Matteo V wrote: ↑
Wed Jun 05, 2019 3:37 am
sergei Scotland wrote: ↑
Tue Jun 04, 2019 9:28 pm
Obviously edging more creates a bigger /taller "keel" out of the back and centre of the board as board sinks deeper. Is it that this increase in submerged surface is bigger in the middle of the board than at the tail (because if the curves edge) and this causes center of lift/water pressure to move forward? So force pushing up and upwind at say 45 degree angle gets bigger around center of the board quicker (with increase of edging angle) than at the tail thus generating rotating moment around center of mass which is closer to back foot than front foot?
Ha-ha that's a long question!
But if this works as described for whatever reason this is wonderful news to me as this is exactly how a skateboard or snowboard works, does it not?
I am at a bit of a loss on this one. I will try to come back to it later if you want me to
As a kid I was doing radio controlled gliders. My instructor was an aero engineer.
He suggested we should design a new glider. He explained how to make sure glider is stable going forward - the center of pressure on a side projection of a glider outline must be on top of center of mass or behind. For a glider on top or very slightly behind is ideal so glider goes straight without any input but turns very easily as soon as there is a small turning force. So we made a drawing of the my preffered outline on paper with square grid and counted number of squares to the front and to the back of required center of mass position (which is known for a given wing profile).
It appeared that my preferred outline had too many squares at the back which meant the glider will be weathewaning too much and will be too difficult to turn. So I decreased the size if the vertical tail to even the numbers. The glider was direction-neutral as a result and was reacting very clearly to entering an area of lift with one wing. When say right wing entered area of thermal lift glider would turn away from the area - turn left-by itself indicating lift to me.
All I needed to do was turn right to enter the area. Glider went up!
This example tought me to think about center of pressure as a point where the force generated by push of air or water is applied.
Now we know that any physical body will rotate /turn if a force is applied at any point but center of the mass. I. E. if we have a bar or a board with a rider and apply a force the item will always rotate unless the force applied goes rught through the centre of the mass. For a bar it is straight in the middle obviously.
So to go straight without turning a kiter must adjust the total centre of mass to be exactly over the centre of pressure.
If centre of pressure moves the board starts turning.
Matteo - obviously you know all of this - I am terrible at explaining the obvious!
What I am trying to say though is that there are 2 ways to turn without using sudden body twists (like a glider) . One is move centre of the mass, the other is move center of pressure (forward or backward).
Obviously as you pointed out edging causes board to turn upwind without changing mass distribution. So there must be a change in the position of center of pressure forward. This seems to be happening because of board sinking more with increase of edging angle. The deeper board sits in the water the more forward center if pressure moves. With board fully underwater it will be obviously in the middle. With front out of the water somewhere closer to the back obviously.
By changing edging angle we control sinking and position of center of pressure.
As the force created by pressure of the water is (partially) directed upwind it causes board to turn upwind when centre of pressure is in front of the centre of mass and downwind when center of mass is in front of centre of pressure.
Just my 2 cents.
I guess one does not need to know all of this to ride a kite, but I hope it will help me to stop my board going upwind too much next time I attempt a waterstart and my board starts curving upwind too much by itself. As you said I need either or both - move center of the mass forward and/or decrease of edging angle to move center of pressure backward. Now I know why, at least in my head!
Edit :moving center of mass forward when board is almost flat on the water can probably move centre of pressure forward too by board making more contact with water along the length of the board. Which might just eliminate effect of moving centre of mass forward. Tricky.