Naca 63a012

I'm just finishing a new hydrofoil for my i14 and need to work out the working range of angle of attack (it has a fully adjustable AofA while sailing like most of the 14 footers run) To do the maths I need the Coefficient of Lift for the NACA 63A012 profile from zero to 10deg AofA (it's symmetrical so +ve or -ve)...Anyone seen the graph on the net anywhere - or maybe have it on file? Many thanks, Penfold

 

Just out of curiosity how are you doing the foil attachment to the rudder-in two pieces?

 

The rudder blade has the section profile of the foil (at the CL) cut into it and the foil is then bonded and of course secondary bonded... Getting the whole thing square is the tricky bit but we have a jig to ensure proper alignment during bonding, and hopefully the of that bond is good enough for the secondary bond - which is vac bagged. Fair, fill, paint, polish, sail...!

 

http://naca.larc.nasa.gov/reports/1945/naca-report-824/naca-report-824.pdf, pg 421.

Better yet, get XFOIL and run the numbers yourself, for your Reynolds numbers, etc. There's also a graphical front-end called Profili that many like to use.

 

Parameters

Here are the key parameters of the class rules and the net outcome I am trying to achieve. I14 class rules only allow 1 lifting hydrofoil surface and the maximum area of that foil is 0.14m2. In practical sailing terms the only place to put this 'foil is attached to the rudder blade, about 400 to 500mm under the waterline.

The net outcome however, is that I want to generate maximimum lift (I want to be able to get around 110kg) at around 10kts (upwind mode) and 50 to 75kg negative at 15 to 20kts (downwind). I have sort of settled on around 120mm chord with a span of 1200mm, semi eliptical (straight trailing edge) planform and aspect ratio, making the max thickness around 14.5mm - which is good for from the loading stiffness point of view, and have made a blank (I am a partner in a high volume carbon fabrication business. Making foils in foam core with pre preg carbon skins is easier for me than most I am happy to say!)

Generally at all hull speeds I want to be able to generate lift in one direction or other for minimum drag - so consider the foil to be cruising in the + 6 to -3 degree A of A range as normal... but of course in certain cases the minimum of drag ie zero lift will be essential and in some wind direction/sea conditions this will be with the boat planing, just we won;t be able to use foil lift due the buoyancy of the bow - or lack of it!

At this time I want to be able at this time to confirm a chord profile, plan and aspect ratio and calculate the range of movement of the foil to start building the foil control system. The idea of an assymetric foil very much appeals but I am worried about low zero lift drag. Also there is some discussion here about whether dihedral will help stabilise the boat (a very good thing at times) or whether it will add weather helm if the boat heels (very very bad!)
Thoughts?!!!!

 

Moth site

In case you don't know about it this is probably the best compendium of practical monofoiler info on the internet:
Australian Moth Class Forum http://www.moth.asn.au/forum/index.php?sid=b2984b019982eb0292321c902664a9d2

 

So you have a maximum lift coefficient of 0.7 and a minimum lift coefficient of -0.14. With an aspect ratio of 12.7, the lift curve slope will be around 0.087 per degree, so the angle of attack range about zero lift will be from -1.6 to 8.0 degrees. Thickness ratio at the root is 12%.

Negative angle of attack doesn't mean negative lift. It's not unusual for a section to have a -3 deg zero lift angle of attack, so you could be operating throughout this range and still be producing positive lift. It all depends on the incidence - the angle at which the foil is mounted. That's why it's easier to work in terms of lift coefficient or angle of attack measured from zero lift, then figure out the geometric incidence at the end.

So what you're really saying is you expect a change in angle of attack from minimum to maximum of about 9 degrees. That's consistent with the lift range you specified.

The minimum drag may not be at zero lift, it may be at a modest positive lift coefficient even when you include induced drag.

In the figures below, I've shown three candidate sections, the NACA 63-012, H105, and H110 sections. The first polar plot shows the two-dimensional section characteristics from XFOIL. The second polar plot shows the 3D drag for the 12.7 aspect ratio (I assumed e=1) of your foil planform.

The H110 has 3 counts less drag at zero lift than does the NACA 63-012, and even when induced drag is included has less drag out to a lift coefficient of 0.12 than the NACA 63-012 has at zero lift. Its minimum drag is 4 counts less than the NACA 63-012's. So using a cambered section is not an impediment to having low drag at zero lift.

The symmetrical section has about 5 counts less drag at the maximum negative lift condition than does the H110. As expected the symmetrical section is better in this condition, but is a 6% gain there worth much larger losses over the rest of the operating range?

At the positive lift condition, the difference is as much as 40 counts between the H105 and NACA 63-012, and 30 counts between the H110 and the NACA 63-012. At the high-lift coefficient of 0.7, the H105 and H110 are indistinguishable and about 20 counts less than the NACA 63-012.

Dihedral looks to me like it would move toward lee helm going upwind regardless of whether you have positive or negative dihedral. The boat is making leeway, and let's say you have a positive lift on the foil. For positive dihedral, the leeward panel will be loaded more and the windward panel less. This will produce a net force to windward. However, it will also mean a reduction in effective span, and increase induced drag - picture the case where the windward panel is completely unloaded and all the lift is carried by the leeward panel. You're basically picking up rudder area when you add dihedral, just like a V-tail aircraft.

Even without any geometric dihedral, you're going to have some hydrodynamic dihedral effect due to the interference between the rudder and the foil. Rudder lift to windward will result in an increase in lift on the windward panel and a decrease in lift on the leeward panel, especially for the inboard portions. This will be similar to having a rudder that is deeper than its geometric depth. You would have to use something like a vortex lattice or a panel code calculation to figure out what all these interference effects were doing to your drag, etc.

Anhedral may be useful if there's any possibility of the foil broaching the surface when heeled.

 

I'm going to build 2! An H105 and the 63012 i started a few days ago (almost finished anyway) That way I can interchange them til I'm happy which is quicker/wider performance range - Mind you, there's no rule to say I can't pick from between them on the first day of a regatta anyway so I'll probably always take both and get some good weather info to make the call on. I'll put some picks up in the next few days so you can see the 63012 fitted. Thanks again

 

I'd love to hear how this comes out. It would be really good to get comparison data like this.

 

Have you been sailing them yet?

 

We've been so busy in the factory in the run up to the new year break, and also working on our new website - www.carbonology.com - that we haven't had time to put it all together. However, the 63012 is finished and grafted into the rudder blade, and the new rig has finally arrived from the builders so we should be ready very soon. Watch this space - or catch the first sail on the news page on carbonology.