Aircraft Propeller Blade Angle Measurement

Abstract

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The primary step in blade repair is the accurate measurement of blade thickness, blade angles, width, face alignment, and length. The dimensions are then documentation on each blades examination record and verified against the minimum satisfactory repair specifications recognized by the manufacturer. Blade repair entails surface grinding and re-pitching, if required. Sporadically, blade straightening is necessary, as well. The manufacturers requirement states certain acceptable limits within which a spoiled blade may be cold set straight and returned to airworthy state. Specialized tooling and accuracy measuring tools allow pitch changes of below 1/10 of one degree. To guarantee accurateness, face configuration and angle measurements are taken repetitively during the repair procedure. Blade angle is the angle linking the chord line of a propeller blade segment and a plane at a 90 degree angle to the axis of propeller rotary motion. The propeller is under ATA chapter 61. This practice is done to restore the propeller pitch to factory settings. Any variation in blade angle from factory setting will have an effect on motor load. A raise in blade pitch may lead to motor overwork and probable damage to the motor. Motor load ought to be cautiously checked subsequent to any modification in blade pitch to ensure safe limits (United States Federal Aviation Administration, 24).

Background

A propeller blade is an airfoil, which thrusts the airplane through the air by translating the rotating force of the engine into thrust. Propellers are twisted to maximize the functioning of the propeller based on changeable operating setting. Wooden propellers were used more or less entirely on business and personal aircraft before World War II. All through the 1940s, steel propellers were fashioned for military use. Present propellers are made from heat-treated, high-strength, aluminum alloy forgings. New compound materials are being made use of in applications where mass and weight are important. Propellers are characteristically designed with 2 to 6 blades. In general, propellers with more than 3 blades are utilized mainly for twin-engine airplane or single engine airplane using engine with horsepower ranking more than 900SHP. These blades have a propensity to be shorter for more fuselage clearance and better ground clearance. Multiple blade propellers also produce higher, less obnoxious sound frequency. Blade angles are important in producing the power that thrusts the airplane into the air (McCauly, 1).

Necessary equipment to perform the measurement

Protractor (angle measurement);

A computer and a scanner;

A pencil;

Water soluble paint and felt pen;

Cardboard;

Digital camera.

Tracing the Edges

A fairly simple method to obtain the propeller blade angle measurement with standard accuracy is the non-detrimental tracing technique. First and foremost, the propeller blade was connected to a block in order that its axis is at right angle to a level table. Now, some pieces of variable cardboard were set up to some extent longer than the radius of the propeller blade- three pieces were used. Then a water soluble paint and felt tip pen were used to mark the leading edging of the propeller blade. Promptly, a cardboard piece was put vertically on the board and pressed against the edge of the propeller blade before the felt tip color dried. Executing the same at the irregular edge, results in two traces on the cardboard. Markers were added to the boards to make out unlike radial segments. Lastly, a graph of the planform of the propeller, which was simply formed by attaching the third cardboard piece against the lower surface and by drawing the outline of the propeller blade on the cardboard with a pencil, was required. Some markers were added for the same radial positions to this sketching.

Some simple trigonometric math was done. Small table with 5 columns for the key edge height hl and trailing edge height ht, the chord length c, the trailing edge - key edge difference dh and the blade angle ? was made.

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For every radius station the local chord length c from the planform graph was measured. More so, the space between leading edge trace hl and cardboard edge were also measured. Measurements of the elevation of the trailing edge trace above earth ht, were also taken. The difference dh = hl ht was then calculated. Finally, the blade angle ? = arcsin(dh/c) was calculated.

Slicing the Propeller

Regrettably, a more or less ideal technique to determine the measurements of a propeller blade angle is destructive. The technique is very straightforward: starting at the tip, a sample propeller was cut at the radial station of concern. The cut was painted white paint and a photograph taken, as well as a pin in the axis of the propeller blade. A slide or a paper print was used to determine the blade angle ? with regards to the axis in addition to the airfoil outline or at any rate the utmost width and the curvature. In actual fact, it is still probable to dig up the airfoil coordinates pretty perfectly from such a photograph by scanning the picture into a computer and drawing the figure with spline curves.

Using Templates

This technique is extremely comparable to the slicing technique. However, as an alternative to cutting the propeller blade into portions, small, flat patterns are used. These templates were fastened to the waxed face of the propeller blade by means of rapid setting polyester putty. These patterns were photographed and then acted upon with the slicing technique. A flatbed scanner and a computer can be used to scan the templates as an alternative to using photographs. The templates were fashioned from plywood. Take note that the template should look like the propeller piece very much, leaving a tiny gap of approximately 2 millimeters. The templates were fashioned in two fractions, in order that they could be divided into a lower and an upper section. The lower section should match flush to the table to have a position for the propeller blade angle. This technique has been applied effectively to examine full scale propeller blades and is fairly perfect.

Comparison of Results

An assessment of slicing and tracing technique indicates that the chord distribution can be determined quite precisely by means of the tracing technique. An extrapolation is suggested close to the tip, if the tip is rounded in shape. The chord length can be determined very precisely with a standard slide gauge.

The inaccuracy in the propeller blade angle due to tracing technique can be greater than theoretically anticipated. Adjacent to the tip, the tracing technique asserts blade angles, which are almost four degrees lower than the results of the slicing technique. Adjacent to the hub of the blade, the divergence may be as great as eight degrees. Considering the leading edge radius decreases the inaccuracy from one to two degrees. The major basis for the error is almost certainly the accurateness of the height dimensions ht and hl, which could be enhanced by more cautious work. Because of the small measurements an inaccuracy of 0.5 mm can lead to a blade angle inaccuracy of more than a few degrees.

Safety precaution

One should ensure that any time he or she touch a propeller the magnetos are off. It is preferable that one keeps the key in his or her pocket. Even then one must make the assumption that the engine could initiate. That indicates that one should keep all parts of his or her body out of the propeller blade arc. In the case the engine starts, the non-pilots in the aircraft would be unlikely in a position to help. Therefore, having a competent pilot at the controls would be the safest alternative.

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