Traveler Position In Light Wind

[RichH]

On a strict mathematical basis, actually the air flow on the 'top' of a foil arrives at the exit (leech) **LATER** than the air flow across the 'bottom' .... all due to the aerodynamic 'circulation' flow. This 'lateness' is what causes the vortex 'after' the foil, and that vortex is what causes the (mathematical) 'circulation' flow (as a 'balancing' reaction). However at the leech, the speed from the top must be equal to the speed from the 'bottom' (kutta condition) or you dont get 'aerodynamic flow'.

I think the (your) wrong premise of that the air flow 'on top' speeds up to 'catch up' is whats causing the difference/difficulty .... the flow across the 'top' does not ever 'catch up' because air in aerodynamic flow below 'sonic velocity' is essentially an incompressible. It speeds up and slows down but it doesnt change density. The pressures changes because of the SPEED (Bernoulli) changes, but the density (static pressure) never changes !!!!!!1

The flow on the leeside speeds up but principally only at the area of upwash (luff) ... and doesnt catch up to the flow across the bottom; in fact, the air across the 'top' arrives at the 'exit' (leech) later than the flow across the bottom --- and such causes a vortex to form - the vortex causing the 'circulation flow' as a resultant of the formation of the vortex.
Also, airflow at or near normal atmospheric pressure (subsonic flight regime) is considered 'incompressible' ... the low pressure (Bernoulli) is developed by the *faster* speed of the molecules BUT the molecules dont get 'farther apart' (due to incompressibility) ... only 'faster' in that region.

The "Origins of Lift" paper describes a simple experiment ... suggest you do this as it will be very apparent visually of exactly what is happening. More startlingly, do the experiment with a jib/main and see what happens to the 'slot'.

Aero and hydrodynamics are NOT 'intuitive' sciences. Even with the "origins of lift" paper, I suggest that you shed ALL preconceived notions of 'lift' ... and then re-read the paper with a mind that is completely free of the 'popular' but wrong notions of lift (totally forget for a moment what you learned in high school about 'how a wing works').

Circulation flow is the most difficult to understand ... I would explain it as the flow patterns when ALL the 'relative' motions/speeds of the sail/wing are reduced to zero (for mathematical simplicity) .... then you can more easily understand the premise of 'circulation' ... once you do understand that there is a vortex 'following' the foil/wing/sail and the circulation 'balances' the vortex .... because of the viscosity of the fluid !!!!!! Viscosity is the 'key' as without viscosity, there can be NO lift (D'Lambert's Paradox).

Reread (maybe several times) the "origins of lift" paper ... but do so without any preconceived notions of 'lift'. Like I stated, aero and hydrodynamics cannot be explained by 'intuition'. Only once you firmly grasp the vortex and the circulation, then your intuition will be correct vis a vis wings, sails, foils ..... and 'lift'.

 

[rgranger]

Whew.... Homework on a listserv. Damn.

Thank Rich I'll give it another read.

 

[Witzend]

Some other things that need to be consider in light air.
1) Rig tuning: in small boat racing such as a J22, loosening the tension is very important.
2) Sea condition: having your sails too flat in light air with choppy seas will kill you. You need to develop some power by moving your draft forward.

 

[COOL]

Question?

When sailing upwind and the winds are light where should the traveler be positioned?

Thanks

Answer: Slightly to windward of centerline so that the boom end
is on centerline with the mainsheet is eased enough to maintain
the proper amount of twist.

 

[ffaubert]

On a strict mathematical basis, actually the air flow on the 'top' of a foil arrives at the exit (leech) **LATER** than the air flow across the 'bottom' .... all due to the aerodynamic 'circulation' flow. This 'lateness' is what causes the vortex 'after' the foil, and that vortex is what causes the (mathematical) 'circulation' flow (as a 'balancing' reaction). However at the leech, the speed from the top must be equal to the speed from the 'bottom' (kutta condition) or you dont get 'aerodynamic flow'.

You're losing me here. This is from the Kutta condition (part of the Kutta-Joukowski theorem).

"One of the consequences of the Kutta condition is that the airflow over the topside of the airfoil travels much faster than the airflow under the underside. A parcel of air which approaches the airfoil along the stagnation streamline is cleaved in two at the stagnation point, one half traveling over the topside and the other half traveling along the underside. The flow over the topside is so much faster than the flow along the underside that these two halves never meet again. They do not even re-join in the wake long after the airfoil has passed. This is sometimes known as "cleavage". There is a popular fallacy called the equal transit-time fallacy that claims the two halves rejoin at the trailing edge of the airfoil. This fallacy is in conflict with the phenomenon of cleavage that has been understood since Martin Kutta's discovery."

This is stating that the topside travels FASTER than the underside and arrives at the trailing edge BEFORE the air from the underside.

Is what you are stating happening when the starting vortex is still at the trailing edge? This would make sense since the vortex curls up towards the topside and flows backwards (did any body else do the bath tub experiment?) showing that the pressure from underneath is greater than the pressure on the topside.

However, as the speed across the topside increases, the starting vortex is stripped away from the trailing edge and the Kutta condition is in effect.

Am I on something of a right track?