How Wings, Sails, Keels, and Rudders Really Work.

[Hermit Scott]

Roger(referring to 14 or so posts ago) If you do a free body diagram of a chain you do add the forces up, it's just that every other one is negative.

All wings have a hump on top that is forward on the wing, if the wing is a piece of plywood tilted up the 'hump' is the leading edge of the plywood. This hump causes a pressure difference.
tsm web have you ever seen a wing where the hump is in the back?
My observation is it's the pressure difference that counts not the mass transfer.

 

[Hermit Scott]

Airplane wings don't create thrust f=m*v^2, they create lift by a pressure differential.
There is a localized low pressure above the wing. If you could move the air above the wing upward to create the low pressure volume of air above the wing it would lift all the same. That the air goes slightly downward behind the moving wind is completely incidental to the pressure difference. The air can move sideways, up, down it doesn't matter as long as it gets out of the way of the top of the wing.

 

[Todd Smith]

Another note, we have gone way too far into fluid dynamics to be able to discus these topics easily and clearly on a web forum. In person we could use paper and pen to illustrate our points, but it's just too hard to explain in words alone. Also we haven't really constrained our conversation to just 2D airfoil theory, but have expanded to 3D wing theory. You really have to finish the 2D conversation before moving on to 3D.

So I'm gonna quit for now, before I become this guy:

We have been civil though

Todd

 

[Hermit Scott]

I love it.

 

[Roger Long]

I've spent a bit of time looking at section data.

That's why you mostly see airfoils with curved bottoms. There are good reasons for them to have curvature on the bottom.

It would have been better for me to show a more typical airfoil in the original animation. Since that part was schematic, it was simply drawing convienience. If I can find the original CAD files, maybe I'll update it. It won't change the flow patterns in the drawing although it would change them near the lower leading edge at a finer resolution than the animation.

If you look closely through those section books, you'll see that some sections have a small residual amount of lift at zero AOA. That's because the measured AOA is based on a geometric convention that can not account for the flow around all variations of wing shape.
The difference between AOA and Effective AOA, at least in this crude discussion, is that little bit between 0 and the angle where Cl = 0 on those sections.

Adding to the confusion is that most wings have some twist for stall control reasons and the 3 dimensional aspects of the circulation that ends up as a tip vortex causes actual AOA to vary even more from the geometric definition along the wing.

 

[tsmwebb]

tsm web have you ever seen a wing where the hump is in the back?
My observation is it's the pressure difference that counts not the mass transfer.

Define back, define hump . All really good laminar sections have max thickness aft. Basically foil sections make lift while they get thicker and recover energy as they get thinner. The larger the percent of the foil that can be dedicated to creating lift compared to that that recovers energy the more efficient the section gets as a whole. So, "good" foils carry their thickness aft. The NACA 67(sub1)1-215 is an example of a section that is widest aft of 50%. The problem I see here is that we have gotten a long way from what lift is to specific ways of creating lift. I'm not sure how helpful that is. For us foil builders there are cook-book answers and for real (tm) engineers there are lots of equations and simulations and fluid tunnels describing how much lift you might get from some object. I think it bears repeating that may be created by airfoils but lots of other things create it, too.

--Tom.

 

[Rick Webb]

I Thought it Would Go to All of This

A long time ago I was taught you need to know only this to fly an airplane.

Areodynamics;

There are:

gravities
lifties
thrusties
dragies

To go faster there have to be more thrusties than dragies.
To go higher there have to be more lifties than gravities.
Dragies do not like the thrusties or the lifties and gang up on them whenever you get more of them.

Weather;

Do not fly into clouds, some of them are full of rocks.

That was it for Ground School.

Once in the air, if you pull on the stick the sky gets bigger and the ground smaller, and if you push in on the stick the sky gets smaller and the ground gets biger. Keep them the same size.

The rest of it is clutter than occupies space in the head that could be used for somthing useful.


Y'all have fun I am going sailing.

 

[TimCup]

Okay, Mr Herring-

First, thanks for putting the previous topic/next topic on top AND bottom of each thread, as I asked months ago. It saved me from alot of brain damage today. Roger, I tried. I really did. But I'm never gonna be an engineer, and if you'll just tell me I can sail well without all that info, I'll live blissful in my ignorance!

cup

ps- And Phil, MY thread was moved to the lounge?!?

 

[Phil Herring]

Re: Okay, Mr Herring-

Cup -

Happy holidays to you! What thread was moved?

 

[tsmwebb]

If you look closely through those section books, you'll see that some sections have a small residual amount of lift at zero AOA. That's because the measured AOA is based on a geometric convention that can not account for the flow around all variations of wing shape.
The difference between AOA and Effective AOA, at least in this crude discussion, is that little bit between 0 and the angle where Cl = 0 on those sections.

I don't mean to be snarky but everybody defines AOA w/respect to cord. Many foil sections that are intended to be used as wing sections also have data with flaps down. In that condition it is typical for them to develop a substantial percentage of their max lift at zero AOA w/respect to cord. Are you suggesting we use different definition of AOA for each change in CL curve? As I keep saying, I'm with you up 'till that last paragraph. I just don't see what it brings to the table. How does it help explain lift? And, the only way I think you can make it work as an argument is to introduce a not very useful and totally unique definition of AOA...

--Tom.

 

[Hermit Scott]

NACA 67(sub1)1-215 What is that? Just post a pic of it if there is one available. Google can't even show images of what that is.

Hump(scottapedia)- the part of the wing that is farthest from the ground when flying level.
Level(scottapedia)-not changing altitude or gravitaional potential
Back(scottapedia)the other side from the front,how do you not know what the back is?lol example, if you were standing on top of a tall building and you got hit by an airplane wing flying level, you would be hit with the front of the wing. If you were standing on a tall building and a plane was backing up and it's wing hit you, you would be hit with the back of the wing. Also the hump on a camel's back is pretty much in the middle if the camel's back IF the hump was moved more toward where poop comes out, I would say that the hump has been moved back. Are there any wings where the hump is moved toward the back?

 

[tsmwebb]

NACA 67(sub1)1-215 What is that? Just post a pic of it if there is one available. Google can't even show images of what that is.

Hump(scottapedia)- the part of the wing that is farthest from the ground when flying level.
Level(scottapedia)-not changing altitude or gravitaional potential
Back(scottapedia)the other side from the front,how do you not know what the back is?lol example, if you were standing on top of a tall building and you got hit by an airplane wing flying level, you would be hit with the front of the wing. If you were standing on a tall building and a plane was backing up and it's wing hit you, you would be hit with the back of the wing. Also the hump on a camel's back is pretty much in the middle if the camel's back IF the hump was moved more toward where poop comes out, I would say that the hump has been moved back. Are there any wings where the hump is moved toward the back?

LOL!

I'm sitting in the pilot house of my boat in La Paz, Mexico with a junky 'net connection so, I'm not going to scan any pictures and upload them for you. So there But, here it is on the web (I hope):
http://books.google.com/books?id=DP...ng+sections&ei=RnJVSe2rEZSyMNSZqd8F#PPA627,M1

Yes, there are wings (and props) where the hump is toward the back.

--Tom.

 

[tsmwebb]

http://books.google.com/books?id=DP...ng+sections&ei=RnJVSe2rEZSyMNSZqd8F#PPA627,M1

--Tom.

Woops I don't think that url worked. Anyway, if you scroll around there you should find it. It's on p 683 in my copy.

And to take care of other stuff: Roger, I think you've been editing! I'm ok with the last two pp of your site now (I bet you'll sleep better knowing that )-- though I still am not quite sure what it brings to the party.

--Tom.

 

[ITMaster]

Interesting post for me Roger. The easiest way for me to understand it, without getting involved with Rheological properties of Newtonian and non-newtonian, dilatent and psuedoplastic fluids and shear and stress and viscosities of fluids is to get into the cockpit of the 172 and start climbing until I do a full power on stall. Then and only then, do I get a real clear understanding of the concepts of lift and angle of attack .
Thanks so much for an interesting discussion and for helping to bring some awareness of what makes our boats move .

 

[CalebD]

Thanks so much for an interesting discussion and for helping to bring some awareness of what makes our boats move .

I'm pretty sure we all can agree that it is wind (sometimes hot air) that makes our boats move! Also, on non-windy days it is some kind of fuel that makes our boats move! The AOA thing is pretty intuitive regarding the sails or your hand riding the air outside of a moving car.
It has been interesting to watch the scientific community here discuss the formulas and physics while still remaining civil to one another (I would have expected no less decorum from this brain trust).

Happy Holidays (belated or not) to all.

 

[Roger Long]

I'm pretty sure we all can agree that it is wind (sometimes hot air) that makes our boats move!

Well, that's the nice thing about sailing. When I had these discussions back when I was flying, the question was asked, "What gets a plane up in the air and keeps it there?"

Answer: MONEY!

 

[Bill Roosa]

Lift or side force from a keel

So after reading all the above I'm left wondering; is this "lift" the keel producing, and presumably the rudder also, going to make my boat go faster or just keep me from heeling a lot?
I was under the impression that given a symmetrical foil and low angles of attack that the lift was perpendicular to the cord. If that is true then the "lift" vector has no forward (or back) component and does not help to accelerate the boat. That is left up to the sails.
I can't change the keel but I can change the sails so hows about a discussion about how to maximize the forward force my sails make so I can blow some hatch covers off those other boats.
I have found that understanding how all the forces (rudder, keel, jib, and main) both in the direction of travel and athwart ships key to making my boat move fast when others are aslant.

 

[Todd Smith]

Re: Lift or side force from a keel

Well, lift is defined as 90 degrees to the flow direction, not chord line. So lift on the keel might make it go faster, but just a little since the angle of leeway is small. And lift on the keel makes the boat heel more. But it prevents the boat from sliding sideways, which is the keel's primary purpose other that as a place to put the ballast.

Likewise the sail's "lift" is 90 degrees to the apparent wind direction and the "drag" is inline with the apparent wind. I have this neat little drawing on my pad here, but it's not on the internet.

When on a beam reach the sail's lift is all drive forward and the drag heels the boat. Close hauled, the lift is more sideways than forward and makes the boat heel a lot.

Generally a sail's lift is greater than it's drag.

Go faster by maximizing lift with minimal drag, there's an entire forum devoted to that on this board.

Todd

 

[Hermit Scott]

The best thing I am taking from this thread is the 'are you coming to bed?' cartoon.

Secondly, I am glad to hear about the air NOT meeting back up on the back(camel poop) side of the wing. That is interesting that my calculus based physics book and fluid dynamics books were wrong. I wonder if the patched elbow sport coat wearers will ever correct that.

 

[Todd Smith]

Where and when did you study those fluid dynamics book ? Because all the ones I ever read had it right.

Todd