Interesting reading for a rainy day

[DaveGherardini]

I came across a couple of pages of interesting reading last nite.

Its funny you know, theres always sombody who says every thing we are taught is all wrong. Some of this stuff is way out there and some is very intersesting. This is just for those interested in other peoples opinions.
Im not bashing the author or trying to get them bashed. Just thought some of you mite find this stuff interesting.

Dave G


Airfoil Lifting Force Misconception
in K-6 Textbooks

http://amasci.com/wing/airfoil.html


O.K., How Do Wings REALLY Work?

http://amasci.com/wing/wingwork.html

[Ollie]

The production of lift by a wing is a complex event that can be partially explained by several theories that do not contradict each other. The misconception is that it only takes one theory to do the job. It takes Newton, circulation and Bernouli to describe how lift works. In fact, Bernouli doesn't pertain to the airfoil itself but to the shape of the airfoil plus the shape of the boundary layer surrounding it. Boundary layer theory itself requires the consideration of viscosity and scale. The big mistake is to try to oversimplify what happens.

Its like Plato's story of the group of blind men that encountered an elephant. The one who got the elephant's trunk said it was like a snake. The one who had a leg said it was like a tree trunk. The one who got the tail said it was like a rope. The one who got the tusk said it was like a spear. The one who got an ear said it was like a fan. None were wrong but each was only part of the reality of an elephant.

[HarryC]

NASA is fairly clear on this, take a look at

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

scroll down to the lift section and follow the links for incorrect theories of lift, especially the one about half venturis.

Also the Royal Air Force's manual states quite explicitly that lift is the reaction to altering the direction of the flow, i.e. downwash.

None of this denies Bernoulli, it just shows up some very wrong applications of Bernoulli's Law. Bernoulli lived before humans flew and said nothing about wings. He discovered a specific instance of Newton's general law of the conservation of mechanical energy. In other words if you don't put energy into, or take it out of a fluid, then the sum of its mechanical energies remains the same. So Bernoulli discovered that if one of velocity or pressure is altered, the other alters proportionally in the opposite direction. The powerful word here is velocity. Not speed, but velocity. Velocity is a speed in a particular direction, if you keep the same speed but alter direction, you have altered your velocity, and hence accelerated. Apply that to Bernoulli's law. If the fluid is made to change direction, its velocity has been altered and therefore there must be a change in pressure. So if a wing turns the direction of flow, a velocity change and pressure change will occur, according to Bernoulli's specific application of Newton's general law. That pressure is felt as the reaction to the downwash.

So Bernoulli is integral to the Newtonian action-reaction of the downwash explanation of lift. It is the stories about equal path length and half-venturis that are hogwash!

H

[DaveGherardini]

I learned from text books many years ago about creating lift (i thought) with airfoil shapes to build a wing. I always wondered how in the heck a fully symetrical wing could generate lift if the boundry layers on top and bottom both meet at the te at the same time. (thinking in the differance of speed theory)Excuse me for my moderate terminogly and terrible spelling but im just a common man who loves airplanes and building rc models. Anyways, some of this actually does explain my douts. I assumed that the symetrical wing created a nuetral boancy or equal pull on both top and bottom. I does point to the fack that Ollie points out that theres a hole lot more going on to make lift than one would think. (Someone like me anyway) I have found the info at the nasa site there really interesting to. Thanks for posting the link H

Dave G

http://www.geocities.com/xstratus/index.htm

[Ollie]

Dave,

The difficulty starts with the fact that the air flow over a wing is invisible. We can't see directly what is happening. Not only that but our intuition about the flow is often deceiving. For example, a symmetrical airfoiled wing which is operating at an angle of attack to produce lift does not split the flow at the leading edge as one might expect. Instead the flow splits at the bottom surface a little aft of the leading edge. The flow over the top of the wing first has to reverse direction and flow back around the leading edge. In another example, the flow over the top and bottom of the wing isn't straight back except near the wing root. The farther out on the span, the more the flow over the top is angled to the center of the wing and the flow over the bottom is angled to the tip of the wing. Where the flow over the top and bottom meet at the wing tip, a vortex (swirling motion) trails from each wing tip. The two vortices and the circulation about the span of the wing feed the up wash in front of the wing and the down wash behind the wing. I doubt very much that this is what the casual observer thinks is happening when he sees a wing producing lift.

[DaveGherardini]

Ollie

My brother told me one time back after i showed him a plane i built from a kit. The plans called for vortex generators for the wing tips. I asked him why in the heck would i want to generate vorticies on the wing tips? He laughed and said "Your not creating vorticies dummy your directing them". This makes a hole lot more sense to me now. What your describing is exactly what he was talking about. Ive built a lot of models and never take it into consideration. I ve learnd from experance that a knife edge trailing edges all around the plane will have less drag , Round tips are better that a flat rib profile tips, ect..and my planes will have a wider envelope(stall to top speed?) Im definatley going to try to picture the vorticies in my head to get a better handle on what they are doing. I might have to build me a wind tunnell with smoke so i can study it better. Well another project to do. Thats ok though, Im really curious now. It might be a waist of time but i will learn something im sure.

Dave G

[Ollie]

Dave,

If you do build a wind tunnel you will be retracing the history of what the Wright brothers did to design the first successful man carrying aircraft just 100 years ago. It is a worthy effort. Curiosity is a great motivator. They did make a "mistake" in interpreting the wind tunnel results. They ended up with a model airplane like airfoil with too much camber and not enough thickness because they didn't take into account the effect of scale factors associated with the flow. Fortunately the "mistake" didn't block thier success.

To me the wonder of the last 100 years of aerodynamics is that men have been able to mathematically model the complexities of air flow around aircraft to the point that they can accurately predict the performance of their designs. There are now computer programs that are like virtual wind tunnels that will run on your PC and even let you fly your design on a simulator to see how it handles before you build it.

[HarryC]

Originally posted by Ollie
Instead the flow splits at the bottom surface a little aft of the leading edge. The flow over the top of the wing first has to reverse direction and flow back around the leading edge.

Get near a full-size plane and you can see the actual point below the leading edge where this happens. The stall warning switch is a small metal plate sticking out from underneath the leading edge. As long as the airflow is pushing it down it is off. If the point where the airflow separates to go top and bottom goes beneath the stall warner, it blows the switch upward and switches it on. So the position of the stall warner shows you where the stagnation point is at high angles of attack.

H