Titanium Foil

[zfennell]

I'm sorry.
Did your rig last any longer than the video?
Any projections for the future?
-bill

[Hawaiis]

I'm sorry.
Did your rig last any longer than the video?
Any projections for the future?
-bill

It lasted a couple more minutes after the gopro self shut off.
I will rebuild with wood, so it will not sink.

[ronnie]

I'm sorry.
Did your rig last any longer than the video?
Any projections for the future?
-bill

It lasted a couple more minutes after the gopro self shut off.
I will rebuild with wood, so it will not sink.

From fiddling around with that Foilsim program using an ellipse with camber/bend in it as the cross-section (for a twintip).
It seems that the 10 to 1 ratio for chord length to thickness seems to come out optimum for angles of attack around 3 degrees. Fatter foils seem to require higher angles of attack and have lower lift/drag ratios.

The angle of attack seems to decide how much bend to have in the ellipse and whether the bend makes the ellipse more convex on the top (which seems to be when the angle of attack is below 2 degrees) or more convex on the bottom [-0.2% camber seems to be the most effective figure](from 2 degrees and up). You can get lift/drag ratios just under 14 to 1.

That assumes the Foilsim program is accurate of course.

I can't find a program that draws the ellipse, but I guess producing an ellipse with a 10 to 1 length to thickness ratio would be easy enough and then draw an arc with the major axis curved down 0.2% at the centre. Then at each point along the major axis, you move the top and bottom surfaces down by the distance the arc is below the major axis and you get your foil shape.

Its a bit counter-intuitive to use a shape that seems like it should be the other way up, with a more convex top surface, but that's what the Foilsim program says about the ellipse. The camber figures do seem to be very small for the large changes in L/D that the program says result from small changes in the camber.

The good news is that a flat plate is quoted as a 13.85 lift/drag ratio at a 2.32 degrees angle of attack, so the ellipse doesn't seem to have any notable advantages based on that. But it doesn't change the figure if you vary the thickness of the plate, so its more of a theoretical figure I think. The other odd thing is that if you use a curved plate, that -0.2% camber figure is quoted as optimum again, so that seems a bit strange.
There is a lot of change you can input to the ellipse shape and the results do vary in accordance whereas with the flat plate all you can seemingly change is the angle of attack.

[Hawaiis]

I'm sorry.
Did your rig last any longer than the video?
Any projections for the future?
-bill

It lasted a couple more minutes after the gopro self shut off.
I will rebuild with wood, so it will not sink.

From fiddling around with that Foilsim program using an ellipse with camber/bend in it as the cross-section (for a twintip).
It seems that the 10 to 1 ratio for chord length to thickness seems to come out optimum for angles of attack around 3 degrees. Fatter foils seem to require higher angles of attack and have lower lift/drag ratios.

The angle of attack seems to decide how much bend to have in the ellipse and whether the bend makes the ellipse more convex on the top (which seems to be when the angle of attack is below 2 degrees) or more convex on the bottom [-0.2% camber seems to be the most effective figure](from 2 degrees and up). You can get lift/drag ratios just under 14 to 1.

That assumes the Foilsim program is accurate of course.

I can't find a program that draws the ellipse, but I guess producing an ellipse with a 10 to 1 length to thickness ratio would be easy enough and then draw an arc with the major axis curved down 0.2% at the centre. Then at each point along the major axis, you move the top and bottom surfaces down by the distance the arc is below the major axis and you get your foil shape.

Its a bit counter-intuitive to use a shape that seems like it should be the other way up, with a more convex top surface, but that's what the Foilsim program says about the ellipse. The camber figures do seem to be very small for the large changes in L/D that the program says result from small changes in the camber.

The good news is that a flat plate is quoted as a 13.85 lift/drag ratio at a 2.32 degrees angle of attack, so the ellipse doesn't seem to have any notable advantages based on that. But it doesn't change the figure if you vary the thickness of the plate, so its more of a theoretical figure I think. The other odd thing is that if you use a curved plate, that -0.2% camber figure is quoted as optimum again, so that seems a bit strange.
There is a lot of change you can input to the ellipse shape and the results do vary in accordance whereas with the flat plate all you can seemingly change is the angle of attack.

Any recommendation on bidirectional twin tip foil shapes?

[ronnie]

[quote="Ronnie]


From fiddling around with that Foilsim program using an ellipse with camber/bend in it as the cross-section (for a twintip).
It seems that the 10 to 1 ratio for chord length to thickness seems to come out optimum for angles of attack around 3 degrees. Fatter foils seem to require higher angles of attack and have lower lift/drag ratios.

The angle of attack seems to decide how much bend to have in the ellipse and whether the bend makes the ellipse more convex on the top (which seems to be when the angle of attack is below 2 degrees) or more convex on the bottom [-0.2% camber seems to be the most effective figure](from 2 degrees and up). You can get lift/drag ratios just under 14 to 1.

That assumes the Foilsim program is accurate of course.

I can't find a program that draws the ellipse, but I guess producing an ellipse with a 10 to 1 length to thickness ratio would be easy enough and then draw an arc with the major axis curved down 0.2% at the centre. Then at each point along the major axis, you move the top and bottom surfaces down by the distance the arc is below the major axis and you get your foil shape.

Its a bit counter-intuitive to use a shape that seems like it should be the other way up, with a more convex top surface, but that's what the Foilsim program says about the ellipse. The camber figures do seem to be very small for the large changes in L/D that the program says result from small changes in the camber.

The good news is that a flat plate is quoted as a 13.85 lift/drag ratio at a 2.32 degrees angle of attack, so the ellipse doesn't seem to have any notable advantages based on that. But it doesn't change the figure if you vary the thickness of the plate, so its more of a theoretical figure I think. The other odd thing is that if you use a curved plate, that -0.2% camber figure is quoted as optimum again, so that seems a bit strange.
There is a lot of change you can input to the ellipse shape and the results do vary in accordance whereas with the flat plate all you can seemingly change is the angle of attack.

Any recommendation on bidirectional twin tip foil shapes?[/quote][/quote]

I don't know how accurate the Foilsim program is. Certainly the thicker a flat plate is, I would expect the lower the lift/drag ratio should be.

At the thickness you are making the wooden foil, I would think a chord to thickness ratio of 10 to 1 would be a good starting point.
The 0.2% camber seems to be too small to me to be manufacturable by hand and I suspect the program isn't that accurate that it makes a noticeable difference from a standard ellipse shape, so I would think a standard ellipse shape would be a good starting point.

The round nose should make it less sensitive to small changes in angle of attack, but it would be nice if there was some way to create something like a 'tail' effect.
Maybe something like having a small flat on the front and back with a lot of small holes joining the front flat to the back might work. A small V cut either side of each hole would funnel water into the holes and spread it out at the tail but that is just pure speculation. It is something that could be tried and then plugged up if it didn't work.

[Hawaiis]

Ronnie:
Thanks,
Since the front is also the rear when bidirectional, I think you are right the sharp edges will be have less resistance at a cost of more drag at higher angle of attack. Can you show a simple drawing on drilling a channels? I have a hard time visualizing.

Mahalo

[ronnie]

Ronnie:
Thanks,
Since the front is also the rear when bidirectional, I think you are right the sharp edges will be have less resistance at a cost of more drag at higher angle of attack. Can you show a simple drawing on drilling a channels? I have a hard time visualizing.

Mahalo

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It would look like this but there should be an identical horizontal line below the holes as there is just above them.
I have seen 12" 3mm wood drill bits advertised so I think that would be a good size. Maybe a 5mm flat on the nose of a 20mm thick ellipse which would be 200mm chord, but you could go to 25mm by 250mm.

If you make the foil by bonding one board on top of another, it would open the possibility of creating the holes during the bond. Ideally the holes should be curved so that when the leading edge is up, the water is turned downward smoothly as it passes through, which should add some lift.

Fencing the flat sides of the foil might help.

It would only be something you might try if you found the ellipse worked well but got a lot less efficient at higher speeds.

[Hawaiis]

Someone found my Titanium bidirectional Foil, It was up current of where I lost it, guess the current at the bottom is opposite of what's on the surface. Thanks everyone.
It look like the wood and Chinese Made screws failed.
Ronnie:
Are you saying to drill parallel holes into the foil?

[ronnie]

Someone found my Titanium bidirectional Foil, It was up current of where I lost it, guess the current at the bottom is opposite of what's on the surface. Thanks everyone.
It look like the wood and Chinese Made screws failed.
Ronnie:
Are you saying to drill parallel holes into the foil?

Yes.

With an ellipse of about a 10 to 1 chord/thickness ratio it should have maximum lift/drag at about 2.3 degrees angle of attack, so that isn't a big angle for the water to change direction.

If you made the ellipse foil and then planed a 5mm flat on the nose and tail of the ellipse. Then you would have to line it up exactly to drill through it and I'm not sure the drill would not 'wander' off line as it passed through.
Like I said, if you make up the foil from wood where you have bonded two boards together surface to surface, then the holes would be best put in at the bonding stage. A layer of glue would almost certainly cause the drill to wander if you were to drill through the bonded area.

The idea is that the water hits the nose and some of it passes through from the nose of the foil to the tail. When it comes out the tail, it tends to fill the volume where you would expect the most turbulence and drag. Then when you go the other direction the tail becomes the nose and the same thing happens, so you are using the water through the foil to make the ellipse work more like an aerofoil shape in both directions.

I'm totally guessing that it might work, so it could be a complete waste of time to do it. If the elliptical foil didn't work well enough without them, then maybe it would be worth trying the holes if you were going to scrap the foil otherwise.

[zfennell]

ronnie.
that idea is pretty clever.
using additional surface area to redirect the flow is pretty common.
for example.
the slot between the jib and main in a sail boat
slats on the leading edge of airplane wings
flaps on the trailing edge of wings.

but the best example is the addition of extra blades in a propellor or turbine stage to redirect as much flow as possible into the desired direction.

in turbo-machinery, the concept is called "solidity ratio"
adding blades will increase the thrust per unit area until you reach a tipping point where the extra drag and interference between blades exceeds any potential lift benefits.

i think what you are suggesting can be done, but this particular approach will add an awful lot of surface area and associated friction before you see any benefit.

you folks have done a remarkable job so far and i hesitate to criticize any attempt to try something new. But if you're selling this idea to Hawaiis, i thought he deserves another opinion before committing time and effort.

Regarding the foilsim app: Thanks I've been playing with it and having fun.
i did notice the few quirks you mentioned.
dont take the l/d ratio too literally at 0 deg camber for your ellipse. The drag numbers appear to come from a lookup table of experimental data that is not quite symmetrical at zero deg.
test your results for pos AOA and neg camber with mirrored values of neg AOA and pos camber.
they dont match.
the flat plate, as you have surmised, has no theoretical thickness, so the presence of thickness slider is misleading even though it appears to be inactive.

the curved thin plate is the same way , but does allow for camber inputs.
thin plates , as you have mentioned , can have very good performance.(especially with camber)
compared to 'fat' foils , even rounded leading edges make pretty sharp traing edges.

if you wanted to simulate thickness in a plate, you may have better luck reducing the thickness in an ellipse to 3% . but its still not the same.

thanks again.
-bill