By Roger Long
Youre making your preflight inspection of the airplane and almost casually run your hand along the leading edge of the propeller blade. A finger catches. Closer inspection reveals a small nick in the prop. Its no big deal. Or is it?
Its a hard thing to do, grounding the airplane on a beautiful weekend for just one little nick in the prop. It is less than 1 percent of the blade sectional area so, if strain is proportional to the area it is distributed over, there should be only an insignificant decrease in strength. Why not go flying?
The simple reason is that nothing bears weight or strain without bending or stretching. When a fly lands on the Golden Gate Bridge, it sags, a little. It is this stretching in response to strain that results in small nicks magnifying the stresses in a propeller far out of proportion to their size.
Imagine a prop blade at rest. Now paint lines in both directions that form a square grid on the props surface. Here is what you get.
Now start the engine up and advance the power to maximum takeoff power. The centrifugal force of the turning prop subjects it to loads that can be more than 10 times the weight of the aircraft. Whether made of aluminum, wood or high-tech composites, the prop stretches in response.
Note that the squares have become rectangles.
This level of stress is the reason a broken prop blade can result in the engine tearing itself off the mounts. When the blades are intact each one balances the forces created by the others. Remove that balancing force and the blade is suddenly not anchored at the hub.
Lose the weight of the engine, and the airplane becomes uncontrollable. A prop failure is far more hazardous than a crankshaft failure. Think of 10 airplanes hung above your head by that small piece of aluminum. Would you want to see a nick in it?
The stresses created within the prop blade are far more significant that you might realize. Put a greatly magnified nick in your grid-marked prop.
And bring it up to takeoff RPM.
Once again, each metal particle in the blade is being forced out by the centrifugal force of the prop turning. It is also being pulled outboard by the particles it is attached to. The nick, however, interrupts this chain. The metal to the left of the nick is no longer being pulled outboard by the weight of the metal to the right. It actually experiences less outward pull, so it stretches less.
Each particle is also held in place by the metal next to it. The metal to the right of the nick is no longer restrained so it stretches farther outboard. The metal is thus being pulled in different directions right at the base of the nick. Added to this strain is the fact that the basic load already increased by the smaller cross section of metal.
The slanted lines of the grid reflect the fact that the metal particles are all bound together. The metal to the right and left of the nick would actually like to slide as blocks. This creates a line through the blade, parallel to the leading edge and running right through the base of the nick, where there is significantly different strain above and below. These strain differences are in opposite directions right and left of the nick and it all comes together at the bottom of the notch.
Fortunately, the prop blade is somewhat overbuilt and operating with a safety factor that, of necessity, is quite generous by general engineering standards. However, the geometry of the stress concentration can multiply the strainaround the base of the notch many times.
In many structures, such as a riveted airframe, a crack may not significantly weaken the structure. The crack relieves the strain on a highly stressed point, but the rest of the structure can take up the load in a better-distributed form.
In a homogenous structure like a prop blade however, the geometry of crack propagation is such that increasing the size of the notch will immediately increase the amount of stress concentration and strain at the base of the notch. It is a vicious circle and the critical metal particles are those in the microscopic area right at the base of the V.
This is not the classic explanation of stress concentration. The usual example is of a member being stretched with one end fixed and a load at the other end. The lines of stress have to flow around the nick and get crowded together. Since stress is a function of the amount of area it is distributed over, the same stress in less area raises the strain.
The change in direction of the stress lines also raises local forces trying to separate molecules of your prop. Just think of pulling on a straight wire vs. pulling on one with kinks in it.
These factors are also at work raising stress levels around the nick. The differential in stretch described above is just adding to the local stress. Now add bending, because the prop blade is fixed at the hub and being pulled forward over its whole length. Bending stretches one side of the prop and compresses the other. The metal around the nick may be taking a triple hit.
The more brittle a material, the more critical stress concentration is. Aluminum is fortunately fairly forgiving in this regard. However, it has other characteristics that are cause for concern. When stressed beyond a certain point, even a small number of times compared to steel, it undergoes permanent changes in its properties that can weaken it and make it more susceptible to cracking.
The probability that the prop blade would have departed the aircraft over this weekend would have been very low. The level of risk would probably not have been out of line with other general hazards of flight. The stress and strain on a prop is not steady, however. It flexes, whips and stretches microscopically with every cylinder firing. Even in a few hours of operation, the magnified strain at the base of this nick could create changes in the metal that would persist even after the shop files the nick into a long smooth shape that will stretch evenly.
Flying an airplane until a qualified person can dress out a nick is always a judgment call. That judgment must be guided however by the knowledge that the geometry of the way materials behave under strain can magnify the effects of a small nick way beyond what you might estimate intuitively just by looking at it.
If you doubt that, next time you open a bag of potato chips, try pulling first on the side that doesnt have the notch.
Also With This Article
Click here to view “Anatomy of a Prop Failure Accident.”
-Roger Long is a commercial boat designer, a private pilot and Maintenance Officer of the Bald Eagle Flying Club in Portland, Maine.
