Boats and foils comparison

Stingray~

Super Anarchist
12,249
3,341
PNW
Wouldn't that have more to do with the vertical component of gravity acting on the resultant rotational force of RM vs sail loading?
Oh, absolutely!
 

Glad you brought that up Victor! I had completely mistaken the component of gravity’s vector.. 

 

Stingray~

Super Anarchist
12,249
3,341
PNW
Wouldn't that have more to do with the vertical component of gravity acting on the resultant rotational force of RM vs sail loading?
Oh, absolutely!
 

Glad you brought that up Victor! I had completely mistaken the component of gravity’s vector.. 

 
Ventilation wouldn't be the cause of pitting, that happens due to cavitation. I too would be interested to see that pic if you can find it.

Cavitation does often begin right at the leading edge, as that where there's a massive drop in pressure.
I think cavitation on the upper side of the leading edge is unlikely in these foil designs at high speed.  It is more to be expected as sheet cavitation at takeoff speed.  At high speed, I'd expect the foils would be designed so that bubble cavitation occurred more toward the middle of the chord.  

Cavitation on underside of the leading edge could be happening at high speed, but I'd think they'd design their sections so that wouldn't happen, either.  A little trailing edge up flap deflection, compensated for by a little bow-up change in pitch attitude would cure it.  

 

MaxHugen

Super Anarchist
OK, here is my take on the anhedral foil righting moment debate:

View attachment 432975

The second graph shows vertical force, and it's quite confusing at first. It shows the same thing that at low cant angle, the outside half contributes minimally, and almost all the vertical lift is carried by the inside. You might think this is wrong (I did first), since the angles should work the opposite way compared to horizontal forces, the outside half is much closer to horizontal - shouldn't it generate the vertical lift? The way it works out is that at those low cant angles, the total lift generated by the outside wing half is minimal.
Couple of questions, as I'm struggling with the 2nd graph...

The Vertical force must be equal for leeward & windward wings when the foil is at 0° cant.  Given that the foil arm to foil cant diff is 42°, then why does the graph show equal FZ at 60° foil arm cant, instead of at 42°?

Also NZ's T foil is shown at 67.5°, which is a foil cant of 25.5°.  However, the data from the ACWS mostly show NZ with a median upwind cant of 22°?

 

MaxHugen

Super Anarchist
I think cavitation on the upper side of the leading edge is unlikely in these foil designs at high speed.  It is more to be expected as sheet cavitation at takeoff speed.  At high speed, I'd expect the foils would be designed so that bubble cavitation occurred more toward the middle of the chord.  

Cavitation on underside of the leading edge could be happening at high speed, but I'd think they'd design their sections so that wouldn't happen, either.  A little trailing edge up flap deflection, compensated for by a little bow-up change in pitch attitude would cure it.  
How would you avoid a large low pressure spike just after the stagnation point on the leading edge?

 
Is the lack of the low pressure at the LE due to the very small LE radius?

Would you mind posting the dat file, so I can have a play with it in XFoil?
The coordinates and polar data are in the zip file attached to the post.

The leading edge pressures come from having used inverse design to shape the section so they don't occur at the lift coefficients used for high speed.  A small leading edge radius would normally be expected to cause a pressure peak, but in this case it is the result of having fine-tuned the leading edge.  In qualitative terms, I like to think of the flow around the camber line superimposed on the flow around the thickness distribution.  The camber line would have an infinite velocity at the leading edge except at the ideal angle of attack.  The thickness distribution would have zero velocity at the stagnation point.  So what the inverse design is doing is achieving exact cancellation of the infinite velocity with the zero velocity.  But for that to work, the two have to be precisely co-located.  If there is any mismatch, then you'll get a pressure peak.  It's not something you can do by eye or just choosing the leading edge radius.

Another way to think of it is driving away from a stop sign.  If you turn immediately as you step on the gas, you can make a sharp turn before the car accelerates very much.  But if you wait and accelerate first, then you can't turn as sharply.  The flow is doing something similar to that.  With the stagnation point at the right place, the leading edge radius can be small because the flow turns first then accelerates.  But if the stagnation point is away from the leading edge, it accelerates away from the stagnation point and then has to make the sharp turn, resulting in a pressure peak.  If you don't know where the stagnation point is, then you have to use a large radius to prevent a pressure peak.  But if you can precisely place the stagnation point where the curvature is high, you can get away with it.

Takeoff is the challenging condition for the leading edge.  If the leading edge suction peak is controlled at takeoff lift coefficients, then a pressure peak on the upper side of the leading edge won't be a problem at high speed.  That's why I think it's unlikely cavitation will occur there at high speed.  Where leading edge cavitation becomes a problem at high speed is when the leading edge is cambered for the takeoff condition and then is too cambered for high speed so a pressure peak occurs on the lower side of the leading edge.

 

MaxHugen

Super Anarchist
The coordinates and polar data are in the zip file attached to the post.

The leading edge pressures come from having used inverse design to shape the section so they don't occur at the lift coefficients used for high speed.  A small leading edge radius would normally be expected to cause a pressure peak, but in this case it is the result of having fine-tuned the leading edge.  In qualitative terms, I like to think of the flow around the camber line superimposed on the flow around the thickness distribution.  The camber line would have an infinite velocity at the leading edge except at the ideal angle of attack.  The thickness distribution would have zero velocity at the stagnation point.  So what the inverse design is doing is achieving exact cancellation of the infinite velocity with the zero velocity.  But for that to work, the two have to be precisely co-located.  If there is any mismatch, then you'll get a pressure peak.  It's not something you can do by eye or just choosing the leading edge radius.

Another way to think of it is driving away from a stop sign.  If you turn immediately as you step on the gas, you can make a sharp turn before the car accelerates very much.  But if you wait and accelerate first, then you can't turn as sharply.  The flow is doing something similar to that.  With the stagnation point at the right place, the leading edge radius can be small because the flow turns first then accelerates.  But if the stagnation point is away from the leading edge, it accelerates away from the stagnation point and then has to make the sharp turn, resulting in a pressure peak.  If you don't know where the stagnation point is, then you have to use a large radius to prevent a pressure peak.  But if you can precisely place the stagnation point where the curvature is high, you can get away with it.

Takeoff is the challenging condition for the leading edge.  If the leading edge suction peak is controlled at takeoff lift coefficients, then a pressure peak on the upper side of the leading edge won't be a problem at high speed.  That's why I think it's unlikely cavitation will occur there at high speed.  Where leading edge cavitation becomes a problem at high speed is when the leading edge is cambered for the takeoff condition and then is too cambered for high speed so a pressure peak occurs on the lower side of the leading edge.
Thank you. Bit by (very slow) bit, I'm understanding this better. The "stop sign" analogy was very helpful!

I use XFLR5, a GUI for XFoil. I've tried to use the Inverse Design feature but haven't really got it to work... may be because I've been using the 'recommended' 100 points, whereas I see you are using ~260.  I shall try that, to see if I can improve my unprofessional foil shape. :)

 
Thank you. Bit by (very slow) bit, I'm understanding this better. The "stop sign" analogy was very helpful!

I use XFLR5, a GUI for XFoil. I've tried to use the Inverse Design feature but haven't really got it to work... may be because I've been using the 'recommended' 100 points, whereas I see you are using ~260.  I shall try that, to see if I can improve my unprofessional foil shape. :)
I think Xfoil's default is 160 points.  I typically either use that or 200 points.

Working around the leading edge in MDES mode is VERY sensitive.  I often work with an angle of attack a little greater than the one I'm interested in, round off the peak that results, and then go back to the intended angle of attack.  It also helps to shape just a portion of the pressure distribution at a time, instead of trying to do the whole thing.  I find that an inverse design iteration tends to increase the thickness, so I have to go back to the geometry design mode (GDES) to reset the thickness.  Which, of course, partially reverses some of the change I'm trying to accomplish.  So it takes a number of iterations to whittle away at the shape until it does what you want. 

 

zillafreak

Anarchist
799
168
Newport Beach
Personally, I think a multihull is the way to go, rather than a ballasted monohull.  There are a lot of reasons for this, among them having maximum righting moment to accelerate to takeoff, and not having to lift ballast.  
So why is the current class of foiling monohull the fastest yet? Size for size the current the class faster than a multihull of similar size/power. Possibly the fastest boat in the world next to Sailrocket.

 

MaxHugen

Super Anarchist
I think Xfoil's default is 160 points.  I typically either use that or 200 points.

Working around the leading edge in MDES mode is VERY sensitive.  I often work with an angle of attack a little greater than the one I'm interested in, round off the peak that results, and then go back to the intended angle of attack.  It also helps to shape just a portion of the pressure distribution at a time, instead of trying to do the whole thing.  I find that an inverse design iteration tends to increase the thickness, so I have to go back to the geometry design mode (GDES) to reset the thickness.  Which, of course, partially reverses some of the change I'm trying to accomplish.  So it takes a number of iterations to whittle away at the shape until it does what you want. 
I have finally been able to make some changes, using a copy of your foil. Does appear that I just didn't have enough points. I'm using the "mixed inverse" mode so that I can modify one surface at a time, and yes, I'm now doing it incrementally. 

I have yet to work out how to adjust the leading edge using the XFLR5 interface, but having your foil as a starting point, I can try tiny adjustments there to see if/where I'm going wrong. :)

A question pls: why is the forward half of the lower surface designed so that it produces no lift?

 
I don't think you can really separate out vertical and horizontal in this way. 
You don't have to, but you can. If you don't factor then out, then you have to work with a single lift vector that is in the direction of the cant angle (or 90 degrees to it?)

In your calculations you have separated out the vertical that have different magnitudes but then combined them assuming they are on the center.

Sure the horizontal component will also move down, but by a different amount unless the cant angle is 45. But more importantly by a different distance to the other horizontal forces, which are centered much further away up the sail.

Separating out horizontal and vertical forces is handy as they need to balance differently. Both need to sun to zero rotational force, as do vertical absolute forces. But absolute horizontal forces need not balance as the boat can have leeway... Oh and forward drive if we go 3d

 

Latest posts




Top