Our Web Wisdom this time is from Don Stackhouse, posted
to the EFLT list in response to a query about leading edge shaping:
Q. I would think that a blunt LE would always stall later
than a sharp one. I am not sure about the violence (i.e. suddenness)
of the stall though. Don, and others, could you please help?
A. First of all, I'll disclose my personal bias. IMHO, the advice that a "sharper" leading edge always results in better stall behaviour is at best a gross oversimplification and at worst outright balderdash. However, there are also cases where a leading edge can be too blunt with regards to stall behaviour, as well as with regard to a number of other properties. Now, with that safely out of the way, we can move on to a more detailed examination of the problem.
There are a whole book's worth of factors involved in the stall characteristics of a given airplane, and there will be no "one size fits all" answer possible. The stall characteristics of the basic airfoil(s) are important, but so is the planform, twist distribution, airfoil variations along the wing, Reynolds numbers along the wing, tail design, types of manoeuvres during which the stall behaviour is a concern, and a bunch of other factors as well.
It is very possible to design a wing with an airfoil that is known to have truly wicked stall characteristics by itself, and yet have an airplane with very benign stall behaviour. For example, the very popular (in full scale light aircraft) NACA 23012 has truly awful stall characteristics. It's painful to look at its rather bizarre Cl vs. alpha plot (that's lift coefficient vs. angle of attack). Almost no rounding off at the top just before the stall (indicative of little or no stall warning before the actual stall break), with a precipitous massive drop in the lift coefficient right at the stall, so abrupt that the plot shows a vertical dotted line for that part of the curve. The tiny amount of lift coefficient that's still left after that initial drop continues to decline steeply at angles above the stall. This is truly an airfoil with malevolence in its heart. By the way, it also has a reasonably blunt leading edge, although I doubt that sharpening it would help. Yet, many airplanes use this airfoil, some with rather abrupt stall characteristics, and others with very gentle stall characteristics.
In addition, there is the question of what exactly constitutes a "sharp leading edge". A very thin airfoil can have what looks like a "sharp" leading edge, yet in proportion to the total thickness of the airfoil, the leading edge radius can be similar to the proportions of a thicker airfoil with a more blunt-appearing leading edge. In that case is that a "sharp" leading edge or not?
Just to confuse the issue even more, there's the case of that Spitfire Mk 22 quarter-40 pylon racer I designed a few years back. It had very little washout, a root airfoil that was over 11% thick and more than three times the chord of the tip (and therefore three times the Reynolds number), and a tip that was less than 3% thick. There was a non-linear variation of other airfoils along the span, but the taper in absolute thickness (not percentage thickness, since the wing was elliptical) was linear. The leading edge radius of the root was larger than the tip in absolute measurements, although it was probably sharper in proportion to the total thickness. Yet, the root stalled first, in large part due to the details of the airfoils involved. The leading edge radii of these airfoils played only a very small part in determining that behaviour.
Let's back up a little and discuss what happens to the flow along the upper surface of an airfoil. At the leading edge (from the airflow's point of view, which is almost never the same as the leading edge at the chord line, the way we humans tend to define it) there is a stagnation zone. This is where the air runs so directly into the leading edge that it can't decide whether it's easier to go over the upper surface or down along the bottom surface. It can't decide, so it just sits there, a little pocket of stagnant air. Any air coming at the wing from above it goes over the top of the airfoil, and any air approaching the wing from below the stagnation zone goes under the wing.
The size of the stagnation zone depends in part on the leading edge radius. A sharper leading edge radius presents less of a blunt wall to the airflow, cleaving through it cleanly (at least at normal angles of attack) with a small stagnation zone and relatively little disturbance to the airflow. In comparison, a fat leading edge radius generates a larger stagnation zone, and sort of bludgeons its way through the air like a berserk telephone pole. I'll let you guess which tends to have better drag.
As you pull the nose up, increasing the angle of attack, the stagnation zone moves downward on the leading edge, perhaps even a little way onto the lower surface. As long as the sharp leading edge is still aimed pretty much into the oncoming flow, this isn't a problem. However, at some angle the stagnation zone moves so far down that the flow actually has to work its way back around the leading edge radius to get to the upper surface, and at that point that sharp leading edge becomes a problem. The flow can't negotiate that sharp bend, so it separates. For very minor cases there is a possibility that the flow can reattach further back on the upper surface, forming a "separation bubble" on the upper surface that has the effect of increasing the leading edge radius (from an air molecule's point of view), as described in another post to this thread. However, the range of angles of attack ("alpha") where this can occur tends to be very limited. In most cases the flow will separate and stay separated over the entire upper surface. This is a classic "leading edge stall", where the separation begins at the leading edge, which normally results in the entire airfoil losing nearly all of its lift all at once.
This can be delayed by increasing the camber in the forward portion of the airfoil, which has the effect of cranking that sharp leading edge downward and keeping it pointed into the oncoming flow at higher angles of attack. However, when you add something in one place you usually take something away somewhere else. In this case that sacrifice is in high speed performance. The increased camber in the leading edge can cause the lower surface airflow to separate at low angles of attack, resulting in a big increase in drag at high airspeeds.
Another option is a fatter leading edge radius. This makes that path around the leading edge easier to negotiate, but can create other problems. The specific angle of attack where separation occurs becomes less well defined, so the effectiveness of things like washout become less predictable. Worse yet, the relatively violent accelerations imposed on the air as it tries to get around this big, blunt object can waste a large amount of its kinetic energy, which then creates problems further aft of the airfoil. And even if those don't cause problems, we're still talking about a leading edge stall, with its undesirable "all or nothing" effect on lift.
What we often really want for gentle stall behaviour is a "progressive trailing edge stall". This is where the stall separation begins at the trailing edge and gradually moves forward as the alpha increases. The USA-35B airfoil used in the Piper J-3 Cub is one example of this. The airfoil never really completely quits flying, there is still something left even at angles well above stall, and there is also plenty of warning as the stall approaches. The Cub uses this airfoil on the entire wing, along with some washout, plus the beneficial effects of the constant-chord wing planform, which has a natural tendency for the stall to begin at the root.
To understand trailing edge stall, we need to understand some
more about the upper surface flow, particularly a phenomenon with
the rather intimidating engineering name of "adverse pressure
gradient".
(to be continued)
If you can't wait for the next instalment, or would like to read some of Don's discourses on other model aviation matters, go to Don's web page at http://www.djaerotech.com/ and browse the "Ask J and D" section.