I General expression of wind force on a sail....

[tech99]

It's not that lee helm is "not favoured", lee helm leads to potentially catastrophic positive feedback during a gust:

I understand this mechanism when sailing with lee helm, but another mechanism giving negative feedback is this:-
Boat turns to leeward.
Angle of attack of rudder is increased.
Boat turns to windward.
In a similar way, an aeroplane having CG ahead of CP, tail plane pushing down, is stable and behaves like this:-
Nose goes down
Angle of attack of tailplane increased
Nose comes up.
One difference I can see from the aeroplane case is that for the 'plane, weight is constant. For a sailing boat, the lateral force is variable.
I have tried by experiment to see if a weather helm or lee helm condition gives better course stability and the results have been so far uncertain.

 

[vinnie78]

As for understanding and calculating lift force. It is essentially a calculation of gravitational forces applied. An objects gravitational weight, must be equalized and overcome for lift to take place. Sails can be used in such a manner to assist in this. A sail tilted drastically could catch air and help raise the vessel. Usually however a hydrofoil has been designed with a large enough surface are to provide required lift on its own. The wind force can be focused as a propulsion force.

If added power is required. Rather than a traditional engine. An inlet hole on the forward hydrofoil can feed large gpm pumps. The pumps then can direct the flow through water outlets on the back of the rear hydrofoil. This powered propulsion acts like a jet boat.

However wind power is our current focus.

 

[DaveE]

As for understanding and calculating lift force. It is essentially a calculation of gravitational forces applied. An objects gravitational weight, must be equalized and overcome for lift to take place. Sails can be used in such a manner to assist in this. A sail tilted drastically could catch air and help raise the vessel. Usually however a hydrofoil has been designed with a large enough surface are to provide required lift on its own. The wind force can be focused as a propulsion force.

If added power is required. Rather than a traditional engine. An inlet hole on the forward hydrofoil can feed large gpm pumps. The pumps then can direct the flow through water outlets on the back of the rear hydrofoil. This powered propulsion acts like a jet boat.

However wind power is our current focus.

I thought lift was the force normal to a foil caused by mass flow over/around that foil, without regard to forces from other phenomena.

It isn't common in physics to define forces (and other things) and include multiple mechanisms, although that's what's necessary to solve most problems. For example, no one says Newton's laws of force are $\vec F = m \vec a + m \vec g$.

Gravity certainly isn't essential in calculating the lift from a fin keel when the boat isn't healing, yet there is a real hydrodynamic force there that most everyone calls "lift".

 

[vinnie78]

Well lift force is the force of air surrounding the foil for aviation. So if pressure force is measured as psi. Lift is calculated as Lift per square inch of surface area. The Lift needs to counteract gravitational weight for Lift of an object to occur.

 

[vinnie78]

There are a number of forces that are not recognized. Yet we deal with the reactionary result. Centrifugal force for example. It isn't an actual explainable force at the moment. Yet put an object in the middle of a spinning Marry go Round and you see the result.

In aviation the concept of lift changes. As a wing design changes. So does the value of lift change. When a design is chosen. Then the size required to provide proper lift can be made. Required Lift for a kite is next to nothing, compared to required lift for a 747. The variable beyond adjustable lift force. Is always the gravitational weight of the object. If gravitational weight isn't counteracted. An object sits in the wind rather than being able to fly in wind.

 

[vinnie78]

I believe your trying to understand lift in aviation better.

I will try to explain the principle.
So the wing design causes the wind flow to move over the top of the wing at a faster rate. Compared to the flow of air under the wing. The increased speed causes an are of lower pressure above the wing. Not being a closed system like in hydraulics this is the case. The reaction is the same as when you draw on a straw. You lower the ambient air pressure inside the straw. The greater ambient air pressure on the remaining fluid being greater. The fluid is pushed up into the straw for you to drink. With a wing the same effect is expressed. Lower pressure above the aircraft raises up and achieves lift.

If the lift value is greater than gravitational weight of the object.

 

[berkeman]

I will try to explain the principle.
So the wing design causes the wind flow to move over the top of the wing at a faster rate. Compared to the flow of air under the wing. The increased speed causes an are of lower pressure above the wing. Not being a closed system like in hydraulics this is the case. The reaction is the same as when you draw on a straw. You lower the ambient air pressure inside the straw. The greater ambient air pressure on the remaining fluid being greater. The fluid is pushed up into the straw for you to drink. With a wing the same effect is expressed. Lower pressure above the aircraft raises up and achieves lift.

https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

Discussion thread: https://www.physicsforums.com/threa...e-wing-work-a-primer-on-lift-comments.845786/

 

[vinnie78]

So your asking or saying?

Air density changes a wings lift value. I fly in colder more dense air, I get better lift and max elevation from my aircraft. Hotter less dense air, I get less lift and a lower max elevation. Overcoming oxygen requirements, should I fly in different environments such as another planet. Density of air will change my lift. The atomic weight of mass may change due to gravitational differences requiring more lift overall.

With hydrofoil lift.
A positive nose down attitude from rear wing just behind center of gravity is best. Rear wing carrying most of the weight. The forward wing will require a drastic angle of attack increase to counter act expected Stern over Bow effect.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

So your asking or saying?

My point was that how a wing produces lift is fairly complicated, and has been discussed at length here at PF. By giving you that reference I wanted to be sure that you had good information to go on in the future.

After some Moderation chores, the thread is reopened.

 

[cjl]

I believe your trying to understand lift in aviation better.

I will try to explain the principle.
So the wing design causes the wind flow to move over the top of the wing at a faster rate. Compared to the flow of air under the wing. The increased speed causes an are of lower pressure above the wing.

Yes... ish. This often leads to further misunderstandings though, such as the equal transit time assumption (which is just flat out wrong), or the belief that "bernoulli" and "Newton" are somehow two separate effects that compete to explain lift. In reality both are effectively conservation statements and in any (reasonably incompressible) flowfield, the bernoulli relation will hold everywhere around the wing, the lower pressure region on top will be flowing faster (in keeping with the bernoulli relation), and you can completely calculate the lift if you know the velocity field around the wing just by applying bernoulli and integrating the resulting pressure around the wing surface. However, you can also entirely calculate the lift by measuring downwash behind the wing and applying a Newton's 3rd relationship to relate the upward force on the wing to the downward momentum flux applied to the flow. These are not competing explanations, they are both just part of the overall physics, and, importantly, neither actually gives you a way to figure out what that lift or flowfield looks like in the first place.

Getting a more complete explanation of exactly why a wing directs flow downward effectively and creates a high velocity region above the wing requires discussion of viscosity, circulation around the wing (which is what that ring of arrows is showing in the diagram on the prior page, as was already correctly surmised), and the fact that a sharp trailing edge effectively enforces the location of the rear stagnation point in the flowfield.

This complexity and common confusion is a large part of why boneh3ad wrote that post years ago in the first place, so as to have an easy article to link when people ask about lift generation here.