Fin Keel vs. Full Keel

[Hiracer]

Originally Posted by keelbolts

Pitts are low powered? I've seen 'em fly & I assumed they must be fairly high powered.

I don't really know, but I didn't think they were jet powered and therefore not comparable to your example.

I don't think these symmetrically shaped wings rely solely on power to take flight. Otherwise, why even have the wings if they didn't produce lift? With zero lift, the wings would just add drag and decrease power. They have got to be there for a purpose and that purpose is lift, symmetrical or not.

[Hiracer]

Originally Posted by keelbolts

I think that's what I've been saying. I believe that here's a difference between an object creating lift & being lifted. Am I mistaken?

That's the difference between a rocket and an airplane. Rockets use power only and dont' have wings. If she has wings, she's getting lift from the wings, simple as that. Otherwise it makes zero sense to have wings.

[Hiracer]

I did a quick google search.

Seems some gliders can fly upsidedown.

Doesn't that prove the point? How else do you explain that? The upside down wing is producing lift. There is no other explanation beyond pure magic.

[bob perry]

Keel:
Yes, you are mistaken.
The symetrical keel by virtue of the angle of attack created by leeway does produce lift. This is not a matter of speculation. High performance keels tested in wind tunnels and can be optimized for lift over drag. Remember, no leeway means no lift. The angle of attack provided by the leeway angle allows the symetrical foil to develope lift.

Some racing boats have used twin bilge keels that use an asymetrical foil. This has been very effective but those keels must be able to be retracted into the hull. Having the wrong foil down for a given tack can be very detrimental to performance.

None of this is new or controversial. You might enjoy digging a little deeper and looking at the development of AC boat fin keels. They have played with a variety of thickness ratios and are critical to thickness due to their extreme high aspect ratio and short chord coupled with the tremendous loads put on the fin by the huge ballast bulbs they carry.

[keelbolts]

What we need here is an experiment. I believe, though I may be wrong, that if you put an asymmetrical wing in a wind tunnel an blow wind directly at it, it will try to rise. If you do the same with a symmetrical shape, it will not rise. One creates lift and that makes it rise. The other will not rise unless it is placed at an angle to the oncoming wind. Again, one creates lift, the other is lifted. It's a small difference in wording, a big difference if the plane you're in looses it's lift.

Almost anything will lift if you push on the bottom.






So let me wrap up the most important parts of what I've said or heard here.
Santa does the heavy lifting. The Easter Bunny lays the eggs.


You gents have a great weekend. Thanks for helping me exercise my brain. I'm outa here.

[bob perry]

Uhhhh,

Keel:
The leeway angle bot boat makes provides the angle to give the keel an "angle of attack". Just "pushing on the bottom" is probably how a lot of old keels worked and we call that lateral pressure. You get a lot of drag with that and that's why flat plate keels are so innefficient.

[Hiracer]

Originally Posted by keelbolts

one creates lift, the other is lifted. It's a small difference in wording.

There is no difference, other than the passive versus active verb. From a physics standpoint, they are both 'lift' albeit produced slightly differently.

More significantly both, symmetrical and asymmetrical wings, work better with high aspect forms. This comes full circle to full keel versus fin keel. The higher the aspect, the more 'lift' or 'lifted,' whichever verb you prefer, comes into play. This is why--symmetrical or asymmetical, it doesn't matter--you don't see full keel type wings on airplanes. The aspect is too low, the leading edge to short, and the lift too low. Airplanes of all types have long leading edges in order to get enough lift. You put a full keel type wing on an airplane, asymmetrical or symmetrical--it doesn't matter, and it will not fly for lack of lift.

[bob perry]

I think the fun of these discussions is that when they develope we all can learn from the views expressed. I find that the more I try to explain something the more I learn about it.

It's Friday. The sun has finally come out.
Now if I can just get those fish to bite.

Hi:
Isn't a flying wing the ultimate "full keel" plane?
I mean, it's all "keel".
How much fuller can it be?

[Hiracer]

Originally Posted by bob perry

Hi:
Isn't a flying wing the ultimate "full keel" plane?
I mean, it's all "keel".
How much fuller can it be?

Bob,

Go fish. You're killing me.

[cal40john]

Try this:

http://www.arvelgentry.com/magaz/How...Gives_Lift.pdf

He starts by showing how a flat plate generates lift.

He's a Boeing engineer, if he's wrong, don't fly.
He has some other interesting articles as well.

John

Originally Posted by keelbolts

What we need here is an experiment. I believe, though I may be wrong, that if you put an asymmetrical wing in a wind tunnel an blow wind directly at it, it will try to rise. If you do the same with a symmetrical shape, it will not rise. One creates lift and that makes it rise. The other will not rise unless it is placed at an angle to the oncoming wind. Again, one creates lift, the other is lifted. It's a small difference in wording, a big difference if the plane you're in looses it's lift.

Almost anything will lift if you push on the bottom.






So let me wrap up the most important parts of what I've said or heard here.
Santa does the heavy lifting. The Easter Bunny lays the eggs.


You gents have a great weekend. Thanks for helping me exercise my brain. I'm outa here.

[Hiracer]

" . . . and early man-made gliders, even with flat, uncambered wings also flew. "

Air foil shape clearly aids lift, but is not a prerequisite for lift.

[Hiracer]

More from Arvel Gentry:

"Both the sail and keel act like wings generating lift and drag, as explained by New Mathematical Theory of Lift, but the action, geometrical shape and angle of attack (aoa) of the sail and the keel are somewhat different. The effective aoa of a sail in tacking is 15-25 degrees and that of a keel 5-10 degrees, for reasons explained below. The aoa of the keel is also referred to as the leeway, the difference between the direction the boat is pointed and the actual direction of travel. "

* * *

"The heeling force from the sail is balanced by lift from the the keel in the opposite direction."

* * *

"Assuming that the effective speed relative the air of a sail is 10 m/s at aoa = 15 and the speed of the keel/boat through the water is 3 m/s at aoa = 5, we find (using that the density of water is about 800 times that of air) that the sail area can be up to 25 times the keel area. In practice, the ratio is typically 7-9 with a traditional full-keel and 10-15 with a standard modern fin-keel, while a modern Americas Cup boat reaches beyond 25."

* * *

The shape of a sail is different from that of a wing which gives smaller drag from the windward side and thus improved forward pull/drive, while the keel has the shape of a symmetric wing and acts like a wing. A sail with aoa = 15-20 degrees gives maximal pull/drive forward at high heeling/lift with contribution also from the rear part of the sail, like for a wing just before stall, while the drag is smaller than for a wing with L/D ~ 3 at aoa = 20, with the motivation given above.

* * *

The L/D curve for a sail is thus different from that of wing: at aoa = 15-20 L/D > 6-10 for a sail, while L/D < 3-4 for a wing. On the other hand, a keel with aoa = 5-10 degrees has L/D > 6-10. A sail at aoa = 15-20 thus gives maximal pull at strong heeling force and small drag, which together with a keel at aoa = 5-10 with strong lift and small drag, makes an efficient combination. This explains why modern designs combine a deep narrow keel acting efficiently for small aoa, with a broader sail acting efficiently at a larger aoa."

Why It Is Possible to Sail - a knol by Claes Johnson

[Dick Pluta]

Let's get back to basics. An asymmetrical airfoil in NOT necessary to provide lift. I once saw a model of Snoopys flying doghouse and it FLEW! When an airfoil of any shape, including a flat plate (or the floor of a doghouse), is presented at an angle of attack a turbulent low pressure area is created on the upper surface. High pressure on the bottom, low pressure on the top equals lift. Asymmetrical airfoils only provide more efficiency in the customary orientation. If you fly an asymmetrical airfoil upside down it is simply less efficient. There are thousands of aircraft from Pitts Specials to F15s flying with symmetrical airfoils because they need to spend a lot of their time in attitudes other than level right side up flight.

Like performance aircraft, keels need to work both "right side up" or "upside down" or more correctly on port and starboard. Conversely, look at the hulls on a Hobie cat. Each is a mirror image foil because the boat is designed to sail on one hull. For a Hobie it works. No keel, yet lots of lift. Unless you plan to fly a hull on your mono or to sail on only one tack you need a symmetrical keel. Does a symmetrical keel give the greatest lift possible? No. Does it give equal lift on both tacks? Yes. Just one more compromise.

One final comment. Like the keel, the sail is an airfoil. Unlike the hand out of the car example, the boat isn't blown by the wind. It is sucked (or lifted) upwind. That's why you can sail into the wind.

Dick Pluta
AEGEA

[bob perry]

There are countless boats with flat plate keels, Star class, Snipe class. For many years the standard centerboard was a flat plate with a bull nose leading edge and a pointy trailing edge. It does work. It does not work very well.

[s/v Jedi]

To the full keel freaks in this thread: look at the picture: this is Jedi and obviously with a shallow draft but fin keel. You also see her bow while you would normally only see her stern disappearing quickly over the horizon... even when going upwind, no matter how full your keel is! ;-)

ciao!
Nick.