Oh, boy. Tip stalls!
Over the years the entire subject of tip stalls has taken on ever increasing amounts of fear based on hearsay. It's probably worth a little bit of discussion.
Why does aviation fear a "tip stall"?
Primarily because full size airplanes (especially earlier ones with piston engines and tail wheels) were normally flown to a near full stall condition during the landing flair. I can still remember my instructors in the Cessna 120s and 140s admonishing me to "hold it off 'til it stalls" just prior to touchdown. Especially with tail draggers this was important because (unless the pilot was purposely trying to make a power on "wheel landing) if the airplane wasn't essentially completely stalled at touchdown the main gear would hit first, the tail would drop and instantly the wing would be producing enough lift to get the thing airborne again. Landings similar to that seen with Ringmasters on hard surfaces would often result . (for the same reason, by the way!)
Because of that reality, it was very important that the "tips" of the wing didn't stall first. This, very simply, was because the pilot's ability to control the roll axis of the ship depended on the part of the wing with the ailerons (the outer half, or tips) to continue flying to assure control during the touch down.
This was generally accomplished structurally by tweaking the tip upward at the trailing edge a bit to "wash out" the tips relative to the root. The result is that the tip is always at a slightly lower angle of attack than the rest of the wing. Thus, when the nose is pulled up the rest of the wing will stall (reach critical angle of attack) before the tips and, therefore roll control will be retained until in the landing roll.
The subject of tip stalls has become almost a mantra among our RC brethren because they are faced with very much the same sort of problems. Landing on short strips with tail draggers raises exactly the same spector. When you throw in the fact that the pilot doesn't have an airspeed indicator or angle of attack indicator to inform him of an impending stall you quickly realize that they must take the problem into serious consideration both during design and building phases and when flying.
We CL stunt guys don't really have those sorts of problems because we just don't do a whole lot in the roll axis. If our ships stall they are going to drop the nose and either regain their lift and fly out of the stall --or they're going to run into the ground. they aren't going to engage in any roll activities because we've got a tether on the inboard wingtip.
Thus, we really don't need to give a lot of thought to control of tip stall even though it has been part of the holy grail since the early days of stunt (don't recall who first came up with the "problem"). That was where the whole concept of a large percentage section at the tip came from, the logic being that a thicker section would stall later than a thinner version of the same airfoil. Probably true but, in the practical sense, not a big deal for us.
The truth is that under power (especially in our modern era) the whole thought of a stall due to inadequate airspeed (the usual type of stall we think about) is pretty farfetched. It can certainly happen on the glide and that is a subject for a different discussion, but the need to avoid "tip stalls" on today's stunters is pretty much theoretical and not particularly germane.
Now, that is not to say that you can't force a stunt ship to stall in a variety of ways. The classic case (difficult but doable with the right set of trim conditions) would be the accelerated stall where the tail authority is such that it can drive the wing to an angle of attack at which it will no longer produce lift (beyond critical angle of attack).
This can be done regardless of airspeed which may be a bit of a surprise to some. Although it is common to discuss "stall speeds" and all full size aircraft flight manuals publish them for all flap positions and weights, it is not the airspeed that causes a stall. The stall is the result of the wing's angle of attack being in excess of that at which airflow will remain laminar to the surface. This angle for a given airfoil is known as the "critical" A of A and if the wing is driven to such an angle it will stall regardless of the airspeed of the craft to which it is attached.
If you couple such aggressive control inputs with a poorly designed airfoil (too thin or too sharp) you can, indeed, make a wing stall. This is exactly how, by the way, the full scale or RC pilot produces the stall necessary to perform a snap roll. Every watch a Pitts or Christian Eagle go flashing off the end of the runway after take off and all of a sudden snap off a 360 degree roll? (by the way, this is an extreme example of the roll/yaw couple about which Brett is quick to talk in trim conversations)
If you watch carefully you'll see that the first thing that happens in that maneuver is an aggressive positive pitch change. The wing stalls (despite the obvious pretty high airspeed), the pilot socks the rudder to the yaw axis and the airplane performs a horizontal 360 degree spin. As the wings return to level the pilot relaxes the back pressure and the airplane immediately resumes its previous.straight ahead flight path. Stall! Snap! Recover! Almost as fast as you can type it.
At any rate, with modern powerplants, good airfoils and reasonable wing loadings stunt pilots don't really need to design with an eye towards stalls of any sort during the pattern and they certainly don't have to give more then lip service to where any stall might occur along the wing span.
Sure, you don't want to "ask for" a stall at the tips by making the leading edge real sharp at the tips while it's nice and blunt at the root. But, even if you do, the result will be lousy maneuvers because that part of the wing isn't producing any lift. That's all.
Ted
Topic: Designing for windy conditions ? (Read 1565 times)
