Auto-Flight
I’ve attached a photo of the formula and it’s derivation (if anyone wants to check it) for the relationship that needs to hold between the lengths. Your point about the friction in the model is a very good one, so I’d go for ‘high’ loads in the sheet and mechanism, by placing the sheetpoint near the end of the boom. Furthermore I’d recommend ensuring that the mechanism’s range of travel covers a reasonable amount of the sheeting spectrum, the range of 30° to 60° between the boom and the centerline would be good for close-hauled sailing. You’ll have to select some operating range of angles for the sensor foil as well, say +/-30°. From that you can figure out your L_rod—how long does the rod have to be so that when it travels from +30° to -30°, the sheet moves enough to get the boom from 30° off-center to 60° off-center?
Marco, your fingers are moving faster than your mind this morning! 😉 Attaching to the end of the boom will need least force, not most. Mid-boom: more force, less throw. Also, typical boom angle for close-hauled is 5-30 degrees, not 30-60, is it not?
Johannes, you may need either a vang, or sprit on your sail, so that the auto-flight doesn’t need to pull down as well as in. This is more than a geometry issue (you could re-route the line to pull downwards and inwards if it were) It’s an issue of total force required from the auto-flight. We want to minimize force necessary to alter the boom angle.
BTW, congrats on the super-simple AoA adjustment. So simple, yet I never imagined it. Nice!
Dave
xDD You are quite right about the mainsheet tension and the sheeting angles Dave.
Johannes, you may need either a vang
There is a vang, but i had not attached the rubber band when i took the photo in my last post.
I believe the adjustments the sheet needs to keep the ama flying is very minor, and to get a small range, and exact control of the sheet i use a short top of the foil/blade. I can move it up and down very easy, so i hope i can find a good spot on the blade by trail and error. I have to start somewhere.
I think the blade should be very high aspect and deep so the variations due to waves is less/negligible.
I hope i can test it today, within a couple of hours…
Cheers,
Johannes
It is set up and ready to fly, but my wife has the car…... No sailing until tomorrow.
There is about 15 degrees of range for the boom, and i can easily change the range by moving one or more attachment points on the foil or the boom. There is a lot of different things that is easy to tweak. The tension set up between the the vang and the rubber band that set the vertical angle of the foil at rest. The AoA of the foil. The immersion of the foil at rest, weight distribution between lee and wind-side, etc etc…. It is a veritable playground for anyone who likes to tweak their boat.
All i need is a hand full of rubber bands and some cable ties….
Cheers,
Johannes
There is a vang, but i had not attached the rubber band when i took the photo in my last post.
I believe the adjustments the sheet needs to keep the ama flying is very minor, and to get a small range, and exact control of the sheet i use a short top of the foil/blade. I can move it up and down very easy, so i hope i can find a good spot on the blade by trail and error. I have to start somewhere.
I agree that the range can be small—on the wind. Larger—though with less force—off-wind.
I think the blade should be very high aspect and deep so the variations due to waves is less/negligible.
I hope i can test it today, within a couple of hours…
I fully agree. If you have problems with the gadget working properly; ie: it runs hard over as soon as the ama comes up or stays loaded until stall then pops out—either will likely be traced to the blade providing too large a portion of the side-force generation. Your main hull is nicely asymmetric, but if you have problems, consider adding a clip-on leeboard on the main hull in order to reduce the proportion of side-force generated by the auto-flight.
Here is what Tom Spear said about this:
The total hydrodynamic side force is basically constant, because it has to oppose the side force from the sail rig. As the boat heels, the blade will start to emerge from the water. This doesn’t reduce the side force on the blade, though because it still has to produce the same force as before. Instead, the leeway angle increases to maintain the same force. But the force does become more concentrated toward the tip of the blade because the top of the blade is out of the water. This means the moment about pivot will increase as the boat heels, tightening the sheet. Just the opposite of what was intended.
Once the boat heels enough for the tip of the blade to stall and lose its grip on the water, then it will slip sideways and ease the sheet. This will act like a safety valve to dump the sheet just as the boat starts to capsize. The ama will drop, the blade will catch, and sheet in again for an automatic recovery.
In waves, the behavior could be unsettling, as the boat repeatedly lets go and then abruptly slams back into operation.
Please note: This is a different Tm Speer quote than shown here earlier Monday.
Dave
If you have problems with the gadget working properly; ie: it runs hard over as soon as the ama comes up or stays loaded until stall then pops out
This is also a reason i wanted a shorter lever arm on top of the foil. A longer travel for the sensing foil-blade with more precise control and more torque controlling the sheet should help making the system (more) stable and easier to “tune” to any condition.
I don’t think this kind of system can handle too varying circumstances, but should be tuned to handle small variations as good as possible. I don’t think a pure mechanical system will be “set once - cross the Atlantic flying the ama while reading a nice book”, but more of a “helping hand” able to keep the ama flying 10 - 20 % more of the time then without it (10 - 20 % of the time crossing the Atlantic, as an example).
Now i have the car, but no wind…......
Cheers,
Johannes
Hey Johannes,
I’ve attached a photo of the formula and it’s derivation (if anyone wants to check it).....
I note that you don’t have a factor for the lift coefficients of the sail and the foil, so the assumption is that they will be the same. In reality, the lift coefficient will probably be lower than that of the sail, as discussed previously, which will increase the size of the foil needed. Other factors missing are an allowance for friction in the system (efficiency factor) and an allowance for hysteresis effects. This last factor affects how fast you will be able to re-sheet the mainsail. At the end of the day though, factors will vary depending on the conditions and the boat heading so It’s all a bit indeterminate.
Mal.
I think why I struggle to accept that this system will work is because, as described with the vertical paddle sheeted directly to a conventional mainsail it borders on being a perpetual motion type of system. The energy required to drive the system is derived from the sail and the system controls the amount of energy that can be derived from the sail. For example, In the extreme case where the the paddle leaves the water, the paddle dumps the sheet and the ama returns to the water, we now have a situation where the sail is feathered, so there is no energy to drive the system. You would have to rely on the boat drifting sideways to provide enough force to re-sheet the sail. My guess is that will never be enough to overcome the friction and you would have to reset the system manually. To get the system to be able to re-sheet the sail, it will probably need to ge geared down so that the sail is never fully released, the downside being that this limits the amount of control that the system can provide.
My guess is that at best it may work to a limited extent (in an on/off kind of way) in some conditions given a lot of fine tuning. I’ll just put that out there for the sake of argument. 😊 Looking forward to seeing Johannes model in action!
Mal.
Thanks for taking a look at the equation Mal; you are right about the points you mentioned.
If the system crashes, then you are relying on the inertia of the boat to get everything set again. If the boat slows so much the mechanism no longer works, then you are right in that the helmsman would have to take the reigns and get the boat going again. There’s no problem in that though, and so long as the boat stays more or less on course I think inertia will generally be enough to get everything going again.
Marco
Bad day for testing Auto Flight - Youtube
Its so calm, i can´t even feel the wind with water on my cheek or forehead…..
Why is there no wind today?!?!? :(
The weather gods can’t stop you—they can only delay you. 😉
I have tested the autoflight device today. There was not enough wind to fly the ama, but i could see the device working and get a feel for the power, friction and range produced by different speeds, different AoA etc etc…
My summary is that it is possible to create a very simple “sheet to tiller” kind of mechanism for autoflight, but it is very sensitive and due to the two different mediums with their very different behavior (different size waves, wave patterns crossing each other, frequent changes in wind speed and wind strength etc) is is not really useful for cruising. I can be wrong about this and i recommend every one else interested in this device to try it out. I guess there is more margin of error on a large proa, due to the increase in friction compared to the available power at smaller scales.
I will probably test this mechanism again but with some changes. The sensing foil needs to be POWERFUL (i´m sorry for screaming here) and be very high aspect with a very deep immersion to make it less sensitive to waves and turbulence near the surface - which leads me to another though about this:
With a much bigger foil with deep immersion, why not make a simple mechanism that adjust the lifting force of the foil in the first place. Some kind of balanced pendulum sensing the angle of the crossbeams and control the lifting force of the foil by adjusting its AoA directly?
With the foil already there, i think there would be less total drag by using the lift to keep the ama flying in the first place, and not increase the sensitivity exponentially by mixing the AoA and lift with a foil and the continuously changing power from the driving foil (sail) in the air.
My first thoughts after the first 45 minutes of playing with a extremely simplified and rough interpretation of this device…
Now i´m of to buy a 1 meter long 2 X 50 mm flat bar of 6063-T6 aluminum….
Cheers,
Johannes
One more picture and a short video of the device doing its thing.
Cheers,
Johannes
Johannes,
It looks to me like the sheet is rigged the wrong way around. It should be connected to the foil under the hinge tube rather than above it ( if i’m interpreting the photo correctly). The force due to leeway should sheet the sail on and the sheet should be released when the foil lifts out of the water.
Mal.
As it is now the foil will sheet the sail harder when fully submerged, but once the ama starts flying the foil will loose some of its pull on the sheet, releasing it gradually until the sail is depowered and the ama drops back down. Hopefully i can find a sweet spot somewhere where there is balance and the ama flies at a stable height.
The wind has been very strange and useless today. I´m waiting for a steady 5 - 7 m/s, preferably from the east or southeast. I need a good stable wind to evaluate this setup. I was on my way to buy some aluminum for a new foil but my wife just looked at me in the way only an angry woman can do. The kind of look that says “NO.” and you know it is in your best interest to forget about aluminum at the moment…
I will probably make a new plywood foil after midsummer.
Cheers,
Johannes