Maintainers Corner: Care and Feeding of your Propeller
CARE AND FEEDING OF YOUR PROPELLER (Part 1)
All propellers, regardless of the material from which they are made, should be regularly and carefully inspected for any possible defect. Any doubtful condition, such as looseness of parts, nicks, cracks, scratches, bruises, or loss of finish should be carefully investigated and the condition checked against repair and maintenance specifications for that particular type of propeller. If the propeller is damaged beyond the repair limits established by the propeller manufacturer, and a replacement is necessary, install the same make/model approved or alternate as specified in the equipment list, applicable FAA Aircraft Specification, Type Certificate Data Sheet (TCDS) or Supplemental Type Certificate (STC).
Carefully inspect aluminum propellers for cracks and other flaws. A transverse crack or flaw of any size is cause for rejection. Multiple deep nicks or gouges on the leading edges or face of the blades is cause for rejection. Blending out a nick or scratch by rolling with a screw driver is an unacceptable practice due to the fact that if the dent or scratch has high ridges, you will actually be folding the metal thereby creating additional stress points.
Propeller Hub-Fixed Pitch
Inspection procedures require the removal of the propeller spinner for examination of the prop hub area. Cracks may be present in the hub area between or adjacent to bolt holes and along the hub pilot bore. Cracks in these areas cannot be repaired and require immediate scrapping of the propeller. Propeller attach bolts should be examined for looseness or an unsafetied or cracked condition. Cracked or broken bolts are usually the result of over torquing. Correct torquing procedures require all bolt threads to be dry, clean, and free of any lubrication before torquing.
Corrosion may be present on propeller blades in varying amounts. Before performing any inspection process, maintenance personnel must examine the specific type and extent of the corrosion. Corrosion, other than small areas (6 square inches or less) of light surface type corrosion, may require propeller removal and reconditioning by a qualified propeller repair facility. When intergranular corrosion is present, the repair can be properly accomplished only by an appropriately certificated propeller repair facility. Unauthorized straightening of blades, following a ground strike or other damage, can create conditions that lead to immediate blade failure. Propeller tip damage will sometimes lead maintenance personnel to consider removing damaged material from the blade tips, however, propellers are often manufactured with a particular diameter to minimize vibration. Unless the TCDS and both the engine and propeller manufacturers specifically permit shortening of the blades on a particular propeller, any shortening of the blades would probably create an unairworthy condition.
CARE AND FEEDING OF YOUR PROPELLER (Part 2)
Due to the nature of wood, these propellers should be inspected frequently to assure airworthiness. Inspect for defects such as cracks, dents, warpage, glue failure, delamination, defects in the finish, and charring of the wood between the propeller and the flange due to loose propeller mounting bolts.
Whenever the propeller is removed, visually inspect the rear surface for any indication of cracks. When any defects are found, disassemble the metal hub from the propeller. Inspect the hub bolts for wear and cracks at the head and threads, and if cracked or worn, replace with new equivalent bolts. Inspect for elongated bolt holes, enlarged hub bore, and cracks inside the bore or anywhere on the propeller. Before installation, touch up with varnish all places where the finish is worn thin, scratched, or nicked. Coat the hub bore and bolt holes with some moisture preventive such as asphalt varnish, and track and balance the propeller. If the hub flange is integral with the crankshaft of the engine, final track the propeller after it is installed on the engine. In all cases where a separate metal hub is used, do a final balance on the propeller with the hub installed before tracking.
New Wooden Propellers
Inspect the bolts for proper torque after the first flight and after the first 25 hours of flying. Thereafter, inspect and check the bolts for proper torque at least every 50 hours. No definite time interval can be specified, since a bolt’s proper torque is affected by changes in the wood caused by the moisture content of the air where the airplane is flown and stored. During wet weather, some moisture is apt to enter the propeller wood through the holes drilled in the hub. The wood then swells, and because expansion is limited by the bolts extending between the two flanges, some of the wood fibres become crushed. Later, when the propeller dries out during dry weather or due to heat from the engine, a certain amount of propeller hub shrinkage takes place, and the wood no longer completely fills the space between the two hub flanges. Consequently, the hub bolts become loose.
In-Flight Tip Failures
In-flight tip failures may be avoided by frequent inspections of the metal cap, leading edge strip, and surrounding areas. Inspect for such defects as looseness or slipping, separation of soldered joints, loose screws, loose rivets, breaks, cracks, eroded sections, and corrosion. Inspect for separation between the metal leading edge and the cap, which would indicate the cap is moving outward in the direction of centrifugal force. This condition is often accompanied by discoloration and loose rivets. Inspect the tip for cracks by grasping it with the hand and slightly twisting about the longitudinal blade centreline and by slightly bending the tip backward and forward. If the leading edge and the cap have separated, carefully inspect for cracks at this point. Cracks usually start at the leading edge of the blade. Check the trailing edge of the propeller blades for bonding, separation, or damage.
CARE AND FEEDING OF YOUR PROPELLER (Part 3)
Propeller Tracking and Vibration
To ensure smooth power plant operations, first start with a properly installed propeller. Each propeller should be checked for proper tracking (blades rotating in the same plane of rotation).
Propeller Tracking Check
The following is a simple procedure that can be accomplished in less than 30 minutes.
Remove one spark plug from each cylinder. This will make the propeller easier and safer to turn. Rotate one of the blades so it is pointing down. Place a solid object (wooden block that is at least a couple of inches higher off the ground than the distance between the propeller tip and the ground) next to the propeller tip so that it just touches. Rotate the propeller slowly to see if the next blade "tracks" through the same point (touches the block). Each blade track should be within 1/16-inch (plus or minus) from the opposite blade’s track. If the propeller is out of track, it may be due to one or more propeller blades being bent, a bent propeller flange, or propeller mounting bolts that are either over or under-torqued. An out-of-track propeller will cause vibration and stress to the airframe and engine, and may cause premature propeller failure.
If a propeller vibrates, whether due to balance, angle, or track problems, it typically vibrates, throughout the entire RPM range, although the intensity of the vibration may vary with the RPM. If a vibration occurs only at one particular RPM or within a limited RPM rage (e.g. 2200-2350 RPM), the vibration is not normally a propeller problem but a problem with a poor engine/propeller match.
Cabin vibration can sometimes be improved by re-indexing the propeller to the crankshaft. The propeller can be removed rotated 180 degrees, and re-installed. Although this may lessen the vibration most likely it will not cure it. Dynamic balancing to the engine is the best way to accomplish this.
The propeller spinner can be a contributing factor to an out-of-balance condition. An indication of this would be a noticeable spinner "wobble" while the engine is running. Inadequate shimming of the spinner front support or a cracked or deformed spinner normally causes this condition.
Interested in saving money and down time on your instruments, avionics, and engine? – consider having your propeller dynamically balanced. Owners who have had this done report a remarkable decrease in vibration, even when they thought they had a smooth running engine to start with. Maximum North has state of the art equipment for balancing and the cost is more than recovered in savings due to wear and damage caused by vibration. Contact Kori for further info at 495-1561.
This information provided courtesy of Kori Ibey of Maximum North Aviation.