Wing mast section, truncated NACA 63015?

Discussion in 'Hydrodynamics and Aerodynamics' started by MichaelRoberts, Sep 6, 2015.

  1. MichaelRoberts
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    MichaelRoberts Junior Member

    In their testing of wind mast sections both Anthony Marchaj ( page 192 in "Sail Performance") and Frank Bethwaite (page 210 in "High Performance Sailing") discovered that the best lift and least drag can be obtained with a section similar to NACA 63015 cut off about half way.

    The sharp trailing edges generate a short turbulence which immediately re-attaches.

    The cut off shape is very different from that which Frank Speer's excellent paper suggests, a teardrop something like NACA0015.

    This rotating mast is for a 18 m / 60 foot catamaran and will be 21 meters long, about 500 mm chord and 180 mm thick. It will be built by laminating carbon fibre over a light ply shell formed over CNC cut ribs. The planform will be curved back and tapered.

    Is there a kind expert out there who could offer some advice on wings mast design?

    Thank you

    Michael
     
  2. PI Design
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    PI Design Senior Member

    Ignoring all theoretical discussions that will doubtless follow, there are very few real examples of boats with truncated, cut off, backs. A Class cats, F18s, NS14s all allow development and all use some form of teardrop or smoothed diamond, rather than a square back.

    There must be some published CFD results looking at this, though I haven't found them.
     
  3. Erwan
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    Erwan Senior Member

    You mean Tom Speer & Franck Bethwaite , I guess.
     
  4. MichaelRoberts
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    MichaelRoberts Junior Member

    oops

    Oops, my spelling mistakes ... I meant wing mast not wind mast and yes I was referring to Thomas E Speer's paper "Aerodynamics of Teardrop Wingmasts". But Frank Bethwaite is definitely not Franck.

    Anyway. I hope this improbable chopped section does make some aerodynamic sense because the lateral second moment of area is better for the same thickness and this improved Ixx helps to meet buckling criteria.

    Thanks for your prompt replies
    Michael
     
  5. OzFred
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    OzFred Senior Member

    There is Frank Bethwaite's Tasar.
     
  6. MichaelRoberts
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    MichaelRoberts Junior Member

    Elongated and cut off

    Thanks for that Ozfred,

    Recently a friend sent me a close up of the mast on an extreme cat called AC40, it has a flat back, a bit wider than the track. So there is a bit of a precedent.

    The reason I am pushing this idea is that even with the truncated section the chord is 550 mm (fore-aft). And this is a lot of area that can't be reefed. The wind force at 2 kPa (hurricane) on 21m x 550 mm is about 20kN, that's 2 tonnes acting through the centroid 10 m up. Thus the overturning moment is 20 tonne meters. The boat can take it but it's a lot of overturning force.

    You might ask why not make a smaller mast. The answer is I can't handle the 14 tonnes of compression with much less cross section area (200 x 550 mm).

    My aerodynamic instinct tells me that a sharp cut off trailing edge will trick the lee side airflow to bridge the gap until it hits the sail (like a car rear spoiler, a gurney flap or the ridges on a frisbee).

    If there's anyone out there with advice about making a one off 21m carbon fibre wing mast please talk to me. Except please don't tell me not to do it - there's no choice, this boat is being built on a shoestring budget and I have found a good Chinese source of high modulus uniaxial carbon fibre. Anyway I like to design and build boats.

    Thanks

    Michael
     
  7. OzFred
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    OzFred Senior Member

    You might also wish to research the work of Wunibald Kamm whose theories have been applied to cars and pushbikes (e.g. Airfoil Development for the Trek Speed Concept Triathlon Bicycle - PDF, 2.8mb).

    I guess the point is that a flat section on the back of the mast need not be detrimental to overall performance. If you need it for structural reasons, then Kamm's principles should at least allow you to minimise the negatives, and perhaps get some positives.

    Naturally I have zero expertise in these things. I wish you the very best, and hope you keep us informed about your project.
     
  8. jehardiman
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    jehardiman Senior Member

    For optimum TE stability in widely varying AoA's (i.e. sailboat mast, fin keel, rudder, propeller, etc.), the base should always be cut off to stabilize the vortex, especially at low c/t ratios. See Hoerner FDD, Sect 3-8 and FDL, sect 2-2.
     
  9. MichaelRoberts
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    MichaelRoberts Junior Member

    Thank you J E H and Ozfred - I'll research both of those references.

    J E you have confirmed my instinct about a small vortex at the trailing edge. Not sure what you mean by small c/t - 550 to 200 seems thick or do you mean to include the sail which at 8% camber has a thickness of around 600 mm and average chord of about 4m

    Anyway, if it's interesting here is a picture of the mast section. Inside the rear channel cars will terminate the full length mainsail battens. These cars allow lateral movement - the sail flops to the lee side to minimise the step at the trailing edge of the mast. Small turbulence then re-attachment - I hope!

    My carbon samples just arrived - now to make a sample and test it to see how it stands up to a few tonnes of compression.

    Keep in touch

    Michael
     

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  10. jehardiman
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    jehardiman Senior Member

    c is for cord and t is for thickness and yes you have a low c/t ratio.

    There are a couple of ways to look at a sail, mostly dependent on what you want get out of your mental/mathematical model. I like to look at a sail as a higher pressure side and a lower pressure side caused by the flow momentum changes (i.e. energy is conserved so changes in flow velocity equate to changes in pressure + losses). The mast is a form that disrupts that flow, possibly changing the shape of the flow patterns in a negative way. One of these changes are the vortices generated by the slope of the aft section of the mast.

    My opinion, supported by data, is that if you can't eliminate the shape change (i.e. smooth foil wing sail) is to critically bind the vortices in an energy hole where they are stable through large AoA changes at the mast. The trailing edge vortices of the sail are a whole other matter and controlled by the overall camber so you don't manifest a te rollover stall situation.
     
    Last edited: Dec 4, 2015
  11. Mikko Brummer
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    Mikko Brummer Senior Member

    Nice description about the bike development, but I would still like to add, that the bike frame tube has no sail behind it...
     
  12. backyardbil
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    backyardbil Junior Member

    I often wonder, in discussions such as these about the intricacies of exact angle of attack of aerofoils (airfoils). That an assumption is made that the natural wind always blows in an exact direction. My own observations and an effect that anyone can see, is that a windvane never remains pointing in an exact direction all the time, but oscillates around quite a few degrees. So any discussion about exact angles of attack etc. must be without much meaning?
     
  13. Petros
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    Petros Senior Member

    coming late to this thread, I have been away from the forum. the original question was the use of NACA 63015 cut off about half way, vs. a teardrop something like NACA0015.

    From some studies I remember reading about some 30 years ago, when the laminar flow foils were suddenly popular for home built aircraft, it was clear there are many times you do not want a laminar foil section. More than a few home builders discovered after they built their sport plane that the behavior of laminar foils changed drastically if the flow was not laminar over the surface.

    If you can maintain a clean and smooth LE on the laminar foil section, it will have lower drag and superior L/D. The problem is that in real world operations, even on an aircraft flying in clear weather, that does not happen very often. the laminar sections that have their boundary layers tripped near the LE are very poor performers, and the old NACA 00xx are actually better performers.

    It is possible to get a really clean surface when building a laminar mast in a well equipped shop, with a lot of hard and careful work. But once exposed to the weather, sun, salt spray, scratches, dings and dents, dirt, rigging mounts, etc., etc. particularly on a recreational cruiser, it will no longer have laminar flow over the mast section, it will perform poorly. Worst of all there could be very drastic changes in behavior. However, a non-laminar foil will still perform well with very little change in behavior with less than ideal surface preparation.

    So if you are building a racing boat that will be always expertly cleaned and maintained, than the laminar 63XXX section would be okay and possible better. But for all other applications I would choose a non-laminar section.

    I think too much time is spend with idealized results from wind tunnel and CFD models, and not enough time by these same researchers out in real conditions. Personally I would choose the NACA 00XX sections for any boat, mast, keels and rudders that I intend to enjoy on the water. And leave the laminar section for those with a lot more money, more maintenance crew, and time to keep it perfect at all times, than I do.
     
  14. OzFred
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    OzFred Senior Member


  15. MichaelRoberts
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    MichaelRoberts Junior Member

    Good morning gentlemen

    Whew, now there is a lot of turbulence in my head, the smooth laminar flow over that parabolic leading edge is now shredded by Petros' expert opinion. All night detached vortices have been whirling away from the lee side of the mast with no hope of re-attaching to the sail.

    Can it be that the shape of the first 5% of the mast-sail surface is so critical that a rough surface can cause a stall?

    Makes me wonder how many more mistakes I've made with the design of this boat - should have had a discussion with you all sooner. There are lots of contentious issues - twin spade rudders each side of the prop - the centreboard foil forward of the mast to offset the lack of lateral resistance of the shallow forebody - and the brushless DC motors energised by two 20 kWh banks of lithium batteries.

    Then there is the problem of getting it off this hill down to the sea ten km away - the only way is a big Russian chopper, or a chinook.

    Do you want to see this boat? Here are two pictures.

    Thank you so much for your responses, please keep up the advice. Now I had better get to the boat shed.

    Michael
     

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