OK, I just said that to get your attention. I've been thinking about this for a while and I can't seem to shake it. Everybody says that mainsails have twist because the apparent wind angle varies, and that's because the true wind velocity increases the higher you are above the water surface. I'm not disputing that concept, but here's something I noticed and can't explain. Because the mainsail is triangular, its chord length varies. from boom length at the bottom to zero (or the width of the headboard) at the top. The sailcloth has the same weight from bottom to top, and the luff tension is about constant from bottom to top. Therefore it stands to reason that the luff of the sail at the boltrope has the same angle to the wind from top to bottom, i.e. there really isn't any twist. Could what we see as "twist" actually be an artificial change in the angle of attack of the chord because its length gets shorter from leech to luff as you go up the sail? To illustrate my point, draw a horizontal section through the sail at the boom (full length of foot). Add the chord of the boom. Now, starting at its aft end, move forward along the curve of the sail (which would be the two-dimensional equivalent of moving up the sail along the leech). When you get about 20% of the way forward along the curve, mark a point. This is about where the aft end of the bottom batten would be if you were looking straight down on the sail from above. Draw a line from there to the luff...the angle of attack of that chord is further aft. Repeat the process at 40%, 60% and 80% of the length of the sail curve (simulating the aft end of each batten). Look at each of those chords...looks like twist, doesn't it. Now look at the luff of the sail. The tangent of the luff entry angle to the wind hasn't changed. So do we really have twist or not? I tried to solve it by assuming a rectangular mainsail (we have triangular mains because we need to clear the backstay and keep the CE low and forward). Gaff-rigged mains are misleading because the aft end of the gaff tilts up and makes it hard to see sail chords. Just so you know I'm not goin' off the deep end, I can look up my mainsail and see the same thing I just described (actually that's what gave me the idea). Any thoughts? Peter H23 "Raven"
Hey Don G, try this...mainsail twist is a myth!!
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Let's Twist again like
Peter A mainsail is not flat. It is not, for instance, a piece of paper or an engineer's triangle. Because it is made of fabric, it has depth (draft), so the even angle of attack discussion in your para. 3 doesn't apply. Sail with the traveler in any position, then vang hard and watch the shape change. Or do the opposite, flat main with vang, pop the vang and all sorts of depth appears. Stu
Peter A mainsail is not flat. It is not, for instance, a piece of paper or an engineer's triangle. Because it is made of fabric, it has depth (draft), so the even angle of attack discussion in your para. 3 doesn't apply. Sail with the traveler in any position, then vang hard and watch the shape change. Or do the opposite, flat main with vang, pop the vang and all sorts of depth appears. Stu
twist
a wing is a rigid structure so the chord of a wing has a somewhat different meaning from what we call the chord of the sail. also, almost all wings are twisted - callled washout by aerodynamicists - from root to tip so that the root stalls before the tip and lateral control is maintained by the ailerons. i'd love to see wind tunnel tests done on sails so we would have real empiricical evidence but what don has been saying makes sense to this fairly green sailor and seasoned aviator.
a wing is a rigid structure so the chord of a wing has a somewhat different meaning from what we call the chord of the sail. also, almost all wings are twisted - callled washout by aerodynamicists - from root to tip so that the root stalls before the tip and lateral control is maintained by the ailerons. i'd love to see wind tunnel tests done on sails so we would have real empiricical evidence but what don has been saying makes sense to this fairly green sailor and seasoned aviator.
Measuring angle of attack
Peter, The sail does twist, but even if your theory were true- that the twist is only imaginary because the cord line of the sail appears twisted as the sail tapers through the draft(belly) of the sail- then the angle of attack has still changed. That is because the angle of attack is measure between the relative wind(apparent wind) and the cord line(the theoretical line from the luff to the leach of the sail). So if the cord line changes, for any reason, then so has the angle of attack.
Peter, The sail does twist, but even if your theory were true- that the twist is only imaginary because the cord line of the sail appears twisted as the sail tapers through the draft(belly) of the sail- then the angle of attack has still changed. That is because the angle of attack is measure between the relative wind(apparent wind) and the cord line(the theoretical line from the luff to the leach of the sail). So if the cord line changes, for any reason, then so has the angle of attack.
Tyvek twist
You mentioned twist is because of the sail being made of fabric. But if the sail was not made of fabric, say of tyvek (paper) or of a metal foil. Would it still have a twist?
You mentioned twist is because of the sail being made of fabric. But if the sail was not made of fabric, say of tyvek (paper) or of a metal foil. Would it still have a twist?
Twisted Thinking!!
Peter, Imagine a perfectly straight line between headboard and clew and call it the leech. Draw chords between luff and leech. Stand at the boom end and look at these chords. They are all parallel. Angle of attack is that between incoming airflow and chords. So, under these conditions, there will be no twist. Now, if you don't want the head of the sail to stall - due to the higher windspeed aloft- you ease the vang and allow twist. I use mid-chord tell tales as well as leech ones. The need for and effect of twist is immediately apparrent.
Peter, Imagine a perfectly straight line between headboard and clew and call it the leech. Draw chords between luff and leech. Stand at the boom end and look at these chords. They are all parallel. Angle of attack is that between incoming airflow and chords. So, under these conditions, there will be no twist. Now, if you don't want the head of the sail to stall - due to the higher windspeed aloft- you ease the vang and allow twist. I use mid-chord tell tales as well as leech ones. The need for and effect of twist is immediately apparrent.
Responses
I'm glad the ball is rolling on this thread. Like Don, my goal is to get people to think more critically about sail trim and why we do what we do. That's how we advance our knowledge of the subject, even if we don't agree. I'm not questioning the existence of twist or its importance, just the way people explain it. I understand that sails aren't flat triangles and I know how and why they change shape. I use twist to very good effect on my boat, which is fractionally rigged with a very flexible mast and an adjustable backstay. I can change mast bend instantly while under way and adjust the shape of the main (including twist) for changes in wind direction and the boat's course. Conventional wisdom says we need twist because true wind velocity increases with height above the water. Since the wind of motion (caused by the forward velocity of the boat and sail) is constant, the apparent wind (vector sum of true wind and wind of motion) moves aft as you go up the mast. Here's where this theory falls short for me. Twist is defined as the change in angle of attack of the sail as you go up the mast. Angle of attack is defined as the angle of the chord of the sail to the apparent wind. The chord of the sail is a STRAIGHT line from the luff of the sail to its leech. Now, here are some things to think about: 1) the chord length changes as you go up the sail simply because the distance from luff to leech becomes shorter. The position of the luff doesn't change, but the leech moves forward. If you draw the chord of the sail at different heights, you will see this (that was my 2-D drawing). The angle of attack increases progressively simply because of the geometry of the sail. 2) the whole "angle of attack" explanation is based on the chord, which is a STRAIGHT line from the luff to leech. That would imply that airflow on the windward side of the sail flows straight from luff to leech. While this is true for hard airfoils like airplane wings, the airflow on a sail is attached to BOTH sides. On the windward side of the sail, the airflow follows the curve of the sailcloth, NOT the chord. The flowlines on both sides of the sail track each other very closely, and are separated only by the thickness of the sailcloth. 3) If the airflow is attached to both sides of the sail, then the angle at which the wind meets the sail is not the "angle of attack" defined by the chord, but rather by the tangent to the luff of the sail at the luff rope. This angle is affected by the shape of the front third of the sail, which in turn is influenced mostly by outhaul tension and luff tension (halyard and cunningham). This is near-constant as you go up the sail. Look at it the next time you're out sailing. 4) If the apparent wind angle really changes as you go up the sail, what good is a masthead wind indicator? It shows apparent wind angle at the location where it does the LEAST good. A sail develops most of its power in the lower half, where the area is greatest. Your thoughts? Peter H23 "Raven"
I'm glad the ball is rolling on this thread. Like Don, my goal is to get people to think more critically about sail trim and why we do what we do. That's how we advance our knowledge of the subject, even if we don't agree. I'm not questioning the existence of twist or its importance, just the way people explain it. I understand that sails aren't flat triangles and I know how and why they change shape. I use twist to very good effect on my boat, which is fractionally rigged with a very flexible mast and an adjustable backstay. I can change mast bend instantly while under way and adjust the shape of the main (including twist) for changes in wind direction and the boat's course. Conventional wisdom says we need twist because true wind velocity increases with height above the water. Since the wind of motion (caused by the forward velocity of the boat and sail) is constant, the apparent wind (vector sum of true wind and wind of motion) moves aft as you go up the mast. Here's where this theory falls short for me. Twist is defined as the change in angle of attack of the sail as you go up the mast. Angle of attack is defined as the angle of the chord of the sail to the apparent wind. The chord of the sail is a STRAIGHT line from the luff of the sail to its leech. Now, here are some things to think about: 1) the chord length changes as you go up the sail simply because the distance from luff to leech becomes shorter. The position of the luff doesn't change, but the leech moves forward. If you draw the chord of the sail at different heights, you will see this (that was my 2-D drawing). The angle of attack increases progressively simply because of the geometry of the sail. 2) the whole "angle of attack" explanation is based on the chord, which is a STRAIGHT line from the luff to leech. That would imply that airflow on the windward side of the sail flows straight from luff to leech. While this is true for hard airfoils like airplane wings, the airflow on a sail is attached to BOTH sides. On the windward side of the sail, the airflow follows the curve of the sailcloth, NOT the chord. The flowlines on both sides of the sail track each other very closely, and are separated only by the thickness of the sailcloth. 3) If the airflow is attached to both sides of the sail, then the angle at which the wind meets the sail is not the "angle of attack" defined by the chord, but rather by the tangent to the luff of the sail at the luff rope. This angle is affected by the shape of the front third of the sail, which in turn is influenced mostly by outhaul tension and luff tension (halyard and cunningham). This is near-constant as you go up the sail. Look at it the next time you're out sailing. 4) If the apparent wind angle really changes as you go up the sail, what good is a masthead wind indicator? It shows apparent wind angle at the location where it does the LEAST good. A sail develops most of its power in the lower half, where the area is greatest. Your thoughts? Peter H23 "Raven"
twist
guys, one reason i wish for wind tunnel tests is that a sail is not an airfoil, it is a wing. the difference, as my brother and i discovered when we invented the wind tunnel and tested ducks in it is that a wing is three dimensional, an airfoil is not. what happens to the airflow at the wingtip, or at the top of the sail, has a major effect on the whole picture. the flow along the span, including the real angle of attack, changes along the span, or in the case of a sail the height, and not a little. one way to see this is the vortices that an oar tip creates in the water as the oar is rowed.
guys, one reason i wish for wind tunnel tests is that a sail is not an airfoil, it is a wing. the difference, as my brother and i discovered when we invented the wind tunnel and tested ducks in it is that a wing is three dimensional, an airfoil is not. what happens to the airflow at the wingtip, or at the top of the sail, has a major effect on the whole picture. the flow along the span, including the real angle of attack, changes along the span, or in the case of a sail the height, and not a little. one way to see this is the vortices that an oar tip creates in the water as the oar is rowed.
Masthead indicator
The masthead wind indicator is extremely important: you need to check it before and while you are raising the main. The rest of the time it is good for giving you a stiff neck or communicating that there is no wind at all (If it is doing 360s, that is a pretty good sign. I once counted 13 consecutive circles in about 30 seconds.) The sails don't care what the true wind is unless the boat is on a trailer in the driveway - or if the it is over about 30 knots, in which case it will be pretty close to the relative wind.
The masthead wind indicator is extremely important: you need to check it before and while you are raising the main. The rest of the time it is good for giving you a stiff neck or communicating that there is no wind at all (If it is doing 360s, that is a pretty good sign. I once counted 13 consecutive circles in about 30 seconds.) The sails don't care what the true wind is unless the boat is on a trailer in the driveway - or if the it is over about 30 knots, in which case it will be pretty close to the relative wind.
addendum
The windex indicates "true" wind when the boat is at rest or heading directly up or downwind, and "apparent" wind any other time. The problem is that the wind meeting the luff of the sails has changed direction some before it actually gets there, so that the tell-tales on the luff are better at communicating what the air is doing where it counts, on the leading edge of the sail, rather than what the air is doing at the masthead. If the tell-tales say the air is "happy" over the sails, then forget what the windex is saying - it is immaterial to what is really going on (air molecule-wise).
The windex indicates "true" wind when the boat is at rest or heading directly up or downwind, and "apparent" wind any other time. The problem is that the wind meeting the luff of the sails has changed direction some before it actually gets there, so that the tell-tales on the luff are better at communicating what the air is doing where it counts, on the leading edge of the sail, rather than what the air is doing at the masthead. If the tell-tales say the air is "happy" over the sails, then forget what the windex is saying - it is immaterial to what is really going on (air molecule-wise).
A picture is worth a thousand words
I searched my files and found pictures of my new mainsail. I wasn't actually sailing that day (note that there's no mainsheet tackle) just checking the set of the sail and playing with the leech tension to see severely it affected the twist (note line at end of boom). For tonight's homework assignment, print the picture attached to this post. You'll see two draft stripes on the sail. If you draw the chord at each draft stripe, the twist in the sail becomes very evident. Remember, I'm not disputing the existence of twist, just WHY it exists. Now, carefully (and honestly) draw a line tangent to the draft stripe at the luff. If you measure the angles of the two tangents relative to the leading edge of the mast, you will see they are parallel. Try it at the two uppermost battens as well. They're full battens and curved just like the draft stripes. The parallel tangents tell me that ALL the air that is about to hit the mast is flowing in the same direction regardless of the height above the water. There is no inherent twist in the air. That's why masthead wind indicators are supposed to be useful (for the record I don't use one either). The twist occurs on the sail because the leech ("TE" for you pilots) moves forward as you go up the sail. It's mostly a function of sail geometry and how we define the "angle of attack". W, which is a concept that we defined, not an inherent property in nature. The fact that the leech is loose, and not rigid like the TE on an airplane wing, doesn't help. The sail material is soft, so as you go up the mast, it becomes easier for the airflow to blow the leech to leeward. That's why "Yellow Pages" and other boats pursuing sailing speed record use semi-rigid "wings" and not regular sails. We tend to forget that the biggest difference between sails and airplane wings is that sail material is soft, while an airplane wing is rigid and unaffected by the airflow. The airflow velocity over a typical airplane wing is much higher than the airflow over a sail. You airmen are probably more familiar than I am with the whole Reynolds number thing and laminar vs. turbulent flow, so I won't go there, but if you look at aircraft development from the days of Sir George Cayley and Otto Lilienthal (see link below, the Wright brothers just solved the power problem) to today's modern composite aircraft, you'll see that airfoils changed shape and wing materials became more rigid (from fabric on wood frames to metal skins) as airspeeds increased. Bottom line, I don't buy the idea that sails develop lift by the reduction in pressure generated on the leeward side by the Bernouilli effect. If this were true, why do loose-footed sails work? Wouldn't the air on the windward side just flow down under the sail and up the leeward side to equalize the reduced pressure? Peter H23 "Raven"
I searched my files and found pictures of my new mainsail. I wasn't actually sailing that day (note that there's no mainsheet tackle) just checking the set of the sail and playing with the leech tension to see severely it affected the twist (note line at end of boom). For tonight's homework assignment, print the picture attached to this post. You'll see two draft stripes on the sail. If you draw the chord at each draft stripe, the twist in the sail becomes very evident. Remember, I'm not disputing the existence of twist, just WHY it exists. Now, carefully (and honestly) draw a line tangent to the draft stripe at the luff. If you measure the angles of the two tangents relative to the leading edge of the mast, you will see they are parallel. Try it at the two uppermost battens as well. They're full battens and curved just like the draft stripes. The parallel tangents tell me that ALL the air that is about to hit the mast is flowing in the same direction regardless of the height above the water. There is no inherent twist in the air. That's why masthead wind indicators are supposed to be useful (for the record I don't use one either). The twist occurs on the sail because the leech ("TE" for you pilots) moves forward as you go up the sail. It's mostly a function of sail geometry and how we define the "angle of attack". W, which is a concept that we defined, not an inherent property in nature. The fact that the leech is loose, and not rigid like the TE on an airplane wing, doesn't help. The sail material is soft, so as you go up the mast, it becomes easier for the airflow to blow the leech to leeward. That's why "Yellow Pages" and other boats pursuing sailing speed record use semi-rigid "wings" and not regular sails. We tend to forget that the biggest difference between sails and airplane wings is that sail material is soft, while an airplane wing is rigid and unaffected by the airflow. The airflow velocity over a typical airplane wing is much higher than the airflow over a sail. You airmen are probably more familiar than I am with the whole Reynolds number thing and laminar vs. turbulent flow, so I won't go there, but if you look at aircraft development from the days of Sir George Cayley and Otto Lilienthal (see link below, the Wright brothers just solved the power problem) to today's modern composite aircraft, you'll see that airfoils changed shape and wing materials became more rigid (from fabric on wood frames to metal skins) as airspeeds increased. Bottom line, I don't buy the idea that sails develop lift by the reduction in pressure generated on the leeward side by the Bernouilli effect. If this were true, why do loose-footed sails work? Wouldn't the air on the windward side just flow down under the sail and up the leeward side to equalize the reduced pressure? Peter H23 "Raven"
Aviation link
For some reason, the link didn't appear. Here it is (hopefully). http://www.wam.umd.edu/~stwright/WrBr/inventors/Lilienthal.html Pter H23 "Raven"
For some reason, the link didn't appear. Here it is (hopefully). http://www.wam.umd.edu/~stwright/WrBr/inventors/Lilienthal.html Pter H23 "Raven"
The cat's out of the bag
I've been a pilot and sailor and it wasn't until last winter when I finally learned that Bernoulli isn't exactly what makes planes and boats fly. I had been taught and believed that the air above a wing sped up over the longer distance and then met up with the lower air at the trailing edge. I was amazed to learn that in fact the air above the wing actually arrives _earlier_ than the parcel of air it was separated from! Air is disturbed well in front of the wing. Circulation has a lot to do with lift. It goes on and on. Just now I couldn't find the page with smoke tunnel photos, but here is a link with diagrams of what wind tunnel photos show. http://www.av8n.com/how/htm/airfoils.html ...RickM...
I've been a pilot and sailor and it wasn't until last winter when I finally learned that Bernoulli isn't exactly what makes planes and boats fly. I had been taught and believed that the air above a wing sped up over the longer distance and then met up with the lower air at the trailing edge. I was amazed to learn that in fact the air above the wing actually arrives _earlier_ than the parcel of air it was separated from! Air is disturbed well in front of the wing. Circulation has a lot to do with lift. It goes on and on. Just now I couldn't find the page with smoke tunnel photos, but here is a link with diagrams of what wind tunnel photos show. http://www.av8n.com/how/htm/airfoils.html ...RickM...
apparent wind
My take on the wind's apparent behavior is that wind speed aloft is greater than on deck, and as the boat begins to move thru the water, the wind direction will apparently come from a slightly different angle at the masthead than at deck level. It is one of those beautiful things about our sport that twist in a sail can help deal with this so elegantly, allowing the sail to have more nearly the correct angle of attack at various heights above deck and across a gradient of apparent wind velocity AND direction.
My take on the wind's apparent behavior is that wind speed aloft is greater than on deck, and as the boat begins to move thru the water, the wind direction will apparently come from a slightly different angle at the masthead than at deck level. It is one of those beautiful things about our sport that twist in a sail can help deal with this so elegantly, allowing the sail to have more nearly the correct angle of attack at various heights above deck and across a gradient of apparent wind velocity AND direction.
Rick- try these URLs for 'modern' sail/aero theory
www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf www.arvelgentry.com .... this is the website of aerodynamicist that CHANGED the worlds thinking about how sails generate lift, etc.
www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf www.arvelgentry.com .... this is the website of aerodynamicist that CHANGED the worlds thinking about how sails generate lift, etc.
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