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Tuesday, November 27 2012 05:59

Prop Talk

Written by  Dale Smith
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 November 2004

Without a doubt, they're the hardest-working, most under-appreciated part of your airplane. Of course, I'm talking about your propeller.

Most of us just think of a propeller as a chunk of aluminum spinning around on the front of our airplane. How wrong we are. Your propeller is one of the most highly stressed components on your airplane. During normal operation, it has to withstand 10  to 20 tons of centrifugal force that is trying to pull the blades right off the hub.

And that's not considering the other stresses put on the blades as they strain to literally pull your aircraft through a sea of air.


Your propeller is routinely subjected to bending, flexing and vibration. And it does all this with little or no maintenance from you. In fact, beyond your cursory glance during your preflight, your propeller never gets a second look, let alone any consideration for a routine overhaul.

And that can be a big mistake.

Since it seems that pilots and aircraft owners have little understanding of what their propellers go through, it's not hard to imagine why they underestimate the value of a propeller overhaul.

"I'd say that first and foremost, the most common misconception among aircraft owners is whether or not overhauls are even necessary," explained Mike Disbrow, Senior V.P., Marketing and Customer Service at Hartzell Propeller, Inc. "Most owners say that their propeller is working just fine, so why should they overhaul it?"

Good question, but most clear-thinking owners would ever purposely fly their engine beyond its TBO—even if it is running smooth as silk. Pilots just don't want to run the risk of an engine failure. So why do they routinely run the risk of propeller failure?

"Propellers don't fail very often," Disbrow continued. "But when they do, it's usually catastrophic. When an engine fails, it rarely causes a pilot to lose control of the airplane. But when a propeller throws a blade or the hub breaks, the imbalance will be so great it will literally pull the engine right off the mount and that most often renders the airplane uncontrollable."

Disbrow theorizes that the reason most pilots don't appreciate the severity of a prop failure is that it is rarely ever listed as the cause of an accident. "Propeller failures are rare and when they do occur they don't often generate much media attention," he said. "So pilots just aren't attuned to the problem. They'll just keep flying on their prop and never give it a second thought... until it starts to leak grease or oil. Then it's into the shop."

Of course by then, what the owner is left with is usually little more than scrap metal. Fatigue, wear and corrosion have rendered all of the mechanical components of the propeller's system useless.

So what should the owner have done? "The typical TBO recommendation on a Hartzell constant speed propeller is divided into two parts: a six-year calendar limit and/or 2,400 hours of operation—whichever comes first. (Yes, there's a TBO recommendation for fixed-pitch props, but we'll get to that later.)

"Hartzell has Service Letter 61 posted on our website (www.hartzellprop.com/product_support/index_support.htm) that spells out all of our recommended intervals for propeller servicing and overhaul," Disbrow said. "We consider these to be requirements, but technically they are only recommendations, because owners operating under Part 91 of the FARs are not required to do any propeller maintenance."

That's probably why a lot of us don't do it.

Your propeller can tell you a lot

The fact is the majority of General Aviation aircraft owners wait until their propeller is damaged or begins to show signs of problems before they take them in for maintenance.

What kind of problems? "The main thing is grease or oil leaks from around the hub," Disbrow said. "Often they are signs that the seals have finally given way or even a hub that's getting ready to fail—if you see grease or oil streaks on the propeller blade, have a qualified mechanic look at it right away."

Another telltale indication of problems is any sudden onset of vibration. "That's a real cause for concern," he added. "If it's sudden and seems to increase with power, land as soon as possible and have it looked at."

Of course, if you spot any dings or dents in the propeller, you need to have those dressed out as soon as possible. Why? Disbrow explained that propeller blades have nodes and the stresses at these nodes are extremely high and generally increase as power is applied or during operation in restricted power ranges.

If you get a ding at one of these points and leave it, it creates a stress riser that can initiate a crack in the blade. And depending on the location of the crack and the overall health of the blade, these cracks can progress quite rapidly.

Disbrow also suggests that you make sure your tachometer is calibrated to avoid inadvertent operation in restricted power ranges. "It's always surprising to us, just how many pilots say that they had some kind of warning of a problem before they had catastrophic propeller failure," he said. "If they had known the signs they probably could have avoided the problem altogether."

Overhaulin'

We all pretty well know what goes on during an engine overhaul, but what does your propeller go through when it finally makes its way into the prop shop? To help give you an idea, Disbrow provided the following outline of Hartzell's approved propeller overhaul processes.

Inspection

When it arrives, the propeller is inspected and a document is prepared that will accompany it through the overhaul process. All applicable ADs, current specifications, and manufacturer Service Bulletins are researched for incorporation during the overhaul process.

Next, the unit is disassembled and cleaned. All parts are thoroughly inspected and those requiring replacement or rework are noted in the documentation.

Many specialized tools and fixtures are required for the proper disassembly and reassembly of propeller components. Many are model-specific and range from massive 15-foot torque adapter bars and 100-ton presses, down to tiny dowel pin alignment devices.

Hubs

Nonferrous hubs and components are stripped of paint and anodized coatings and are inspected for cracks using a liquid penetrant inspection (LPI) procedure. Certain models of hubs are also eddy current-inspected around high-stress areas. This will detect flaws that are below the surface of the material. Magnetic particle inspection (MPI) is used to detect flaws in steel parts.

Additionally, components that are subjected to wear are dimensionally inspected to manufacturer's specifications. After passing all inspection steps, aluminum parts are anodized and steel parts are cadmium plated for maximum protection against corrosion.

Blades

The first step is the precise measurement of blade width, thickness, face alignment, angles and length. The measurements are checked against the manufacturer's minimum acceptable overhaul specifications.

Next, the surface of the blade is ground and re-pitched, if necessary. Specialized tooling and precision measuring equipment permit pitch changes or corrections of less than 1/10th of one degree. To ensure accuracy, face alignment and angle measurements are taken repeatedly during the repair process.

Lastly, precision hand-grinding of the blade airfoil is done to remove all corrosion, scratches and surface flaws. Then the propeller blades are balanced to perfectly match each other and anodized and painted for long-term corrosion protection.

Reassembly

Once the components are completed, it's time for reassembly. Part numbers are re-checked with the manufacturer's specifications. The parts are lubricated and installed per the propeller's particular overhaul manual. Both high- and low-pitch blade angles are set and the assembly is checked for proper operation and leaks.

The reassembled propeller is checked for static balance. If necessary, weights are placed on the hub areas of each "light" blade socket. These weights should be considered part of the basic hub assembly and should not be moved during subsequent dynamic balancing of the engine.

Finally, the propeller's components are safety-wired unless secured by self-locking devices. The finished propeller then receives its maintenance release tags reflecting the work accomplished, applicable ADs and all incorporated service documents. Now the propeller is ready to return to service.

Picking a prop shop

As you can see, overhauling a propeller is a sophisticated and demanding process that requires a combination of specialized training and equipment. It's not something that any maintenance facility can, or will, attempt.

"Many pilots don't realize it, but the FARs spell out clearly what types of propeller maintenance can be done by a licensed mechanic and which need to go to an approved propeller shop," Disbrow said.

"And in my opinion, that's one of the biggest mistakes owners make—when they decide to have their propeller overhauled, they all too often choose a shop based on price. A low price may attract a lot of business, but you may not be getting the quality of overhaul you need."

Disbrow's suggestion to owners: "If it's a Hartzell propeller, finding a factory-authorized shop is as easy as visiting our website. There you'll find a complete list of shops that we know have the equipment, documentation and training to perform quality propeller overhauls.

"If there isn't a Hartzell authorized shop in your area, then I suggest you talk to other owners to find out who they have used, then visit the shop yourself and ask questions," he added. "Just like you'd do with your engine when shopping for someone to do your overhaul. Get information, ask questions and don't let price be your only guideline."

Also, to help in your search Disbrow provided a list of questions that you can use to help narrow down your search. (See "Questions to ask your prop shop," later in this article.)

Fixed pitch props need love too

Up until now all of our talk has been about the needs of constant-speed propellers, so you owners who have fixed-pitch props probably thought you are off the overhaul hook—well, think again.

Your simple fixed-pitch propeller needs just as much care as its complex cousin.

"Pilots think a fixed-pitch propeller is just a slab of aluminum but they're much more than that," explained Ed Zercher, General Manager, Sensenich Propeller Manufacturing Company. "It's not indestructible. Along with fatigue, the biggest concerns are nicks and dings that happen along the leading edge of the blade. If you don't routinely dress these areas they can quickly lead to cracks and possibly blade failure."

Zercher also said that your fixed-pitch propeller has a TBO time, too. "The TBO for one of our propellers is 2,000 hours," he added. "Like the constant-speed propellers, the TBO is just a recommendation unless the airplane is used for hire."

So how do you "overhaul" a fixed-pitch propeller? "The term overhaul is sort of overstating the process," Zercher said. "What we really do is recondition the propeller. We begin with a thorough inspection which includes a check of pitch, blade angle, face angle and the like. Then we resurface the propeller by removing about ten-thousandths of an inch of material. That removes any surface corrosion and relieves metal fatigue."

After additional inspections, the propeller is anodized and painted so it is, in fact, a brand-new propeller. And it doesn't just look new, it flies like new, too.

"Pilots are amazed just how much better their airplane performs with a newly reconditioned propeller on it," he explained. "Many of them say it's like they got a bigger engine on their airplane. They just haven't noticed that over time their propeller has gotten out of its optimum shape. After it's overhauled, it delivers the performance the way it's designed to."

 

Questions to ask your prop shop

The truth is, not all prop shops are alike. There can be significant differences in the sophistication of their equipment and the experience and training levels of the technicians.

To help you create your own prop shop scorecard, Mike Disbrow, Senior V.P., Marketing and Customer Services for Hartzell, provided us with the following list of questions to help you get a better understanding of each shop's capabilities and qualifications:

• Does the shop maintain the required sample program approvals to perform critical required overhaul tasks? Shops need to be audited and approved to perform tasks including the Aluminum Hub Special Process, Hard Chrome Plating, Cold Compression Rolling, Shot Peening and others.

• Are the non-destructive tests performed by a certified Level II inspector? If they say yes, ask to see the certification.

• When are the blade dimension inspections performed during their process? Many shops take the measurements before they grind the blade, but not again after. A dimensional inspection must take place after the grind to ensure that the blade meets the requirements of the propeller manufacturer's overhaul manual.

• Will your aluminum blade be anodized or alodined? While both are permitted, anodization provides corrosion protection that is far superior to alodine.

• What new parts will be provided in the overhaul price? Price quotes can vary greatly from shop to shop. Compare what parts are included in each shop's overhaul kit. Also, insist that you are getting genuine factory replacement parts.

• What procedure does the prop shop use to perform shot peen? Many shops do not use any procedure to control shot angle, intensity, or duration. Shot peen is a critical operation to improve part fatigue resistance. Done incorrectly, it can actually damage the part and reduce fatigue strength.

Dale Smith has been in love with airplanes and flying forever. A prolific aviation journalist, Dale writes for numerous other aviation magazines. When he's not writing fun stuff about airplanes, Dale is also a principal partner in Flying Boat Creative Services, an advertising agency specializing in aviation.

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