vortex generators

Darn you Lester - those numbers seem to make it even more important to try to hit the correct shapes and sizes. WOW !

Wil & Lester — thanks for the response. In the past I have been primarily interested in solid wings for sails, and thus have stayed with low speed aircraft theory… along with whatever info I could get from Steve Clark and a few of the "C Class boys.

Jose - I did pull the following drawing from an article I found about the development of Miss Nylex (U.K. C Class) which showed different thoughts of mast/soft and solid wing sails as they moved through the years of various challenges. I always liked the “idea” behind this sketch, and liked the fact that with the exception of the leading edge, the soft sail similar to the DynaWing concept might work - especially in classes that allow rotating spars. I don’t have a date for this iteration, but I am guessing it was before the DynaWing went commercial. Since the C Class seldom used jibs in later years, this “flap” concept was used to try to speed up and direct flow. Of course, since then they have progressed to solid wings, but every once in a while they seem to drift back to retry old ideas but with a new twist.

Download Attachment: [ FlapWingConcepts.jpg](http://www.rcsailing.net/forum1/data/Dick Lemke/20045695354_FlapWingConcepts.jpg)

My apologies for going off topic from water foils to air foils, but Jose’s post about mast “turbulators” reminded me of this forward flap idea.

Back to the water foils ! [:-glasses] Wil - back to the comment about articulating camber in keels, would something like this flap idea work if installed directly in front of leading edge of keel? Would it help - and would you control flap opening by radio, or if left free to pivot, would it align itself much like a rotating mast does to apparent wind? Only need a limiter to prevent too much opening.

Just a thought begat by comments!

Going further off the topic. Ten years ago when I was racing a 12ft dinghy in Auckland. A pilot fitted wind votexs to his mainsail, they were made out off light gauge ali. Can not remenber what it when.

Back to the wings for a moment - I found the photo I couldn’t find earlier in discussion.

Note the wings and thickness. From what you are saying, this has both drag & lift in fairly high numbers but is desireable, and might be suitable for a “sail replacement” being symmetrical and drag not being as critical, but a similar section used underwater would see little lift and LOTS of drag, thus the thickness would be a negative feature?

Am I interpreting that correctly?


Download Attachment: [ tiny_flight01.jpg](http://www.rcsailing.net/forum1/data/dick Lemke/200456195214_tiny_flight01.jpg)

Hi sailors

What i am saying is easier than these. It’s just a short stripes (like in the photo) all long the mast. (i’m still thinking that, maybe, it’s a stupid idea, because i don’t test it, yet)[:-glasses]

Download Attachment: VGTop%20copy.jpg


Aaaallways look on theee briiiight siiiide of liifeeeee

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by jose

It’s just a short stripes (like in the photo) all long the mast<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Hi Jose

I’m playing with quite a substantial gap between mast and sail on my IOM. You might say that I’m using the mast as a vortex generator for the sail…

As I understand it, the idea of a turbulator is to energise the boundary layer and trip it into “controlled” turbulence. The reason you’d want to do that is if the BL was (1) laminar at the time, and (2) was separated from the surface of the body. Turbulating the BL makes it stick to the body instead of separating.

So the idea of a turbulator is to avoid separation. The reason for that is that separated flow (1) gives more drag than turbulent flow (but this in turn gives more drag than laminar flow), and (2) gives less lift because separated flow indicates the body is stalled to some degree.

If you don’t have separation, then you don’t need a turbulator, and using a turbulator just adds drag. When are you going to get separation? When (1) the body stalls, or (2) it is moving “quickly”. The classic studies indicate that “quickly” involves a Rn of around 300,000 or more, but can be down to as low as 100,000 in certain situations.

For our toy boats, with low Rn, separation due to moving too quickly isn’t much of an issue. So we’d usually only be interested in a turbulator if we have a lifting body that approached stall regularly. This happens on sails most of the time, on rudders whenever the boat changes course, and on fins when operating outside of their “normal” design parameters.

Lester Gilbert

Hello Dick,

I have a busy day ahead of me, so I can’t answer your question with the detail it deserves. It is an interesting question on the surface and perhaps we can re-visit it with some math sometime. But for now, I will need to keep my comments short:

I don’t believe a leading edge flap will be of much use on the keel. Keels generally do not operate at very high lift coefficients. Most symmetrical foils are capable of generating left up to about 10 or 12 degrees of angle of attack (Cl or 0.9 - 1.1) before stalling. Yet most boats operate well below this angle. Could you imagine your boat sailing along with a 10 or 12 degree leeway angle? That would be pretty shocking. Generally, you get your best lift to drag ratio at much lower lift coefficents than this. Take a look at the polar I’ve attached for the NACA0009 airfoil:

Download Attachment: NACA0009_polar.gif

The minimum lift to drag ratio can be found by drawing a straight line from the origin (0,0) that is just tangent to the top of the curve. This point occurs at a Cl of about 0.4. So this is the Cl that you would want to design your keel to operate at. But since this produces about 6 degrees of angle of attack (which can cause a fairly draggy hull), you probably want to bias this a little lower on the polar down to say a Cl of around 0.2 or so. This will give you 3 degrees leeway angle for a high aspect ratio keel.

Now what does this have to do with your flap idea? Flaps are primarily used to create high lift devices. But we don’t want a high lift keel, we want a low lift to drag ratio keel.

However, your idea does have some other merit. the main body of the sail in your drawing is allowed to camber. Cambered foils operate at better lift to drag ratios than symmetric foils. Also, cambered foils can generate their lift at lower angles of attack so the leeway angle of the hull is less thereby reducing the hull drag. now, you may not want to have the keel “snap through” like your sail would because when you are sailing down wind, it would “luff” and that would be very slow. what you want is a proportional lift device. Something that elastically stretces to a cambered shape in response to load. Something like this:

Download Attachment: pat5181678_2.gif

Good luck and we should chat more later when I have more time…

  • Will

Will Gorgen

Hi Lester

If you see this site http://perso.wanadoo.fr/amn.minifoiler/id75.htm you’ll see what i mean. This kind of sail could be the jib (adecuating it for it)and if you see the second photo carefully, you will see the VG stripes on wooden part.


Aaaallways look on theee briiiight siiiide of liifeeeee

The only thing I remember about vortex generators was that when I was flying Lears and Falcons we had to count the silly things and if any were missing look up in the MEL what the max number missing were that would allow us to continue, and if they could be next to each other. Sheeehs… pieces always falling off. I often wonder how many of those things have landed in someone’s back yard.

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by Peter _Birch

I read on a topic today that some-one is has used vortex generators on a r/c boat. Where would the value be in this?
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Vortex generates make the fluid flow near the wall turbulent. This concept is used for plane wings to prevent a laminar seperation which will cause a sudden decrease of the lift (stalling). A turbulent flow has always a bigger drag because of the bigger gradient of the velocity profile du/dy and the resulting bigger wall shear stress!!!

The conclusion is that turbulators will slow down the boat ! [:-basketball]