Propeller Pitch...

From : Don Stackhouse

Regarding Paul Gleeson's 8-5 prop with 2.5 degrees added pitch from his new hub, there's one more factor that the previous posters to this thread have overlooked. The aerodynamic center ("AC") of a propeller blade is located at the 75% radius (i.e.: 3/4 of the distance out from the axis of rotation to the tips of the blades), so this is the "correct" location to measure pitch. This is the standard practice on nearly all full-scale propellers.

In this case, the diameter at the aerodynamic center is .75 x 8", which equals 6". The circumference at the AC is therefore Pi x 6", or 18.85". Dividing the 5" pitch by this circumference gives us the tangent of the blade angle, and taking the arctangent gives us a blade angle of 14.86 degrees.

We now add the 2.5 degree contribution of the new hub, for a new blade angle at the AC of 17.36 degrees. We next take the tangent of that angle times the circumference at the AC of 18.85" (assuming the new hub does not change the diameter), and get a new pitch of 5.89".

The pitch in inches at the tip will of course be about a quarter of an inch greater, as calculated by the other posters to this thread. As Anthony Brindle and Klaus Scharnhorst indicated, the pitch in inches along the blades will no longer be constant.

One of the posters used the term "ideal" with reference to a propeller blade with this type of constant pitch distribution. This is a common misnomer in the model community. The most efficient propeller designs for the vast majority of situations will NOT have constant pitch distribution along the blades. Just as a wing has a lift distribution along its span, so does a propeller blade. Since a propeller blade sees a non-constant airspeed along its span as well as non-constant inflow angles, the determination of the optimum lift distribution along the blade is extremely complicated, much more complicated than the corresponding determination of the optimum lift distribution of a wing. The end result tends to be roughly egg-shaped, with zero at the tip and the center, and with the peak lift values at around the 75% radius (the AC).

Don't get your protractor out yet. We have to figure the inflow velocities (in the direction of flight) at each point along the blade (which are by no means even close to constant in a real propeller), the angle of attack required to achieve the desired lift coefficient at each point on the blade relative to the zero-lift angle for the airfoil at that point, and the difference between the zero lift angle, the chord line and the line at which pitch will be measured at each location along the blade. Most of our pitch guages for props measure the angle of the flat undersurface of the blade, which is quite different from the chord line. Because the flat-bottomed airfoils typical in propellers dramatically change in camber and thickness along the blade, the zero-lift line goes through quite a range of angles as well, which even more complications to the measurement. On full-scale propellers, blade angles during the manufacturing of the blades themselves are checked with templates between the blade and the protractor at each reference station along the blade, so at least the measurement is relative to the chord line.

In Paul's example, adding a constant number of degrees pitch along the entire blade will tend to add more lineal pitch at the tip, and lower amounts at the more inboard locations. The lift distribution along the blade will be shifted outboard. Whether this makes the prop perform better or worse depends on how good the prop design was to begin with, and on how well it was fitted to that particular aircraft. In general, if the original blade designers knew what they were doing, altering the twist distribution of the blade in this manner is more likely to hurt rather than help. It may have a pitch that measures at 5.89", but it will probably not perform as well in most situations as a prop that was designed for 5.89" pitch to begin with. Twist distribution is one of the most sensitive parameters in the aerodynamic design of a propeller blade, and messing with it in a ham-fisted manner will usually not produce the optimum results.

On the other hand, if you're installing this prop on an aircraft with a lot of blockage at the middle of the prop disk, such as a radial-engined WW II warbird model, it's possible that this shifting of lift outboard on the blade could be beneficial.

Props, wings, and airfoils in general are like shoes; the most important single factor in their performance is how well they're fitted to the application.

Don Stackhouse
DJ Aerotech