Hi, Dave--
Now it's getting interesting. I just wish we had a boat designer to chime in here, as we are way over on the aviation side of this thing, I think. As an aviation editor for about 15 years, I got to hop into a lot of aircraft, including the 177RG, which I liked a lot. Our company plane was an M20J and I also did aerobatic training in an 8KCAB, which has the "semi-symmetric" wing airfoil because they figured you'd want to be inverted a lot. (There's one of those still missing, with Steve Fossett in it.) Regards, Dave S. (II) On 3/17/08 2:32 PM, "David Shaddock" <[EMAIL PROTECTED]> wrote: > Hi, other Dave S.-- > Your hunch is correct; laminar flow in water disappears at a pretty low for > the waterline lengths and foil lengths we're discussing. After I wrote my > comments earlier, I went back through the thread and found the link to Bryon > Anderson's excellent article, which explained everything I was trying to > cover in a better fashion, with diagrams too. He mentions 5 knots as the > approximate speed at which we lose laminar flow. That's oversimplifying, > but it gives you an idea. But laminar flow has a lot to do with the NACA > profile--some profiles intentionally move the maximum depth aft in order to > maximize laminar flow; the idea is to keep the lift working for you as long > as possible. My last plane was a Cessna 177RG, which had a laminar flow > wing designed to keep the flow 'attached' for 70% of its surface or better > at speeds of about 170 mph. It took a long time getting off the ground, but > was incredibly efficient in the air. It was very different from my friend's > 175 (we traded for a while so he could get his commercial license), which > had a fat high-lift wing that got you off the runway in a third of the > distance but only provided 2/3 the top speed for the same power and fuel > burn. > > Just because a flow goes turbulent, we're still interested in it and it can > still perform some useful work. Besides, it's kind of a necessary evil; you > can't just provide the part of the foil that gives you laminar flow and then > remove the rest <grin>. The whole profile works together. > > Now, you mentioned vortex generators... They sell dimpled surface material > for airplanes to put on the surfaces where the flow starts to detach, under > the theory that the dimples (like the ones on golf balls, except standing > proud of the surface) generate mini-vortices that help keep the flow (albeit > turbulent) attached. Makes me wonder what this would do on the aft faces of > a keel. I notice my BMW has raised bumps all over the edges and strut for > the side-view mirrors, to cut wind noise by keeping the flow attached so it > can't escape and whistle. > > But let's talk for a minute about porpoises. Capable of 25 knots > underwater, and without a turbulent boundary layer... I guess they don't > understand Reynolds numbers. Apparently they have a paper-thin outer skin > with a thin spongy layer below that covering their real skin. One theory > says they can detect turbulence and adjust their body shape to reduce it by > controlling this soft layer. Dr. Kenneth Davidson studied them heavily and > figured their speed was more due to their streamlined shape and the smooth, > oily skin, but later research does suggest that the softness is apparently > as important as the skin. But I wonder why my inflatable isn't quicker... > Anyway, since I don't see a mechanism for the porpoises to control this fat > layer, maybe it's a passive thing. I don't think I'm ready to plaster my > keel with neoprene to try it out, but it makes me wonder if the flexible > layer couldn't react to impending turbulence and change shape just enough to > keep the flow attached. > > Dave Shaddock > > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts > Sent: Monday, March 17, 2008 12:14 PM > To: [email protected] > Subject: Re: catalina27-talk: Keel Fairing > > > > > > Hi, Dave-- > > That's all true, more or less, but what I have a strong hunch you will find > is that these foils do not give us laminar flow at our speeds and angles of > attack. > > In other words, going to windward, I believe you would find that we are > nearly always in some form of turbulent flow, at some point in the keel > section, unlike aircraft, where we do get laminar flow most of the time, > over most of the wing. > > (BTW, the ratio of Reynolds numbers is 13:1, water versus air. Don't hold me > to it, but I believe this is the practical consideration when modeling > foils.) > > These days, there are inexpensive underwater cameras that could perhaps show > us what our keels are doing. It isn't a fair comparison, but I get a good > look at my rudder and it always looks like it's in some degree of turbulent > flow going to windward. (I have the old rudder, which is an anachronism.) > > Regards, > > Dave S. > > PS-I am very familiar with the root versus tip design concept for beneficial > stall behavior in aircraft, and we could throw in wing twist if we wanted to > complete the picture. And let's not even get started with Whitcomb winglets, > stall fences and stall strips, not to mention vortex generators. > > > On 3/17/08 12:31 PM, "David Shaddock" <[EMAIL PROTECTED]> wrote: > >> The Cessnas and other aircraft sometimes use different foil shapes at the >> root and tip in order to make sure the inner part of the wing (closer to > the >> fuselage) stalls first, making the aircraft dive and regain speed while >> still providing some control out at the wingtips to avoid a spin. This >> isn't an issue with sailboats. >> >> But our keels can still stall--the keel provides windward lift if it > doesn't >> stall, at the expense of some leeward slip. If the keel stalls, you lose >> the lift and you see a lot more leeward slip/skidding. >> >> There are so many NACA profiles that it's hard to imagine anyone using >> something that's NOT a NACA profile--they have tested and published > results >> for hundreds of them, with some having only a tiny variation from others. >> But those tiny variations can make measurable differences, especially > since >> we're operating our profile in a medium 800 times denser than air. I have > a >> book I used for aircraft design purposes that's got everything they had >> published through about 1990. At any rate, selection of the ideal profile >> for a sailboat involves knowing the aspect ratio as well as the target >> speeds. For example, there is a concept called the lift/drag bucket--a >> high-lift keel profile provides a lot of drag, but might be a worthwhile >> price to pay if you're trying to achieve the best VMG in light air, > because >> at low speeds the drag doesn't hurt as much and adding lift while > minimizing >> leeward slippage pays off. For higher speeds, a lower-lift profile works >> better because when the boat is moving faster through the water, you'll > get >> a resultant increase in the actual lift windward and have less drag to > worry >> about--but overall you'll see more leeward slippage. >> >> A bulb at the bottom of the keel offers two things--for one thing, it >> minimizes the tip vortex (which adds a great deal to drag), but mainly it >> helps provide a lot of mass at the extreme draft, which provides more >> righting moment. If the rules allow, you can carry more sail because of > the >> extra righting, and you'll heel less which means more sail upright and >> working for you (although heeling may increase your waterline length on > some >> hulls and raise your speed). If the rules don't allow added sail, you can >> take advantage of the increased righting moment by cutting weight out in >> other areas and you'll accelerate faster. >> >> The profile Tim has picked out for his sportboat is a good one; at the >> speeds he might be getting on a planning boat, he could probably have done >> well with a narrower profile, too, but this way he's covered for a wide >> range of conditions. >> >> A lot of this information is in Steve Killing's book on Yacht Design and >> also in Skene's Elements of Yacht Design--but the later publications of > that >> are much more informed than the early ones). >> >> Dave Shaddock >> >> -----Original Message----- >> From: [email protected] >> [mailto:[EMAIL PROTECTED] On Behalf Of David Shugarts >> Sent: Monday, March 17, 2008 10:52 AM >> To: [email protected] >> Subject: Re: catalina27-talk: Keel Fairing >> >> >> >> >> Hi, Tim-- >> >> I think your summation of it as "like a Chevy" is a pretty good analogy. > To >> go back to the source, I have now heard Frank Butler answer a number of >> sophisticated questions with what sure sounded like naivete to me, so I > have >> a hunch that our factory keel section was a "oh, whatever" decision at the >> time. Then these better keel sections would naturally be an improvement, > but >> only because the bar was set so low. >> >> It would be interesting to hear from an expert here, because I just feel > as >> though the designers of the cool toys are way beyond NACA foils. Or > perhaps >> they really are more about the bulb than the keel section itself. For >> instance, if we could hang a heavy lead bulb on a carbon fiber keel, we >> would probably do it, and we might find that ANY keel foil would be fine > for >> the purpose. >> >> BTW, this link: http://www.hanleyinnovations.com/glossary.html, shows a > few >> cases where the NACA 0012 was used in aircraft, but it also shows that > some >> venerable aircraft (e.g., the Cessna 150/152) had one foil at the wing > root >> and another at the tip (in other words, more sophisticated). Notably, the >> B-17 Flying Fortress had it as the root foil (love that airplane!). >> >> Regards, >> >> Dave S. >> >> >> >> >> On 3/17/08 12:03 AM, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote: >> >>> I don't profess to have any knowledge whatsoever when it comes to fluid >>> dynamics, I have just been going on threads on SA and bits and pieces of >>> knowledge that I've read from different designers. >>> >>> I think that as far as high performance (e.g., sport boats, hulls that >>> will plane >>> off the wind) sailboats are concerned, a bulb on a keel foil is pretty >>> much the >>> name of the game. Certainly heavy displacement and cruising boats will >>> look toward other keel configurations. But the NACA foils offsets have >> pretty >>> much been determined to be the go-to configurations for fast keel struts >>> in the sportboat world. There are a few arguments over whether a 0011 >> section >>> might be faster than a 0012 seciton (with a resulting decrease in >>> strength/robustness >>> to loads, etc) for example, but the 0012 shape seems to be the chevy >>> pickup when >>> it comes to most foil sections below the waterline. >>> >>> These are fairly simple shapes. Pretty easy for an amateur to cut with a >>> hot wire, >>> or for a CNC machine to do it. >> (http://www.youtube.com/watch?v=q7uvq4RlhHM) >>> I can certainly imagine that areonautical designers would have the need > to >>> come up >>> with more complex shapes for specialized, shape-specific demands, > executed >>> at high speed with enormous G-force loads in the atmosphere, and new >>> materials and production techniques would allow for a huge amount of >>> variability when it comes to foil offsets these days. >>> >>> But these are just simple symmetrical foils shapes that you can order up >>> and get made pretty cheaply on-line...I just ordered a 54" piece of >>> spyderfoam cut to NACA0012 sections, >>> for about a hundred bux incl. delivery. It's a dream-world out there now >>> for home boat (or aircraft) builders! >>> >>> tf >>> >>> >>> >>> >>>> My ears perk up here. First, I confess ignorance. Are boat keels based > on >>>> NACA foils, and do they apply to water, as opposed to air? Perhaps there >>>> was >>>> a series of NACA foils intended for water? I just never paid attention > to >>>> that part of things, although I studied NACA airfoils for my own > purposes >>>> many years ago. I vaguely recall a factor called Reynolds Number that >>>> would >>>> govern foils in various media, such as air and water. Can you elaborate? >>>> >>>> Regards, >>>> >>>> Dave S. >>>> >>>> PS--I was just a layman studying the foils at the time, but I went >> through >>>> them all pretty carefully. It seemed to me that they were kind of >> empircal >>>> in nature. I got the impression that the great virtue of a NACA foil, > for >>>> an >>>> aircraft designer of the 1930s or 1940s, was that it was thoroughly >> tested >>>> and predictable. However, it seemed as though a lot of developments of >>>> later >>>> decades, such as the Clark-Y, not to mention variable sweeps and tapers, >>>> variable chords and foils in a given wing, etc., began to favor >> departures >>>> from the NACA foils (except when mere predictability was the goal, as in >>>> vertical stabilizer foils). So, although I later got into aviation >> writing >>>> and was constantly looking for NACA foils, I didn't find many in the >> wings >>>> of light aircraft. In my time, we saw NASA come out with the GAW-1, and > I >>>> have always assumed that later, composite aircraft designers were free > to >>>> work with an infinitely variable foil in mind. >>>> >>>> >>>> >>>> >>>> On 3/16/08 8:40 PM, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote: >>>> >>>>>>>> but they also value every advantage they can get. >>>>> >>>>> key words^, huh? >>>>> >>>>> nice explanation, Chris. >>>>> >>>>> So I guess Compu-Keel is still around? >>>>> >>>>> http://www.compukeel.com/ >>>>> >>>>> odd because you get NACA foil specs on-line for free...but I guess all >>>>> class >>>>> legal keels cant be derived from NACA sections. >>>>> >>>>> tf >>>>> >>>>> >>>>> >>>> >>>> >>>> >>>> >>> >>> >>>
