peterraymond
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C-Class wing design
I have only gotten to page 19 of this thread, but since wings are a current topic I thought I'd jump in. Apologies if I'm repeating recent content.
In message 104 of this thread Tom Speer has three charts dealing with the optimum plan form for a sail. You can always reduce induced drag by making the sail taller, but at some point there is too much heeling moment. Also, the classic elliptical plan form assumes no gap at the base of the sail. Using lifting line theory and with some other assumptions, Tom showed the plan forms that will give you the lowest induced drag for fixed heeling moment and he did this for different gaps at the bottomand different relative luff lengths. Even for the case of no gap, the elliptical plan form is beaten by a taller shape that's more tapered.
If everyone remembers post 104, this is nothing new. But, when I think about how you would design a C-Class wing, I'm not sure if I understand all of the assumptions, or the format isn't quite what seems easiest to me.
A C-Class obviously has fixed sail area and the righting moment you can generate is based on boat and crew weight and that's fixed too. In the design process what can you control? You can change the gap at the foot and you can change the hoist.
If you increase the gap there is more flow under the boom and therefore more induced drag. Also, you lose power at the foot, so you need to sheet in to get back to the same heeling moment.
As you make the sail taller, induced drag goes down while the center of effort goes up. For both reasons you need to sheet out to get back to the same heeling moment.
Tom's graph looks at performance at different gaps at the foot and different luff lengths. What you read off the chart is the induced drag and the height of the center of effort for each optimized solution. I think the relative induced drag on the horizontal axis is exactly what you want, but on the vertical axis, I think it would be nice to see the lift you generate for constant righting moment. It seems like this gets you one step closer to having the numbers you need to figure out boat speed.
However, this is really just the start of the process. In the real world you have restrictions on the wing shapes you can generate. If you twist the wing to minimize induced drag, how do you then twist it to account for different apparent wind angles with height? You can also design this lovely plan form, but if you have a three element wing like Cogito, don't you want the trailing edge of the second element (the flap) and the leading edge of the third element to be straight?
I see how you could have a whole team just designing and modeling the wing.
I have only gotten to page 19 of this thread, but since wings are a current topic I thought I'd jump in. Apologies if I'm repeating recent content.
In message 104 of this thread Tom Speer has three charts dealing with the optimum plan form for a sail. You can always reduce induced drag by making the sail taller, but at some point there is too much heeling moment. Also, the classic elliptical plan form assumes no gap at the base of the sail. Using lifting line theory and with some other assumptions, Tom showed the plan forms that will give you the lowest induced drag for fixed heeling moment and he did this for different gaps at the bottomand different relative luff lengths. Even for the case of no gap, the elliptical plan form is beaten by a taller shape that's more tapered.
If everyone remembers post 104, this is nothing new. But, when I think about how you would design a C-Class wing, I'm not sure if I understand all of the assumptions, or the format isn't quite what seems easiest to me.
A C-Class obviously has fixed sail area and the righting moment you can generate is based on boat and crew weight and that's fixed too. In the design process what can you control? You can change the gap at the foot and you can change the hoist.
If you increase the gap there is more flow under the boom and therefore more induced drag. Also, you lose power at the foot, so you need to sheet in to get back to the same heeling moment.
As you make the sail taller, induced drag goes down while the center of effort goes up. For both reasons you need to sheet out to get back to the same heeling moment.
Tom's graph looks at performance at different gaps at the foot and different luff lengths. What you read off the chart is the induced drag and the height of the center of effort for each optimized solution. I think the relative induced drag on the horizontal axis is exactly what you want, but on the vertical axis, I think it would be nice to see the lift you generate for constant righting moment. It seems like this gets you one step closer to having the numbers you need to figure out boat speed.
However, this is really just the start of the process. In the real world you have restrictions on the wing shapes you can generate. If you twist the wing to minimize induced drag, how do you then twist it to account for different apparent wind angles with height? You can also design this lovely plan form, but if you have a three element wing like Cogito, don't you want the trailing edge of the second element (the flap) and the leading edge of the third element to be straight?
I see how you could have a whole team just designing and modeling the wing.
