Chuck, would you elaborate on this? Why is "The trailing edge is the most important attribute of the airfoil... "?
Thanks.
Bill
Bill,
Apologize for the tardy reply. Been a busy boy lately. I'll be at Watkins Glen this weekend and then in Norwalk Ohio the next. If you want to see REAL nitromethane powered motors drop by!
Anyway... when I was a young man flying models all my teachers told me about "Bernoulli's Principle" and how that was how a wing created lift. I ate it all up and then went on to college to become an aerospace engineer. I started to understand that Bernoulli didn't really apply to real airplane wings. Look at most modern airliners - the bottom of the wing is longer than the top - whoah! After many years I finally had the epiphany that aerodynamic isn't really about classical fluid flows, what it really is is a special branch of thermodynamics.
Bernoulli's equation will not predict the lift of an airplane wing. I looked at it and tried to figure out why. Yes, there is a spanwise component of the flow on a real, 3D wing it can't account for but something else was missing. So I drank a few more jars of shine and pondered it and then it hit me. Bernoulli does not account for viscosity!
That's...HUGE. How big? I'll tell you. One of the things we aero guys love to talk about are Reynolds Numbers. We use them to compare the airflow of a 6 inch chord wing to a 10 foot chord wing (or funny car body!). We can do that because the Reynolds number is dimensionless. We know that if an airfoil section is operating at a specific Reynolds number it will behave the same regardless of the chord. The length of the section is part of the number, so if we want to use a small test section compared to the actual wing we can increase the velocity of the air or use compressed air in the wind tunnel to increase it's density, or both. This way, a small section will behave the same as a full-size transport aircraft's wing and we can get meaningful data.
(Yes, I'm old enough to have used wind tunnels and not computers.)
So why the rant about Reynolds numbers? Here's the kicker: The reynolds number represents the ratio of viscous forces to the airstream's momentum. Read that again.
And that's when it all came clear to me. The air's
VISCOSITY is a main driver in how an airfoil works. So I explored it more. Where does the viscosity have the greatest effect? At the trailing edge! Once that veil was lifted, I began to understand it all. The trailing edge is sharp, when the wing starts to move if there is an angle of attack the air gets delayed at the trailing edge due to viscosity. A vortex is created and the flow field gets distorted and bingo bango, you get the circulation and the net result is the air is shed at an angle at the trailing edge. The net change in momentum up or down through a control volume around the wing times the mass flow rate of the air is equal to the lift. It really is that simple. If you change the angle of attack the air's viscosity changes the flow field and increases or decreases lift. When you finally get a high enough angle of attack, viscosity overtakes momentum and the flow becomes turbulent.
You can search about "shed vortices" if you want to understand it more.
So, even though it's been many years since I gave up competitive flying I still build a lot of models and put in a lot of flights content in my search for that perfect pattern. And every ship I build now I find ways to make the TE sharper and sharper and they keep flying better. My best results have been to split the TE on the flaps and elevators, glue in a 1/4" wide strip of 1/64 ply and profile them to a knife-edge.
There is a classic experiment you can do to visualize this next time you have a bowl of soup. Put the spoon halfway into the soup, it's an undercambered airfoil. Now, start to move the spoon. Watch how the flow warps at the trailing edge and moves forward.
Chuck