This is a quote from Great Planes about the Super Decathlon 40 "The problem
some modelers have had is that they tip stall the airplane immediately after
takeoff."
This is on essentially a straight plank wing. I find it unlikely
that the stall isn't occurring inboard on one wing and the loss of lift
causing that side of the wing to drop -- even though the tip has not stalled
yet.
From : Don Stackhouse
Yes, on a constant chord, constant airfoil wing (also called a "plank" wing or a "Hershey bar" wing), the stall tends to begin at the center in level, un-yawed flight, with the tips being the last to go as the stall progresses smoothly outwards from the center. However, this is not necessarily the case in this particular example.
There are probably several things occurring here.
First of all, calling this a "tip stall" in this case might not be exactly correct. That term gets misused a lot. Many things get called tip stalls, not the least of which is the rather common problem of a "loose nut on the end of the control stick".
I'd bet that the stall in this case actually starts somewhere around the middle of the left wing panel. In any case, the stall is not symmetrical between the left and right halves of the wing, so the lift between the left and right wings is no longer equal, which therefore results in a rolling departure to the side that has lost the most lift (probably to the left, in this case). Just about any significantly non-symmetrical stall that results in a pronounced drop of one wing during the stall break tends to get labelled a "tip stall", even if it's not the tip itself that was stalled.
The P-factor from the prop is probably causing a yaw to the left, just as if there was some left rudder deflection. This makes the left wing act like it has a little bit of aft sweep, and the right wing act like it has a little bit of forward sweep. This tends to steal some of the lifting work away from the left root and give it to the left tip, with just the opposite happening on the right wing.
An inexperienced pilot is likely to counter this yaw with aileron instead of rudder. The resulting aileron deflection has the effect of creating wash-in on the left wing and wash-out on the right wing. The adverse yaw from the ailerons makes the yaw even worse, requiring even more aileron deflection to counteract it.
In addition, the yaw causes a differential pressure between the left and right sides of the fuselage, which slightly decreases the lift of the inboard portion of the left wing and increases the lift of the inboard portion of the right wing (this is one reason why high-wing airplanes can sometimes act as though they have plenty of dihedral, even though they actually have zero dihedral). This adds even more workload to that poor, abused left outer panel, and also requires additional right aileron deflection.
The torque from the motor tends to roll the airplane to the left. This also requires some more right aileron deflection.
You now have an airplane that (due to P-factor) thinks it has some left rudder deflection, has a greater load on the left wing due to yaw, torque, wash-in on the left wing and wash-out on the right wing due to aileron deflection, and reduced lift-making ability on the left wing root due to fuselage effects. If you then stall the airplane in this operating condition, guess which way it's going to drop a wing!
Most modellers seem to believe that when one wing drops in a stall, the tip of the wing has stalled before the root.
Could be the tip, or (more likely) it could be the section of the wing just inboard of the tip, where the ailerons usually are located. My own planform analysis software gives me a map of the local lift coefficients along the span for a given flight condition. By comparing the local Cl's with the Cl capabilities of the airfoils at each of those locations (including allowing for things like the effects of sweep, and of turns), I can see which portion of the wing is going to hit its maximum capability before the others. That is generally where the stall will initiate.
Usually I go one better than that; I design different individual airfoils for each segment of the wing that CAUSE the stall to begin where I want it to. Aeroelasticity (the way the structure deflects due to aerodynamic loads, and the effect those deflections have on the aerodynamics) also has an influence on all of this. For example, I worked on a pylon racer several years ago that needed more washout on the turns than it needed on the straights. If I twisted it to meet the turning requirements, it would have excessive drag on the straights; if I twisted the wing to match the requirements on the straights, it was likely to tip stall in the turns. Also complicating things was a quirk of the rules that resulted in an airfoil thickness at the tips of less than 3%, a thickness at the root of over 11%, and a Reynolds number at the tips that was only about a third that of the root.
Fortunately this airplane pulled about 17 G's in a typical turn, so there was a significant amount of deflection in the wing due to that. By using a combination of five different airfoils along the span, plus using Finite Element Analysis to design a wing structure that deflected into a specific amount of additional washout at 17 G's loading, I was able to get the root to stall before the tip at all flight conditions, and without paying a drag penalty for excess washout.
Yes, it was a huge amount of work, but in this case the extra effort was worth it. For a simple sport model intended only for boring very tiny holes in the little patch of sky over my backyard, I would probably use a much simpler approach.
I (and some others) think that the stall rapidly progresses from the root to the tip, but the root always stall
first (highly tapered swept wings and retreating heli blades excepted).v
No. Depending on the design of the wing (planform, twist distribution and airfoil distribution along the span, etc.) and the details of the flight condition (airspeed, altitude, bank and yaw angles, turn radius relative to wingspan, propwash effects, etc.), a stall can originate almost anywhere along the span. Tight turns (such as thermalling turns in a very light weight RCHLG) are especially tricky, because of differences in the airspeed at different locations along the wing. If you can make an airplane light enough (and therefore its turning radius small enough), you can force just about any wing design to stall the inboard wingtip. My own spreadsheets that I've developed for aircraft design include calculations for turning radius vs. bank angle, and for the resulting Reynolds numbers and Cl's at the root and at both tips. These numbers become essential when I determine the final airfoil sections and twist distribution at the very end of the wing design process.
Stall fences work by confining the spanwise flow and preventing a stall from propagating from the root to the tip.
Yes, that's correct. This is especially important on swept wings, where spanwise flow can cause a region of separated flow on one part of the wing to spread to other portions of the wing.v
If the tip was stalling first, this would not help.
That's correct. However, in cases where they're using fences, they're also using other techniques such as plaform/airfoil tuning, washout or stall strips to cause the stall to initiate inboard.
Not entirely. Stall fences, etc. do work in the right situations, provided that the rest of the wing design is done properly, and that the stall fences, etc., are properly integrated into that design.
As with most things in airplane design, it doesn't usually happen by accident. To make it work right, either you have to be extremely lucky (not a good strategy to follow if you want consistent results!), or else you have to do your homework.
Don Stackhouse
DJ Aerotech
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