That's really well-put, Keith. A lot of the variations in the NACA symmetrical foil shapes relate to the station location of the maximum thickness. Moving the max section after improves laminar flow and keeps the attachment longer, but at a cost of reduced lift in total and with a benefit of reduced drag (partly less induced drag due to less lift, and partly less skin drag due to reduced turbulence). But the stall characteristics change, too--where a gentle curved profile like the NACA0012 (if I remember right) stalls an airplane gently, with a shudder, I think that a profile with a more aft maximum section tend to break more abruptly--which makes sense if you imagine the turbulent transition moving forward along the profile--in the profile with an aft max point, it will stop lifting at about the time it hits that point, which will be early than a profile that has its max section more more forward.
Back to planes for a sec--there are a lot of laminar flow wing sections in experimental aircraft (they work nicely with ultrasmooth composite wings, and are seldom seen with aluminum riveted construction, which is why the C177RG is kinda rare in production aircraft from the '70s, when our C27s were being built). But laminar flow depends so much on a perfect surface--a heavy rain or flying into a cloud of insects can really wreak havoc on your lift. I guess that's all the more reason to polish the heck out your keel if you've put effort into promoting all the laminar flow you can get. Another curious item--Steve Killing mentions a feature used on some high-performance keels called a dillet--like a fillet, except it's inverted; a groove designed to induce local turbulence and convince the water flow to reattach itself to the keel. Tim could have a groovy keel on his sportboat. Cool. Dave Shaddock -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Sneddon, Keith Sent: Monday, March 17, 2008 1:56 PM To: [email protected] Subject: RE: catalina27-talk: Keel Fairing Thicker sections stall later, given the maximum thickness at the same %chord. This is because the corner you are asking the flow to go around is more gradual with a thicker section, so the flow can stay well ordered. A thicker section not stalled has more drag than an equivalent but thinner section not stalled. BTW, the flow stalling with AOA is not a discrete event, but rather starts at some distance behind the nose of the section on the "high side", and moves forward with increasing AOA. A partially stalled foil still generates lift, but is very draggy, where a fully stalled foil (like a dingy rudder hard over) really only works as a brake, and doesn't generate much lift at all. Keith Sneddon -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Monday, March 17, 2008 2:45 PM To: [email protected] Subject: RE: catalina27-talk: Keel Fairing >>>A fatter profile, all other things >>being equal, will generate more lift at a given angle of attack, but may >>also stall earlier and will create more drag-partly due to increased wetted >>area, frontal profile, and the induced drag that comes with lift.<<< whoa whoa whoa. I THOUGHT the fatter sections would: - generate more lift thru a wider range of angles of attack - GOOD - create more drag- BAD - STALL LATER- GOOD whereas, thinner sections would: - generate less lift thru the same range of angles of attack - BAD - create less drag - GOOD - stall earlier - BAD as defined above, you are saying fatter sections: - generate more lift - GOOD - create more drag - BAD - stall earlier - BAD and thinner sections: - generate less lift - BAD - create less drag - GOOD - stall later - GOOD so it seems like, the way you have delineated the trade-offs, there is much less incentive to go with a thicker foil. Please understand, I'm not trying to argue here, I'm just trying to grasp the concepts! I also understand that if the boat is planing downhill at 15 kn the thicker section is going to be more forgiving! So I guess my question is, once again, dont thicker sections stall LATER? thanks, tf
