I’m new here, and I have a question about sailmaking
I want to make my own sails with the use of a camber board. But there’s one thing that caught my attention, and that’s the so called setback angle. Is it really necessary for a good sail to cut the panels with a setback of 70 degrees? Why not straight with 90 degrees? what are your opinions about this matter?
Thanks for now,
What is the “setback angle” please? I’ve not heard this term before.
I second that? I have an idea of what you mean, but im not sure.
Are you refering to the angle of the panels to the mast by any chance?
Yes, what I mean is the angle between luff en seams.
See also picture
It’s all about aligning the stable part of the cloth with the load patterns. The panel configuration we commonly see, where the panels are at 90% to the leech, was developed for woven sail cloths, such as Dacron.
Of course these days we have non-woven sail cloth for our models, and the benefit of aligning the panels to the leech is no longer there (but it still looks good).
Check out this photo from the IOM World Champs just finished - you’ll note that the mainsail panels are parallel to the luff.
It’s a little of subject, but do you know what material/clothing the sails in your picture are made from?
I can think of three reasons to make this angle different from a “simple” 90 degrees. One is to align with the load path as Murray notes, and so somewhere from 85 degrees at the foot to around 70 degrees at the head would seem sensible for an IOM.
Two is to allow you a longer path for the sail shaping (broadseaming) during construction. I guess you are likely to be able to be a little more accurate if you are trying to broadseam a join 450 mm long, say, rather than 350 mm long.
Three is, for a radically small angle such as 45 degrees, you can have the seam introduce boundary layer turbulence over the sail surface. There might be some advantage to stimulating such turbulence, since it helps with reattachment of flow if you are at or near stall. (The sails are set at or near stall for most of their windward working life… (smile))
Have you got any research on the boundry layer turbulence effects at the sort of Reynolds numbers we are talking about? That would be very interesting.
I’ll see what I can lay my hands on. But it is the “dimpled golf ball” effect – the dimples create turbulence in the b/l which promotes re-attachment after separation and so lessens drag so the ball flies further…
Edit: For example, from http://en.wikipedia.org/wiki/Boundary_layer:
“At lower Reynolds numbers, such as those seen with model aircraft, it is relatively easy to maintain laminar flow. This gives low skin-friction, which is desirable. However, the same velocity profile which gives the laminar boundary layer its low skin friction also causes it to be badly affected by adverse pressure gradients. As the pressure begins to recover over the rear part of the wing chord, a laminar boundary layer tends to separate from the surface. Such separation causes a large increase in the pressure drag, since it greatly increases the effective size of the wing section. In these cases, it can be advantageous to deliberately trip the boundary layer into turbulence at a point prior to the location of laminar separation. The fuller velocity profile of the turbulent boundary layer allows it to sustain the adverse pressure gradient without separating. Thus, although the skin friction is increased, overall the drag is decreased.”
Nobody knows what material these sails are made from?
I’ve read a lot (as much as I have found time to read) about this with regard to radio sailplanes - particularly the information available on the internet and my local library from Michael Selig (who has done a lot of work on low speed airfoils - and I’m sure you a very familiar with his work). My thinking has always been with regard to the underwater appendages, and not the sails. I must look into this more. My gut feel would have been that at our very low airspeeds (over the sails) applicable to monohull models, we would not see a measurable difference from the introduction of a turbulator to the sail foil. I’d always assumed the other variables, and the turbulence introduced by the spar, would be much more significant. I’d love to have the time, and equipment to do some testing.
This sort of thing could be of particular interest to those interested in mutlihulls and wing sails.
Sorry Folkert - getting a bit off topic.
Some of the sailmakers have information on their sites about the fabrics and weights they are using. You may like to check the following examples:
http://www.sailsetc.com/new/downloads/Sailmaking_Notes.pdf - this PDF file provides great information compliments of Graham Bantock / Sails Etc.
Yes, Selig’s “Airfoils at Low Speeds” is very relevant. On my “Books” page, I note “… of immense importance because the testing was done at low Reynolds numbers, exactly the sort of numbers we sail with. The only reason to buy the book is to be able to read the five or six pages that talk about flow separation, separation bubbles, separation hysteresis, and trip turbulators. This information is still not fully appreciated in our sport.”
My gut feel would have been that at our very low airspeeds (over the sails) applicable to monohull models, we would not see a measurable difference from the introduction of a turbulator to the sail foil.
Well, it is the combination of speed, length, and density which are important (ie Reynolds number), and which probably make tripping the flow over the sail (in air) possibly even more significant than tripping it over the fin (in water).
Selig feels that for Rn at or below around 60,000 we are in a regime which is not well understood and in which bubbles and separation thrive. So for example, let’s have our IOM sail at 1 m/sec with true wind speed around 3 m/sec. Rn for the mainsail would be around 3 x .25 (sail chord in m) x 68500 = approx 51,000, and for the fin around 1 x .1 (fin chord in m) x 877000 = approx 87,000. So in fact it is the sail that is experiencing the lower Rn regime…
the turbulence introduced by the spar would be much more significant
Yes, I am sure you are right, so I wouldn’t go about making my setback angle 45 degrees just yet for the mainsail (smile). But the jib? Hmmm…
Hi Lester -
I am curious if you are saying you advocate a sail chord of .25 - or was that just a number you pulled to demonstrate the difference between water and air for purposes of illustration?
I have been trying to find a good source for optimum (or heck, even average) chord depth to use for a solid wing. Unfortunately, everyone (anyone) you ask always has a varied theory on whether to be fat or thin. Currently I have started building some ribs for the wing, but have stayed around NACA 1500 which “seems” to produce some fairly “fat” sections - maybe just my imagination.
On the other hand, I keep hearing that the thin wing will have a much less forgiving angle of attack, and it will be hard to keep in the groove. Some advocate a thicker wing to allow a bit of mis-match AoA. On the big boats, one can “feel” when you have it dialed into the “groove” but with these little ones, standing on shore and having only electronic attachment to the boat, one cannot “feel” when the optimum angle is obtained.
My first set had a 6 foot wing, tapered, with a maximum boom length of 14 inches. Head was 6 inches as I recall - numbers are at home. I have since scaled down the overall height to about 4 feet (48 inches) mainly due to raw material sizes available, but have maintained the same foot length. I feel (yet to be determined) I can still generate the same “horsepower” as the tall wing, but it may be more useful since it will be down lower and have less tendency to heel. I am still fighting a hopeless feeling about making the wing too thick or too thin. It will have a trailing edge flap (one top one lower) so I can fool with the total camber as needed - but am seeking guidance and suggestions on the leading forward section of the wing. To date, I have stayed with “slow-flyer” theory, which would be a thicker leading edge section. Do you see any reason to go/stay thinner?
I don’t mean to hijack this thread, but last few posts are heading in the direction of what I was seeking. Any comments or opinions appreciated.
ummmm - this is a solid wing “sail” - not “mast” to which I am referring.
Yes Lester, I too had calculated the rig as operating at a lower Rn than the foils. As you’ve noted, it seems to get a bit fuzzy at these very low numbers. What we need is a big sponsor for a PhD study!
Dick, the number Lester used in his example works out to be a little over 8% - which I feel comfortable with in an IOM mainsail. I wish I knew something about chord depths as they may suit a wing sail on a multi - but I don’t. You’d be operating at higher Reynolds numbers, so the information relevent to slow flying RC sailplanes - particularly test results at high angles of attack, would be a good starting point. You’ve probably already studied that.
It is the chord of an IOM mainsail at or around its centre of effort…
I have been trying to find a good source for optimum (or heck, even average) chord depth to use for a solid wing
I assume you mean wing thickness (chord being the measurement from leading edge to trailing edge). Marchaj’s book “Sail Performance” devotes an entire chapter to the topic… but doesn’t give much help in deciding a thickness measure…
… have stayed around NACA 1500 which “seems” to produce some fairly “fat” sections
I guess you mean NACA 0015, ie a symetrical foil with a thickness of 15%. That is a “fat” foil IMHO…
I keep hearing that the thin wing will have a much less forgiving angle of attack, and it will be hard to keep in the groove
Yes. But it depends on what you want to do. If you seek maximum VMG, then my feeling is that a thinner foil is needed, to allow you to sail closer to the wind and to generate less drag while you are doing so. On the other hand, if you seek maximum speed on a beam reach, or a more relaxing sail without having to worry about angle of attack, then a fat foil will do that better…
I feel (yet to be determined) I can still generate the same “horsepower” as the tall wing, but it may be more useful since it will be down lower and have less tendency to heel
Well, the air moves slower lower… But you are talking about aspect ratio here, and the main advantage of high aspect ratio is efficiency – a better lift to drag ratio. Again, it depends on what you want to do and the winds in which you want to sail. If you need speed and pointing ability in light winds, then high aspect ratio is what you want. On the other hand, if you don’t want to have to lie ashore when the wind is breezy and you don’t care too much about induced drag, then a lower aspect ratio gives you a wider sailing envelope.
From Dick’s response, I’d assumed he was talking of camber - but re-reading the original, Dick was probably correctly describing chord. I should have known better.
Awww Crap - maybe I should just go home and start over tomorrow. Trying to do work and also write a post. NOT a Good idea! :banghead:
Boy Lester - am I glad you were able to decipher what the heck I was trying to post. Perhaps it’s your academic background? Trying to decide exactly “what” a student was trying to write?