Hello, I have a question about the location of the 'chord line' of an airfoil.

This may seem like a simple question at first but I think the Jeppesen pilot manuals may be in error (or I am naive and something more profound is going on). You may be able to clarify the discrepancy. The Jeppesen training books as well as your website refer to the 'Chord Line' of an airfoil as "...an imaginary line through the airfoil running from the furthest forward point on the leading edge to the furthest aft point on the trailing edge." This is what I remember from previous learning. But when I read in the Jeppesen books about propeller efficiency, the definition of an airfoil chord line seems to change. In the Jeppesen books and elsewhere, the chord line of a propeller is not depicted as an imaginary straight line from the leading edge (traveling through the airfoil) to the trailing edge, but rather it is depicted as the 'blade face' or a line somewhat tangent to the underside of an airfoil where the relative wind strikes that underside... It must be correct, but why does the location of the chord line change from wing airfoil to propeller airfoil?

From : Don Stackhouse

It doesn't. Technically the chord line for a prop is the same as for a wing, or for any other airfoil-shaped surface, running from the furthest forward point on the leading edge radius to the furthest aft point on the trailing edge. It's the longest line that will physically fit within the perimeter of the airfoil.

Some very old references from the days of old airfoils like the Clark Y use a tangent to the underside and call it the "chord line", but this is incorrect according to the modern definition of the term.

However, in some cases it's common to "cheat" on this definition. Prop airfoils in the outboard portions of the blade in particular are often quite flat-bottomed or even undercambered, with relatively sharp leading edges, so that a tangent to the underside is nearly parallel to the chord line.

To measure blade angles from the true chord line would require a template for each location on each different design of blade that you wanted to measure. When inspecting individual blades themselves for proper twist during manufacture it's normal to do exactly that, using a full set of templates for each type of blade to find the true chord line of each reference location along the blade.

However, when assembling the prop, the various pitch settings are set with reference to a tangent to the underside of the blade, typically at a standard reference radius about 3/4 of the way out from the hub. The assemblers have to deal with a wide variety of different props every day, and having to sort through a bunch of templates for the right one for the reference station for that particular prop just complicates the job, adds labor time, and also introduces additional chances for error. The settings for each type of prop assembly take this into account, so that the blade angles in the manual for that prop correspond to measurements taken from a tangent to what folks in the prop industry refer to as the "face side" of the blade (the curved front surface of the blade is called the "camber side"). As long as the measurements are taken the same way as the angles were established for the manual, this works just fine.

There's another reference used in the initial aerodynamic design of the propeller called the "zero lift line". This is the angle of attack of the blade that results in a lift coefficient of zero, and it's typically a few degrees above the chord line (in other words, representing a few degrees negative angle of attack) for a typical cambered airfoil. For a symmetrical airfoil in normal subsonic airflow it is the same as the chord line. Since the airfoils along the blade are still undetermined during the design of the blade, it's easier to use the zero-lift lines along the blade for initial design purposes, then work out the corrections for the twist relative to the local airfoil chord lines to make the official engineering drawing of the blade after the final airfoils are determined.

On model airplane props it's common to measure pitch from the flat "face" side of the blade, which can result in some real aerodynamic oddities in the prop's performance. Some brands of props in the past have advertised having "true helical pitch" (meaning in their case that the pitch in inches relative to the face side of the blade was exactly constant all along the blade, which in itself is generally not the best way to get max efficiency from a prop in the first place!). However, the thickness and camber along the blade varies, and with them the location of the zero lift lines along the blade. A prop with true helical pitch relative to the blade face will almost certainly NOT have true helical pitch from the point of view of the airflow passing across it. Of course, since true helical pitch isn't usually a good thing anyway, it's not really the important parameter in the first place. Also, because there is often a very significant difference between the tangent to the face side and the line for zero lift, the true pitch of the prop from an aerodynamic standpoint could be quite a bit higher than the number you get by measuring the pitch from the blade face. This could throw off your calculation quite significantly if you're trying to analytically match the prop to a particular model at a specific flight condition. A lot of things that get lots of mileage as "marketing hype" end up being quite bogus and potentially confusing when considered logically from an engineering standpoint. Caveat emptor.

The bottom line is that although there is only one true definition of "chord line", there are many ways to establish a reference on an airfoil from which to measure angle of attack. What matters in the long run is that whatever reference is used to manufacture and use that flying surface is properly correlated back to the reference used when the flying surface was designed.

Don Stackhouse
DJ Aerotech