sail aerodynamics

You and Tom crack me up.

Increasing span without changing chord, increases area, increases AR, reduces required Cl and and thus reduces induced drag and increases form drag.

Increasing span without changing area , reduces chord, increases AR, the Cl remains the same, but the induced drag is reduced and the form drag remains the same.

Increasing AR without changing span, reduces area, increases required Cl and the induced drag remains the same and the form drag is reduced.

Is that about right?

I have no idea why the two of you insist on considering span independent of area (and thus AR). It is misleading to an extreme.

Changing span and increasing area at the same time is reducing one drag producing factor while increasing another.

Changing AR (span) with fixed area changes only one drag producing factor.

Yes, you nit pickers will point out the obvious error ... at some speeds Re limits gains from increased AR.

Can we at least agree that L/D max occurs when lift exceeds drag by the greatest margin?

That leads to the conclusion that when form drag (shape and area of both surfaces) and induced drag are equal, maximum L/D is reached.

Increasing span alone increases form or profile drag in two ways. The obvious increase in wetted surface due to the increase in effective area x 2 *and* the increase in drag due to the increased thickness/chord ratio required to keep the foil from breaking at the root. At some point increasing span increases drag. What you have pointed out is that AR does not always reduce induced drag, while failing to note that the drag force in your halve the chord example *is* decreased ... thus I am quite correct; increasing AR increases L/D

To start a reply with a smug "no so" and then to proceed to ignore the fact that I very clearly qualified the increased AR/Induced Drag/Area relationship is offensive.

I have worked on the L/D problem with a span limit constraint and with an area limit constraint. I cannot think of span as different or independant from AR. I think I understand the realtionships pretty well.

I agree completely that any increase in L/D is probably due to the mast being in the slot ... a benefit that the aftmast rig does not enjoy.

Cheers,

Randy

PS. The AoA on a sail cannot always be increased to provide the required lift ... unless you don't expect to sail to weather very well.

 

Because when considering sail airfoils, it makes the most sense to consider the span fixed, and just let the chord vary. That way you nearly decouple both the heeling moment and the induced drag from the sail airfoil problem.

 

I guess I should be more specific about which rig of I'm talking about- I'm thinking about a rig with a vertical mast in back, and 2-3 jibs in front of it. I'm assuming storm ji(b)s to balance things in the stronger stuff. Advantages I see for the wing mast are

Lift at small aoa
less drag when feathered (or near feathered)
adding to system lift when needed.

I'm trying to imagine your #1 criticism-

the cut of the mainsil could be different
the overlap could be different
what degree of choke of the slot would be too much for you? (Dr. Smith is trying to say something to me through Dark Matter, but it's too faint to hear clearly...)

I'm wondering at what size a smaller wing mast becomes useful, but the wierdness of the sub 100,000 re region seems to me like it would to come into play. In a most delightful way, nae doot.

Paul

 

I agree. But the way you put it makes it sound like changing span is a good drag reduction method.

Increasing the span of either or both foils (aero and hydro) tends to increase the heeling moment so keeping the heeling moment constant pretty much removes adding span to reduce drag as an option does it not?

For a fixed span solution span loading becomes problem to solve, You and Tom have probably been looking at this for years.

Back 20 some years ago I met Tom Selig (sp?) when I was designing low Re RC sailplanes and had the honour of building a few wings with his profiles. The last I heard he was still doing low Re research ... I wish I was still in contact and could infect him with the sailing bug.

One thing seems pretty certain to me. A masthead rig with sails bound by the forestay and backstay might be a bad choice. No design I'm aware of has evolved to that planform.

Thanks for the reply,

Randy

 

Randy- there was a high altitude variant of the the Spitfire close to the end of WWII that had long pointy (pinhead in sailing parlance) wings, very much (to my musical eyes) like a sail. Probably different because it was a wing. But I've always wondered why they did this, as it seems to go against the grain, so to speak.

Paul

 

It was to primarily to reduce the wing loading while leaving most of the wing unchanged. Lower wing loading gives greater max altitude capability.

 

Is this where I jump in and say ... but lift = weight how does increasing area let the plane fly higher?

Just kidding!

 

Does it count as evolution or desperation?

 

Age is a rotten thing ... Michael Selig

 

Engineering issues. The wingtip on the Spitfire was attached to the main structure. Any change of the main structure would be a very major redesign, but making the tips smaller or larger was a reasonably straightforward exercise. The most practical way to add a little more area to a Spitfire wing was to make larger wingtips, and the pointy end about the only solution that wouldn't be totally bizarre...

This page seems good
http://spitfiresite.com/reference/variants-technology/2008/04/spitfire-wings-02.htm

 

Very nice! Thanks for digging it up.

 

Thanks, Guys!

 

There is another issue at play with the wing tip I think. Increasing the area outboard would raise the stress on the main spare at the root, meaning a major redesign of primary structure. But if you increased the total span only, without increasing the area you will not increase the wing root loads as much, and you greatly improve the climbing performance. With aircraft the span loading (total weight over total wing span) is directly related to climbing capacity with any given engine power. I think this was a way of improving the climb performance of the Spitfire without having to redesign the wing or re-engine the aircraft. If you increase span without increasing the wing area you end up with pointy wing tips.

Something missing on sail design I think is consideration of L/D. Though it has been mentioned here a few times, there tends to be little consideration for looking at the total L/D of the rig design. There is always consideration for max CL and max AOA before stall (best pointing) perhaps because these are important in a sailboat when at certain extreme edges of the envalope (and in racing that would give you an advantage when sailing there), but I wonder if better overall performance will be improved with better L/D, not just Clmax. All of the CFD models appear to ignor L/D max, yet in an aircraft or sail plane, the best speed and best distance is acheived at best L/D. It seems to me this would be an advante in a sail boat as well.

Most sailing rigs, both modern and traditional are very draggy, and only in a few instances have I seen any consideration for reducing the total drag of the rig. All of the focus is always on max Cl and max AOA. Multiple element foils (and sail plans) achieve much higher Cl max, but ALWAYS at the expense of L/D max. On an aircraft Clmax is only used for landing or in a max g manuver (not your typical flight condition), and extra drag on landing is helpful in controling your flight path angle. I suspect the thinking is that in a sail boat you always want the max speed for any given wind speed, which means achieving the max CL under all conditions (unlike an aircraft where the total life at any given speed or flight condition has to equal the weight of the aircraft). So drag is ignored on a sail.

But on a sailboat when you are at CLmax, you will be fighting against the higher drag. In all points of sail except going down wind high drag kill your performance. It will slow the boat, it causes more heeling moment (reducing the efficiency of both the hull/keel and the sail) and increases the slip or leeway. It seems to me if you have a rig design that gives say 9:1 L/D instead of the more typical 3:1, you would have 2/3rds less drag for the same amount of lift, that means less heeling moment and better windward performance. Going down wind of course the drag direction is the same as your lift direction, so it is the only place where the drag is beneficial. But even then if you generate more lift for any given amount of drag, even your down wind performance could be improved.

Best L/D is a complex relationship with the lift curve and the induced and parasitic drag relationship and there is no simple rule of thumb for determining where it occurs. The best L/d doesn't occur at minimum drag, nor when induced and parasitic drag are equal as someone speculated. It just all depends on where each of the curves land on a plot of total drag and total life vs. angle of attack. It is usually plotted and the ratio calculated at each point of lift (or AOA).

I am planing on experimenting some day with a small rig that would optimize L/D rather than clmax and see what I get. Needless to say, it seems to me that minimizing all the drag on the rigging, the deck, the mast and the sails all will improve performance, and having the highest practical aspect ratio (to reduce induced drag) all will be useful. I suspect that better being able to control both camber and twist of the sail will also yield better performance. This means a single element "foil" or sail.

Any thoughts on the L/D performance of the rig deign vs. CLmax?

 

Put your flame suit on.

Sailing upwind (anytime the apparent wind is forward of the beam) is a Max L/D *of the entire system* problem.

TSpeer and others have been looking at L/D max for years. At the risk of misquoting, it may be that L/D max of the sail plan cannot be used.

On an aircraft wing, span loading and root chord load do not produce a roll moment, the forces are mirrored by the other wing.

On a sailing vessel the sail plan produces a roll moment (heel) that must be balanced by righting moment. The hydro foil acts to *increase* the heel moment. Thus sail rigs must maximize L/D of the sail plan without increasing the heel or roll moment. The lift force available at L/D max with the roll moment constraint is not enough to drive the boat to Vmax.

You cannot just double the span (height of rig) and keep the boat from tipping over. As you point out, sailing boats are draggy things. All the deck gear, rigging, people on deck, lifelines etc add to drag. Max L/D of the aero half of the problem must include all the aero drag, not just that of the sails. It may well be that on many boats that the parasitic drag is so high that even near Clmax for the sails, the induced drag is lower than the parasitic drag, and L/D max cannot be reached at all.



Cheers

Randy

 

At the risk of sounding atavistic, L. Francis Herrescoff, on page 21 & 22 of The Common Sense of Yacht Design describes an experiment (in 1925) with 3 identical hulls of the Developmental Class, 19' Length, 125 sq ft of sail. Smooth seam Spanish cedar planking like a rowing shell. "One had a conventional sloop rig, round mast , shrouds and all. One was cat rigged with a double luff sail: the third was a cat yawl with both sails double luffed.... from what little racing we did it seemed apparent that the sloop with the round mast was fastest in moderate wind and a choppy sea. The cat was fastest in light wind and a smooth sea. The cat yawl seemed much the best in strong wind, particularly to windward, and won the most points in the races.."

Apart from some multis (and you might argue that the slim hulls smooth out motion and chop resistance), how much has changed?

L. Francis was working for Starling Burgess at the time, who had been an "aeroplane designer" and they were "wind resistant conscious" which would make sense, as minimizing drag was big at that time. He then goes on to talk about double luff sails on an iceboat and "tacking to leeward".

The problem with maxing L/D has been messed with for a long time, with some progress, but the sloop keeps on keeping on... Maybe we are now getting to a point where L/D max is important from a practical standpoint?
Although round masts are still hanging in there.

Paul