Profile to F21 lifting rudder

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newt2u
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Profile to F21 lifting rudder

Post by newt2u »

I am not a fan of flat plate rudders, as they tend to stall out under high load, this separation of flow can cause loss of control. Although I have not had problems with my F21 to windward, I did have to fight the rudder a bit with the spinnaker up on my last sail of the season, so I have been thinking about solutions. The current rudder design has a flat plate of around 7mm profiled to a point at the leading edge. I intend to add a NACA 0010 profile to the plate using a core of hard blue foam encased in fibreglass and epoxy resin with some carbon fibre on. the leading edge for strength. I got a model aircraft shop "Cloud Models" to cut me the cores from the plan below and they are arriving tomorrow.I've had some experience with the materials as I have built large scale model gliders in this way.

Here is the existing rudder:
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And here is the plan I used for cutting the cores. I will have to trim the cores on the bottom edge but it was easier to specify to the cutter with the top and bottom both orthogonal to the leading edge.
rudder.jpg
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I will update the thread as I progress on this little project.
Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Joined: Wed Sep 11, 2019 11:24 am
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Re: Profile to F21 lifting rudder

Post by newt2u »

Blue foam cores arrived today.
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So I stripped back the rudder plate to bare aluminium...
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... and test fitted the cores. They fitted perfectly and will only need a small trim around the tip of the rudder which is angled and rounded.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Re: Profile to F21 lifting rudder

Post by newt2u »

First side of the profiled core epoxied to the plate, with an assortment of weights to hold it down while it cures. I am using the blank it was cut from over the core to maintain pressure over the curved surface.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Location: UK

Re: Profile to F21 lifting rudder

Post by newt2u »

First side is now cured to solid (not fully cured yet), so flipped over and laid down the second side. You can see that as the original plate's leading edge was profiled to a point I will have to fill and build up the leading edge in order to get the correct profile.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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drcscruggs
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Re: Profile to F21 lifting rudder

Post by drcscruggs »

Looking good!

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newt2u
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Re: Profile to F21 lifting rudder

Post by newt2u »

Thanks Dr.

Today the epoxy is fully cured so I removed the backing cutoffs and trimmed the foam to the tip shape of the blade with a hacksaw and then rounded off the tip, as well as the square edge at the root. I used 180 paper but you have to be careful with the foam as you can deform it by being too aggressive. Not a big problem though, as any small dings can be filled later.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Location: UK

Re: Profile to F21 lifting rudder

Post by newt2u »

As I said in the earlier post, the leading edge (LE) of the blade, as well as to a lesser extent the trailing edge (TE) were profiled, this means that they were not flat, so there was no bonding over about 2-3cm at the LE and 0.5cm at the TE, there was also a ding in the blade at the TE.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Re: Profile to F21 lifting rudder

Post by newt2u »

So I needed to fill the gaps but at the same time ensure there was good bonding around these areas. I used a filler made from epoxy with the addition of glass microbaloons. This filler is strong and yet can be sanded relatively easily. I mixed two pots, the first at about 3:1 balloons to epoxy by volume to keep the filler loose, so it flowed into the gap, the second I made about 5:1 so that I could build up the profile. I find this a nice filler to work with as the epoxy allows it to flow into a smooth finish. I will probably need another layer of filler when this has cured in order to build up the LE a bit more. The photos show the result after application with very little tooling and still flowing.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Re: Profile to F21 lifting rudder

Post by newt2u »

By the time the glass balloon filler had cured to a solid it had flowed some more into the cracks. Next job was to add some height to the LE so that I could sand this down to the required profile. For this I used some fairing compound mixed about 10:1 with epoxy. This filler I used to build up the leading edge to an approximation of the NACA 0010 profile and will sand back using a couple of templates.I may need to add more filler in places during that process but will probably use car body filler as it will cure quicker.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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newt2u
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Re: Profile to F21 lifting rudder

Post by newt2u »

Filler was all properly cured today so I set to profiling the leading edge. I used a detailed power sander on the high points to get the right level then lots of hand sanding with 180 paper. There were a couple of low points that I had to build up some more but overall a nice finish. You can see both glass balloon filler and fairing compound filler in the first picture.
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I used car body filler for for these small patches, which goes off in 20-30 minutes, so then back to finishing the sanding to profile.
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Rockin - F21 twin drop keels - located Milford Haven
Wintered in Shropshire where I am when I’m not sailing

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