sail aerodynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by Guest, Mar 21, 2002.

  1. brian eiland
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    brian eiland Senior Member

    I remember devouring that book about 45-50 years ago, but I've since lost my copy years ago,...and some of my memory as well....ha..ha

    PS: Doug, sent you a private message about that book
     
  2. Petros
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    Petros Senior Member

    nice diagram from Marchaj's book.

    It appears to me that in points of sail, if the drag is reduced, forward thrust would be improved. Reducing the drag would benefit all sailing conditions, even if not at max L/D.

    the only place where there might be a benefit to high drag, is when sailing dead down wind. But usually it is faster to jibe downwind anyway, taking advantage of the extra thrust that can be generated when at an angle to the wind.

    Interesting observations about an ice boat. Since the speed record for ice boats is over a hundred miles per hour, I always thought that the speed record for "soft" water boats have a lot more potential for much higher speeds. We just need a way to make the hull in the water have a lot less drag.
     
  3. DCockey
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    DCockey Senior Member

    Wings and other aerodynamic devices on race cars have been fixed for 45 years or so. The best tradeoff has to be determined between high downforce for maximizing speed around corners and low drag for maximum speed on the straights.
     
  4. SukiSolo
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    SukiSolo Senior Member

    Nice to see your copy of the Marchaj book, is about as faded as mine Doug....;)
     
  5. Mikko Brummer
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    Mikko Brummer Senior Member

    Wow! What a great posting that really sheds light on what is important and what is not when it comes to sails. I have not seen this anywhere before, thank you.

    However, it's good to bear in mind that lift isn't everything when it comes to sailboat performance, drag is important, too. Increasing aft camber will increase drag. Wish you could produce a similar dependence for drag ;-).

    Still, it's safe to say that in light winds (when not fully powered), more lift nearly always means more boatspeed… then we are talking about CLmax, and viscous effects become very important. To the sailor, lift equals "power". When not constrained by righting moment, and not yet at hull speed, even a little more drive turns into more speed, increasing apparent wind and again increasing speed.

    I share your dislike for the term "leech return", but camber is perhaps a too sophisticated word for sailors. They would say draft or fullness. Entry and exit is also used, but they refer rather to the local chord than the boat centerline. I think your pdf emphasizes how leech return, defined as the angle between the boat centerline and the sail leech exit, does make sense: If you imagine a round entry, draft forward, open exit sail, sheeted further in towards the centerline, or a shy entry, draft aft and closed exit sail, sheeted further outboard, they will produce a similar lift, as long as their "leech return" is similar (angle of the last 18% against the app wind angle ;-).

    Thus, the coach boat looking at your sail trim (or spying on competition's sails) has every reason to eyeball leech return, rather than where the boom lies in relation to the boat, or the fullness or even the twist of the sail - leech return will best tell how much power the sailor is using.
     
  6. markdrela
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    markdrela Senior Member

    A much more physically significant definition of "leech return" is the angle between the sheet leech area and the relative wind, which is "alpha" in the pdf.

    The angle between the sheet leech and the boat centerline is of course more easily gauged by eye, but it has no special aerodynamic significance.
     
  7. BobBill
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    BobBill Senior Member

    I can sort of see it, but have always been under impression the luff was the lifter...have to chew on it for a bit, I am dense.
     
  8. Petros
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    Petros Senior Member

    lets see if we can confuse you further: on a typical foil 80 to 90 percent of the lift is from the front 20-30 percent of the chord length (look at the pressure distribution plots to confirm this). However, altering the angle of the last 20 percent will affect the up-stream flow in profound ways that will alter the lift. That is why BTW, both flaps and ailerons, as well as other control devices such as elevators and rudders, on aircraft are always on the trailing edge. There are leading edge devices too, but these are more to delay the stall (so it allow more lift) at low speeds (landing) rather than as a control surface. this is why the profile at the LE is so critical to generating lift, and why the TE is the best means of controlling that lift, and a means to control the aircraft.

    to summarize, when you alter the flow over the last part of the foil by changing the angle, you will greatly affect the flow upstream over the LE, and gratly alter the amount of lift.
     
  9. BobBill
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    BobBill Senior Member

    ros

    Petros, much thanks, that explanation fits my ancient views and makes sense...fills in the blanks, so to speak and much thanks again for taking the time to add the comment.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    I agree, but even the true wind direction is difficult to see, not to mention the relative wind ;-).
     
  11. Mikko Brummer
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    Mikko Brummer Senior Member

    Seen it, been there...;-). In the early 90ies, we tested a "2D" carbon wing in the wind tunnel (to evaluate Finn mast profile candidates) - the windtunnel engineer was a race car fanatic, and he suggested we try a gurney flap. In 2D, the effect was devastating, as shown by the graphs. You can also see that the effect is pretty similar to adding some camber - I recall someone (Mark Drela?) stated somewhere, that if a wing profile needs a Gurney flap it was badly designed in the first place.

    The smoke test on the profile reveals nicely turbulent structures of the flow. In the video, at 24 frames/s (I guess VHS was something like that, if not 12 fps interlaced), and to the bare eye the smoke is just white smoke, but with a proper camera and with the flash on, you get to see the turbulence. We discovered this by accident, later when developing the film.

    Of the Finn mast profiles, the NACA4 shape was the best (adding also most area), and the Pacman shaped (with sail recessed) the worst, if I recall. We had pressure taps laminated inside the profile and also measured the pressure distributions. The data was used to calibrate a flow separation scheme included in our VLM program at the time, MacSail.
     

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  12. Mikko Brummer
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    Mikko Brummer Senior Member

    A couple of years later, we went on to test the Gurney flap in a 3D model of a 470 dinghy. In 3D, in the leech sweeping forward as they do on triangular sails, the flap lost much of its charm, but it still had some effect. Our boys used it in the Olympics -96, were very close to winning gold but unlucky and 4th in the end. In 1997, they won the Worlds with the spoiler-sail, as we called it.

    The spoiler/flap was controllable with the leech line, so you could turn it off/on. It was no good in very light winds, at its best between 8 to 12 kn, and in heavier winds of course "off". The spoiler was tedious to make, on the limit of being class legal or not, and soon forgotten when the crew stopped racing after winning the Worlds. Every now and then, I come to think of it as I look at the Volvo race, for instance - it could be efficient for jib reaching and fetching that you have plenty in the round the world race.
     

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  13. brian eiland
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    brian eiland Senior Member

    Airfoil Sections for Soaring, etc

    This discussion gets ever more interesting. Regrettably I have a number of home chores that need attention before I can spend more time reading it all.

    But just a basic simple question at this point. If as postulated the front portions of the airfoil are not that important, its the aft portions that determine the lift,....THEN why all of this experimenting over the years with NUMEROUS different airfoil sections??
    ..for just one instance...
    http://soartech-aero.com/V3.htm

    ....many other IMAGES
     
  14. Doug Halsey
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    Doug Halsey Senior Member

    Great stuff, Mikko ! Thanks for sharing it with us.

    A couple of questions :

    1 - Were the Finn masts you tested fixed in space, or were they free to rotate (at least in line with the boom) ?

    2 - What were the Reynolds numbers in those tests & how would you characterize the turbulence level in the tunnel ?

    3 - How did that flow separation scheme in your VLM work out ?
     

  15. DCockey
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    DCockey Senior Member


    Airfoils generally have thickness with the surfaces separated by an aerodynamically significant distance. The thickness distribution as well as the camber affects the velocities and pressure along both the top and bottom surfaces of the airfoil. As long as the flow stays attached to the airfoil the pressure difference across the airfoil and thus lift is primarily affected by the camber, with the aft portion of the camber having a larger effect on lift then the forward portion. The thickness distribution in conjunction with the camber however does have a significant effect on the individual velocities and pressure on the upper and lower surfaces which affects the boundary layers. Differences in boundary layer characteristics can lead to differences in viscous drag and also differences in separation at higher angles of attack. (At higher speeds compressibility effects also may be important, and at sufficiently high speeds shock waves can occur. These effects are strongly influenced by the thickness distribution as well as the camber.) Airfoil design is about determining thickness distribution and camber which keeps the flow attached and minimizes drag over the desired angle of attack range while meeting other constraints (thickness for structure, etc.)

    Conventional sails are "thin" membranes of virtually constant thickness, usually a mast/furler foil/wire or rope which serves as the leading edge. The primary way to affect the velocity and pressure distributions on the sail is by changing the camber of the sail. (This assumes the mast/furler foil/wire or rope, and camber near the leading edge is such that the flow just aft of the leading edge either remains attached or quickly re-attaches.) There is no thickness distribution to change the influence the velocities and pressures on each side of the sail.
     
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