NACA Foil Alternative?

[PJake]

I've designed and built a rudder for an ocean racing kayak based on the NACA 0012 section. My rudder is 9" long, 4" wide, raked at 40 degrees from vertical (to shed weeds). Reynolds number is around 500,000. The rudder was hand shaped from a balsa core then fiberglassed and varnished. I cut out a template for each end of the blank to guide in shaping but hand shaping is not my strongest skill so there are shaping errors. The thickness is based on the thickness of my rudder shaft. 90% or more of the time the rudder has no angle of attack so minimum drag is important. Angle of attack for most maneuvers is 10-15 degrees or less so I'm looking for a stall angle of 20 degrees or more.

Based on some internet research I'm led to believe that the NACA section has a relatively high drag. Can anyone recommend another section that would perform well given my needs that might have a lower drag? I'm especially interested in possibly a symmetrical (fore and aft) section because it might be easier to shape. Also, how sensitive is the drag for the NACA section to errors in shaping?

Obviously this isn't aerodynamic but the principles are the same. . .

Thanks in advance!

 

[GregLocock]

Investigate a laminar flow section (ie max thickness at 60% chord). I'd guess that there is a minimum thickness/chord ratio you want to look at, and it might well be fatter than you think.

Some Wright brother type tank-tests might give you a level of expertise greater than your competitors.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

i'd agree with greg's suggestion, your reynold's number is pretty low (as you'd expect with even a fast moving kayak); but then there are a bunch of other factors coming in to play (the water/air surface, the flow field round the kayak)

unfortunately these profiles look quite tricky to form (i'm willing to guess I'm less handy than you are !). and i think they'd be pretty sensitive to profile deviations.

but the good news is that i'd expect they'd have better stall performance (they generally have a well rounded nose).

again, like greg, testing is probably the best way to optimise. one thing i've learnt in the past is that you need to carefully about the wax you use ... a really shiny finish may look nice but won't perform the best.

are there rules about where you place the rudder ? this might be something to optimise too. how about different rudder shapes (like a double rudder, maybe with an endplate) ? tho' i'd have thought you'd get more "bang for your buck" by improving your paddle blade design.

good luck !

 

[btrueblood]

Why not retract the rudder except when you need to turn? I'm assuming most of the race is a straight line, with a single turn at the midpoint of the course?

I usually agree with Greg, but have to differ here: I'd avoid the laminar flow airfoil; it's low drag characteristics are much more sensitive to profile and surface finish defects than the old NACA symmetric foils (not sure if there even is a laminar-flow symmetric foil, but I could certainly be wrong), and the low drag benefit (compared to other airfoils) disappears at higher angle of attack (which, if you retract the rudder, will be the only place the rudder will be used). Also, if memory serves, the stall characteristics of a laminar flow foil are worse (stall is sharper, lift/drag ratio worse) than for other profiles (again, check me on that).

Rb's suggestions sparked this idea - how about two retractable rudders, one at the bow and one at the stern - the turn rate (yaw rate) would be higher, even though the drag would increase. Would be interesting to see if how quickly the boat turns vs. the speed it is making as it exits the turn makes a difference in course times. There's probably an optimum turn rate for any given rudder configuration, which means you could fix the angle of attack of the rudder, and drop it into the stream when necessary (or if it can't be retracted, to have a simple two-position mechanism that would hold the AOA at either zero or "best turn" angles).

 

[amorrison]

I assume you have talked to these people and there cohorts so you will not "reinvent the wheel".

http://www.humanpoweredboats.com/Photos.shtml

hpva? ihpva?

Google-Patents - option?

 

[swearingen]

At the annual Workboat show in New Orleans this past December, I saw a rudder that, when deflected, changes its shape and chord length to a curve that is conducive to the desired turn. I know this is pretty complex for a boat such as yours, but this would allow you to make the rudder almost wafer thin. In the low Reynolds number area, this would get rid of much of your form drag and reduce your parasitic drag slightly. Then, when deflected, it would change shape to prevent separation and maintain laminar flow.

 

[PJake]

Swearingen - that sounds amazing. Do you have any link to something like that. You're right that it would be too complex for my needs but it sounds great. A morphing rudder....

 

[GregLocock]

That's neat. I'm not even sure that it is really hard to do.

Yes there are symmetrical laminar flow airfoils. I don't know if it it would be a good idea, as you say the high AoA performance is probably rubbish.

On the yacht I sail, which was designed to win races by someone who does a lot of that sort of thing, the rudder's shape doesn't look anything more special than whatever is necessary to fair in the tube that forms its main spar, at about 50% chord. I've noticed the rudder stalling quite a few times, easy to do if the main's traveller is too high. We do retrim the sails to maintain a neutral helm, I'd be intrigued to see some tank tests showing whether a slightly unbalanced trim, with some helm, is faster in some circumstances.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[backward]

Hi,
I think you're looking in the wrong directin. water being 12 times denser than air look at.supersonic sections the thicknes to cord as low as possible. It will cavitate at high angles of attact but at that point you're into a ton of hull drag anyway.
I would use stainless shim stock .010-020 and fold tight to form the leading edge and about 50% cord slip in a .125 rod and close up the trailing edge and ends with overhanging end plates at the same angle as the waterline.
Backward

 

[GregLocock]

Water is much denser than that, but the flow is way subsonic. Speed of sound in water is at least 3 times faster than in air, and I think it is 10x

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Canoe2you]

Hey PJake

Your choice of NACA 0012 is actually a very good one. The NACA profiles perform very well in the 500,000 RE range.

The existence of laminar flow at the water surface is highly unlikely due to particles and wave turbulence. This leaves a laminar section without it's main benefit of lowe drag. However it's disadvantages such as low max lift and stall hysterisis would still be there.

Get back on the sanding block and make a good NACA foil rudder and you wont be sorry. If you want the lowest drag just reduce the size of the NACA rudder about 20% compared to what you would need for a typical laminar section with it's lower lift.