Thanks for the great insights into your work so far. I knew I had seen the concept floated some months ago but I couldn’t remember where.
You mention a concern I hadn’t thought enough about “The 30/70 relationship of area around the pivot axis doesn’t change, so weathervane control isn’t effected.” This combined with your con #3 may save me a few failures.
I have to remember to base my rig’s fore to aft ratio (for weathervane effect) on the line extended from the leg of the Z member that goes into the mast step. It’s so obvious now that you have pointed it out but I think I would have missed this otherwise.
One additional con for me may be exacerbated by the fact that my main boom does not slant up as you mentioned earlier. The bend in my Z member will put the jib even lower when the boat is broad reaching or running. This can be corrected if there is benefit to the concept.
I had assumed that counter weights could take care of any unwanted rotation in swing rig so I am not too concerned.
Do you have any feel yet for what degree of aft rake will be beneficial on a run? I may be able to do some quick and dirty experiments in this direction due to quick change Z member in my rigs. I should be able to prebend a Z member to one side such that when the rig is sheeted in the mast will angle to starboard for instance. As I let it out for a broad reach or run with the main to port the rig will angle back. By changing the angle of bend in the Z member I can try different angles of aft rake. Of course this will only work on one tack and it will not cause the mast tip to move to windward like a raked step would so I’ll only get half the information. But it may let us know how much angle we need to see a benefit if it exists. Any thoughts?
I had rather hoped that with the swing rig I might end up with two mast steps. The first would be vertical for use in light wind with a standard Z member. The second would be located in front of the first and raked aft so that the insertion point for the acute Z member could be as little as one mast tube diameter ahead of the first. This could be easily retrofitted to existing foam hulls.
Hey Brent - I think that just bending the mast pivot of a regular rig will not give you a sense of having an aft raked mast tube in the boat. The movement of this system is really odd, and as I mentioned in the previous post I am not sure its worth the trouble just yet.
My rake angle is in the neighborhood of 10 degrees. This may be too much, but I derived this as a starting rake by splitting the difference between my Bantam’s conventional rig’s slightly forward tilting mast and the angle of the jib stay. Also, a more radical rake angle to start with should show any hidden problems earlier. Brent, if you want to undertake developing this system I would try a less angled rake, perhaps 7 degrees or 5 degrees as a starting point. You will more likely have some early success at these rake angles.
And yes, the section of the boom forward of the mast pivot does rotate downwards when the sail is let out. To correct for this I make my Z bend wire in two parts rather than just attach a straight boom to the Z itself. I silver solder a stub to the aft side of the mast pivot even with the first bend in the wire for the main boom to slide over. This allows me to angle the horizontal section of the Z bend upwards without changing the main boom’s relationship to the boat when beating upwind. Its more work and involves some more advanced skills but it has been a satisfactory and lightweight solution to potentially sticking the front of the rig in the water. It is however no longer a simple rig as the McRig was originally conceived.
For my rig I found that the raked mast tube (where it exits the deck) has to be about 6/10s of an inch forward of of the straight up mast tube’s location. This can be quite substantial seeing how many Footies seem to have only about 4 inches of boat from the mast to the bow.
Can anyone give me some insight into the pros and cons of various jib area to main area ratios? My reason for asking is this…with a swing rig and a Z member CE position in relation to the mast is mostly irrelavent. Taken to one extreme you could consider the McRig to have 100% jib to 0% main (or vice versa).
Since I have the freedom to choose this ratio what are some suggestions for ideals? My current “A” rig is about 69% main 31% jib. Perhaps I should be looking at something quite removed from this for overall rig efficiency. Thanks in advance for any input.
Niel, on the previous subject, I’ll try something in the 5-7% aft rake range to see what results I get.
Perhaps it may be irrelevant where the sail CE is relative to the McCormack rig pivot, but the relative areas in front of and behind the pivot certainly aren’t.
I haven’t actually tested this, but if you were to have the same areas fore and aft, a wind coming over the beam wouldn’t know which way to turn the rig and it would tend to flutter between the two positions. This would make it difficult to head into the wind reliably, and even more difficult to get out of irons when you find yourself there. Too large a jib area seems to manifest itself most when trying to tack at low speed, it becomes quite difficult.
Most of my McCormack rigs seem to have settled down to the magic 30%:70%, or a Main to Jib ratio of 2.33. This seems to give the most reliable handling characteristics and, of course, a reasonable amount of sheet load reduction.
Of course you are correct regarding the importance of ratio of sail area in front of and behind the pivot axis in order to achieve the required weathervane effect.
Perhaps the easiest way to visualize what I’m asking about is to imagine maintaining the exisiting profile of a successfully balanced McRig but splitting the single sail to form a main and a jib. As long as the sailplan/profile remains the same the balance for rig CE and weathervane effect will not be largely changed by this split. If the rig and it’s associated pivot axis worked well before the new sloop rig should also balance and pivot well. The result will be a swing rig.
In fact this is how I designed my first swing rigs. I started with McRigs which can be built from scratch in under an hour to establish the sailplan and location. Once I was satisfied I used the profile to define the sailplan for the swing rig.
What I want to know now is whether there are advantages/disadvantages to a relatively larger or smaller jib component in a sloop rig.
Look at Handley-Page slots - and make sure the jib FILLS the foretriangle - i.e. comes back to the mast. If you can arrange it so that a little positive leech rounding gives you a small geometric overlap (i.e. a projected overlap with the sail on the centreline), so much the better. :zbeer:
It seems very well thought out, but I am having difficulty reconciling his recommendation of setting the Jib at some 20/25 degrees more than the Main. Most of my references seem to suggest more like only 5/10 degrees.
Also he doesn’t mention using a smaller camber on the Jib relative to the Main - this does seem to have a noticable effect.
Something that Brent wants quantifying is the area of Jib relative to the Main, and you effectively referred him to aircraft LE Slats. In yachting terms this would seem to imply the need for only a small Jib. But the difference here is that your average monoplane does have only one wing, with no chance of being able to add one forward. The LE slat is then a small but effective way of bending the airflow to keep it attached as long as possible over its solitary areofoil. With our yachts however, we already have the luxury of 2 sails and the question then becomes one of how best to orientate them most efficiently
To this end, what my admittedly only graphical experiments have shown to date is to set the Jib with about half the camber of the Main, and then adjusted the Jib angle such that its exit angle is tangential to the back of the Main. This seems to suggest a 10 deg. Jib for a 5 deg. Main with the difference slowly reducing as the Main angle increases. This is for IOM type sails were Jib and Main have similar foot lengths.
Would it then be correct to tell Brent to go for as big a Jib as possible -consistent with other parameters allowing, such as weathercocking - to take advantage of the extra power that the Jib would supply whilst at the same time ensuring that the angles are right to reduce turbulance over the Main ?
Hi FF - Actually the jib to main ratio of a swing rig is not really as straight forward as it sounds. On my swing rigs (which I started developing in the US circa 1989 and have sailed with exclusively in the M and 36 Classes since) the ratio of 28/72 only pertains to the sail area, not how that area is deployed. The real consideration is leverage, that is the location of the c of e for the jib and the c of e of the main relative to the pivot point, and that is really what determines the amount of weathercocking and hence control of the swing rig.
The jib and main angles you site sound about on. The jib angle is related to the jib’s camber though. If the trailing edge of the jib is in too tight, or there is too much camber, or there is not enough twist in the sail then it will backwind the main. Conversely, if the jib is angled off the centerline too far and/or there is either too much twist or a hard leach then the boat won’t show much speed because the sails aren’t working together. A set of telltales on the mainsail even with the 3/4 height of the jib will help tune the rig to avoid stall.
Most swing rigs use much larger foot lengths for the main than the jib. If the jib and the mainsail both have similar foot lengths in a fractional arrangement then the jib’s c of e moves forward relative to standard practice. This can be done but you would probably have to reduce the jib’s area in relation to the main to something like 25/75. With my swing rig designs for the larger classes that I mentioned the jib foot averages about 2/3rds of the main foot length. And that is probably large compared to Roger’s M rigs.
Just about all swing rigs on this side of the pond do not overlap the jib, but leave a slot similar to that used by conventional rigs. Overlapping the jib would require a level of precision in set up and a level of control over the sail shape that is not practical for one servo to manage. With our boats all indicators seem to be pointing to the averaging factor, which is why Brett’s McRig is so popular.
:zbeer:FF - your analysis of aircraft is incorrect. There is a small and generously inglorious class of tandem monoplanes. The most signal example was the French Flying Flea. Aircraft have had the option of having two lifting surfaces but have generally chosen not to take it up.
At this pint can I suggest a convention (the symbols are more or less standard in most classes to which the point is relevant.)
I = Foretriangle height measured deck to intersection of forestay/jib luff with mast.
J= Foretriangle base. Measured horizontally from jib tack point to fwd side of mast.
P = Mainsail hoist
E = Mainsail Foot
BAD = Boom Above Deck
LP = Longest perpendicular of headsail, i.e. distance luf -> clew measured at rightangles to luff/forestay.
I have not had time to look very closely at the article you cite. It is certainly non-trivial. I have two initial impressions. The first is that, as Niel rightly says, the relationship between overall rig base (J+E), J and sheeting angle is complex, as is the problem of camber. If we take an ‘ideal’ airofoil section of the entire rig and try to superimpose sals on it to apprximate the ‘ideal’ shape you will see what I mean. A you correctly surmise, I do view the whole thing as a single foil with a slot. I pointed Brent to HP slots because (a) there is an extensive literature and (b) they are very versatile: the exit from the slot on a Messerchmidt Bf 109 wing exited at around 30% of chord - which accords (aaagh!) quite well with yacht practice.
The experimental apparatus in the article is fascinating. I had not come across it before. I think I shall make one. It does strike me though that calibration may be difficult. It must be grossly prone to surface tension effects in the working space. Any of you rral engineers have any idea how to linit this/compensate it out. :graduate:
A bit of an oversight on my part there. You’re obviously correct in saying that weathercocking is a function of moment rather than just the straight areas. I guess the saving grace is that there is no absolute value for all boats, although I suspect that when a workable ratio is achieved for one boat then it should hold for other rigs on that same boat.
I have all of my McCormack rigs saved in a spreadsheet so I will have to go back and see if the moment correlation is noticably better than the plain area one - just another job for the pile :rolleyes:
I guess it must be a moot point as to whether the Flying Flea is a tandem mono plane or a high stagger biplane. Either way it does seem close to our Jib and Main yacht sail arrangement. I wonder how they overcame the early problems they experienced with stick back nose down effects ?
I’ve not been able to find any illustrations of the ME 109 LE Slat arrangement. Your saying that it exited the wing at 30% chord puzzles me a bit. Most slats rely on air travelling externally to help maintain a boundary layer, but to say it exited the wing suggests it was at some time travelling within the wing. These exit ducts are not at all obvious on the photos Google throws up.
My Blue Peter style cornflake packet mock-ups clearly illustrate the importance of matching the Jib and Main angles. I find that it is possible to achieve a half decent looking entire rig with any number of boom angle sail camber combinations. What is not at all obvious is which ones are best. I suspect it all boils down to acceptable Jib angle of incidence and I don’t really understand what is required here.
Flying Fleas were eventually rehabilitated (too late for the majority of builders) by increasing the “gap” so that the venturi-servo effect became manageable (within the pilot’s strength). Descendants of the Pou are still popular as homebuilts in France, and still carry the HM (Henri Mignet) type number.
ME109 slats is a fairly large subject, as different Marks had different equipment, and I think some of them dispensed with the slats completely.
I will have a quick check, but my memory says that:
Early 109s had automatic Handley Page LE slats, (which are aerodynamically deployed) - they would pop out close to the stall and restore airflow over the top surface of the wing. Asymmetric deployment was a problem in combat and could spoil the pilots whole day. http://109lair.hobbyvista.com/techref/systems/control/slats/slats.htm
Incidentally, I believe that Messerschmidt purchased the licence for the HP slats and maintained the commercial deal right up to the outbreak of war.
A little reading suggests that later Bf109s (Fs and Gs) still had slats, but markedly different - now I wonder about the Buchon - more to look at!
Sorry to ramble, but someone mentioned planes:D
andrew
I think the fact that there are so few Mignet type aircraft confirms that the tandem wing configuration is not the answer to the search for more lift. The yacht Jib is apparently a workable equivalent for sailing vessels where it is obviously far easier to actually see when the slot is closing up too much by observing a crease down the Main. Also, the lack of the third axis makes it less catastrophic if you get it wrong.
The Me 109 info is interesting, even if it did come from a modelling site. Perhaps Angus was thinking of something else when he said that the slat exites 30% along the wing chord.
I guess we ought to close this subject as we are getting decidedly off topic.
The “concept” of a jib and main with a slot is simply carried on to the next step by solid wing sails with flaps that (in a braod sense) function to make a slot similar to what you create with a jib acting on the main.
Obviously the next step in a swing rig is to “upgrade” to a solid wing where the flap is integral to the unit, and the whole thing rotates much like a softsail swing rig to adjust for angle of attack. The softrig being a bit easier to tune and stay within the “powerband”
Guys some of you will no doubt recall that I did try this slot idea as can be seen from the photo of my bottle boat at the Birkenhead meeting with Bill Hagerup havin a deko at my set up.
Trouble was it was excellent across the wind and down wind being easily as fast as some of the other boats but lousy at going up into wind.
Try as i might, even to the extent of stiffining the leech of the jib with carbon fiber it would still not head up to wind like a normal rig.
If you think you have an answer then I am listening because I still think its workable - just I did not get it right
Rgds
AndyT
It looks as if my ageing brain is beginning to forget tings even when stone cold sober!
It cannot have been the Bf 109 - but I have very clear recollections of a cross section of the wing of some well-known high performance aircraft of c.1939-45 with what was essentially a ducted wing with the duct controlled by an aerodynamic device on the leading edge. Whether it is stricylt correct to call this an HP slot (as opposed to slat) is probably a moot point.
Surely the fundamental point is that these aircraft devices inject air intio the low pressure, high volocity airstream just before the point at wbich (it is believed that) separation will occur. A sensible 'guess is at about the ppoint of maximum thickness - or a little ahead. Hence the conventional ratios.
Note that although other proportions can be used, the reason for doing so is generally not dynmamic efficiency. For example, the very big foretriangles and ‘ribbon’ mainsails of the last boats to the RORC rule were purely the result of a rather freakish way of measuring sail area. And they were certainly not efficient: I once sailed briefly on a boat with a 170% genoa (LP - 1.7J). She had been the year before’s RORC Class 1 champion and she came with a very comprehensive ‘manual’. According to the book, even in the lightest weather, she never carried the 170% genoa to windward - the light 150% was faster. The 170% sail did pay for itself on a close fetch, but the real reason for having it was that it brought about another 1.5 m of spinaker width - free in terms of rating.
[QUOTE=Angus;47366]It looks as if my ageing brain is beginning to forget tings even when stone cold sober!
It cannot have been the Bf 109 - but I have very clear recollections of a cross section of the wing of some well-known high performance aircraft of c.1939-45 with what was essentially a ducted wing with the duct controlled by an aerodynamic device on the leading edge.
>snip<
Andy i was going to try a rig like that with my V-12… you may have just saved me some time… I did plan on putting a squared off main sail like yours but I think I’ll stick with a triangular jib…
I always thought the the leading edge of the jib was important, maybe with having it vertical instead of angled gave you too much lee helm??? and har to control…
I once tried a square shaped semi foil McRig variant. I used a symmetrical depron foil for the first third of the area and sail material for the remaining two thirds. I now suspect the biggest problem with this type of rig for windward work on a Footy may be a lack of twist. The Fathead swing rigs I’m using now generally carry a fair amount of twist in the main.