Aviation From My Rocking Chair: A Healthy Fan Up Front Is a Good Thing

Howdy, troopers. Welcome to another installation of “Ricks Playground” (none of this airplane stuff qualifies as work!) This month let’s discuss composite propellers; including their operating stresses, advantages, and disadvantages. I’ll also cover some inspection items you can add to your preflight.

If you look at propellers at rest, it’s hard to imagine what stresses they endure while in operation. There are five forces which act on a propeller…all at the same time, including Centrifugal force, Thrust Bending force, Torque Bending force, Aerodynamic Twisting moment, and Centrifugal Twisting moment. Of these, Centrifugal force (which tries to pull the blades from the hub) is the greatest. Centrifugal force is related to RPM in that as RPM increases, force is increased exponentially.

A typical metal propeller blade load is 25 tons of centrifugal force at the root with a minimum of 20 tons. If a propeller fails catastrophically, not only is there danger from the unrestrained blade, but, according to Newton’s law of Momentum Conservation, the force the blade is subjected to normally is transferred to the system when the blades depart the aircraft. It is this amount of force (20-25 tons or more) that can rip engines from mounts and cause severe structural damage.

Now that I’ve scared everyone, I’ll remind you that failures for any propeller are quite rare. Composites do, however, have certain advantages over metal, the first being their much reduced weight. This increases efficiency because less horsepower is needed for the same thrust. Secondly, composite propellers do not shrink dimensionally after rework. They always are the same size after repair. A metal propeller has material removed until it becomes under-dimension and unusable. The real danger of undersized metal props is their susceptibility to resonance. Tap a metal prop and hear it ring like a bell. Each time material is removed, it changes the natural pitch or frequency of that ring. If a prop finds a sympathetic frequency to respond to, the tip can deflect up to 6’’ which very quickly causes failure (this is also the reason some propellers have a red arc within the normal green RPM operating range) A prop within dimensional limits can’t do this.

In order to keep the fan up front healthy, there are some things we need to check on a regular basis. Start with a good visual inspection looking for:

(1) Delaminated areas caused by impact damage or water/oil/grease contamination. Suspected de-lams will need to be Tap tested with a quarter. Good material will have a sharp, light sound. A delamination will sound like a dull thud.

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(2) Loose nickel or stainless steel leading edge protection. Some manufacturers allow re-gluing, but most require returning the prop for overhaul.

(3) Missing paint or coating due to erosion. Areas where coating is missing could allow fluid to enter and saturate the composite. Fiberglass deteriorates very quickly when exposed to oil and grease. Kevlar degrades slower but just as surely. If water contaminates the laminate and freezes, the expansion of ice will cause delaminated areas.

(4) Propellers exposed to high heat (140-150 degrees f) can delaminate between plies. Check with the propeller manufacturer for inspection and repair criteria.

(5) Composites are susceptible to lightning strikes…especially carbon fiber. Several methods are used to protect propellers from this type of damage including, metal spars, erosion sheaths, and special metal coatings. Make sure they are in serviceable condition.

Well, troopers, it’s time for this ol’ cowpoke to head for the bunk house, and as always I thank you for your support. Writing this column is the best education I could ever get.

Rick Vaux

TC4130