What effect on the characteristics of an airfoil does the position of the maximum thi

[RobZ]

What effect on the characteristics of an airfoil does the position of the maximum thickness have?
For example how are two airfoils different if one has the max thickness at 40% chord and another at 33% chord, or even 30% chord?
Rob.

[BMatthews]

In the old model books the rule of thumb was that the further ahead the high point the later and softer the stall would be. This assumed some fairly routine airfoil shapes.

But really you're not all that extreme with a 40% high point. I though you were thinking of the 60% and more back max thickness points such as found on the NACA laminar flow series of airfoils.

Back to the more model style stuff though....

For example an Eppler 474 is quite hard to stall. Some buddies used this airfoil for control line combat on my recomendation and found that it worked far better than the previous "standard" which had been the Phil Granderson Tadpole Special. The Eppler was thinner but had a shape that the air preffered so it stalled at a higher angle of attack then the TS. They did fairly well with it for a couple of years.

These days you'll find the max thickness points or the highest point on an airfoil are all over the place within reason but where the high point is located is a function of both the thickness profile and the camber. What really allows the airfoil designers to break some of the old "rules" is the ability to study the pressure distributions that are able to be calculated with such programs as Xfoil and the Eppler program. By avoiding sharp changes in the pressure curves and controlling the slope of the pressure curves they can give us airfoils that work with shapes that would normally be considered unwise to try. Such as the Selig 1233 heavy lift airfoil.

However along with this comes the need for extreme accuracy in duplicating the shapes in many cases. Variations of as little as a couple of percent on some of the more extreme shapes will totally ruin the performance promise. And on airfoils of this sort do not even THINK about using open structures covered with flexible films. The sag between the ribs and spars will completely ruin the performace again.

[GaryO]

Get one of the airfoil programs, Tracfoil or Profili and do enough experimenting to answer your questions.

[banktoturn]

What effect on the characteristics of an airfoil does the position of the maximum thickness have?
For example how are two airfoils different if one has the max thickness at 40% chord and another at 33% chord, or even 30% chord?
Rob.

Hi RobZ,

There are probably several effects. One pretty important characteristic of the point of maximum thickness the point of minimum pressure (on the top surface) will be close to the maximum thickness. This is important because from the leading edge to the minimum pressure, the pressure is constantly decreasing, along the surface. Thus, from the leading edge to this point, the air on the surface is flowing toward lower pressure, which is desireable. After the minimum pressure point, the air encounters an 'adverse pressure gradient'. The adverse pressure gadient tends to cause separation and/or transition to turbulence. One component of a wing's drag can be reduced by delaying the transition to turbulence, which can be achieved by delaying the onset of the adverse pressure gradient. If you look at some of the various laminar flow airfoil sections, you will probably notice the the point of maximum thickness is relatively far back. This doesn't mean that every airfoil section with a rearward point of maximum thickness will achieve laminar flow or low drag, unfortunately.

banktoturn

[RobZ]

Thanks Bruce and banktoturn.
Rob.