Attaching I
n November we stepped through the process of selecting a propeller for your airplane, and, ass u m i n g you've decided on a prop, it's time to install it. The process of installing a prop depends on the type of prop (wood, metal, or composite, and fixed-pitch, variablepitch, or constant-speed) and the type of crankshaft your engine has (with a flange, splines, or tapered, and whether it's suitable for a fixedpitch or constant-speed prop). Regardless of these variables, installing your prop properly is critical to safe flying. Using the wrong type hardware, torquing the bolts improperly, rolling an O-ring, and improper safetying can all create major problems. When installing a new prop it's absolutely essential that you follow the manufacturer's instructions, use only new hardware, use the proper torque values, etc. Once you've correctly installed the propeller, you should inspect it regularly for damage and corrosion.
Installation Because propeller installation depends on the type of prop and crankshaft, we'll start with the basics of installing a fixed-pitch prop on a flanged shaft, a tapered shaft, and a splined shaft. Then we'll address the installation basics for a constantspeed prop. Fixed-pitch flange—A large m a j o r i t y of h o m e b u i l t s m o u n t a wood, metal, or composite fixed-
pitch propeller on an engine with a flanged crankshaft. With a few variations that depend on the prop's composition, the installation is relatively simple and has a few dos and don'ts. Most manufacturers provide a howto guide. If they don't, seek the assistance of an airframe and powerplant (A&P) mechanic to check your work. If you're installing a wood prop, you'll need to use a faceplate to protect the wood (see Figure 1). Large washers will not do the job. During installation it's important to compress the wood to its maximum— without damaging or crushing the wood. Obviously, using the proper size bolts is important. And so is a torque wrench, which is the only way to tighten the bolts to the values the m a n u f a c t u r e r gives. Sensenich Propeller Manufacturing has a good installation guide, and some of the points that follow are taken from it. Inspecting the crankshaft's flange for damage and corrosion is your first step. Correcting any defect you find before placing the prop on the flange is your next step. Also, confirm what fasteners the flange accepts. Some have threaded inserts that accept bolts, and others have dowel pinholes that require bolts and nuts. Before you attach the prop to the engine, be absolutely certain that the magneto switch is turned OFF. Always treat the propeller as though the engine is going to start when
you move it because many engines have accidentally started with the mag switch in the off position. To make sure the engine can't start, and to make the propeller easier to more, remove one spark plug from each cylinder. And chock the wheels to keep the airplane from moving. Mount the prop on the shaft in a position most convenient, usually the 10 o'clock—4 o'clock position when you're facing the engine, and then install the faceplate and bolts. If you're using a propeller spinner, m o u n t the spinner bulkhead or bracket before installing the bolts. Tighten the bolts slightly so the faceplate and propeller are snug but the wood has not been compressed. Next, check the track of the propeller by r o t a t i n g the prop and watching where the tips are in relation to a fixed object on the floor. You can also clamp a stick or rod to the wing or fuselage so that it just barely touches the prop blade. (Again, make sure the mag switch is turned off and you've removed a plug from each cylinder.) Move the blades slowly and make sure that each tip tracks within 1/16 inch of the other, using the fixed object as your reference. If the track is off, correct it by tightening the bolts nearest the blade tip that is forward. Moving diagonally across the bolt circle, tighten the prop bolts in small increments. Continue to check the prop's track while you're tightening the bolts, and be sure t h a t you
Because propeller installation depends on the type of 56
DECEMBER 2000
ROTATION
tighten the bolts on opposite sides of the blade centerline evenly. Sensenich recommends that you loosen the bolts after tightening them to relax the wood. After an hour of relaxation, re-tighten the bolts using the process just described. After tightening the bolts to the proper torque value, safety-wire them (see Figure 2). Safety wire should be attached between two bolt heads or nuts, depending upon which is used. This is a better procedure than safetying all bolts with one continuous wire. Installing a metal fixed-pitch propeller is similar to installing a wood prop, but you don't need the faceplate. Fixed-pitch tapered shaft—On older model, low-horsepower engines you'll occasionally find a tapered shaft. To properly mount a propeller on the crankshaft you'll need a propeller hub. Again, inspecting the shaft for corrosion, cracks, and other defects is your first step. Usually a tapered shaft has a keyway (see Figure 3). Finding cracks within the keyway's corners isn't uncommon, and most A&P mechanics recommend using a dye penetrant inspection to discover them. Also inspect the hub, and A&P mechanics again recommend a dyepenetrant inspection. After these parts pass inspection, place the prop on the hub with the
DRAIN HOLE
METAL' TIPPING .019"
BRASS MONELSTAINLESS
STEEL
THICK METAL FACE PLATE 3/16" -1/4"
PROTECTED LEADING EDGES
FIGURE 1
TONY BINGELIS SKETCHES
WOOD PROPS - PROTECTIVE FEATURES DESIRED TRACKING BOTH BLADES
NOTE - IF PRACTICAL, REMOVE TOP SPARK PLUGS
IGNITION SWITCH OFR
• ANY SUPPORT 58
DECEMBER 2000
TYPICAL BLADE TRACKING SET-UP
FIGURE 2 A correctly Installed propeller where bolts are safetied two at a time.
blades in the proper position relative to the keyway. Usually, the engine manual gives the proper position, and you tighten the bolts using the same cross-circle process described previously. When the prop and hub are one, mount the assembly on the crankshaft. Make sure the key is properly installed in the keyway, and use a retaining nut to hold the hub in place. As you install the retaining nut, compress the snap ring that is part of the assembly and push it in with the retaining nut until it snaps into
its groove. (When removing the
prop, the snap ring serves as a FIGURE 3
"puller," actually pulling the prop from the tapered shaft when you unscrew the prop-retaining nut.) Tighten the retaining nut to the torque value specified by the manufacturer. To achieve the desired torque you may need to use a 3-foot steel bar that fits into the holes in the hub-retaining nut. You can calculate the amount of force to be applied to the bar depending upon the recommended foot-pounds of torque. After tracking the prop and making any necessary corrections, install a clevis pin or small screw that safeties the retaining nut and keeps it from vibrating loose.
Fixed-pitch splined shaft— Radial engines commonly have a splined crankshaft, a series of splines and voids with one master spline that is larger than the rest. There's also a master spline on the prop hub so that it will fit on the shaft in only one position. As before, thoroughly inspect the shaft and hub before installation. Mount the prop on the hub just as you would a taperedshaft installation. To mount the prop-hub assembly on the engine, use propeller cones to center the hub on the shaft (see Figure 4). Usually, the rear cone, which is closest to the engine, is made of one piece of bronze, with a cut to allow for flexibility during installation. Put the rear cone in place before you heft the prop into place. When installing a propeller assembly on a splined shaft you must ensure that the position of the propeller on the hub in relation to the master spline is correct. The engine manual should provide instructions for this procedure. The front cone is a matched set (with serial numbers that confirm their pairing) made of two machined steel pieces that to fit together. It goes on the shaft after the prop, and the retaining nut goes on last. After tightening the retaining nut to the proper torque value, install a clevis pin or small bolt to safety it. If the hub is loose on the shaft after you've tightened the retaining nut, it means the f r o n t and rear cones "bottomed" during the installation, and the f r o n t p a r t of the
FIGURE 4
KEYWAY THREADS
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cone has bottomed on the shaft's splines before it contacted the hub's seat. To correct this problem you may need to add a spacer. If you're new to installing a prop on a splined shaft, I recommend that you seek the assistance of a mechanic who's performed this task several times. Constant-speed props—There are several variations of the installation process for constant-speed propellers, but the normal setup requires oil to flow from the engine to the prop hub to control the blades' pitch angle. During the installation you must insert an O-ring or s i m i l a r device to keep the engine's lifeblood (the oil) from leaking. Use extreme care when performing this step to properly place and not damage the O-ring. Constant-speed props also require a governor. Because there are a number of variations of the installation process, follow the manufacturer's directions and recommendations. Seek the guidance of an experienced mechanic if you have no prior experience. Installing a constant-speed prop is not an overly difficult procedure, but it can be quite involved.
Final assembly and checking
Once the propeller is in place, you may want to install a spinner to smooth the airflow over the propeller assembly and into the engine for cooling and to improve the airplane's appearance. A spinner is usually attached to a mounting bracket or a small bulkhead. In either case, you need to include them in the propeller installation process. You'll probably purchase a spinner kit for your propeller and airplane, and this might require you to drill holes in the bulkhead. You can also build your own spinner. Tony Bingelis gives a good explanation of this process in his book Firewall Forward. Before you attach the spinner to its b u l k h e a d or bracket, doublecheck the propeller installation for proper safetying, etc. Once all the components are in place, check the
prop's operation at the proper static rpm, which is simply the maximum number of revolutions the engine can produce at full power when the aircraft is stationary. You should find the prescribed static rpm in the engine specifications. It's important to have an accurate tachometer to determine whether the engine has achieved the proper number of revolutions. If the static rpm is lower t h a n prescribed, your engine may not develop adequate thrust for takeoff. If it's too high, the engine may exceed its redline limit. If you have this problem, it's possible that the propeller you selected is not adequate for your airplane.
Inspection and care
Regular inspections are the key to safe, reliable propeller performance. Obviously, if your airplane is experiencing engine roughness or vibration, one of the first places to look is at the propeller. If you have any doubt about the propeller's airworthiness—do not fly the airplane. Have a professional take a look and make any corrections needed. Inspecting the condition of the blades, hub, spinner, and prop controls should be part of every preflight and pre-takeoff inspection. Inspecting the blades is critical, and you should look—and feel—for any damage (nicks and dings) or defects (scratches and cracks). If you discover any of these, correct them before flying the airplane! Wooden props—In addition to cracks and nicks, look for glue failure, evidence of delamination, and
warping. Sensenich recommends that owners remove the propeller if
they find any of these defects: • Cracks in hub bore • A long, wide, or deep crack parallel to the grain of the
wood • A deep cut across the wood grain • A separated lamination • Oversized or elongated hub bore or bolt holes
• An appreciable warp (discovered by inspection or through rough operation) • An appreciable portion of wood missing • Obvious damage or wear beyond repair Wooden props usually have a fabric or metal sheathing and metal tipping, so look for wear and damage in these areas as well. Composite props—Damage to a composite propeller should be fairly obvious. Delamination is a possibility, along with cracking, and damage to the outer surfaces. Inspect composite props in accordance with the manufacturer's recommendations. Look carefully for small hairline cracks in the gel coat found on some composite props. Vibration generated by a propellerengine m i s m a t c h often causes these spider cracks. If you find a crack in the prop's base material, do not fly until the manufacturer inspects the propeller. Metal props—Nicks caused by rocks and other debris are the most common type of damage to a metal prop. Corrosion can also be a problem. FAA Advisory Circular 20-37D, "Aircraft Metal Propeller Maintenance," gives an excellent guide for inspecting and correcting metal prop problems, and it stresses the importance of discovering—and removing—nicks. AC 20-37D r e m i n d s pilots to "have nicks removed promptly and properly whenever they are found, for a nick may develop into a crack during normal propeller operation. And, if the crack develops far enough, the blade will fail. Nicks are most critical in the outer 18" of the blade, and a small nick may be as detrimental as a larger one. Removal of a nick is not preventive maintenance—it must be accomplished by either a certificated mechanic or FAA approved repair station." Corrosion may be present on a propeller blade or hub, and discovering it warrants further investigation. Light surface corrosion may
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not be serious, but a knowledgeable mechanic should inspect it. Corrosion may require removal of the propeller and repair by a certificated propeller repair facility. General inspection and operation—To properly perform more thorough periodic inspections of constant-speed propellers, and some fixed-pitch installations, you'll need to remove the spinner. I want to emphasize that whether you find damage or defects during a preflight or periodic inspection, it should be cause for concern. Understand that you should not make any repairs to a propeller. Leave that for the experts at the prop shop. After the initial flight with your new propeller, recheck the security and torque of the prop bolts. Thereafter, check attaching bolts, the hub assembly, etc. at least every 50 hours of flight or annually. A few more things for you to consider. Do not use the propeller as a tow bar to move the airplane. Be careful where you run up the engine—don't do it in areas where you might pick up rocks, etc. The same applies when taxiing. If you ever have a prop strike, immediately consider the propeller as unairworthy until an expert inspects it.
Propeller safety
The propeller is the most dangerous part on your airplane, and the FAA has an excellent publication about propeller safety, Advisory Circular 91-42D, "Hazards of Rotating Propeller and Helicopter Rotor Blades." Emphasizing the danger of propellers, it says: "It is particularly tragic that propeller and rotor to person accidents, along with airmen, have included bystanders, passengers, and children among the injured persons. Propeller and rotor to person accidents differ from other aircraft accidents in that they usually result in fatal or serious injury." W i t h t h a t i n m i n d , here a r e some safety p r e c a u t i o n s you should remember: Make sure the area is clear of 62
DECEMBER 2000
people and objects before starting the engine. Shut down the engine for boarding or deplaning passengers. Treat all propellers as though the ignition switches are turned on. Use wheel chocks and parking brakes before starting or hand-propping an engine. Hand-propping an engine equipped with a starter can be convenient when faced with a low battery or faulty starter, but it exposes personnel to a possible accident. To preserve safety, remedy the problem with ground power or by replacing the starter. Only people with experience should handprop an airplane, and a reliable person s h o u l d be in the cockpit to manipulate the switches and hold the brakes. Before shutting down the engine, test the magneto ground by briefly turning the mag switch off and then back to both. If the engine "stops" for the moment the switch is off, the magnetos are properly grounded. If it continues to run, one or both of the grounds are defective and should be repaired. Before moving a propeller or connecting an external power source to an aircraft, be sure that the aircraft is chocked, the mag switch is turned OFF, the throttle is closed, the mixt u r e is at IDLE-CUTOFF, and all equipment and personnel are clear of the propeller or rotor. F a u l t y diodes in aircraft electrical systems have caused starters to engage when external power was applied regardless of the switch position. The propeller is what generates
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the thrust that makes flight possible, and given this important role, pilots should treat the propeller with respect during installation and operation.
Resources Here are a number of sources and information about propeller selection, installation, and safety. Those with product numbers are available on the EAA website at www.eaa.org or by calling 800/564-6322. All
FAA documents listed (and all FAA documents in general, including regulations, FAA orders, and handbooks) are available on CD-ROM from Summit Aviation at www.summitaviation.com or by calling 800/328-6280. • Firewall Forward by Tony Bingelis—F13950 $24.95 • Aircraft Propellers and Controls (Jeppesen
Training Book) • Airframe and Powerplant Mechanics Powerplant Handbook, AC 65-12A • AC 20-37D: Aircraft Metal Propeller Maintenance • AC 43.13-1B: Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair • AC 91-42D: Hazards of Rotating Propeller and Helicopter Rotor Blades • Propeller Operation and Care—FAA-P8740-21 • www.sensenich.com • www.hartzellprop.com • www.mccauley.textron.com
