Setting Blade Pitch for Sensenich Composite Propeller

Sensenich two-blade ground-adjustable propeller

The Sensenich two-blade composite propeller for Rotax and Jabiru engines is light-weight, strong, and very efficient. 

For our Zenith 601HDS plane with a Rotax 912 engine, the ground-adjustable propeller delivers strong take-off performance as well as good cruise speed.

Smoothness throughout the entire rpm range rivals that of more complicated and less efficient three-blade propellers. 

The secret to outstanding performance is a very precise setting of the propeller pitch so that the angles of attack of the two blades differ by less than 0.1 degrees. 

Such precision can be achieved with a high-quality propeller protractor where accuracy is guaranteed to be +/-0.05 degrees for each blade, since then total error between blades is +/-0.1 degrees. 

The same accuracy can be achieved with basic workshop tools and parts. This post shows how.

A word of caution. Sensenich supplies detailed instructions for installation of the two-blade composite propeller. Make sure you follow those instructions while installing or repitching the propeller. 

The discussion below enhances the section that covers the setting of blade angles.

Preparation

Secure the nose gear wheel with a chock so that it cannot move at all. 

If your plane has the nose gear on the tail, as some people like to say, secure both main wheels with chocks. The idea is to prevent the plane from moving even a bit when you later adjust propeller position against the compression pressure of the engine.

Rotate the propeller into vertical position. Make sure that, standing in front of the propeller, you only rotate the propeller counterclockwise to avoid air being drawn into the oil passages of the engine by the oil pump. 

Later, we sometimes move the propeller back and forth a bit, but never more than 1/8 of a turn. Such small movements do not cause induction of air into the oil system.

Create Surface
 
Create behind the propeller a flat, even surface parallel to the floor, size about 18 by 10 inches. Use a piece of plywood or stiff cardboard. 

With wood blocks or bricks or whatever else is handy, raise the surface so that it is 3 inches higher that the tip of the bottom propeller blade. 

In our case, we created the surface with a cardboard box that just happened to have the right height.

Position the surface behind the propeller as follows. Standing in front of the plane, the surface should start at the leading edge of the bottom blade and extend 18 inches to the left. 

Slide the surface forward so that the blade when moved slightly back and forth just passes in front of the surface, say with a uniform gap of 1/4 inch.

Tape paper to the surface so that it is uniformly covered. We need the paper for precise marking. A good choice is 8 1/2 by 11 printer paper.

Mark Blade Position

Position the propeller so that the trailing edge of the bottom blade is vertical. 

Mark the position of the trailing edge on the paper. 

From now on, blade measurements are carried out while the trailing edge of the blade currently at the bottom is at that same mark.

Record Blade Angle

Place a straight edge of length 18 inches or more onto the surface and slide it against the blade as shown below.

Straight edge positioned against bottom blade

The back side of the blade is ever so slightly curved. Position the straight edge against the blade so that the two small gaps at the leading and trailing edge of the blade are equal in size. 

If you are willing to do a bit more work, grind the straight edge a bit so that it fits snug against the propeller. Remove grinding marks with sandpaper. For protection of the propeller, cover that portion of the straight edge with insulating tape. Mark exactly where the straight edge touches the propeller. Due to this change, you no longer need to judge the size of two small gaps.

Draw a line along the backside of the straight edge. 

When you remove the straight edge, you just have the line on the paper. In the photo, note the barely visible mark on the vertical portion of the paper, at the trailing edge of the blade. 

This mark defines the vertical position of the bottom blade.

Line indicating position of straight-edge backside

Check Second Blade 

Slowly rotate the propeller counterclockwise until the top blade is in the bottom position. 

Place the straight edge against the backside of the blade and check whether the line on the paper is exactly at the backside of the straight edge. 

If the line is not in that position, check if it is parallel to the backside or not.

If the line is exactly at the backside, you have a perfect match of blade angles and zero tracking error, and you are done.

Correct for Tracking Error

If the line is parallel to the backside, then you have identical blade angles, and the gap between line and backside is the tracking error of the blades. 

Check whether the tracking error is within the tolerance allowed by Sensenich. If so, you are done. If not, take remedial action as described in the installation manual.

Change Blade Angle

If the line is not parallel to the backside, then the two blades do not have matching angles, and one of the blades must be rotated a bit. 

You can estimate the angle error by the rule that a 1/16 inch deviation over a distance of 18 inches corresponds to a blade angle error of 0.2 degrees, which is far less than the error of 0.5 degrees allowed by Sensenich. 

But such a deviation is easily measured and accounted for by rotating a blade, so you can eliminate such an error.  

The process stops when you have a true match of the angles under an acceptable tracking error. 

The deviation will be below 1/32 inch, corresponding to an blade angle difference of less than 0.1 degrees. 

Recording Blade Angle

In our case, the propeller can be removed from the engine only by first removing the two blades. Thus, the position of the blades is lost and must be restored in the subsequent reinstallation. If you have that problem, too, you must record the propeller pitch before removing the blades, as follows.

1. Rotate the propeller so that the blades are horizontal.

2. Use a plumb bob at the end of the blades to mark two points on the floor. Connect the two points by a straight line. This line corresponds to the axis of the propeller. We want to record the propeller pitch relative to that line.

3. Arrange the paper on which you are recording the propeller pitch so that the edge of the paper is parallel to the line on the floor. Since we use a box for support of the paper, we simply fix the paper to the box and then line up the edge of the box with the line on the floor.

4. Draw on the paper the line with a straight edge as described earlier. This gives you a precise recording of the propeller pitch. 

5. When you re-install the propeller, position the blades tentatively and carry out the above process. The line on the paper is then correct for the desired blade angle.

Test Flight

Be prepared for a big grin when you fly the plane with the adjusted propeller. 

There will be virtually no vibration throughout the entire rpm range.