Last time Don Stackhouse discussed the effect of leading edge shaping on the stall characteristics of a wing, pointing out that, although this was a significant factor in the leading edge stall, what we really need is a wing with a progressive trailing edge stall characteristic. Read on and be enlightened!
As we all learned when we studied Bernoulli's theorem in science class, the air speeds up as it flows over the curved upper surface of the wing, and this causes its pressure to decrease. However, it then has to decelerate as it approaches the trailing edge, causing its pressure to increase again, so that at the trailing edge the speed and pressure are once again reasonably similar (although not exactly the same!) to what they were ahead of the wing.
This means that the air has to accelerate over the first part of the wing (ahead of what I'll call the aerodynamic high point), and decelerate over the last part of the wing. As it accelerates, it is gaining speed, and therefore its pressure is dropping. If you were to step into the shoes of an air molecule in that zone, you would see that the air pressure behind you was higher, and the air pressure downstream, the region you were heading for, was lower than where you were now. You're flowing from an area of high pressure into an area of lower pressure. The change in pressure, this "positive pressure gradient" is helping you along.
Then you reach the "aerodynamic high point" of the upper surface, or what appears to an air molecule as the high point. It's probably a little ahead of what we think of as the high point, as listed in the coordinates for that airfoil, if the airfoil is at a positive angle of attack. From this point on, the flow is decelerating, and the pressure is now INCREASING. The rate at which it is increasing depends in part on how steep the slope of the airfoil's surface is at that point, but in any case it means that the area you are flowing into has a higher pressure than where you are now. It is fighting your progress towards the trailing edge. For you to continue flowing towards the trailing edge, you have to fight against this steadily rising pressure, this "adverse pressure gradient".
If you have enough kinetic energy to overpower the rising pressure, you can overcome it and reach the trailing edge. It's like a dip in the road for a bicycle. The downhill slope you encounter at the beginning of the dip is like the positive pressure gradient the air molecule sees as it flows over the forward part of the airfoil. However, after passing the bottom of the dip on your bicycle, or the high point of the airfoil for our air molecule, we depend on our speed and the kinetic energy it represents to allow us to coast back up out of the dip in the road, or for the air molecule to overcome the adverse pressure gradient and reach the trailing edge.
If the air molecule runs out of kinetic energy, it loses the battle with the adverse pressure gradient and separates from the airfoil. The steeper the surface of the airfoil, the worse the adverse pressure gradient becomes. Meanwhile, the air is gradually losing kinetic energy through skin friction as it flows along the airfoil surface. If the airfoil is large enough and/or flying fast enough for the air molecules in the "boundary layer" near the wing's surface to be turbulent (i.e.: high Reynolds number), then they can receive some fresh infusions of energy from the layers above, sort of like a bike rider pedalling a little as they try to coast out of the dip in the road. Turbulators can force this laminar-to-turbulent boundary layer transition to occur to some extent, although at very low Reynolds numbers the turbulated flow can revert back to laminar further aft on the airfoil. Vortex generators (spanwise rows of little vanes) are used on some full-scale aircraft for this same purpose. They can also be seen on the vertical fins of some multi-engine airplanes, where they help improve rudder authority for fighting asymmetric thrust if one engine fails.
However, if the slope of the surface and the resulting intensity of the adverse pressure gradient are just too great, even that extra kinetic energy from a turbulent boundary layer can't prevent the flow from separating.
That slope the air molecule encounters at any given point along the surface depends on the airfoil shape and on the angle of attack. Most airfoils have convex upper surfaces all the way to the trailing edge, which means the slope is steepest and the adverse pressure gradient is the most severe right at the trailing edge.
If the airfoil is operating at very low Reynolds numbers, where the boundary layer flow tends to be laminar over the entire surface (no mixing between layers to bring in fresh energy from the higher layers), then the energy the boundary layer starts with at the leading edge is all that it has available to get to the trailing edge. It's like a bike rider who is not allowed to pedal at all while coasting uphill out of the dip in the road. By the time it gets to the trailing edge it will have the lowest energy left of any point on the airfoil, with the least remaining ability to fight an adverse pressure gradient. This is why good low Re airfoils tend to have flat or even slightly concave upper surfaces aft of the high point, and a high point that is well forward so that the slope of the airfoil aft of the high point (and the resulting adverse pressure gradient) is kept as gentle as possible.
Such an airfoil tends to be efficient, but it does hurt the stall characteristics a little. If we shape the aft part of the upper surface so that the adverse pressure gradient at any point along the surface is matched to the air's remaining supply of kinetic energy at that point, then the tendency for the air to separate is approximately equal over that entire section of the airfoil. This means that when it does separate, that entire region of the airfoil tends to all separate at the same time. It's a trailing edge type stall, but it is not progressive. A huge chunk of your lift goes away all at once.
The leading edge radius plays a part in this. If a gradual, trailing edge type stall is desired, then there needs to be enough leading edge radius to make sure the leading edge flow doesn't break down and separate before the flow over the trailing edge gives up and quits. However, the high accelerations over a severely rounded leading edge can waste some of the boundary layer's energy right at the beginning of the airfoil, leaving less available to fight the adverse pressure gradient over the aft portions of the airfoil. Also, the high accelerations in the beginning of the airfoil mean higher peak airspeeds at the high point, and therefore a greater amount of decelerating that must be done after the high point (and therefore a more intense adverse pressure gradient). This is one case where there are definite penalties to being too sharp and to being too blunt! Excessive thickness in the airfoil, regardless of the leading edge radius, causes similar problems. At low Re's, thick airfoils are generally a bad idea. There are adverse pressure gradients on the lower surface as well. Too much thickness and/or camber can result in separated flow on the top, bottom or both at all angles of attack, causing major lift losses and drag penalties. If the flow over the aft part of the upper surface is separated, then only the remaining part of the upper surface is making lift, while the separated portion is busy making gobs of drag. That very thick airfoil with the fat leading edge radius might not stall until it gets to a very high angle of attack, but its total lift and L/D at that point will probably not be as good as a thinner airfoil with fully-attached airflow at a lower angle of attack.
I've seen cases in small R/C models where 6% thick was still way too much. Designing good airfoils for very low Reynolds numbers is extremely tricky work. This is why I'm especially skeptical of rules, for park flyer/indoor classes in particular, that try to specify minimum cambers or thicknesses for airfoils, typically with the intent of guaranteeing some level of low speed performance. At the Re's where these models operate, such rules can actually accomplish the exact opposite of what they were intended to do!
OTOH, there can be handling problems associated with making the adverse pressure gradient region too gentle. Besides the obvious one of ending up with such well-behaved flow after the high point that it's the leading edge flow that separates first, putting us right back at the problem of an abrupt leading edge stall characteristic, and the problem I mentioned above of the entire aft airflow separating all at once, there's also the issue of shift in the center of lift if the trailing edge of the airfoil does stall first. If we lose the trailing edge flow, then we lose most if not all of the trailing edge's contribution to lift. This means that the center of the remaining lift of the airfoil is now forward of where it would be if the entire airfoil was still working. This tends to pull the nose up further, increasing the angle of attack even more and potentially making the stall even worse. We end up with a gentle stall characteristic in the airfoil itself, but depending on the overall design of the airplane we could end up with something that wants to rear up and bite its tail as the alpha approaches the stall angle! The tail design and location are both major players in this particular issue.
Other devices can help an airfoil's resistance to stall, such as slots or slats in the leading edge. These create an opening in the lower surface that syphons some high-pressure air from that region and ducts it up and out through a nozzle directed aft along the upper surface just behind the leading edge. This injects a jet of very energetic, high-velocity air into the upper surface boundary layer, giving it the energy to stay attached to a higher angle of attack and a greater amount of lift. Slotted or Fowler flaps do the same thing for the airflow over a flap. The Lockheed F-104 Starfighter had a row of nozzles in the hinge gap of the wing flaps. These took very high pressure bleed air from the compressor section of its jet engine and squirted out a sheet of supersonic air over the upper surface of the flaps to energize the flow and make them more effective. The downside of these devices is that when the stall finally does come, it tends to be more abrupt and violent than the airfoil would have without these devices.
It's not necessarily good to have a totally gentle stall. It's possible to get into a mushing condition without realizing it. Something with a well-defined gentle stall break but only a minor loss of lift at that point is better. On our old Monarch '94, the second in our Monarch series of R/C hand-launched sailplanes, the stall break in the early prototypes was almost nonexistent. It was possible to get into a mushing stalled condition without realizing it until too late, costing lots of wasted energy and altitude. We redesigned the root airfoil to have an abrupt separation at stall over about the last 20%, giving us a well-defined but gentle stall. The airfoils Joe and I designed for the Roadkill series models are designed to have a gradual trailing edge stall characteristic, but enough of a well-defined stall break, as well as other handling cues that show up before the stall break itself, so that you will know if you're flying too slow.
If you would like to read more of Don's discourses on other model aviation matters, go to Don's web page at http://www.djaerotech.com/ and browse the "Ask Joe and Don" section.