UK-Cherub Forum
Cherub Chat => Tech Chat => Topic started by: Clive Everest on September 20, 2014, 09:10:59 PM
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As many of you will be aware we have been planning to fit foils to our boat following the nationals.
Watching Graham and Eddie over the last year this looks like a lot of fun in a very Cherub way.
It probably also offers the biggest potential jump in performance of any obvious development.
It seems that the press cannot get enough of foiling developments it could really raise the profile of the class. Every magazine and online publication seems to be full of it since the last AC. Most of this is happening in new one design cat classes. By been one design they are snookered as they cannot make themselves better once the limitations of their design have been established.
The A class cats are foiling as a development class but with rules that were written to stop foiling and as a consequence they make it harder and more expensive than it could be.
The Cherub is one of the very few proper development classes left and offers the opportunity to lead the fundamental development of future sail boats in a real competitive environment.
It would appear to me that the biggest challenges with putting a Cherub on foils will be light wind take off and staying on-board. Speed will probably not be an issue. The Kiwi R classes have already shown that it can be done.
Clearly the Moth is the benchmark foiler. Compared to them we have similar sail area to weight (without spinnaker). We have plenty of righting moment – more than they do for the rig and weight.
However our hulls are only one foot longer and three times the width carrying twice the weight so getting to take off speed will be hard.
We therefore need a very wide foil that will carry the weight at relatively low speed with low induced drag. I also hope that a large foil will also stop the top end speeds being too mental which I like.
To stay on board we are going to need very good height control. Last year at QM we all saw Riot Vans potential but the crashes need to be eliminated. We do not have graham and Eddies bounce.
The solution that we are about to try on A + E has a large lift foil mounted between 2 parallel centerboards. The foil is 1.45m span and 0.23m^2 area. We were going to make it even wider but this width allows us to roll the boat on the trolley.
By having the lift foil span the 2 parallel centerboards we avoid all the structural issues of the T joint.
The wand control is able to adjust the angle of the entire foil not just the flap. I am hoping that this will give good height control. I chose this arrangement for these reasons.
Dan and Roger have done a superb job this week of making a main foil in MDF moulds that Paul Croote machined for me. Their main comment has been that the main foil was massively easier and quicker (cheaper) to build than the flapped Moth foils. They now want to convert their Moths to a similar system.
Pictures attached.
The centerboards are very lightly forward of the old ones. This extends the wheel base improving pitch stability but hopefully will not upset the balance.
The centerboard cases are oversized to make it easy to drop the centerboards. They were made by laying up a sheet of carbon with a return that can be set into the foam of the hull.
Each case weight 300g. I have not re weighed the boat but the total additional weight added to the hull is between one and two kg.
We have used the remains of Roland's broken Demon rudder to extend ours to close to the length it was before we drastically cut it down.
Weather permitting we will launch tomorrow. I don’t think that we are going to try and race Bart’s Bash thought.
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more pics
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Good luck tomorrow,
R
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Clive,
I like the plan, good luck tomorow.
I have always liked the idea of the main foil rotating, and using the twin boards does solve the problem of the centre axle. I have always thaught that with the moth flap foils in full lift config it must be quite high drag, and moving the main foil as a whole shoudl be able to give a greater range of lift adjustment.
Does the wand drive the main foil directly, or does it use the secondary foil like a servo tab?
Do you expect much latteral movement of the center of pressure of the main foil with a change in alpha?
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Hi Phil,
The wand drives the foil directly.
The center of lift on the cambered foil does move aft as the speed increases and the Cl goes down.
The tail plane is there to control / neutralise this.
without it the theoretical push rod forces would increase from 100N at 8kts to 800N at 18kts.
The pitch of foil on the tail plane can be adjusted ashore.
There may be some advantage from allowing the push rod force to increase a bit with speed. It will mean that the wand stays very attached to the water and any elasticity will make the boat fly lower at speed which is attractive.
The intention is that the center of lift is always aft of the pivot so there is direct force through the control linkages and backlash is not an issue.
I would love to use the wand to directly control the tail plane / servo tab.
It would mean that the foil would trim to a constant Cl as you flew through chop with no input from the wand / control system. The control gain could then be set much lower giving a smoother ride.
At the moment I can not see how to route the control through to the trim tab and do not have faith in it then having enough authority to dominate friction in the main foil bearings. One step at a time.
The wand and the control tubes through the fore deck are carbon golf sticks brought from the scrapie for a pound. As a sailor there is something very satisfying about cutting the head off a golf stick.
live
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As a sailor there is something very satisfying about cutting the head off a golf stick.
Brilliant
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how dit go? how dit go?
debs is going to be really pissed as she was hoping for some housework doing... do you realise this may mean me getting out the jig saw again!!! Clive this is not very helpful, how dit go???????
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Sorry to be slow replying. Helen had organised an evening of culture. I could have spent the evening in a dark quiet room.
We got to fly. The control system is no where near good enough. lots of big crashes.
Upwind was easier than downwind. Had some moments showing promise and plenty that will give food for thought.
Thanks to Harriet of harbour Chandlers for having a camera to hand.
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It's much better than house work Ade.
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Why am I getting warning message:
Warning: topic is currently/will be locked!
Only admins and moderators can reply.
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Too much awesome content perhaps. Nice Helmets!
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Great work Clive, hopefully Diana and i will be down to Hayling soon to see you in action. Helmets look like a sensible idea!
just unlocked the topic (according to the log, it was you who locked it!)
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Impressive stuff Clive.
Quick question for you. Can you explain the purpose / function of the extra winglet, trim tab thingy behind the main foil for me? I couldn't quite work it out from your original posts.
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Without it the centre of lift of the main foil would move aft as the speed increases due to the fact that it is a cambered section.
This would put too much force on the push rod.
The phenomenon would be familiar to anyone old enough to have sailed an original windsurfer regatta.
In a gust the load would all go to the back hand and the front would be positively pushing up.
modern sailboard sails are flat and gutless but easy to handle.
The tailplane creates a down force that increases with speed as its AoA becomes more negative taking load off the push-rod allowing the wand control to work.
I think that the control issues that I had were due to the AoA of the tailplane being modified by the main foil in front of it resulting in insufficient load on the push rod and a sluggish reduction in lift as the boat rose.
It is difficult to see exactly what is happening when looking for a clear flight path through the rigging.
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Looks awesome Clive. Will we see it at QMSC????
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Thanks Simon,
Unfortunately we have a family commitment that weekend.
The pictures do not show how much of the time it was not flying.
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Best of luck with this, interested to see how it turns out. The warning about posts could be because you made several posts in a short period of time.
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The tailplane creates a down force that increases with speed as its AoA becomes more negative
Thanks, I think I get it now. A much smaller foil, but on a longer lever so able to increasingly counteract lift from the main foil as speed increases. So in theory, could this be the basis for a completely automatic system which finds it's own equilibrium without any wand controls at all, other than just a spring on top of the push rod to put some angle into the main foil at low speed?
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Bloody good so far. Well done. Lots of really interesting stuff going on here. Do you think the foil could even be fixed with the tailplane on a parallelogram fixing that could effectively reduce lift as speed increased. Maybe more drag but early days... The boat looks good in the still photos. How easy is righting after a capsize? Any thoughts on a dihedral foil working in this set up if height is a worry?
Joe has hidden my jig saw!!!!!
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Close Neil, it is not counteracting the lift from the main foil but counteracting the pitching moment.
It means that the combination remain balanced at different angles of attack.
If you had a symmetrical section it would not be needed but you would not be able to generate that much lift. Symmetrical sections have minimal pitching moment.
You will always need a wand, the foil itself does not know how far it is below the surface.
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Hi Ade,
Tell Joe to speak to Alex.
I think that a dihedral foil is only going to help if it is surface piercing so that the amount of lift reduces as it comes out of the water.
This is how the flying cats are getting stable flight.
The changes in lift with a change in height is not that great compared with what can been done with a wand. They also suffer from changes in pitch causing significant changes in lift.
I am sure that the wand is the better system. look at how well the Moths fly.
I just need to make it work.
The static pictures give a better impression than the reality at this stage.
Righting from a capsize did not seem to be a problem. You have a ladder to climb and with my weight the boat pops up easily. I had several chances to test this.
Dan and Rog made the trailing edges sharper than I would have liked.
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I see (I think!), the extra wing is counteracting the rotation of the main wing and taking a lot of load off the pushrod, but there is still a load of lift being directed through the main axle and up the daggerboard.
Also, a system which trims out the main foil with increasing speed is not the same thing as a system which trims out lift with increasing height off the water, hence why the wand system needs to be attached to the boat, and not directly to the main foil I suppose.
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That's it.
I think that at the moment the tail plane is taking to much load off the push rod and the main foil is not following the wand. I think that this is because I have not properly allowed for the deflection of the flow over the tail plane from the main foil..
I have also not allowed for the hydrodynamic torque that is generated when trying to trim the foil.
Fortunately it is easy to adjust the aoa of the tail plane and not a big task to make a bigger one.
Hopefully there is no problem putting bigger forces through the push rod.
If the weather is cooperative We will try again this weekend.
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I await with baited breath. It's so wonderful to have a thread about foiling which is backed up with substance and a good serving of 'doing'!
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Hi Clive,
The boat looks awesome, and personally I think that it flew first time out is a massive achievement. I also really like the twin booms on the tailplane, very reminiscent of a p-38. It will be great to see the progress made over the next few months.
Mike
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Nice one Clive...maybe I'll start getting my act together!
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Been looking at the tailplane on your main foil and thinking about the effect on stability. If the boat drops suddenly wouldn’t the upward flowing water increase the angle of attack on the tail pushing main foil to decrease its angle that then makes it drop quicker? (assuming any slack in the system allows it)
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The tail plane should be doing much the same job as the tailplane on an aircraft.
It controls the position of the total center of lift.
An aircraft does not suddenly pitch down if it drops slightly.
I am not claiming to have all the answers. certainly it was not controlling in a way that I would like.
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We had a chance to try and fly again on Saturday. Conditions were close to perfect. 12 to 20 knots and flat water.
After a bit of adjustment of the tail plane we got a sensible load on the wand and it started to fly better. We both suffer from vertigo and lack of oxygen at altitudes great than 1 foot.
Alex has managed to upload a bit of headcam footage.
http://m.youtube.com/watch?v=UrxeTYlTJ78
Clive
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quote of the year!
"stop being scared...they'll think you're a girl!" - Clive at 3:15
great work of the foil, looks nice and stable.
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Top footage gents, you make it look easy!
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That looks like a top sail you had. Alex was loving it. Is that his Christmas present sorted then?
other Qs;
Were there any mothies about to gauge speed differences?
What was the for and aft trim like, crew wise?
Whats your feeling on kite useage?
Well done. R U still smiling?
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Hi Ade,
It was pretty good. We were a bit scared of it. On the first sail 2 weeks ago the control was not working and it bit us pretty hard a couple of times.
Hopefully we can make it work and gain confidence and learn how hard you can push it.
It will not be anything like as fast as a Moth. I don't want to go that fast. 49er speed would be fine.
It was pretty stiff fore and aft and will only get better with speed. We are trapezing as far forward as we can get to put most of the weight on the main foil. Alex is on the front of the wing half past the shroud. In the lulls he was going in forward of the shroud. if we pitch pole he is already past it. It was a much more comfortable trapezing position than normal for me as I was not being pulled forward as you are when at the back. Downwind you can see him at times leaning onto the foredeck.
We will definitely need a kite. It may well be much flatter and smaller than normal to allow the apparent to go further forward.
We had a new kite at the nationals that was flatter than before as I felt the deep entry was already restricting the speed downwind. For the foils I am planning to restrict how far out the bowsprit goes to make it more of a code 0.
I was going to cut down the old kite but if anyone wants to buy it I might get a new one made. I was talking to Mike Lennon about more stable polyester kite cloths, it would save a lot of time and be a better job than my sewing would allow and would not destroy a sail that is still in good condition.
Can't wait for next weekend. Hoping for a lot of global warming this winter.
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Clive, this looks amazing.
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Do we have an interesting development suitable for the Dinghy Show?
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The weather has not been suitable the last few weekends.
Hopefully this afternoon will be better.
I have 3D printed some clip in pole bearings.
The intention is that we use a cut down kite with the pole only extended half way out.
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It has finally warned up enough for us to come out of hibernation and try flying again.
The winter mods seem to have worked. The boat no longer leaks and the foils are white so we can see the weed.
The conditions were perfect yesterday.
We even had a chance to line up against a 14 unwind.
Unfortunately there is a bit of water on the lens so Alex's video is not super clear.
We're hoping that it will continue to blow a steady 12 knots for the rest of the summer.
Clive.
https://youtu.be/-y-FDQVSId4
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Amazing. Well done Clive and Alex. Fast, reliable and very steady. Pack spare pants in 15-20 knots of breeze.
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Wow, that is fantastic. Great effort. Will have to try and make it down to see for myself when Graham doesn't drag me up to Brightlingsea at the weekend.
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After a few weekends of unsuitable winds (not 11 knots) we got out again and had a chance to fly again and try a small flat 9m code zero kite.
https://youtu.be/XAyU8LjikBg
Clive.
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Looking good.
How fast and what happens on the corners?
Are you flying this weekend?
More importantly who was the music by?
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Top speed yesterday was 15 knots.
We will try and use them at Weston. If the weather is blatantly unsuitable (not 11 knots) we will take them off. Corners can be quite slow. we have not got anywhere near a foiling gybe but did stay on the foils most of the way through a kite drop.
Alex is racing Green power cars at Goodwood next Sunday so I will be sailing with Georgia on the Sunday. She has only been in the boat a few times and never with the foils.
You need to ask Alex about the Music.
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Alex has the definition of a good life!
AI comps, racing cars and a Dad who builds a foiling Cherub for him.
Well I will have my fingers crossed for 11knts on Saturday.
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It looks awesome Clive. You say you're slow in the corners, but how slow were moths when they first went foiling? I imagine it's just a question of maturing the design and developing the right techniques.
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A bit more flying at the weekend with a new top speed of 18.4kts.
Alex has posted a bit more with a few failed gybe attempts.
https://www.youtube.com/watch?v=o0yrJBhxOfU
Clive
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Very nice! great to see some good development testing going on.
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We tried putting the camera on the bowsprit at the weekend for a different view.
https://www.youtube.com/watch?v=_l7PtVJQRJg
Had hoped to break 20kts but maxed out at 18.6
Clive
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Looking really good Clive. It looked like you were still up on foils for most of the drop, how are you finding manuvers?
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Hi Phil,
It is too early to really know if manoeuvres are going to be expensive.
We have only just got to the stage where straight lines are working.
A week ago we did our first club race, we seemed to be a lot faster than the 700s and B14 but did not finish far in front of them.
Tacking is slow, mostly because I am very cautious going into the tacks. It is wobbly as it comes off the foils. As I move back to give Alex room (he seems to need more every month) the rudder foil can stall first leaving the bow high in the air. Once round it comes back onto the foils smoothly.
We managed a foiling gybe for the first time which will help a lot.
We had a couple of laps where we lost loads at the leeward mark by misjudging lay lines and coming in, off the foils, very slowly.
Will we see you at Mumbles?
Clive
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Hi Clive,
It certainly sounds like some interesting development you are doing there, I would really like to see it but unfortunatly won't be coming to Mumbles. Getting us, a boat a 2 yearold and grandparent babysitters to the other end of the country is just a bit too much.
Phil
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For those that did not see too much of us at the nationals:
https://www.youtube.com/watch?v=toRQnW1KZ4g
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Crikey. That looks like a bit of a game changer.
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Wow, that is really impresive you and Alex are making the boat look really stable and sorted. Have you moved the mast fwd?
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Mast step has gone forward 450mm.
Gave a lot more room in the cockpit until Alex grew.
Nick has the old sails and the centerboard.
New main is 1m bigger and jib smaller.
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The fact you have engineered the whole boat from scratch is highly impressive. Did you design and make the actual foils yourself Clive? Do you think this level of foil construction would be feasible for a reasonably skilled Cherub home builder?
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The rudder is the original Demon T foil with the shaft extended thanks to a bit of a broken rudder from Roland.
I made the centreboards. Rog Angell made the main lifter.
None of them were hard to make.
One of the many advantages of this system is there are no structural T joins and no moving flaps.
It is much easier to build than the moth system.
The main foil assembly is effectively 3 planks.
You could shape bits of wood and over laminate.
The laminates are quite thick at about 2mm so you do get better accuracy using a female mould. The moulds I used were CNC cut in MDF. The layup was all wet. Using MDF with prepreg is risky as the mdf has a habit of changing shape as it dries out.
I plan to try taking plaster moulds off 3d printed blanks.
The blanks could be hot wire cut foam. This is accurate enough for competition RC gliders and very cheap.
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Clive (all),
Is the development now at the stage where we could publicise it - and see if foiling will do for the Cherub what it did for the moth? We could summarise the above into a short article to accompany the video and send to Sailing Anarchy/ Y&Y (perhaps they would do a fuller article in print?) with a link to this thread if people want to get involved. We also need to think how to make it easier for people to get into foiling Cherubs (and Cherubs in general) building on your 'flat pack' approach. We started in Cherubs due to the publicity around the 05 rules and the female Olympic trials, perhaps foiling could be the next catalyst
If we are going to go foiling I believe we need to have a momentum and critical mass to the foiling fleet. Having just one or two boats foiling and cleaning up at events will fairly quickly end up with a fleet of one or two boats at the events which I'm sure nobody wants.
Thoughts?