Biplane and Tandem Theory Meet?

Anyone know of a good resource or thread here or elswhere that deals with the where and how of where biplane theory meshes/morphs/interacts with/to tandem wing theory?

I'm doodling an IC two mast design, with one of the masts moving moving from side to side (or perhaps both). In his biplane theory, Munk messes with stagger and separation to a point, but doesn't take the next step that I could see to tandems, assuming tandem interaction is different, which it seems to be from some NASA & academia stuff I www'd, so, understanding what conditions are doing as far as interactions depending on placement of the two airfoils (as far as dynamically moving them from a biplane to a tandem arrangement) are to me as murky as this last clause of this sentence is becoming. There.

Are there clear separate, different, equilibriums defining the two conditions, and what happens in between the two conditions, as, say when moving two foils from a tandem relationship to a staggered biplane relationship? Is there a blended condition? Or does the atmosphere catch on fire?

Paul

 

I'm asking specifically to determine if there are any configurations to avoid, whether the two sails should be different sizes or the same size because of different resultant lift because of system interactions, and what might be the optimal separation between the two rigs. This approach seems to have the advantage of balance, as well as increasing offwind speed, as long as upwind VMG can be kept competitive. From what I've read tandem and biplane theory have different requirements as far as optimising lift and balance of and in the system, but since I'll be able to change the placement of the sails in this system, it seems knowing some general theory would help, in order to initially place the sails more effectively depending on the relative wind condition and direction.

Paul

 

reply

Biplanes allowed early designers to get a lot of lift area with little structural weight ie they have small spans and the inter wing rigging wires make a strong structure without needing much wing strength.

Aerodynamicly speaking the two wings on a biplane interfere with each other. The main reason is that in the gap between the wings, the upper wing is creating a high pressure and the lower wing is creating a low pressure. The net result is more drag and less lift than a single wing of the same area.

The best aerodynamic situation for a biplane is where the interference is reduced as much as possible, i.e. have the largest gap possible and a negative stagger (lower wing is forward of the upper wing). Both of these assist in getting the wing air flows are out of each other's way.

I presume that the same analysis would be relevant for sails if you put two masts abeam of each other on eaither side of a cat (note that this would not be a good thing), however the forward / aft configuration of the foils on a yacht means that they have little relation to biplanes aerodynamics as I understand it.

 

You can use vortex theory to get your head around this problem. First forget that the wings have a span for a while and just assume it's all 2D. Then you place a vortex at each wing. The vortex looks like the one you get when you pull the plug out of the bath tub. It spins in circles quite fast at the centre where your wing is and gradually gets slower the further you move away from the centre. The direction in which the vortex rotates is determined by your wing. The vortex should move in the direction of the airflow on the upper low pressure surface (accelerate it) and slow it down on the low pressure surface. If you have two wings you have two vortices and they will interact with each other. In the classic biplane configuration the vortex of the lower wing will make the airflow move faster around the upper wing. The upper wing will have the opposite effect on the lower wing and slow the airflow down. The more you separate the wings the smaller the effect because your bathtub vortex reduces in speed quite quickly the further you get away from it. If you have one wing behind the other like on a normal aeroplane where you have the wing and the elevator at the back you get a different interaction. The wing will produce downwash where the elevator is. Because of this you have to increase the angle of attack of the elevator to achieve the same lift as you would without a wing in front.

By the way there is a graphical method that you can use to draw the flow lines by hand. It uses horizontal lines for the incomming flow and circles for the vortex. Where they intersect you determine a number by counting. The same numbers are connected by the flow lines. You only have to space the horizontal lines and circles accoring to the "strength" of the flow. if I could remember the fancy name for this method you could probably googel it and find out about it.

Cheers,
Karsten

 

Paul, You mention that you are doodling with a two mast an arrangement on an IC. Seems very unusual? However are the sails in a biplane arrangement as on each hull of a catamaran, eg. Radical Bay design, or in a fore and aft arrangement as in a schooner?
The amount of flow interaction varies approx with the inverse of the separation, in both cases.
If the two sails are staggered both laterally and longitudinally then a blended arrangement must occur in practice. Some high speed catamarans have been designed with such a staggered arrangement. I am not familiar however with any maths that describe the exact relationship, but do have experience firsthand with a tandem arrangement of sails on a 6 metre sailboard.

 

Thanks for the replies;

tc42- your reply would agree with Munks paper, if your course is upwind, but as you head deeper, and drag becomes more beneficial, does the tandem arrangement become more powerful? Does spacing rigs apart as far as possible reduce this potential (if it's there)? Is this what Mari Cha's design team is exploiting, or is the schooner rig an artifact of atavistic engineering restraints? And when your really deep, does positive stagger start looking better?

Karsten- Thanks! Is this the electrical analogy? I'm Googling forthwith.

frosh- It is unusual now, but until Uffa pulled his rule stunt, the two mast rig was the usual thing, and actually enshrined in the rules, at least in America (!). So the ethnologist in me started wondering if there was an impirical reason that the two masted rig came about- balance? Some of the canoes didn't have rudders- only a skeg, and steered with the sails and occasionally, a paddle.
Be interesting to find out any race results between sloops, unas, schooner, ketch, and yawl canoe rigs, assuming you could determine the courses sailed.

You sailed one of thise tandem boards? Cool! I did too. Didn't go upwind for love or money. Tandem arrangement though.

Paul

 

Karsten-

I've been circling away, and is there circulation around the system as a whole (both foils), and does this effectively create one 'foil' that has in sum a lower AR, which might be better offwind? (At least Marchaj argues that lower AR is better offwind).

Paul

 

"So the ethnologist in me started wondering if there was an impirical reason that the two masted rig came about- balance? "

All the Canoe stuff I've read indicates that early on the cat ketch was chosen so you could lie/sit/sleep in the cockpit which is hard to do when there's a sloop mast in the way. There's also a mention (from Sandy Douglass) that the cat ketch remained after the cockpit was abandoned because there was no way to get under the boom in those very tippy pre-Uffa canoes....you couldn't go around the back like we do today. Lower C of E must have been a factor too. Perhaps it's significant that the beamier and more stable UK canoes had sloop rigs more often even before Uffa?

Re tandems. Division III was the tandem division for course-racing boards. They ended up like massive Div II round-bottomed boards. Charly Messmer (world champ) wrote a section of a book on them and said that the front sailor should keep his rig as upright sideways as possible, but raked aft. The aft sailor raked his rig to the bow and to windward. Seen from the side, there was a considerable overlap, but the "slot" was quite wide side-to-side.

I think the old Brit Chance Equation had a raking mizzen for the same effect.

The good tandems were considerably quicker around a course in typical Euro conditions than a singlehanded board, which is pretty impressive considering a 12'9" Div 2 board with 7.4m sail is almost always faster than an International Canoe or Moth (even in 6 knots, and upwind).

I'm not sure if that helps, but that seems to be the closest experience to what you're looking for. There may be an old copy of Messmer's book on ebay and it contains a few pics and description of the technique.

 

CT 249, some interesting history in your posting, especially regarding the Div 3 sailboards. I was well acquainted with the Lechner Div 2 and have witnessed their amazing performance in light/moderate winds.
Paul, I have seen photos of the pre Uffa canoes on the web and though I do not have any performance information I do agree with the reasons given by CT 249 as well as one of my own.
This era of canoe probably did not plane in the modern sense, (probably did in really strong winds downwind) and therefore the amount of power applied by the mainsail(s) was much less influential on boatspeed than with a modern IC.
Handling issues associated with the different configurations were likely more important than L/D which is the modern holy grail.
As far as balance was concerned you would definitely find minute sheeting adjustments of the two sails in a tandem arrangement would allow quite good control of the course sailed on any one particular leg of a race course.
Paul, I designed and built the tandem sailboard. It has a daggerboard that protrudes below the hull more than 1.1 metres when fully down. The rear fin draws about 70cm. and stays fully down all the time when sailing.
It goes upwind around the same as a Mistral One Design and is really fun to sail for two experienced sailboarders on all angles to the wind, even winged out on opposite sides on a dead run!

 

Paul I think you are right. You can add the two vorticies and effectively you end up with a lower aspect ratio wing with all it's advantages and disadvantages. I'm sure that there is also an electical analogy. The vortex behaves like the magnetic fiel around a conductor.

Cheers,
Karsten

 

frosh-

Were you ever at the Pan Ams (Kailua Bay) in the late 70's, early 80's?
We may have crossed paths. I was obsessed with Div II boards, had a CRIT 650 & Div II, Ken Winner, lusted after the Alpha fattie, designed and built some lunatic roundies. Always thought the Davidson was the best. Any way, did you put mast tracks on the tandem?
If so, did you mess with them much while sailing?


CT 249-

I always thought Div. III was a rumour only. Cool. Even decades later. ahem, So.... sleeping in narrow tippy boats might have had something to do with split rigs. I think you're probably right, but I always like to think of some lone genius out there noticing pure performance outside of culturally enforced 'practical' chimera. Sigh. And I'll ask you the same question as I did of frosh- any mast tracks on the Div. III boards? Anybody mess with how close/ how far apart the rigs should have been?


Karsten & frosh & CT 249-

So maybe how close or far apart the rigs are in tandem is more important than stagger? Esp given Messmer's overlap advice? (There's a name I hadn't thought of in a long time!) Which seems to mimic some schooner rigs which showed overlaps galore. (Didn't VDH try a canting mizzen on his Beeg Red Ceegar?) Maybe moving the rigs fore and aft is more effective? Close for downwind, separated as much as possible for upwind? Might make Munk (wherever he is) happy.

Paul

 

Paul, I was at Kailua Bay in May 92 on holiday, never sailed in the Pan Ams but would have loved to. I still think they have been replaced by a faster but inferior concept, the Formula board.
My Tandem has two 8 in. tracks that you can't adjust while sailing. Clew of front sail is about 1 to 2 ft. from rear mast if both rigs are upright, depending on sail boom length.
We have found by trial and error that when sailing with apparent wind ahead of the beam (which is mostly) then the front sail is best slightly undersheeted and the mast upright laterally. The rear sail is then slightly oversheeted and the mast raked a bit to windward. This creates a slot between the two.
I could not readily source pedal driven adjustable mast tracks, but they would have been worthwhile, even if only to get the sailors weight therefore the CG further aft on reaches. You have to realise that mast position dictates the CG on a tandem in the fore-aft dimension. We get thrashed by other sailboards on the beam reach, I think primarily because of inability to reduce wetted hull surface enough, rather than inefficiency due to sail interactions. It may not be possible to really measure the practical result of sail separation without two full length pedal adjustable mast tracks on a tandem.

 

Paul, as one of the early US canoe gurus put it, "A canoe that cannot be slept in is nothing but a hollow mockery"; cruising was deeply entrenched in early canoes (and, it seems, the reason they were so popular).

I believe that pre-Uffa UK canoes did plane, basing that on things like the 1890 canoe "Snake" and contemporary descriptions of her charging around with her bow out of the water. However the US boats were more like Moths, going fast without planing as far as I can see. Douglass says they went like a cat hull, slicing rather than planing. Then again, planing is a messy field; as Julian B says, the more you look into it the harder it is to define.

I'm not sure if any DII boards had mast tracks at the time when Div III was active. I was at the D2 worlds in '85 and I think masts were still fixed then. We've always been meaning to have a D2 regatta but haven't got around to it yet; they are still very quick for their sail area.

I sailed against the world's best two non-foils Moths a couple of years ago and found even in 5-8 knots the Lechner with 7.3 or IMCO 7.4 was just as fast as the Moths, and of course in a breeze the board is gone.

It's interesting to see that with the Exocet Kona, Starboard Serenity and Tabou Windstyler, longboards are back in fashion. Here in NSW, we still race Windsurfer One Designs, the Mk2 version of the original board....up to about 8 knots we're quicker than the RSX which has 25% more sail. So much for 30 years of "development"!

I'm not sure about tandem overlap. My vision of the ultimate tandem was one mast, with one sailor acting as mast hand and the other as sheethand, both on wings for better leverage and on a massive D2 hull.

 

frosh, for what it's worth, I sailed on a tandem funboard way back when, and the masts were pretty far aft. I can't remember whether it had concaves or no. When planing, the forward sailor was standing out above the water (!) The front mast had a track, the back one didn't. Resulted in a few bruises. I don't think anyone was serious about it, mostly doing it for the thrill of sailing in the air. Water starts were interesting, and then I heard the thing broke while jumping- well, while landing. Would have been interesting to know how high it got. We never did experiment with mast placement, other than best guess- was more of a reaching machine. Have you thought of putting a couple of tracks across the board? It's great that you're still messing with the concept- any pics available? I only wish I could still windsurf, but shoulder injury and surgury have put an end to it. To get back to topic, do you see any positives in a split rig for an IC?
CT 249, longboards aren't back here. Pity. The Mk2 Windsurfers came with the aluminum boom no? My yellow factory MkI second had the teak booms- you had to fall in the water once in a while just to keep them wet enough to hold on to. Slippery while dry! The board was pretty soft, not fair, and kind of slithered through the waves, but given the small amount of money I had when I arrived at the factory, I was lucky Hoyle gave me anything. My Ken Winner had a track. That thing was great in bigger air. (But then it was developed for the Pan Ams in Hawaiian conditions.) The 650 did, can't remember if the D2 did. Mine did, but on most of them, I was messing with really round forward, and flat aft, so I needed to move around some. Never did get it quite right, as in medium conditions the better Div I boards would get past. Light air was good as long as the water was flat, but smaller boards would smoke me in the heavy stuff- esp. my brother's F2 Lightning, which was the first perfect board in many respects. If you were under 170 lbs. I wonder how some of the dedicated race boards like the Alpha Race would do against the RSX? All those concaves. Sometimes I think it would be good to go back and look seriously at the design of boards at that time- everthing was happening so fast that a lot of good ideas must have been lost in the shuffle. Like Div. II. I LIKE your ultimate tandem idea. I put wings on a couple of my experiments just to vang the centerboard, and they worked GREAT!! But, the idea does lead to IC's, I think. Sounds like you see no advantage to a split rig though.

Paul

 

One or two masts?

Hi Paul, There is no contest between a single mast or tandem rig if maximum performance is the main criteria, and maximum sail area is limited. For cruising, sleeping in etc, I think a ketch might be the easiest to handle and most comfortable.