From: http://www.djaerotech.com/dj_askjd/d.../wingfin1.html

A finite span wing in upright flight has more pressure on the bottom of
the wing than on top. Lift is always measured perpendicular to the local
airflow direction, and drag is always parallel to it. Once you know
these two bits of information, it's not hard to understand how winglets
work.

The higher pressure air under the wing wants to spill around the wingtip
to try to fill in the low pressure area on top. This flow results in a
tip vortex trailing aft from the wingtip, like a horizontal tornado. You
can see these vortices at the wingtips of a jet fighter during a high
lift maneuver in sufficiently humid air, or at the tips of an airliner's
flaps during a landing approach in wet weather. The energy extracted
continuously from the aircraft to make the air swirl like that is what
we call induced drag.

As you probably recall from our previous discussions of induced drag,
it's at its worst when we're trying to make lots of lift with relatively
little airflow. This means that slow flight (low speed, low mass flow,
high lift coefficient) is one of the worst cases. This also means that
the intensity of the tip vortices will be highest at these kinds of
flight conditions.

Now we need to talk about "helix angle". If you understand the pitch of
a prop, you're already familiar with it. Helix angle is one way to
measure how far something rotates compared to how far it travels forward
in the same time. The blade angle of a propeller blade is nearly the
same (minus its efficiency effects and local angle of attack) as its
helix angle. A wingtip vortex has a helix angle as well. This angle will
be nearly parallel to the airplane's direction of flight when induced
drag is low, but twist up into increasingly greater angles relative to
the flight direction as we slow down or pull more "G".

If we have a significant amount of induced drag, and a correspondingly
stronger tip vortex, then the flow at the wingtip will not be parallel
to it, but rather at an inward angle on top and an outward angle on the
bottom. This is where the winglets come in.

If we park a lifting surface in the middle of this angled air flow, it
will develop lift perpendicular to the angled air flow. The resulting
lift will be angled forward, and the forward component of that lift will
be producing thrust. The lifting surface (i.e.: "winglet") will also be
producing drag of its own, including both parasite and induced drag.

If the drag the winglet produces is less than the forward component of
its lift, then there will be a net thrust applied from the winglet to
the aircraft. This thrust actually represents some of the energy in the
tip vortex, harvested from the vortex by the winglet and given back to
the aircraft. That's it. That's all there is to it. It's so simple!

OK, now the catch. How do we maximize that thrust? This is where it gets
complicated. If you increase the angle of attack of the winglet by
increasing the "toe-in" angle, then it makes more lift force (which
should theoretically increase the forward component of that lift), but
it also makes more drag force. Depending on the specific situation, this
could increase, decrease, or not change the net thrust of the winglet.
It's going to depend on a lot of factors, including the flight condition.

This last item is particularly critical. Because the amount of induced
drag, and the helix angle of the vortex decrease as you increase
airspeed, the energy available for "harvesting" by the winglet decreases
as you fly faster. Meanwhile, the parasite drag of the winglet is
increasing. Eventually you get to a point where the total drag of the
winglet is equal to the forward component of its lift, and at that point
the winglet produces zero thrust. This is called the "crossover
velocity". At airspeeds higher than the crossover velocity, the winglet
adds to the aircraft's total drag, and you would be better off without it.


Cheers

Navvie wrote:

From: http://www.djaerotech.com/dj_askjd/d.../wingfin1.html

This is from a model airplane web site, and some of the terminology for
hydrodynamics is rather different. For one thing, we don't talk about the
"helix angle" of underwater foils. For another, attempting to get a net
forward (or windward) lift component from keel winglets isn't going to work
because they have to sail on both tacks.

If an airplane were designed to spend 50% of it's time flying upside down,
you'd see some very different wing section and winglet designs... in fact, you
can look at some stunt plane designs and see...

Fresh Breezes- Doug King

DSK wrote:

Navvie wrote:


From: http://www.djaerotech.com/dj_askjd/d.../wingfin1.html


This is from a model airplane web site, and some of the terminology for
hydrodynamics is rather different. For one thing, we don't talk about the
"helix angle" of underwater foils. For another, attempting to get a net
forward (or windward) lift component from keel winglets isn't going to work
because they have to sail on both tacks.


Yep, that's why they are on both sides.

Cheers

.... attempting to get a net
forward (or windward) lift component from keel winglets isn't going to work
because they have to sail on both tacks.

Nav wrote:
Yep, that's why they are on both sides.

So, are you saying that keel winglets *do* have a net windward lift, considered
seperately from their contribution to the keel's lift?

Fresh Breezes- Doug King

"Navvie" wrote in message
...
From: http://www.djaerotech.com/dj_askjd/d.../wingfin1.html

A finite span wing in upright flight has more pressure on the bottom of
the wing than on top. Lift is always measured perpendicular to the local
airflow direction, and drag is always parallel to it. Once you know
these two bits of information, it's not hard to understand how winglets
work.

The higher pressure air under the wing wants to spill around the wingtip
to try to fill in the low pressure area on top. This flow results in a
tip vortex trailing aft from the wingtip, like a horizontal tornado. You
can see these vortices at the wingtips of a jet fighter during a high
lift maneuver in sufficiently humid air,

How can you see this in New Zealand? They don't have any fighter aircraft.
Plus you can see it in very dry air, like all the time at Davis AFB is
Tuscon.

or at the tips of an airliner's
flaps during a landing approach in wet weather. The energy extracted
continuously from the aircraft to make the air swirl like that is what
we call induced drag.

As you probably recall from our previous discussions of induced drag,
it's at its worst when we're trying to make lots of lift with relatively
little airflow. This means that slow flight (low speed, low mass flow,
high lift coefficient) is one of the worst cases. This also means that
the intensity of the tip vortices will be highest at these kinds of
flight conditions.

What happens when , at low airspeed, one injects air into the airflow?


Now we need to talk about "helix angle". If you understand the pitch of
a prop, you're already familiar with it. Helix angle is one way to
measure how far something rotates compared to how far it travels forward
in the same time. The blade angle of a propeller blade is nearly the
same (minus its efficiency effects and local angle of attack) as its
helix angle. A wingtip vortex has a helix angle as well. This angle will
be nearly parallel to the airplane's direction of flight when induced
drag is low, but twist up into increasingly greater angles relative to
the flight direction as we slow down or pull more "G".

If we have a significant amount of induced drag, and a correspondingly
stronger tip vortex, then the flow at the wingtip will not be parallel
to it, but rather at an inward angle on top and an outward angle on the
bottom. This is where the winglets come in.

If we park a lifting surface in the middle of this angled air flow, it
will develop lift perpendicular to the angled air flow. The resulting
lift will be angled forward, and the forward component of that lift will
be producing thrust. The lifting surface (i.e.: "winglet") will also be
producing drag of its own, including both parasite and induced drag.

If the drag the winglet produces is less than the forward component of
its lift, then there will be a net thrust applied from the winglet to
the aircraft. This thrust actually represents some of the energy in the
tip vortex, harvested from the vortex by the winglet and given back to
the aircraft. That's it. That's all there is to it. It's so simple!

OK, now the catch. How do we maximize that thrust? This is where it gets
complicated. If you increase the angle of attack of the winglet by
increasing the "toe-in" angle, then it makes more lift force (which
should theoretically increase the forward component of that lift), but
it also makes more drag force. Depending on the specific situation, this
could increase, decrease, or not change the net thrust of the winglet.
It's going to depend on a lot of factors, including the flight condition.

This last item is particularly critical. Because the amount of induced
drag, and the helix angle of the vortex decrease as you increase
airspeed, the energy available for "harvesting" by the winglet decreases
as you fly faster. Meanwhile, the parasite drag of the winglet is
increasing. Eventually you get to a point where the total drag of the
winglet is equal to the forward component of its lift, and at that point
the winglet produces zero thrust. This is called the "crossover
velocity". At airspeeds higher than the crossover velocity, the winglet
adds to the aircraft's total drag, and you would be better off without it.


Cheers

If set up with dihedral or when heeled, yes they can -but its small as
their area is small.

Cheers

DSK wrote:

.... attempting to get a net
forward (or windward) lift component from keel winglets isn't going to work
because they have to sail on both tacks.

Nav wrote:
Yep, that's why they are on both sides.


So, are you saying that keel winglets *do* have a net windward lift, considered
seperately from their contribution to the keel's lift?

Fresh Breezes- Doug King

Oz wrote:

On Mon, 02 Feb 2004 11:27:49 +1300, Nav
scribbled thusly:



DSK wrote:


Navvie wrote:



From: http://www.djaerotech.com/dj_askjd/d.../wingfin1.html


This is from a model airplane web site, and some of the terminology for
hydrodynamics is rather different. For one thing, we don't talk about the
"helix angle" of underwater foils. For another, attempting to get a net
forward (or windward) lift component from keel winglets isn't going to work
because they have to sail on both tacks.



Yep, that's why they are on both sides.

Cheers


Which chucks you whole spiel out the window.
Ever see a jet with winglets that extend to the lower side of the
wing?



Never flown inverted on a 747-400 no. Come to that I've never flown on
any inverted jet, but if they were flying both sides up with equal
frequency you can bet there would be winglets on top and bottom... It
really isn't all about the end plate effect you know. Look at AC
winglets -its all about controlling the parasitic drag from the keel tip.

OK?

Cheers

The Professor wrote:


What happens when , at low airspeed, one injects air into the airflow?


Depends on injection flow rate and position doesn't it? If it's a fart
people may complain.

Cheers

Nav wrote:
If set up with dihedral or when heeled, yes they can -but its small as
their area is small.

I'm not saying you're wrong, but the wing keel designers I've talked
with have never mentioned it. First of all the winglet is mostly
horizontal. Here is where Jax's favorite, the 'sine function' comes
in... if the winglets produce a net lift, then it is going to be mostly
up or down unless the dihedral and/or heel angle gets very large, like
greater than 45 degrees.

When you get done emailing Phil Bolger and the SAYRA race officials, you
can email Evan Gatehouse, Stephen Baker, and Paul Kamen to ask them
about it. They all hang out on other sailing forums and are quite
knowledgable about this sort of thing.

Of course, one of the most important functions of a wing keel, which you
forgot to mention, is that it gets more ballast weight down low. A model
airplane designer would not be concerned with that, though

Fresh Breezes- Doug King

Sorry,

But the wings are an AC keel are not intended to function of an aircraft
winglet. The function of the aircraft application has been effectively
described here, the the wings on a keel have a primary function that is
different. The tip vortices are pretty much confounded buy the huge
bulb (much like an aircraft tip tank will do).

The wings are actually designed to provide additional lift to windward
that is not dependent on the keel blade attack angle.

That is why they pay so much attention to the attack angle of the wings
during the set up and trials. They have to play out a dual between more
angle for more lift to weather when heeled and less angle for less drag
with the boat upright going to leeward.

As the current IACC rules have not allowed this to be trimed underway,
they only get to set it once per race, and if you listen to the
discussions going on the background, you will here them talk about the
adjustment as three-quarters or one-half degree.

Matt Colie (certifications available on request)

Nav wrote:



Oz wrote:

On Mon, 02 Feb 2004 11:27:49 +1300, Nav
scribbled thusly:



DSK wrote:


Navvie wrote:



From: http://www.djaerotech.com/dj_askjd/d.../wingfin1.html



This is from a model airplane web site, and some of the terminology for
hydrodynamics is rather different. For one thing, we don't talk
about the
"helix angle" of underwater foils. For another, attempting to get a net
forward (or windward) lift component from keel winglets isn't going
to work
because they have to sail on both tacks.



Yep, that's why they are on both sides.

Cheers



Which chucks you whole spiel out the window.
Ever see a jet with winglets that extend to the lower side of the
wing?



Never flown inverted on a 747-400 no. Come to that I've never flown on
any inverted jet, but if they were flying both sides up with equal
frequency you can bet there would be winglets on top and bottom... It
really isn't all about the end plate effect you know. Look at AC
winglets -its all about controlling the parasitic drag from the keel tip.

OK?

Cheers