This probably isn't going to help. . .OK - help me win a bet
On page 29 of "Successful Sunfish Racing" by Derrick Fries, all three boards are shown in the correct sailing position. The Shadow board is shown with the straight long edge forward.
Isn't this in the Sunfish Bible?
My speculation is the Shadow board got such bad reviews from racers and recreationalists alike, and was superceded in short order by the Barrington Board, it wasn't worth mentioning beyond the above photo caption. No one I know who's sailed the other styles has ever tried to replace one in kind. They eagerly upgrade to either the Barrington or composite.Derrick Fries says that regatta results indicate that the Barrington board performs best with the swept back edge forward and the old board is best with the long edge forward. He makes no mention of the shadow board but since the shadow board also has a swept back edge wouldn't this also be the best forward edge? This contradicts the photo I mentioned in his book!
Back to square one!
Alan Glos;131998 My question is: for the tapered board said:Well...here is an explanation that'll make you win that bet. I am a pilot and have finished my qualifications to fly for an airline, just have to graduate college soon. I have taken many of classes on aerodynamics, which is basically hydrodynamics. Bernoulli's principle that applies to our explanation of how lift on a wing is created, was derived from his studies on pressure and flow in plumbing back in the day. Increase velocity, decrease pressure and vice versa, and engineers manipulate the curvature of the wing surfaces to create lift using this principle, among a few others.
Anyways, the Sunfish's daggerboard is shaped like a wing, with both sides having the same curvature (neutral stability) This is not really the same for your generic aircraft...usually you see this design on aerobatic aircraft. Not really creating lifting forces to one side or the other (ideal for a daggerboard shape). You manipulate lift by changing the angle of attack on these types of wings (pitching the aircraft up/down)
You want the straight edge forward with the tapered edge facing towards the stern. The idea here is you create the smallest aerodynamic footprint so to speak. Ever see those giant vortex's that come off the wingtips of airliners? They create drag, and lots of it, especially when the aircraft is heavily loaded, slow, and in a clean configuration. That wing is working hard to keep the aircraft in the air. They are so strong and create so much disturbance to the air that the Boeing 757 has an FAA mandated minimum 5 mile following distance ATC must follow for separation of aircraft (due to it's particular wing design). They have been known to roll small airplanes who get too close! Nothing like going for a barrel roll 200 feet above the ground when you didn't induce it!
Point being...you can reduce the size of these vorticies by creating a smooth entry into the water and a smooth exit, which is what the tapered edge does. The tapered edge is similiar to the elliptical wing the WWII British Spitfire used and the modern kitplane Silence twister uses. Aerodynamically they are almost 100% efficient, in theory anyway. So if you want to go fast, straight edge forward. You will reduce your drag more than putting it in backwards, which would just leave you with a larger disturbance off that nice squared edge on the tip.Hope ya win that bet!
BC
Ever see those giant vortex's that come off the wingtips of airliners? They create drag, and lots of it, especially when the aircraft is heavily loaded, slow, and in a clean configuration...
BC
Here's a link that may help show how the shape affects turbulant flow. Winner for low drag seems to be the elipitcal shape. Airfoil shape gets really complicated.
--->You're talking about two different things here. Elliptical is the overall wing shape, viewed from top down. the shape of your airfoil is your lateral cross-section, it's not a wing shape type as viewed from above.
Strange to say, leaving the trailing edge of a centerboard square produces the lowest drag. The square edge produces a vortex that the water flows around. When we flew RC planes years ago, going from a rounded edge to a square edge on the alerions increased the roll rate an increadable amount.
--->Perspective 1: i'm curious where you found that a square trailing edge is the best idea for lowest drag. The vortex is something you don't necessarily want, unless I am not taking something into consideration for hydrodynamics, which I would like to find out. From my 7 years of flying and 170K education at ERAU and UND, my understanding is the vortex is where the two layers of flow between the upper and lower surfaces of the shape meet. The pressure difference, with the bottom layer of flow being of a higher pressure relative to the upper layer, causes it to twirl around the top and make the vortice (which in this case may not apply due to the mirrored curvature of both sides of the daggerboard) It's an effect that robs the wing or daggerboard in this case, of useful surface area or effectiveness on the tip.
For example: If you fly an airplane closer to the ground, the bottom layer of flow cannot flow around to the top of the wing, thus reduced vorticies, so you can fly an airplane close to the ground below Vr (rotation/lift-off speed) but above Vs (stall speed), and if you try to maintain that airspeed above roughly one wingspan above the ground, there is a good chance you will stall and settle back onto the runway...this is because the wing loses some effective surface area on the tip due to these vorticies, among other factors involved. Your induced drag (by product of lift) goes down within one wingspan and increases the higher you go above that. This is called ground effect and is a great example of how vorticies rob your airfoil of performance.
--->Perspective 2: Now it is possible that having it in with the tapered edge forward could be beneficial. There still will be a more abrupt re-attachment of flow on the square edge of the tip, which could cause some turbulent flow and create the vorticy you speak of. But the fact that the tip is tapered and the entire edge has a more of a sharp entry than the straight edge, it may be better for a less disturbed entry and more speed. The thicker the airfoil, the more air molecules pack up on the leading edge, even more so with the straight edge. This is called a stagnation point. But when you have a taper or sweep in the wing, the air molecules roll off the edge towards the tip so to speak, thus the aircraft is allowed to travel at a faster airspeed than the airfoil "thinks" it is going. That is how engineers are able to trick physics so wings do not have to be ungodly heavy and overly re-inforced to support the loads in flight. Though, tapered and swept wings need a higher airspeed than a straight edge wing of similiar size to maintain flight.
So this could mean having the daggerboard in backwards could mean some low speed upwind issues? Maybe? Not sure..Anyways...basically, it is possibly that there will be less resistance and more speed inserting the daggerboard, tapered edge first. Viewed from the bottom of the tip, the daggerboard has a similar shape to the "swede-form" hulls that fast kayaks use. that is a sharp, but gradual entry with a fast but smooth exit. I used to own a twin kevlar kayak that had that shape, it was a rocket and could glide in smooth water with very little effort. I think we all know that the best shape for this daggerboard would be a elliptical one. Maybe i'll have to build one out of fiberglass and try it...<----
---> as far as the roll rate on your R/C models...are you talking about the aileron being part of the wing tip as well? as in half the span of the trailing edge? because in that case, you are just increasing the surface area and effective surface area on the aileron by deflecting more of it into the relative wind. Then again I haven't seen the thing, so i'm taking a best guess. <---
Anyways...thats my perspective...I think that there is some testing in order this weekend!!
BC
Right, that's what I was referring to in my earlier post.A point to consider...possibly...just thinking out loud here.....if by putting the daggerboard in backwards, I wonder if you end up moving the center of pressure/effort, that is acting on the daggerboard to provide lateral resistance going upwind, further aft a few inches, causing the arm (distance between the daggerboard and rudder) to be shorter, and decrease rudder effectiveness? hmmm...
OK guys, my mind is doing flips and barrel rolls with all the physics discussion but I'm getting the message that the straight edge was intended to be the leading edge which answers the original bet question. However, I also get the message that in real sailing conditions of waves, heeling, etc. the boat performance may be better with the board in "backwards". Fries' book says the same thing when he speaks of actual regatta performance.
It's a scientific fact, not a "consensus".For a given foil shape (cross section) for a centerboard, is it better for the thicker part to be toward the leading edge, or the trailing edge?
I think the consensus is that the thicker part should be forward.
In general, I don't see why the/a bottom edge should be at a non-zero angle to the flow. In the context of previous-generation Sunfish centreboards, I'd go for a longer leading edge, just to get a nicer cross-section at the tip. But as there seems to be considerable disagreement, the difference can't be very big.For a given foil shape, should the leading edge be longer, shorter, or the same length as the trailing edge?
As the foil is always attached to the boat while sailing, isn't it pointless to speculate how it would sail "by itself"?When the foil is attached to a boat:
Due to the dynamics of the boat, does the ideal configuration change from that of the foil by itself?
Any difference in that sense can't be very significant. A daggerboard doesn't "pop up" in that situation, it hits the back of the centreboard trunk... and the faster you go, the bigger the damage.is a centerboard with the short edge forward and large radius chamfer more likely to be popped up when running aground as opposed to one with the long edge forward and a small radius chamfer?
Nitpicking again... but you actually do get lift either way, you just get more of it with less drag and less chance of stalling if you orient the foil the way it was intendedto get lift from a foil shape board the thick edge goes forward.
I figured someone would nitpick this, but I didn’t feel like taking the time to type up the whole thing on my phone. Next time I’ll be more accurate.Nitpicking again... but you actually do get lift either way, you just get more of it with less drag and less chance of stalling if you orient the foil the way it was intended
_
To add another wrinkle to this debate…
I have raced and sailed Sunfish for many years; and I also have a Super Sunfish setup. In my efforts to find additional kits and parts as well as learn what I can from various articles - the question arose regarding which centerboard is best, and which edge forward is better, and that led me here. So here is a post with questions and gratuitous Super Sunfish citations.