Loop Keel

[Roger Long]

Re: Foils and lift

Spoken like a true engineer. You can CALCULATE lift without considering the displacement but you can't HAVE it. You can't have a pressure differential without creating flow in a free system and you can't have flow near an object without creating pressure differentials.

You say "practically NO displacement" for great spans. That is still some and the span makes up for the small amount.

As soon as you bring "cause" into these lift discussions, they start to go off track. That's where the "Eggs don't cause chickens." quote comes from.

 

[Hermit Scott]

Todd, I would have to say that your statement one is somewhat in error, or airplane wings would be shaped a different way if the shape didn't help produce lift.
Air foils that aren't shaped like airfoils are terribly inefficient.
They taught us in fluid dynamics that the shape and angle of attack affect lift. However they also told us that the particles meet aft of the wing. That can be proven one would think just by observation. Why would all the text books claim that the particle meet back?
Saildog, That makes sense about the boat not travelling straight in the water, causing the angle of attack to push the boat into the wind. I didn't know sailboats travelled cocked at an angle while under sail, since I have never sailed before.
There were a few things I couldn't accept in the physics book though. They taught us that gravity acted over an endless distance. I don't believe that. Another thing I laughed at was that my cardiovascular system happened by accident. lol
I had to add this, this one is hilarious...that all the oil we pull out of the ground is made of dinosaurs. Yeah, right!

 

[DanW9494]

Sort of like twin keels

Todd, I read your link. Section 3.8 explains my points.
The loop keel is about 3 times longer than a typical keel. It has to be narrow to avoid wetted surface area. It has to prevent stalling despite a complex shape, since that is a BIG drag.
To get maximum lift from a long, narrow foil with good stall characteristics and low drag, you think sailplane, as in asymmetric foils.
Multihulls use asymmetric foils for their hulls, centerboards and daggerboards because the angle of attack is better for windward lift than the boat's long axis angle of attack.
The leeward side of the hoop keel loses leeward lift while the windward side of the keel is increasing windward lift when the boat starts to heel. Thus it gives dynamic stability with a ridgid frame, no moving parts to fail. I don't think it works as well as the link I posted above which has a sliding symmetric foil that can extend to leeward and provides lift by it's angle of attack.

 

[Todd Smith]

airfoil shape

Todd, I would have to say that your statement one is somewhat in error, or airplane wings would be shaped a different way if the shape didn't help produce lift.
Air foils that aren't shaped like airfoils are terribly inefficient.
They taught us in fluid dynamics that the shape and angle of attack affect lift.

I was trying to point out the incorrectness of the idea that it is the asymmetric shape of the airfoil that causes lift. A symmetric airfoil or a flat board can both produce lift.

The shape affects how much lift the wing can produce and how much drag gets created.

However they also told us that the particles meet aft of the wing. That can be dis-proven one would think just by observation. Why would all the text books claim that the particle meet back?

There, I fixed that for you

No text book used in an aerospace college will say that the particles meet back up at the trailing edge. That's a story told by people who don't know better or don't care.

Forgive me if I miss read the subtle sarcasm in your post.

Todd Smith

 

[Ed Schenck]

This is an interesting thread. Kind of old news(see link) but still interesting. I'm more a software engineer(retired) but sail and used to fly. So I am really enjoying the discussion. But someone will have to explain one theory that keeps popping up here. The one about "Eggs don't cause chickens".


http://www.boatdesign.net/forums/sailboats/radically-different-yacht-keel-loop-keel-16265-4.html

 

[Roger Long]

But someone will have to explain one theory that keeps popping up here. The one about "Eggs don't cause chickens".

Internet lift discussions usually degenerate into this argument, "Is it Bernoulli (the difference in pressures due to varying flow velocities) or Newton (the "downwash" of air)?" People line up on each side and argue for their favorite dead white male. It's silly. The two are inextricably linked aspects of the same overall circulation field and the object that created it. Arguing which "causes" lift is like the proverbial question, "Which comes first the chicken or the egg?

It's vaguely true to say that eggs cause chickens but it's grossly misleading explanation of poultry.

 

[Hermit Scott]

Todd, We just grazed the surface of airfoils and lift in fluid dynamics. There was no sarcasm in my statements, I really don't know anything here for sure. I did mean by observation one would be able to tell what's going on with air flowing over a wing, I wasn't biasing that statement one way or the other.
My professors were wrong about alot. But airplane wings lift planes off the ground whether the engineers believed the particles met or not. Engineers and scientists are better at quantifying things than explaining them. Like I have said before scientists can absolutely quantify gravity but still have no idea what causes it.

Higgs, do you have any pictures of your Nassau 34 up on a website anywhere. I like the look of your boat , but i couldn't find ANY pics of it on the internet.

 

[Rick Webb]

OK, Now I finally Understand How the Lifties Work

One of you smart guys explain ground effect to me.

 

[Roger Long]

One of you smart guys explain ground effect to me.

The circulation field (air moved by the passage of the wing) extends a surprising distance above and below it. The presence of the ground alters the circulation because air can't flow through the ground. The flow when near the ground has the effect of increasing the angle of attack, and thus lift, without the pilot having to change the pitch angle of the aircraft. This increase in angle of attack is also not accompanied by as much increase in drag as it would have been it the pilot had raised the nose. The effect is that it feels like the plane had flown into a cushion of air or suddenly lost some weight.

Wings are much easier to understand when you visualize the air as being still, as when an airplane is flying on a calm day, and the wing moving though them. When you re-animate tracings of wind tunnel photographs as I did for an article I wrote, you see that the the air is actually moved forward under the wing briefly. This seems impossible at first blush but it's just a difference in flow velocity from the perspective of someone in the aircraft.

As a wing goes by, the air is moved forward below it, upwards in front of it, and back along the top. Angle of attack is essential to this. The differences in flow velocity result, through the Bernoulii effect, in the pressure differences that are the sole focus of many popular discussions of the subject. These air movements are brief, think of the way a boat carries wake waves along with it even though the water doesn't move along with the boat. The net effect of the air movement is downwards accelleration of a mass, the air, which exactly equals (at least in straight line flight) the weight of the aircraft. The corresponding pressure differences on the wings are also equal to the weight of the aircraft. Neither the air movement nor the pressure differences can exist without the other in the case of an object in free fluid.

Later: I found the circulation animation file. Here it is. You need to click to the full size to see it in action. You'll understand wings and sails a lot better if you watch this for a while. It was made from tracings actual wind tunnel photos that had pulsed smoke streamers so the frames could be moved to show the wing as it it were moving through still air.

More complete explanation now available here:

http://home.roadrunner.com/~rlma/Circulation.htm

 

[Todd Smith]

Todd, We just grazed the surface of airfoils and lift in fluid dynamics. There was no sarcasm in my statements, I really don't know anything here for sure. I did mean by observation one would be able to tell what's going on with air flowing over a wing, I wasn't biasing that statement one way or the other.
My professors were wrong about alot. But airplane wings lift planes off the ground whether the engineers believed the particles met or not. Engineers and scientists are better at quantifying things than explaining them. Like I have said before scientists can absolutely quantify gravity but still have no idea what causes it.

I wasn't sure if you were biasing the result one way or the other. Or how in depth you had studied airfoils and fluids.

To speak to quantifying rather than explaining, I take a utilitarian view. If I can understand a phenomenon in a way that allows me to use if for my needs (either design or piloting or trimming sails), then I don't really need to understand "cause". I'm in Roger's camp that "eggs don't cause chickens". Except on this concept of air displacement

Teachers often teach things that they themselves don't understand fully. Sometimes the teachers do understand, but don't think they can teach the real explanation, so they simplify the lesson to give their students a partial understanding.

Merry Christmas everyone, too bad the snow is melting
Todd

 

[tsmwebb]

...
1) airfoils ( wings, keels and sails) produce lift because of Angle-of-Attack
NOT because of shape. ...

Many foil sections produce lift at 0 angle of attack (eg all NACA 6#-n## foils where n >0). I think it is unfortunate that so many discussions of lift are illustrated by streamlines around a foil section. First, it tends to make people forget about plan form which is very important and second, it implies that special sections are needed to create lift. Lift is developed by everything from hatchet shaped super-cavitating propeller blades and wrinkly swimmers hands to glider wings and space shuttles... Lift is used in nature by a great many creatures that move in a fluid from mahi mahi to mosquitoes to maple tree seeds. Lift can be developed using spheres and cylinders and plates and prisms. Sails, which are essentially 2d, develop lift. Lift is a force that is perpendicular to a stream and drag is parallel to a stream. Lift and drag can be created in lots of ways and described in lots of ways. It seems to me that most discussions of what lift is pretty quickly turn into discussions of ways of developing lift. Many folks then confuse the methods with the force. So, just because airplane wings develop lift it doesn't follow that you need an airplane wing to create lift. A maple seed will do just fine.

--Tom.

 

[Rick Webb]

Ok Two More Questions

Why is ground effect more pronounced in rotory wing aircraft than in fixed wing?

If you need a airfoiil shape to create lift how does an airplane fly upside down?

 

[Todd Smith]

Shape

Many foil sections produce lift at 0 angle of attack

Sorta true, depending on how you define the 0 angle of attach point. But increase or decrease the angle of attack and the lift goes up or down.

I think it is unfortunate that so many discussions of lift are illustrated by streamlines around a foil section. First, it tends to make people forget about plan form which is very important and second, it implies that special sections are needed to create lift. Lift is developed by everything from hatchet shaped super-cavitating propeller blades and wrinkly swimmers hands to glider wings and space shuttles... Lift is used in nature by a great many creatures that move in a fluid from mahi mahi to mosquitoes to maple tree seeds. Lift can be developed using spheres and cylinders and plates and prisms. Sails, which are essentially 2d, develop lift. Lift is a force that is perpendicular to a stream and drag is parallel to a stream. Lift and drag can be created in lots of ways and described in lots of ways. It seems to me that most discussions of what lift is pretty quickly turn into discussions of ways of developing lift. Many folks then confuse the methods with the force. So, just because airplane wings develop lift it doesn't follow that you need an airplane wing to create lift. A maple seed will do just fine.

--Tom.

The shape can be a lot of different things, that is true.

But understanding lift does not require consideration of plan form, designing a wing, keel or sail does.

Todd

 

[Todd Smith]

Ground effect

Why is ground effect more pronounced in rotory wing aircraft than in fixed wing?

This is hard to explain in a short post, but I'll try.

You can split the power required to fly an aircraft into 2 parts.
Induced power, required to create the lift
Profile power, required to pull the aircraft through the air

Ground effect reduces the required induced power.

A helicopter in hover requires a lot of induced power and little profile power, so ground effect greatly reduces the total power needed.

An airplane flying at landing speed requires about the same amount of profile and induced power, so the ground effect is smaller as a percentage of total power required.

That's a quick summary off the top of my hat, so please forbear any in-accuracies. It's also a gross simplification. But the basic aerodynamics are the same for airplanes and helicopters.

If you need a airfoiil shape to create lift how does an airplane fly upside down?

You don't need an airfoil shape, see earlier posts.

Todd

 

[alangbaker]

Todd, I would have to say that your statement one is somewhat in error, or airplane wings would be shaped a different way if the shape didn't help produce lift.
Air foils that aren't shaped like airfoils are terribly inefficient.
They taught us in fluid dynamics that the shape and angle of attack affect lift. However they also told us that the particles meet aft of the wing. That can be proven one would think just by observation. Why would all the text books claim that the particle meet back?

Not all the text books claim that, and it is easily observable that the particles don't meet up at the trailing edge.

There were a few things I couldn't accept in the physics book though. They taught us that gravity acted over an endless distance. I don't believe that. Another thing I laughed at was that my cardiovascular system happened by accident. lol

Sadly for you, reality doesn't care what you accept or don't accept.

 

[All U Get]

Boys,
To keep it simple, my granddaughter's kite will fly if grandpa runs fast enough. If this loop will fly in kite form, that doesn't make it a good idea. With all the debris floating around I would think much of it would be trailing from the bottom of the boat. I at first thought it followed the long axis, but wow. Every Mainer probably has a good spot for a demo, I'd use Port Clyde with lobsta toggles and bouys.
All U Get

 

[Hermit Scott]

Alangbaker, I am proud to be the focus of your first post. I suspect you were lurking for some time but could not resist posting something when you disagreed with me. At least give me a chance to defend my postition. Almost all things that were once considered final, definitive answers to questions become an incomplete or limited explination of things. For instance Newton's La Principia in view of Einstein. Recently, 5 years or so ago, star systems and galaxies were reported by astronomers as not only drifting apart (as previously thought) in space but accelerating away from each other.
Einstein had to add a 'constant' to his equations to make them come out right, because he believed gravity acted over an infinite distance slowing the universe down from expanding. It amazes me how much faith people put into the science of today, when the science of tomorrow tells a different story.

 

[Todd Smith]

For anyone wanting more details on this, I found the designer's web site.

http://www.hmtmarine.com/keel/overview/index.php

Personally, I think it will never work in the real world, even if it might be theoretically possible. The angles and loads would have to be just right and would only be perfect for 1 sailing condition.

Todd

 

[NotCook]

So the Cessna has a slower stall speed and can use a smaller engine because the wing has more lift for the same size wing.

That explains why when I fly the Cessna upside down, it doesn't climb as well!


I understand the design of the loop keel evolved from a design prototype for a crab pot retrieval system. They found the boats were faster and more stable, so they developed another market for it...


Yes, I am kidding - so don't sue me!

 

[newly anonymous]

grounding

I try to go aground at least once a year, just to keep in practice. My last boat had a bulb keel, which was twice as difficult to extricate from our soft bay mud.
As I see it, a loop keel is always going to have to be a bulb keel, to which I respond, "No thanks."

And it's going to be a whole lot of fun on the Pacific coast when you pick up a piece of kelp on that thing.