I have a simple question and am hoping that there are some simple, or made simple, answers.  Is there a benefit to changing root to tip airfoil shape, thickness, or high point to avoid or delay tip vs root stall? 

Does it matter?  If you're getting close to stall then the ship's probably way misbehaving anyway.

Is there a benefit to changing root to tip airfoil shape, thickness, or high point to avoid or delay tip vs root stall? 

I wondered the same thing for years. I even built models with thicker tips but no more. There is no aerodynamic reason to do it in a control line model.

The concept of various wing thicknesses comes from full-size aviation. Full size airplanes typically have ailerons near the tip of the wing and flaps near the root. If tips stall, ailerons become useless and the P-force of a prop can twist the plane into a spin. The solution is to ensure the section of the wing closer to the wing root stalls first thus ensuring that ailerons stay effective through the stall and can keep the plane level.
Designers have two commonly used solutions to solve the tip stall problem. Most commonly employed one is twisting the wing such that the near tip section of the wing has a lower angle of attack than the root section. Free Flight gliders employ this quite a lot. The problem with this approach is that twisted wing has higher drag which is not desirable for fast airplanes. A faster airplane such as Cirrus SR22 employs a different approach: they make wing section near the tip thicker than the root. This is the approach you are thinking of using with a model airplane, correct? There are two flaws in the reasoning though:

1) Our models do not use ailerons. There is no reason for different sections of the wing to stall at different points.
2) Our models do not stall on purpose. One can fly through the entire pattern and NEVER stall. As a matter of fact, stalling in stunt is undesirable. Stalling is unpredictable which kills consistency. Consistently is the name of the game in stunt.

Either way, there is no benefit of designing extra controls to prevent tip stalling.

Does it matter?  If you're getting close to stall then the ship's probably way misbehaving anyway.

More likely me than the plane , but that possibility leads to the question.

and sweep and sideslip.  A rule of thumb is that the farthest-back part of the wing will stall first.  A rectangular CL airplane wing will start to stall in the middle if it's going straight ahead, but on the trailing wing if it's upwind or downwind on the circle.

Huh.  I did not know this.

Of course, the one place I heard about the effect is the Illustrated Guide to Aerodynamics.  Any book that advertises its illustratedness in the title isn't going to be the world's most advanced.

So, does this mean a 747 with an X-29 wing would be safer from tip stall than one with a conventional wing  ?

Not do both, or either 

What I've toyed with with some limited success are LEX methods to increase vorticity at high angles of attack in order to keep the flow from separating inboard and thus keeping a better lift distribution.

On NACA airfoils we typically use in stunt, as AOA increases, the lift increases in a linear fashion up to 10 degrees. After 10 degrees(for 18% airfoil, 11 for 22%), the increase in lift for every degree drops off SIGNIFICANTLY while the drag keeps increasing exponentially. In other words, to try and eek more lift by letting the wing go to higher AOA is a loosing battle. I would look into other mechanisms of increasing lift.

Here's an idea worth checking out: increase wing area but use thinner airfoil to offset increase in drag. Difference in lift between 0016 and 0022 airfoils is negligible. 0016 becomes useless beyond 10 degrees, while 0022 is useless beyond 11. 0016 develops more lift between 5-10 degrees but like I said, the differences are negligible.

On NACA airfoils we typically use in stunt, as AOA increases, the lift increases in a linear fashion up to 10 degrees. After 10 degrees(for 18% airfoil, 11 for 22%), the increase in lift for every degree drops off SIGNIFICANTLY while the drag keeps increasing exponentially. In other words, to try and eek more lift by letting the wing go to higher AOA is a loosing battle. I would look into other mechanisms of increasing lift.

Here's an idea worth checking out: increase wing area but use thinner airfoil to offset increase in drag. Difference in lift between 0016 and 0022 airfoils is negligible. 0016 becomes useless beyond 10 degrees, while 0022 is useless beyond 11. 0016 develops more lift between 5-10 degrees but like I said, the differences are negligible.

Well, not sure I agree with above. For starters, the airfoil data is two dimensional, airplanes are 3 dimensional. THe aspect ratio controls the real angle of attack because of the induced drag.

A low aspect ratio wing with the same airfoil and area of one with a higher aspect ratio will stall at a higher angle of attack.


Also, once the critical angle of attack on an airplane is reached the curve flattens and there is no futher loss of lift ( unless you go into supemanuverability ) due to the reduction in induced drag, so the lift and drag coefficients remain fairly constant around the "knee" which doesn't happen in 2-D flow .

Additionally, if you look at the curves for symmetrical airfoils you'll notice that the flat wing of a Guillows glider has the same curve as a Nobler. 2Pi with the zero-lift AoA at zero degrees.

Yeah, I'll give you we use flaps, but the changes in lift coefficient due to the flap deflection are not calculated as a fucntion of thickness.

SoI guess what I'm saying is airplane wings don't act the same as sections in a wind tunnel.

And you have to remember, that models fly at such low Reynolds numbers that there isn't sufficient energy in the flow to really apply Buckingham Pi to the results and assue dynamic similarity to the wind tunnel results.

Because of this, and the fact that test sections were typically small, it was common practice to pressurize the wind tunnel to increase the air density and push up the Reynolds numbers to numbers relevant to full-scale aircraft design.


Readers digest version: on a stunt ship aspect ratio has more to due with stall than the airfoil. Since all the airfoils we fly qualify as thick the stall will be trailing edge forward so the stall characteristics won't change much. In fact, once wings get thick enough they begin to stall before thinner ones due to the exaggerated turning of the flow required.

As I always say, we fly airplanes, not airfoils, and airplanes are much more complex than airfoils.

Readers digest version: on a stunt ship aspect ratio has more to due with stall than the airfoil.

This is evidence that Readers Digest is not a good place to learn aerodynamics.