The Subject is LIFT

[Rick9619]

What an interesting day!

Boys what a day, we put to sea out of our trusty slip in san diego actually planning to motor aways past Point Loma. The wind freshened and we raised the main and unfurled the jib. Decided what the heck, lets sail out to the Coronados, just inside Mexico, and low and behold about three miles out to sea we come upon a kayak! Arghhh... this guy is from New Zealand.. spends a bit of time here.. and he goes on to say there is a vessel in distress about a mile and a half off the end of his paddle. Dude what the f#ck are you doing out here? Just paddling around mate... Sheesh. So we take a bearing and set up for a close reach in about 15 knots apparent towards good ol mehico. AS we are under way I am thinking about all the posts on lift and how I am applying them. Its really simple, when you trim the boat and say put in 3-5 degrees of weather helm, the keel is actually lifting the boat out of the water. It has a positive angle of attack pure and simple. You can feel it. It was so fun! Screaming along at hull speed on our trusty 336 and scanning the horizon. We found him. Another guy fishing with one motor, one bait tank, no radio, no vessel assist or sea tow, and a crappy battery. We tied off and I gave him one of my two engine bank batteries for a jump, got him fired up and on his way. Tis what we mariners do! Next, my sister who is visiting and on her first sail from Memphis, gets to see the dolphins cruising along with the mother ship! So now, and this is no fish story, I had been keeping my eye on a very tall mast with kevlar sails that had blown by us DURING the rescue and was sporting a very nice kite that is now again in the distance. We are doing just under 8 now sprinting for the inlet and looking over our shoulder. He is on a beat just like us. What a beautiful racer/cruiser. Damn money can be nice sometimes. He chews up the two or so miles he is behind so fast we did not have time to finish our subways... BUT damn if he doesnt run aground.. yes aground on the shoal at the entrance to Shelter Island. I s^it you knot! We are poping champagne and celebrating our victory. Tactics are everything... !!! Oh and jibed around and asked them if they needed help... NOPE and thank you.CheersRick

 

[Bilbo]

Something

If I may be permitted to add a few comments:"A U-2 spy plane has a very high aspect ratio wing. For low aspect ratio, think F-16 or something similar to that. High aspect ratio wings are typically long and slender when viewed from above."With aircraft, it depends upon speed and useage mostly. Yes the U2 plane was not a particularly fast aircraft and it flew relatively high (over 70,000 feet)in very thin air so it neeeded a high aspect (lift) ratio wing and basically flew at a very slight speed over it's (Minimum) stall speed. http://en.wikipedia.org/wiki/Lockheed_U-2The F-16 on the other hand is a fair example of a low aspect ratio but it also has some other things going for it that may complicate things. I would suggest that another design such as the F-102 delat-winged craft be of low aspect. Almost like flying on a flat board but using the board's angle of attack to produce a measure of lift on the top side. Many of the stunt planes such as the Pitts biplane actually use a relatively low aspect ratio airfoil because this gives them the capability of inverted flight with less of an apparent angle of attack for the fuselage. In regards to the pivoting keel idea, doesn't one of the racing boats have a keel that has the capability to pivot only to port / starboard (not changing the angle of attack) as if on a hinge so that it moved it's weight to the windward side to counter the heeling moment? This would keep the boat beter on it's feet and then have less hydrodynamic friction.Canting Keel:http://www.sailtexas.com/modernyachtdesignfinalb.htmlSee photo 3 , 4 etc

 

[Phil]

Canting keel

It looks as though the movable keel gives a longer lever arm to the bulb thereby reducing the weight needed for the same righting moment. It's not for lift, it's for weight reduction. Lift and leeway reduction is achieved by the rear and forward "rudders", both of which have trim tabs.

 

[mortyd]

lift

whatever picture you sent, unless it is a hologram it not a picture of a wing which necessarily must be three-dimensional; i forget if this very fundamental conecpt is introduced in second or thir year aerodynamics, but sure not in sailing 101. what you have blindly stumbled upon is the basic difference in the words AIRFOIL and WING. a little knowledge is a dangerous thing.

 

[Rick9619]

More on lift

Hey Mortyd, good to see ya still on this subject. I suppose we are probably just beating it do death. After all, dynamics is only theory. But Capt'n Bob here is a delimma. I was looking through my ships papers yesterday and came across something interesting. My Hunter 336 comes with and I quote "winged keel" Whats up with that?CheersRick

 

[mortyd]

lift

rick. only a theory managed to get wilbur and orville in the air for hours up to ten thousand feet in the very kitty hawk aircraft once the learned how to fly it before another human being got one foot up. some theory.

 

[Phil]

Another answer

The same fluid dynamics applies to water and air. It's a common misconception that the curved surface of a wing is required to produce lift. Think of it more like draft in your sail, it promotes "proper" air flow, but it's not required. Lift isn't magic, it's just a word for force experienced by an object in a direction perpendicular to the flow. Lift has more to do with direction than with some mysterious process. If you stick your hand out the window of a moving car, the air is forced to flow around it in a manner that depends on the angle of your hand. Forget about theories, you can feel your hand pushed up. That's lift. Your hand is also pushed in the direction of the flow, that's drag. You can certainly bring all kinds of complications into the picture. Reality is messy. It doesn't negate the basic lift concept.So, the answer you seek is that your keel is not flexible. If draft (asymmetry) were designed into it, which side would you put it on to work on both tacks? On the other hand, trim tabs do something very similar, they add the curve you feel you are missing. They promote flow to produce lift more effieciently without so much drag. But they aren't required, as you know from experience every time you go out sailing.ps - The lower flow/higher pressure region is to waterward of the keel. The higher flow/lower pressure region is on the other side. It's really just what you would expect from the forces you know are there.pps - Are you really looking for a more technical description of how flow develops around a keel with a nonzero angle of attack?

 

[Bilbo]

lift

Not 100% sure but my aircraft friends generally say that the amount of lift generated in a typical airfoil wing is not enough to actually keep the aircraft flying. One needs to include the actual AoA of the wing. So in effect the wing of an aircraft is sort of skidding across the air somewhat like a planing hull boat. With this in mind Capt'n Bob's picture doesn't show the typical wind vortices of an airfoil that have a negative air pressure on the top rear of the wing shape.Something to read here:http://amasci.com/wing/airfoil.htmlYes that canting keel isn't really functioning as a hydrodynamic force but more as a counterweight to the heeling action. (The other function of a weighted keel) Since this is the case, these boats use foils in the bow and stern to help with directional stability just as the traditional rudder does as well.-another factor in the mix. Now the winged keel will put more weight down lower without the keel being too deep in the water. It may have some lift action (airfoil shape to the winged section) which may improve boat speed a very small amount as the boat goes faster but that would be very marginal unless the wing is extensive in width. The primary advantage is a shallower draft over a fin or bulb keel.

 

[Ross]

The wind is blowing about 25 here today

anyone who thinks that you need a caarefully engineered shape to produce lift need only try to carry a sheet of plywood a short distance. Lift will be generated when ever the fluid velocity on one side of an object is different then on the opposite side. You can get lift on round objects. Which is why taut ropes will vibrate in the wind. The objective of the designer is to get an acceptable balance between lift and drag. Carried to extreme fin keels could be as long as aircraft wings but finding water to sail the boat in would be a bit of a problem. The fin keel depends upon the forward motion of the boat to develope lateral resistance (lift). In very light air it will make more leeway than a full length keel, but when to boat speed increases the fin keel will generate more lateral resistance (lift)and less drag than the full length keel.I would expect a fin keel to "sail" around her anchor more than a full keel because the hull shape also provides a foil shape that produces lift when the boat is not perfectly pointed into the wind. Actually even then it produces lift that is symetrical port and starboard. The lift becomes asymetrical when the wind does not flow equally down both sides of the boat. Then the boat "sails" off to the side until drag overcomes lift and she "sails" back. A riding sail counters the aerodynamic lift of the hull form.

 

[mortyd]

lift

bilbo. not exactly. the effect of the airflow from the wingtips - idiotically called wake turbulence by the media - is resposnible for much of the lift, and also drag, generated by a wing. i've never seen these wingtip vortices even hinted at in anything but a university level book on aerodynamics book. this is not nit-picking, this is what got the wrights in the air while NOBODY ELSE was even on the right track.

 

[Phil]

Liftless?

mortyd,Are you suggesting that a wing in an air tunnel with sealed end conditions (infinite aspect ratio) and only bound vortices develops little or no lift?

 

[Rick9619]

Yes it could be liftless

Phil, if you put a symetrical.. lets just call it a shape, in a wind tunnel at zero angle of attack ( the relative or apparent wind is parallel to the mean camber line) and controlled conditions, it will not generate measurable lift. If you put an asymetric shape... airfoil, wing, at zero angle of attack, yes you will create lift becauses on one side the distance traveled to the back of the wing is shorter. As Bilbo's friends say... when we slow the object the wing is attached to, we create less lift, So now we increase the angle of attack on the wing by pulling up on the elevator (or dare I say add a little weatherhelm) to keep the lift vector greater than the weight or gravity vector which keeps us flying. On "bugsmashers" or a "hershey bar" wing cessna, vortex generators are installed to keep the wing from stalling all at once so our little general aviator doesnt become another statistic. On the swept wing planes that I haul people around the country on, our wing stalls at the root first by design which gives us a chance to put our coffee down and fix the problem Oh and by the way, the F-16 wing is aerodynamically unstable. Without the 4 flight computers "trimming its jib", the flight would be a hell of alot shorter than our beloved bicycle brothers.CheersRickIf we COULD sail in 100 knot winds... wonder how small the "no-sail" zone would be? Gee..

 

[Bilbo]

Tip vortices?

mortyd ,I was not mentioning anything about the vortices at the extreme ends of the wing tips. Are we off on terminology? I was tring to say this from that web page:"The popular explanation, PATH-LENGTH or AIRFOIL-SHAPE: wings do not deflect air, instead they are sucked upwards because the "airfoil" shape has a longer surface on top. Airfoils are curved on top and flat below, and therefore the air follows a longer path above than below. Air that is divided at the leading edge of a wing must rejoin at the trailing edge. Since the upper surface of the wing is longer, it causes the air to flow faster over the upper surface, which (by Bernoulli's principle) creates lower pressure above. Because lift is caused by the shape of the wing, wings can create lift at zero attack angle. They can create lift simply from path length difference which leads to pressure difference, and no air needs to be deflected. After a wing has passed by, the air does not remain moving downwards. (THIS EXPLANATION IS SERIOUSLY FLAWED.)"I understand about the Wrights. Their advancement in aerodynnamics is actually in the careful scientific study of the airfoil shapes through the use of the wind tunnel. I believe that they also applied this to their propeller shape as well. As for getting airborne, there are some that would say with great conviction that they were not first in powered flight. Not going to argue that one though as it's not relevant. I do believe that their advancements far surpassed the competition very quickly. Rick says: "Oh and by the way, the F-16 wing is aerodynamically unstable. Without the 4 flight computers "trimming its jib", the flight would be a hell of alot shorter than our beloved bicycle brothers."I saw a video on the Blue Angels F-18s recently. Apparently, they fly with a great deal of down trim on their elevator trim tabs so that they have the stick in a constant up force of something like 18lbs pull. This helps to keep the craft more stable with respect to each other's courses. Otherwise with neutral trim they would be constantly playing the stick up and down. It does not make the missons easy.

 

[mortyd]

lift

no, phil. i'm saying it's not a wing. we have neither time nor space to devote ot the first week of aerodynamics 101, no less further courses. if you guys want to believe you can just imagine this body of science or get it from a few diagrams, be my guest; i sure won't have to worry about hitting you in the air. bye.

 

[Phil]

Ok, in that case

RickYes, I suppose I envisioned my wing (uh, shape) with some angle of attack. My point was directed elsewhere. Bilbo I believe was referring to flow lines and turbulence in the aft part of the wing. The curly-cues he was looking for in the diagram represent turbulence or separated flow as opposed to vortices and aren't needed to explain lift. But that led to mortyd's statement about the relationship of lift to airflow at the wingtips. (for Bilbo, these flow lines are close at regions of high flow/low pressure and farther apart at regions of low flow/high pressure) For the benefit of anyone still following this thread, I thought I'd try to get mortyd to clarify his error. I believe what he was thinking of is the relationship between lift and circulation. Circulation occurs at all parts of the wing, not just the wingtips. It's a mathematical description of flow, not an actual movement of air around the wing. Lift is proportional to circulation. This is not to be confused with the resulting actual rotational flows that occur due to the finite length of the wing.I also don't get the hypertechnical distinction between wing and airfoil. Wing is a generic kind of word and airfoil is the shape most people imagine when taking a cross section of a typical wing. It's too bad that tempers seem to be rising, judging from the slights and jabs. I guess I should just drop it, though it is a fascinating subject.This is a sailing forum, you ask? As I mentioned earlier, the same basic dynamics applies to the wing/shape, your sails and your keel, which was I believe the original topic of this thread.

 

[mortyd]

lift

phlogiston, phil. mumbo jumbo. sorry.

 

[Bill Roosa]

forget lift and think vectors

Understanding how the force gets generated is not useful. All you need to know is that it gets generated and what it's magnitude and direction are.I think we all will agree that the keel does generate two forces, a athwart ships force that tries to resist the force generated by the sails pushing sideways and a drag force (the whole hull actually) that resists the driving force from the sails. There is also a force from the rudder that "trims" the keel force.So the two athwart ships forces balance the athwart ships forces from the sails and the "thrust" force from the sails is balanced by the drag force from the hull, keel, and rudder once the boat gets up to a stable speed.Since you want the hull forces to be in a standard configuration called "a little weather helm" you really only have the sail forces to play with.So figure out what "a little weather helm" looks like and then you have a baseline for how to set the sails to get everything in balance.I suggest you draw a boat diagram and play around with it. Some rules to apply:all the athwart ship forces must sum to 0 (port is + and starboard is -)all the forces in the direction of travel must sum to 0 (toward the bow is + and stern is -)pick any point on your diagram and the product of every force times the lenght of a line dropped perpendicular to it to the point you picked (force times distance = toque) must sum to 0. (clockwise toques are +) If the forces don't sum to 0 then the boat will start accelerating in the direction of greater force.

 

[Rick9619]

Phil, Bill, Mortyd, Bilbo

Holy Cow who cares. I always thought we are supposed to have fun here. Everyone was just taking a shot at explaining something that is tough to understand. You dont have to have someone say "you fixed it for me, or I understand". I thought some of the analogies were great and I did take aero and fluid dynamics in real college. Im sittin here chuckling, certainly with no offense taken or meant to be given. I sail because I love it. The particulars of serious racing dont concern me. Why a keel works.. who give a fudge. Which one would be best on my boat? Yeah im listening. I just happen to have an aviation background including 2800 hours in the F-16 so I understand what the Blue Angles and Thunderbirds do to fly formation which they teach everyone to do in flight school. I just thought it was a neat thread and a great philisophical discussion. Heck I would rather talk about boat drinks and such. Shees... cant we all just get along.CheersRick

 

[CalebD]

The short answer to this question:

"where does the lift come from?": the sails, and yes, the keel too. This aint aerodynamics 101 or 599, please. Also, those of you who think you understand all this mumbo jumbo about fluid/gas dynamics from flying airplanes, go fly a kite!

 

[Bilbo]

Yes

Well then never mind guys.............I was just trying to learn something about sailing through a discussion. "If the forces don't sum to 0 then the boat will start accelerating in the direction of greater force."I just read up some in the Chapmans' 64th ed. which displays this very nicely. A very limited summary of pages 275-280:They use terms"Center of lateral resistance" for the combination of the keel and rudder.BTW the keel (and hull)generally points at 2-3 degrees off the direction of travel as caused by the slip to leeward. This 2-3 degrees of attack is where the symetrically foiled keel "lift" comes from."Center of effort" The position of the sum of all the lift and drag forces produced by the sails. (Ahead of the center of lateral resistance)If these two forces are in balance, the boat will travel straight forward and excessive rudder will not be needed but because the designers locate the sails to add a slight weather helm in average wind, the application of 2-3 degrees of rudder is used in the opposite direction.