Lift and Drag: Prevent Common Sail Trim Errors
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- เผยแพร่เมื่อ 17 พ.ค. 2024
- Explains the concepts of lift and drag in sailing and how these concepts relate to common sail trim errors. See our related post at sailzing.com/prevent-common-s...
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Straight forward and simple good video without overuse of terminology! Thank you
Thank you! Check out SailZing.com for more sailing tips. -Rob
Thanks for taking the time to comment!
Great video
Hello,
Fantastic video!! Well done Sir! I learned to sail a long time ago but got busy with the military and life!! Anyway I decided to attend a ASA 101 course an I have it all down except sail trim so this video was perfect!!! Hopefully I can get it down before Tuesday when the class starts!!!
I subscribed to you channel and I'm very impressed with you presentation!!
Fair Winds!!
Doug
Thanks for the feedback!
Great Video, looking forward to future videos.
no
Thanks for this video. from 16:00 you talk about main sail trim and that the upper leech tell tale should move to leeward periodically. I've heard this often and would like to understand: why is that the case only for the upper tell tale and not for those below?
Thanks for the comment! Here's the way I see it: I think the boat's performance would benefit if all the leech ribbons disappeared periodically. However the lower two-thirds of the leech is less reactive to mainsheet tension acting through the boom than the upper leech. You'd have to trim really hard in medium breeze to get the lower leech ribbons to disappear, and then the upper leech would be stalled permanently. One way to get more power out of the lower portion of the sail is to ease the outhaul. This puts some shape in the lower leech, and you may be able to see periodic dancing in the lowest leech ribbon.
Cool. So 11:30, says that I want the leeward telltales flowing smooth (every racer knows that), but that I *don't* want the windward telltales flowing smoothly, I want to see them lifting just ever so slightly.
Worth remembering that the mass of a cu metre of air is greater than 1kg. Therefore a sail is deflecting many kg of air continuously. An easy explanation with which I start early sailing lessons.
This is more like it, but... You are not deflecting too many cu metres in a second, actually not even one! Even if you have a really big sail on your boat you should understand that b/c of the thickness of the sail that´s just a 1000 of a metre or so, that won´t be a lot of cubics and pressure and pressure differences is all about the area and has nothing to do with cubics!
One of the best videos on sail trim!
Just 2 questions:
1.) do we always want our sail to operate at the best lift/drag ratio? I mean at all wind strengths and wind directions? Are there cases where we actually want to operate at minimum drag or maximum lift? Cant find an explanation about this on the whole internet or in the book you refer to (a manual of sailtrim) :)
2.) you show curves for lift, drag, lift/drag ratio vs. angle if attack. How do these curves look like for a “flat” sail vs a a “baggy” sail?
Cheers!
1) You want maximum lift when speeding up because the drag haven't got so big yet (speed isn't so high in the boat). These sitsuation are right after tacking (ofc you need to sheat head sail to right trim after speeding up) AND in waves which are slowing you down when reaching, because in waves, (enough) speed means safety.
Thanks for the response. I think i know what you want to say, but i still dont understand 😄…
I mean i always want maximum speed, not only in waves and after tacking. So should i always go for maximum lift? This should be at a sheet angle just right before the main leach telltales stall.
Also: the aerodynamic drag of the sail and its lift come always as a pair, so as soon you get (max) lift you also get drag. Are you referring to hydrodynamic drag?
As the angle of attack increases, both lift and drag increase, but at some point the lift to drag ratio decreases. So you don't always want maximum lift. Once up to speed, you want to decrease drag, just like airplanes do by raising flaps. Better performance.
In section 6 you say that downwind it is the force of drag that is pushing the boat
As you state earlier drag is a function of lift, without lift you cannot have drag in the terms you define it.
When dead downwind there is no lift, so you cannot have drag. Downwind it is the wind pushing on the sail, nothing to do with drag, it is Newton’s second law. There will be some lower pressure behind the sail increasing the pressure differential, but it is not the primary force.
Thanks for your comment. I did a little more reading on the definition of drag. Most definitions refer to drag as the force in the opposite direction of the relative motion between the object and the fluid. I think this definition fits the downwind sail situation. The sail is still moving upwind relative to the wind. The wind is passing the sail, so in relative terms, the sail is moving upwind. I agree that the statement "without lift there can be no drag" is wrong. Maybe better to say, "lift is always accompanied by drag." You can have drag without lift, but you can't have lift without drag.
SailZing. When the sail is acting as an aero foil then lift and drag are a function of fluid dynamic forces explained by several equations of which Bernoulli is one. Your explanation of this is good, including your explanation of incipient drag as a function of lift.. However when downwind the sail is not acting as an aero foil. It is a vertical flat body in an fluid stream, in this case air. It is a fully-stalled aerofoil if you like. The force on the sail and its resultant motion is simple mechanics as explained by Newtons Second Law. It is nothing to do with drag even though the inertial frame of reference does indeed have the sail moving ‘ upwind’. The boat would move in the same way if you had a large hosepipe spraying.
a jet of water on the sail. There would be no drag induced by the water. I fear you are trying to post-rationalise because changing the video is a pain, however to be accurate and avoid confusing folk, you really do need to change it, or indeed simply edit it out, as in its current form it is wrong and spoils what is in all other regards a good piece of work on this complex and often misunderstood topic.
Savage
whitefields5595
He did say he would not provide the complete, detailed explanation, rather provide a simple explanation that is practical in the sailing scenario. Your description of impulse due to direct impingement of a fluid stream is also an incomplete explanation of all that is going on. To explain fully would require a few semesters of aeronautical engineering.
B
The authors have two wrong scientific approaches: researching the creation of Lift force and Low pressure at upper side of the wing, relative to the ground surface and Earth. I explain the aerodynamic cavitation and existence of Lee side aerocavern, and creation of Aerodynamic force.
If you think this is easy, as you say at 8:40 I have a Q for you! Why are you not making millions of dollar in Americas Cup as a consultant instead of making BS videos?
It´s not an easy thing! That´s why the best hydrodynamics designers and engineers in the world is involved in sailing performance in AC.
Question #3 seems not quite right. As you increase wind speed over the sail surface you increase lift, requiring less camber to produce the same amount of lift but with less drag. Flaps up! Sheeting the vang would seem to create even more lift but with additional drag. Flaps down!
The explanation of lift, differences of pressure and differences of velocity on either side of the sail is questionable. Please look up flow turning & NASA explanation of lift.
Thanks for the comment! I consulted several sources for this video and all the sources point out the various misconceptions about lift, so I was trying to be as accurate as I could without getting into too much detail. A good source was "The Art and Science of Sails," by Tom Whidden. This source states that lift results from both the turning of the air flow and the difference in pressure between the windward and leeward sides of the sail due to the difference in flow velocity. There are some deep nuances behind each of these sources of lift.
@@SailZing For sure there is a lot of nuance. You've certainly done a good job with this video. I've found the best way to accurately understand lift is to relate it to the base three Newtonian laws.
Overall quite informative, but the speaker stuttered about a dozen times -- not very professional
Go somewhere else then