how moths work?

   Ok, so I get the basics of foiling moths; that a wand is pushed back when submerged and the boat is moving, which through some linkages operates a flap on the main foil. Boat goes up, wand is less submerged and goes forward again. Helm then twists the tiller extension changing the rudder foil attack to keep the boat level. Yes?

Also, that you can change the "gain" of the system by moving bits of the linkage so that the ratio of wand deflection to foil angle can be adjusted, also that the foil flap control is non-linear so you can arrange things to create lots of flap movement at "lift off" but reduce the sensitivity once at ride/flight height.

BUT... watching moth videos, I can see that in short chop the wand is flipping back and forth, so is the foil flap wapping up and down too; isn't that quite draggy?  Sure, you want the boat to follow a fixed height above slow swell, but there's no point trying to "track" chop and you don't want the flaps moving about unless you want to alter the ride height.

Seems like you'd need some damping ( low pass filtering) between the wand and foil flap (eg a spring with an oil damper in parallel )..but I don't see any.  

What's stopping the flap from flapping up and down as the wand flips back and forth? Anything?

..and isn't wand deflection (hence flap deflection) going to increase with speed, creating more lift at higher speeds, when surely you need less? Or do they just fly higher at higher speeds?

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NickA Send Private Message Find Member's Posts Really should get out more Joined: 30 Mar 05 Location: United Kingdom Online Status: Offline Posts: 784	Post Options Post Reply Quote NickA Report Post Quote Reply Topic: how moths work? Posted: 02 Feb 21 at 9:42pm
	Ok, so I get the basics of foiling moths; that a wand is pushed back when submerged and the boat is moving, which through some linkages operates a flap on the main foil. Boat goes up, wand is less submerged and goes forward again. Helm then twists the tiller extension changing the rudder foil attack to keep the boat level. Yes? Also, that you can change the "gain" of the system by moving bits of the linkage so that the ratio of wand deflection to foil angle can be adjusted, also that the foil flap control is non-linear so you can arrange things to create lots of flap movement at "lift off" but reduce the sensitivity once at ride/flight height. BUT... watching moth videos, I can see that in short chop the wand is flipping back and forth, so is the foil flap wapping up and down too; isn't that quite draggy? Sure, you want the boat to follow a fixed height above slow swell, but there's no point trying to "track" chop and you don't want the flaps moving about unless you want to alter the ride height. Seems like you'd need some damping ( low pass filtering) between the wand and foil flap (eg a spring with an oil damper in parallel )..but I don't see any. What's stopping the flap from flapping up and down as the wand flips back and forth? Anything? ..and isn't wand deflection (hence flap deflection) going to increase with speed, creating more lift at higher speeds, when surely you need less? Or do they just fly higher at higher speeds?
	Javelin 558 Contender 2574

Daniel Holman Send Private Message Find Member's Posts Really should get out more Joined: 17 Nov 08 Online Status: Offline Posts: 997	Post Options Post Reply Quote Daniel Holman Report Post Quote Reply Posted: 02 Feb 21 at 10:24pm
	All good questions. My limited understanding is that the rudder angle is set and forget by and large - its easier to move ones arse 3" either way. I also think that when the boat is high, the wand can be flopping around vertically whilst not really affecting the flap much, I think due to the cam profile at that angle as it actuates the flap being quite circular. And you can adjust the point at which that happens to some extent with ride height. The flexibility of the wand can provide some compliance (spring rather than damper). The wand length is often adjustable too. So gain as you describe it handled by the distance between wand end, length of wand and cam profile. There is some compliance due to wand stiffness / length relationship. Not aware that there is any damping per se. My limited experience of sailing a posh exocet is that once up and sailing, the wand looks as if its skipping and hopping like a frog in a sock but you don't feel that is really affecting the flap at that sort of frequency, its really serene or at least it is on flattish water. Iguess it shouldn't be responding much to stuff below a certian height, you'd want it to follow bigger wave elevations for obvious reasons. Its bloody clever, I think lots of clever people have put a lot of effort into getting them how they are nowadays.

NickA Send Private Message Find Member's Posts Really should get out more Joined: 30 Mar 05 Location: United Kingdom Online Status: Offline Posts: 784	Post Options Post Reply Quote NickA Report Post Quote Reply Posted: 03 Feb 21 at 6:21pm
	"Its bloody clever, I think lots of clever people have put a lot of effort into getting them how they are nowadays." Too right. Video of an "old" Exocet ( from way back in 2019 ) has control lines for ride height, wand return force and linkage ratio ( gain in my control engineer parlance). If they needed low pass filtering in the control, I'm sure it would be there.
	Javelin 558 Contender 2574

aardvark_issues Send Private Message Find Member's Posts Visit Member's Homepage Really should get out more Joined: 24 Jul 05 Location: Weston-Super-Mare Online Status: Offline Posts: 505	Post Options Post Reply Quote aardvark_issues Report Post Quote Reply Posted: 04 Feb 21 at 1:45pm
	A few people have messed around with some sort of damping. I've had an inline damper robbed from an RC car to have a controlled amount of slop - other people have just straight up introduced slop in the control systems. Long story short, it's faster to have more control and ride closer to the height limits than it is to try and engineer out the bouncing you're seeing in the wand.
	Home of Rocket Racing

dohertpk Send Private Message Find Member's Posts Posting king Joined: 28 Sep 12 Online Status: Offline Posts: 172	Post Options Post Reply Quote dohertpk Report Post Quote Reply Posted: 04 Feb 21 at 9:51pm


dohertpk Send Private Message Find Member's Posts Posting king Joined: 28 Sep 12 Online Status: Offline Posts: 172	Post Options Post Reply Quote dohertpk Report Post Quote Reply Posted: 04 Feb 21 at 9:52pm
	Interesting discussion. It's precisely this level of complexity that makes me think I'll never have the kind of mind needed to own one of these boats!

Rupert Send Private Message Find Member's Posts Really should get out more Joined: 11 Aug 04 Location: Whitefriars sc Online Status: Offline Posts: 8956	Post Options Post Reply Quote Rupert Report Post Quote Reply Posted: 04 Feb 21 at 10:12pm
	I thought the answer was "magic". And if magic is simply science beyond a person's comprehension, then I'd be right.
	Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686

Sam.Spoons Send Private Message Find Member's Posts Really should get out more Joined: 07 Mar 12 Location: Manchester UK Online Status: Offline Posts: 3400	Post Options Post Reply Quote Sam.Spoons Report Post Quote Reply Posted: 04 Feb 21 at 11:02pm
	Sailing anywhere other than directly before the wind is magic to most people
	Spice 346 "Flat Broke" Blaze 671 "supersonic soap dish"

Riv Send Private Message Find Member's Posts Far too distracted from work Joined: 23 Nov 13 Location: South Devon Online Status: Offline Posts: 353	Post Options Post Reply Quote Riv Report Post Quote Reply Posted: 05 Feb 21 at 2:14pm
	Remember double de clutching cars without synchromesh? Now we have all sorts of flawless auto boxes. Anyone can drive them. Same will be true of Moths on the next few years. All the difficulty of the control systems will be designed out and almost anyone who wants to practice the basics will be able to fly easily.
	Mistral Div II prototype board, Original Windsurfer, Hornet built'74.

Sussex Lad Send Private Message Find Member's Posts Far too distracted from work Joined: 08 Jun 18 Online Status: Offline Posts: 360	Post Options Post Reply Quote Sussex Lad Report Post Quote Reply Posted: 05 Feb 21 at 2:26pm