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Fixed-pitch Propellers for Small Aircraft

One of the most critical yet overlooked parts of the aircraft is the fixed pitch propeller primarily due to its reliable simplicity – it has no moving parts. The downside of a frequently neglected propeller is the ever present possibility of blade defects which could ultimately contribute to blade separation. Although the possibility of separation is rare, it could also have disastrous results when these types of defects are not corrected. The best approach to ongoing propeller servicing and maintenance is to keep up on proper blade dressing procedures and to perform the five year corrosion inspection as required by Standard 625, Appendix C (6) (b). This task is critical to the longevity and serviceability of the propeller.

This Standard, pertaining to Fixed Pitch and Ground Adjustable propellers, states: “At intervals of not more than 5 years, the propeller shall be removed from the aircraft and inspected for corrosion or other defects over its entire surface, including the hub faces and the mounting hole bores. While the propeller is removed, it shall also be checked for correct dimensions. However, if defects which require repairs beyond those recommended as field repairs by the propeller manufacturer are found, the propeller shall be repaired by an organization approved for the overhaul of propellers” followed by the Information note: “The dimensional check requirement does not include a check on blade twist. The dimensional check refers to changes in blade dimension resulting from repairs, particularly cropping of the tips. It is intended to ensure that the blade diameter remains within service limits” These inspections are best performed by an approved propeller shop for the best results. The more experienced shops can also address manufacturer bulletins and other improvements at this time. Individual AMO’s and/or AME’s can also perform this work provided that they have adequate equipment and the resources needed to meet the requirements of this Standard. When checking aluminum propellers for proper dimensions, the manufacturer’s type certificate identifies the minimum allowable values for blade width and overall diameter. Once these limits have been exceeded, the prop is deemed unfit for use on certified aircraft.

Many propellers are made of either wood, aluminum, or composite materials. Most fixed pitch propellers used on small aircraft today are made of an aluminum alloy forging and are either anodized or painted to prevent corrosion of the bare metal. Propellers are typically painted matte black or grey on the back side (which is actually the cambered side as viewed from in front of the aircraft). The blade tips are also painted (usually white or red) to provide visible warning when operating. The blade face (flat side) is painted flat black to reduce glare as seen by the pilot (from inside the aircraft). A front mounted prop is called a tractor as seen on most certified aircraft, whereas a rear mounted prop is referred to as a pusher though they are far less common.

In theory, a propeller is essentially a rotating airfoil consisting of 2 (or more) blades attached to the hub which is mounted to the engine crankshaft flange. The function of the propeller is to convert engine torque into thrust (or “lift” as compared to a wing airfoil). The amount of torque is dependent upon the rated horsepower of the engine. Forces acting upon the propeller during operation include centrifugal force (which tries to pull the blades from the hub and is considered the greatest stressor), torque bending force (which acts in opposition to the blades direction of rotation), and thrust bending force (which tries to pull the blades forward - ahead of the neutral plane of rotation). Blade angle is the angle between the blade chord line and the plane of rotation. The blade angle is determined by the propeller manufacturer according to the type of installation. This is where the term “fixed-pitch” comes from although the pitch can still be adjusted at a propeller shop within specific tolerances. The propeller blades therefore have a predetermined Angle-of-Attack (The angle between the blade chord line and relative wind) as determined by the fixed blade angle.

Most fixed pitch propellers are manufactured to accommodate the best all-round flight characteristics for the aircraft however, these prop blades can also be twisted to favor either cruise or climb characteristics. A “cruise prop” has a higher blade pitch and is effective for installations where take-off and climb performance are considered secondary to greater efficiency in cruise flight. A “climb prop” leans more towards a lower blade pitch and is used where take-off and climb are much more critical. Seaplanes are a good example of requiring a prop that is of very fine pitch and longer diameter in order to power itself off of the water.

The propeller manufacturers (McCauley, Sensenich, etc.) determine the most efficient pitch distribution along the blades starting from the hub out to the tip of each blade. Blade stations are measured in inches also from the center of the hub (maximum blade angle) out to the tip (minimum blade angle). This pitch distribution provides for more even thrust loading across the entire length of the propeller. Another term known as geometric pitch represents the mathematical distance that the prop should move through the air with each full revolution. The effective pitch represents how far the propeller actually moves through the air with each full revolution. The difference between these two figures is what ultimately determines the true efficiency of the propeller which is anywhere from 60 to 90 percent for most fixed pitch models.

Safety concerns with small aircraft propellers (when parked) are focused upon treating the propeller as though the “mags are live” suggesting that the slightest movement of the prop by hand could cause it to quickly rotate unexpectedly under the right conditions. Always try to use a tow bar for moving the aircraft and minimize handling of the propeller for this purpose. The prop is the easiest and most accessible part of the aircraft and is often the obvious choice for ground handling the aircraft. With this word of caution in mind, the propeller spinner is also off limits for pushing an aircraft into place. Pushing on the spinner introduces stress to the spinner bulkhead making it vulnerable to cracking. There are still frequent examples of Flying Club staff handling aircraft by the prop in certain situations but it should not be accepted as a routine habit – for anyone.

Regular servicing of the propeller is maintained through regular dressing (or filing) of the propeller blades to remove nicks. These nicks along the leading edge of the blades are inevitable due to the close proximity to loose gravel/debris during ground operations. The aircraft pre-flight routine includes a focused inspection of the propeller blades for nicks or damage. The prop is also subject to gradual erosion best described as many small pockmarks along the leading edge of the blades which do not normally pose a threat to the ongoing serviceability of the propeller. Prop nicks are the result of harmful pieces of gravel striking the prop hard enough to leave a deep cut or blemish on the aluminum blade. A reportable nick is usually something that you can easily feel with your fingertip during your blade inspection. Maintenance personnel should be contacted so that they can repair the blade by dressing it out with an appropriate metal file and blending thee area as per their approved procedures. A properly dressed nick reaches the base of the defect before being blended out. This ensures that the damage will not act as a stress riser during the extreme forces of operation which could lead to the onset of a crack in the material. This could then ultimately lead to blade separation if not treated properly.

Some simple reminders for aircraft operators and pilots include regular inspections (and reporting any suspicious defects to maintenance), regular cleaning of the blades, avoiding ground runs over loose gravel (whenever possible), and for tricycle gear configurations - keeping the nose oleo inflated to spec to prevent the prop from turning too close to the ground. Maintainers can ensure proper blade tracking during routine maintenance checks by referencing one blade relative to the other while being rotated by hand through its standard plane of rotation. The blades of a common two-bladed propeller should track within one-sixteenth of an inch of each other. More complex engine and propeller combinations can also be dynamically balanced to reduce the amount of vibration felt through the aircraft simply through the calculated placement of small weights to the spinner bulkhead. Many maintenance providers with the right equipment provide this service nowadays.

Regular care and maintenance of aircraft fixed-pitch propellers is not complicated but nevertheless requires attention to the few areas of concern that have been covered here. Once again, Keep an eye out for propeller nicks and prepare for the 5 year corrosion inspection when it comes due. Sending the propeller to a local specialized propeller facility like Hope Aero is money well spent for a detailed inspection on the one simple yet critical component that pulls you through the sky.

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