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

[Roger Long]

As promised down in the thread on the loop keel, here is a a summary of a couple articles I wrote for an aviation magazine years ago and some animations that should make understanding aero and hydrodynamic lift easier.

http://www.rogerlongboats.com/Circulation.htm

 

[Hermit Scott]

Oh, I see it now. There is a pressure difference above and below the wing, and THAT causes the wing to produce lift. JJ

Do you have a simulation that shows a wing moving through air the whole time, instead of a thin slice? There seems to be a conservation of mass equation missing in your theory, because it looks like what you are saying is (FORCEupward)=(FORCEcreatedbypressuredifferential)+(FORCEcausedby (Mass of the air X Acceleration) F=F1+F2
But from your own statements above ' the average of pressure differences above and below the wing multiplied by the wing area equal the weight of the airplane' This statement implies the equation looks like F=F1

What do you propose the 'real' lift equation looks like?

 

[Ross]

Roger, This would seem to me to indicate that the wing rides on a self induce wave. This helps me to understand how kites generate lift. I often watch birds soaring on terrain created air currents with a vertical component. The bird can seemingly "hang" in the air with slight trim adjustments for several minutes at a stretch. Watching a large bird glide across calm water on a landing approach you can see the air turbulance disturbing the water surface.

 

[Roger Long]

The pressure difference doesn't cause lift any more than eggs cause chickens. It's something that must take place in a system that is creating lift just as eggs must take place in a poultry farm.

It seems to you that I'm double counting the forces, but I'm not. The force on each link of a chain is the same but you aren't supposed to add them all up.

Ultimately, the "real" lift equation would be the same as same as a rocket engine, M x V squared = M x V squared but the vectors and interactions are so complex that that simplification isn't usable. In a similar way, the pressures on the nozzle and combustion chamber of a rocket engine equal the thrust. For engineering purposes, you can pretend that those pressures are the thrust but the rocket isn't going to go anywhere unless stuff is flying out the back.

Although a simple and crude explanation, and therefore wrong and incomplete in many ways, the original article was vetted by aeronautics professors and used by at least one in his college classes.

Consider this: If you could magically create those pressure differentials without flow, why would the plane stay aloft? Why wouldn't it just pump air upwards, from high pressure to low, and sink faster?

 

[Todd Smith]

Roger,

I just read your article and would agree with it as an explanation to a layman. As an engineer, I feel that the only real explanation of lift is the complete equations, such as:
http://en.wikipedia.org/wiki/Kutta–Joukowski_theorem, just for starters.

Also, there generally is a definition of an "Effective Angle of Attack", that you reference.

And finally, these "explanations" never translate into useful information for pilots or sailors, who need to understand the practical relationships between angle-of-attack, curvature, stall, lift and drag to fly their airplanes or to trim their sails.

Todd

 

[tsmwebb]

What do you propose the 'real' lift equation looks like?

Lift = 1/2 * mass density of fluid * V^2 * Area * Lift Coefficient

As Roger says these discussions often get silly fast. What is lift is an easy question. Why is lift is a hard one. I don't want to get into a shouting match over this. The engineering of wings is well documented and for most of us who just occasionally make a foil a quick glance at Abott and Von Doenhoff is all we need. The only bit of Roger's site that I can't let go without comment is the last paragraph. His angle of attack thing is just goofy. And, of course, any real world wing will be 3d and that opens a whole other tin of worms...

--Tom.

 

[Roger Long]

Also, there generally is a definition of an "Effective Angle of Attack", that you reference.

That jogs my memory. One of the aerodynamists that reviewed my article pointed that out. A lot of water's gone under the keel since I was flying and thinking about this stuff. I'll change the page wording to reflect that.

And finally, these "explanations" never translate into useful information for pilots or sailors, who need to understand the practical relationships between angle-of-attack, curvature, stall, lift and drag to fly their airplanes or to trim their sails

Actually, my second article was considered by many pilots who commented on it to provide very useful understanding of things like why your feet can kill you in an airplane. Stall is not loss of lift. Once a plane is in a fully developed and stable spin, it has just as much lift as it did when it was flying level. Stall is loss of stability in the airstream. Since sailboats are much less free to react, their response to this instability is so highly damped that the stalling behavior is much less dramatic, if it is noticable at all.

 

[RichH]

Lift on any foil acting in a fluid (whether air, water, or any other fluid) is totally depended on VISCOSITY. Without viscosity there can be NO lift. You can approximate lift calculation by either newtonian (F=MA) or strictly by 'energy' (MV^2) methods simply by doing the 'viscous shear' calcs - for foils, for turbine blades, for wings, for your hand held outside of an automobile window.

As long as the fluid has reasonable viscosity and Kutta Condition (see Todd Smith post above) is met, then there develops 'circulation flow' hence lift --- applies to ALL foils - even including an 'upside down' wing/foil where the simplified "bernoulli" explanation totally fails !!!!!
One must remember that sails have essentially NO thickness and therefore become a very special case.

The 'revised' seminal tech paper on lift in sails: http://www.arvelgentry.com/techs/Origins_of_Lift.pdf
http://www.arvelgentry.com/origins_of_lift.htm

PS -. for your own edification, ***DO*** the 'experiment' that is outlined in the 'origins' paper ..... take a dish of water, sprinkle pepper on top and then 'pull' a foil through the pepper flakes. Its going to make you say ---- "AHA! all those sailing books are WRONG".

 

[Ross]

I think that keels and rudders behave like wings but sails behave a little more like kites. If sails truely behaved like wings wouldn't they be more effective if they were built like parafoils?

 

[Roger Long]

His angle of attack thing is just goofy.

Goofy but true.

 

[kloudie1]

Lift Observations

Outstanding stuff, Roger.. As a kid I could envision that same flow when I looked at the soot trails on the fuselages.. The big Pratt & Whitney radials made enough mess that the trail traced out the airflow path above the wing.. We played with some boundary layer control stuff by drilling hundreds of thousands of holes in the top skin of the wings of this airplane. The little turbine engine sucked on the upper skin and REALLY made some lift! It was an experimental unit that Miss State Univ built for the Army back in the mid 60's.

 

[Roger Long]

As long as the fluid has reasonable viscosity and Kutta Condition ...

Ah Kutta. A lot of these net discussions about lift are like the story of the six blind men describing an elephant. The Bernoulli / Newton red hearring is like arguing whether the front legs or the back legs are really the elephant. Kutta is sort of like the trunk. It's pretty essential to having an elephant

PS -. for your own edification, ***DO*** the 'experiment' that is outlined in the 'origins' paper .....

If you look at the animation in my OP link and imagine the air particle movement continuing, you should be able to see in your mind's eye pretty much what is being described there.

 

[tsmwebb]

Goofy but true.

I must have misunderstood what you were getting at. I just opened A&Von D to a random foil (65-418). It has a nice round bottom and top. The angle of attack is defined with respect to a line that goes from the tip of its nose to the tip of it's tail (cord). That line is well within the body of the section. With an infinite aspect ratio at zero degrees it has a CL of about .3 for Rn of about 3 to 9 * 10^6... So, at zero angle of attach the section will create lift. And, FWIW, that CL is well within the drag bucket so it is reasonable to assume it is w/in the intended use envelope. So, what's all this stuff about flat bottoms?

--Tom.

 

[Roger Long]

So, what's all this stuff about flat bottoms?

Many wings, and the one most non-aviation readers have in mind, are flat on the bottom as shown on my animation. The wing on the 172 I used to fly was actually pretty flat on the bottom except near the leading edge.

It is often believed that AOA is the angle between this flat bottom and the airflow, not the mean chord line you were just looking up.

 

[Ross]

Where would one locate the mean chord on a sail?

 

[tsmwebb]

Where would one locate the mean chord on a sail?

The cord of a section of sail is a straight line from the luff to the leach. It is the same as the cord used when looking at mean lines.

--Tom

 

[Hermit Scott]

Why provide a paper on inviscid air flow to support a point, it doesn't exist? With out the math and the corrosponing DIMENSIONAL ANALYSIS anyone's defense of a particular opinion is irrelevent. You might as well talk about how you feel about lift and then base your argument on that.
Learn what a free body diagram is and then sketch it out for me and you will understand that there is nothing magic about it. Assuming the wing is at a constant altitude, there is a force or sum of forces acting upward on the the wing that are equal to a force or sum of forces acting down on the wing(generally this is the weight of the wing). If the difference of pressures times the area equals the weight of the wing than what other forces do you propose there are?
If there were any other forces they must cancel out. Other wise the wing will be rising or lowering.

 

[tsmwebb]

Many wings, and the one most non-aviation readers have in mind, are flat on the bottom as shown on my animation. The wing on the 172 I used to fly was actually pretty flat on the bottom except near the leading edge.

It is often believed that AOA is the angle between this flat bottom and the airflow, not the mean chord line you were just looking up.

I don't know about the foils on a 172. I have built foils including various NACA, eppler and clark sections. I've spent a bit of time looking at section data. Sometimes sections get modified to make them easier to build -- eg I've used 63A0xx sections for rudders because the hollows are removed from the aft section... However, I don't know of any foil sections that are flat on the bottom that are not compromised to make them easier to build. As I type this I have A&VonD on my lap and I can't find a single foil in it with a really flat bottom. I suspect that the reason that you see a lot of drawings of dead flat bottomed foils in explanations of lift is because they're trying to sell the longer route theory and everyone knows that a straight line is the shortest route between two points. Since you're not taking that vile road why not use a more optimal section form? And I'm still confused about the AOA thing. AOA is between cord and flow, effective AOA is a 3d issue mostly related to aspect ratio... Anyway, I'm sorry if I'm missing the forest for the trees here. I generally like your intro to lift it's just that last paragraph that makes me wince.

--Tom.

 

[Todd Smith]

I'll just make a note that AOA is generally defined as relative to the mean chord line, but there is NOTHING magical about the mean chord line, it is just an easily defined geometric reference.

There is no assumption that zero AOA means zero lift.

Todd

 

[Roger Long]

If there were any other forces they must cancel out. Other wise the wing will be rising or lowering.

You've got it but you're not getting it. Everything must cancel out. There are no other forces. There is also magic skyhook for those pressure differentials to attached to. Something has to be moving in the in the other direction (down) for the airplane to stay up against the accelleration of gravity. That something is air.

It's just as true (or, wrong, according to you point of view) to say that the pressure differentials are being created BY the plane being held up as it is to say that they are holding it up. "Cause" becomes a very distracting concept why you are talking about the necessary symmetry for forces.

No, you don't need dimensional analysis to understand this stuff. Many have, and do, understand it without numbers.