UK-Cherub Forum
Cherub Chat => Tech Chat => Topic started by: BenR on May 31, 2012, 01:11:23 PM
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Has anyone ever used the RG-8 section for a t-foil, is seems to give a much higher lift for AOA, max lift at 8 degrees compared with 14 degrees for the popular NACA63-412 section. This should also give a much higher lift/drag ratio not to mention the fact that you can get away with a smaller (cheaper) foil. It is thin at 10.8% but that just gives less drag again as long as you can make it strong enough.
I assume there are all sorts of problems with cavitation associated with this foil but I can't seem to find any good analysis as it was developed for gliders in the 80's and dumped fairly quickly after if was adopted.
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From http://www.worldofkrauss.com/foils/1165 (http://www.worldofkrauss.com/foils/1165)
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It is not massively different from Kevin's section for the E6 rudder.
So whilse we have no experience of this section I can say that the main problem we have had with the similar section is making it with the concavity at the back. Even when you idealise the back edge as a "flat bit" fairing the foil up is fairly tricky. We never completed the home build rudder we tried to make a year ago for that reason.
I'd reccomend having close look at the Daemon foil on Ronin/SK4/Riot for how it has been handled by the pro's.
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I was looking at exactly that page, It stalls later, lifts more, drags less on paper it looks quite good. Just wondered if anyone has any experience with it.
When you compare it to the NACA 63412 it seems to win, but there has to be a reason no-one uses it. As I think Will said on another topic, the h105 seems popular too as it is specifically designed as a low speed hydrofoil. I cant seem to find a dat file for that so it isn't much use to me.
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yeah the concave shape makes it pretty hard to fare, female moulds and prepreg seem like the only way to do it well. Did you ever get anywhere with your double L and cross piece mould?
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My current construction has a concavity at the back. I think it is reasonably fair.
I am fairing in a bulb arond the rudder lift foil intersection with bog but made it too fat initially and now have a lot of difficult sanding ahead of me.
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Have you read any of Tom Speer's stuff?
http://www.tspeer.com/
He suggests the H105 section.
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yeah the concave shape makes it pretty hard to fare, female moulds and prepreg seem like the only way to do it well. Did you ever get anywhere with your double L and cross piece mould?
Little progress due to "other project". Will be digging the L moulds to restart in the very near future as I need a foil for EJ and Evo. If it works, you will be welcome to borrow it.
IMHO I'm not convinced by either the "use pre-preg" or "female moulds reduce fairing" arguement. The latter really depends on how good the moulds are and the ammount of buggeration needed to join top to bottom. I think both are due some serious head scratching for us home builders.
I am fairing in a bulb arond the rudder lift foil intersection with bog but made it too fat initially and now have a lot of difficult sanding ahead of me.
Owch. Enjoy it Phil. Looking forward to getting back to TSC....
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I have H105 as a dxf, and also as a blender 3d model. I got some plugs machined up a few years ago and have some molds in the garage at the moment. They seem to work OK for making foils, I have made a couple so far, and have got better with each one.
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I have the plugs for Phil's mold so if anyone wants to make another set of molds it shouldnt be a problem
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hmmm, tempting, What are the dimensions of the plugs?
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If you want accurate low cost plugs try:
http://www.foamwings.co.uk/Pages/Site/Default.asp
They have a computer controlled hot wire cutter that will cut Styrofoam wings for model aircraft.
You are restricted to linear tapers but the quality is very good (I am sure that competition model glider flyers are pretty demanding) and the cost works out at ~£10 per section.
I have made hydrofoils out of 2 or 3 sections, having taken glass moulds off the foam blanks.
I have also put wooden stringers in the Styrofoam and laminated straight onto them.
By being careful with the temperature control I have even managed to cure prepreg on the Styrofoam.
I suspect that they could cut a one use foam mould. Great for rapid development.
Barry is very helpful.
Clive
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Hi all; does anyone have a T-foil section or a complete T-blade surplus (that they want to sell)? I am planning/thinking of either making, or finding 1 to test lifting sweet dreamzzz arse out the water to gain a bit more speed!
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Ade,
I have an Aardvark spare T + Stock, you're welcome to borrow it to see how it goes, I would only use it if I broke my demon foil.
In fact you wuld have done me a favour if you'd said last week, as I had a hell of a time getting 2 T foils and Eddie in the car .... one of them nearly stayed in Pwhelli !
G
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Sam and I are hoping to run a DT school project for his GCSE around foils then (hopefully) make the foils at the sticky weekend. The school probably have the tools to make the foam core around which to plonk (that's a technical term) the carbon.
I'm very hazy about the internal structure of foils so was planning to start a thread soon with Sam and sort out design etc
We have a plain rudder to convert to T-foil and want to make a long dagger board for Loco.
Fancy joining in Ade? Might persuade the school to do the difficult bit for 2 T-foils and a dagger or two. Roland said 'Do they have a CNC...?' something-or-other when I mentioned it and if they do this is apparently 'good'...yes I am that hazy about the detail :)
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The home build approach is to route out steps which correspond to contours on the foil section. With a parallel section main foil the contours are straight lines. For the lifting foil or a centreboard they are generally curves so you either need to be a whiz with cad or happy to plot points and draw curves.
My new foil appears to work well. It had a slight concavity at the back but I think i got this too deep. it was very lifty but incredibly sensitive to angle of attack that it appeared to be more draggy than our old one most of the time. I filled in most of the concavity with car body filler which has helped. I can always sand this out at a later date if required.
I did find that I had set the lifting foil to lift with the rudder vertical. with the waves at Pwllheli this meant i was running the foil a little negative which meant the rudder was angled back loading up the tiller. i plan to re set the lifting foil angle a bit more nose down to avoid this effect.
We noticed on our tow in on Thursday that the lifting foil bends up at the tips under load. I do not know what happens when the boat jumps off a wave and lands on it. The forces must be immense.
I think my rudder section was a H105 with 200mm cord, 9% thickness and 1m long from the top of the stock. I would have to find out what i used for the lifting foil but it sounds like the RG8 is a good bet. It's just really difficult to rout these thin foil sections from foam with good accuracy. High density foam is easier to rout being denser but medium density foam will work. Span of the lifting foil should fit in the boot of your car. ours is 1m. Any wider and as graham says you have to leave the crew behind. :(
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Phil - Its more likely the H105 section IS your lifting foil since it is an asymmetric section. 9% thickness also sounds a bit on the thin side for a rudder vertical...
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I know this is a bit of a cop out but... With the t foil sections; could 1 side (the underside) be a flat section. As that would make it easier to laminate and 1/2 the build complexity. Or would that section produce too much drag? (thinking of compromise).
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Hi Ade,
It was good to meet you last week. The boys seem to get on well. Alex now wants a mountain board like Joe's.
There is nothing wrong with having a flat under side (for the last 75%).
I have done it in the past and did not see it as a cop out. The theoretical drag penalties are very small.
Making a concave foil accurately without CNC female tooling is pretty unlikely.
I brought some sections from http://www.foamwings.co.uk (http://www.foamwings.co.uk)
and made a mould that went right around the leading edge so that the top surface and the leading edge were all moulded in one. The section that I was making was small and thin and the foil was completely solid carbon. We are used to cored foils but as the sections get thinner solid makes more sense. I have not seen a cored sailboard fin but have seen some that are 70cm long, significantly bigger than each half of our T foil.
Early 14 T foils used stock sailboard fins. If you can find a couple of suitable ones there really is not a lot wrong with using them. They are symmetric so will not creat quite as much lift as an asymmetric section but I do not believe that we should be running them that close to their max lift as the induced drag becomes very significant.
I am also sure that we do run them generating down force. Mine certainly was on the first two days last week.
Clive
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The first two T-foils I built were flat on the underside for ease of construction, and they seemed to work, but were not really the shape I had initially planned.
A few years back I got plugs CNC'd for a H105 rudder blade, from which some moulds were pulled, which I still have.
I think that either Dave C or Tom K currently has the plugs, which might be able to be used again. However they may not be in existence, or decent condition any more.
I decided to go for building moulds from CNCd plugs because if you are targeting a 10mm thick lifting foil, you will be laminating around 1-2mm of carbon on a core of 8mm max thickness, so your skin is at least 20% of the thickness of your final foil. I don't see how you can keep an accurate shape post lamination.
Calculating the shape of the core with a 1-2mm offset at 90 degrees from the surface also takes quite a bit of CAD work.
If you have the option to do some CNC work I would go for plugs rather than core.
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The image attached shows the 1mm projection for the core for our last rudder lifting foil.
Even a super accurate core, is not helping to find the concavity?
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I like Clive's solution (getting a model aero wing place to produce a foam plug to take a mould off...or even build the carbon onto?) but I will start a different thread when Sam's all set up for his project to sort out the design - he needs to be involved at all stages as it is a GCSE project and he's currently still in holiday mode
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The foams that can be hot wire cut for model aircraft are not strong enough to use as a core.
They are however cheap
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My foil is nearly made now but (danger time)... I have been thinking again! (trying to save work and ignorance. etc.. )
I have made the foil from 2 windsurf fins and have belt sanded 1 side flat therefore giving me an asymmetric shape - nearly finished carboned up. blar, blar.
Currently Sweet Dreamzzz has a pivoting rudder and has quite a lot of forward angle when secured in the down position. The helm balance is lovely when sailing so I thought, for now, don't mess - keep it all the same.
Soooo; heres my dangerous bit... 'Its asymmetric, therefore its neutral at slow speeds (with drag) but lifting as speed increases (the glass being almost full attitude) why not test it without adjustment'! (Is this where you all fall off your chairs laughing)
The plan then is; why don't I shove the foil into the trailing edge near to the base at the neutral fixed angle of the forwards leaning blade. The lift will be created by the assymmtric foil when speed is gained (perfect). I know this is a shot in the dark and you are probably all thinking 'daft bugger' just make it adjustable right from the start.
honestly any comments will be greatly appreciated before I endevour with this plan.
However I also think to make it adjustable I will need to make a new stock. further to this...
I was wondering if anyone can remember if the existing rudderstock could have been made at a 'stickyweekend' as it looks to me as if it was knocked together and completed quite quickly. If it was its still doing the job brilliantly. Is there another STICKY WEEKEND planned?
Also I cannot access members area or post photos or do anything apart from forum. Can anyone email me with more contact details.
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There will be a sticky weekend in the nearish future. You are always welcome to head down to Chew for a weekend and make one around sailing. R
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Ade, I can tell you that you can get away without adjusting your t-foil as long as it isn't too windy. On Badgers a while ago the gearbox seized up and we sailed for a few events with it in a fixed position. We probably sailed better than usual as we were just focussing on sailing the boat properly with fore and aft trim rather than worrying about what the foil was doing.
A little bit of positive foil angle would be ok (for reference, the t-foil on The Badger is still set up so that even when fully 'off' there is still at least a degree of positive foil... this is ok up to around 20knots and any sea state then it becomes tricky to say the least). Obviously an adjustable foil would be better in the long run (adjustable stuff is cooler, right?!) but as long as your current stock is not going to explode then I don't see why it shouldn't be ok. From memory, the rudder on sweet dreams was a pivoting foam/carbon stock with a plastic pin through it? (this right Hoppy?)
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Normally you would use your forum login to access the wiki, however this is currently broken and I have not had time to fix it, so currently noone can login.
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Yes it is possible to sail round a course without adjusting the foil and keeping it moderately positive. This is more doable in flat water than in waves. However getting the right angle of lifting foil to the rudder blade may be tricky.
If you have a high lift section it will be more sensitive to angle of attack.
I got the angle of our new foil to rudder a bit wrong for the waves at Pwhelli and had to angle the rudder blade back to keep the bow out. This makes the helm feel heavy. So the moral is angle the lifting foil downwards by 2 or 3 degrees so the rudder blade is never angled back from the vertical.
I will have to do a cut and re glue/re-fair job on our foil to rectify this.
if you went from a fixted foil to a pivoting one you may have to do similar.
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T foil now made. Joe and I going to test next weekend. (no adjustments yet!!!) We are going surfin to Watergate Bay next week so we will both be well used to going 'arse over tit'