. Can you do that without a keel, or will the craft swing into some least-resistance position? * The ability to set the shape of the sail. Easy. > If your ascii sail were such a sail, there would be some movement > downward, though if you mount it on a baloon, i suppose it would be > minimal compared to your movement due to being blown around. If there can be some lateral movement, on the beam if not close hauled, then there is a wedge in the door -- aerial yacht designers can optimize sail and hull shapes to get as much control as possible, there can be races, etc. > How much is a contragrav vehicle like a balloon though? I don't remember > if the rules in vehicle are specific on that, so if you don't put your > vehicle in a background that has precedence cases, you might be able to > pick something that suits your vehicle. > > And there is an other idea. Could you add additional contragrav units, to > influence the wind around your vehicle? I'm assuming Traveller. A world has low-tech, sail-powered ground vehicles, and high-tech, engine-powered grav flyers. It would be logical for sportsmen on such a world to work with sail-powered contragrav. Regards, Onno _______________________________________________ GurpsNet-L mailing list <[email protected]> http://mail.sjgames.com/mailman/listinfo/gurpsnet-l
