Features  / 02.13 /

Prop art

Our path to the sky


Photography by Chris Rose

There is something incredible about a piece of metal, wood, or plastic that can withstand 20,000 to 40,000 pounds of force at each turn, and still boast the esthetics of a world-class sculpture. The propeller is powerful, strong, and pretty enough to be made into art. There is nothing like the sun shining off the huge, polished propeller of a P–51, or the perfect curves of a three-blade workhorse.

What’s most beautiful about the propeller is that it’s our path the sky. And the fact that a fan on steroids can get us in the air is nothing short of amazing.

The propeller is an airfoil, which is to say it’s very similar to the wings. Most light-aircraft wings are pretty much straight slabs, while the propeller has a significant amount of twist built in. But otherwise, they are essentially the same. There is a chord line between the middle of the leading edge and the rear, and an angle of attack between that chord line and the relative wind. As the propeller turns, it “lifts” the airplane forward, just as a wing lifts the airplane vertically.

propellerThe Wright brothers are credited with designing the modern propeller, even though it’s been described for centuries. Their incredibly advanced version included the twist, and according to one FAA estimate was only about 5 percent less efficient than today’s computer-designed models. Modern propellers boast an efficiency of around 80 percent, meaning that only about 20 percent of the received power is lost through friction and other factors.

By far the most common propeller on training aircraft today is made of aluminum, with two blades and an integrated hub. As engines get more powerful, their propellers go from two to three or four blades—or even more—and usually to a constant-speed control mechanism with an oil-filled hub and individual blades. Less-powerful airplanes often use wooden propellers.

The pilot’s concern for the propeller is generally the same regardless of material, although wood does require some extra attention. Preflighting the propeller requires getting physical with your machine. It helps to take an overall look at the shape of the blades to make sure nobody pranged the airplane and just put it back on the line. A view from five feet away should reveal any unusual twist or bend.

Then you must get close and run your hand down the length of each blade, especially on the leading edge. Cracks, and the resulting blade failures, often originate from small nicks here, so it’s important to make sure any small nicks or cracks are addressed by a mechanic. Finally, take a look at the back of the propeller for damage, usually caused by rocks or other debris.

In most airplanes the propeller won’t be perfectly smooth, and that’s OK. The trick is determining the difference between some peeling paint and a real problem that requires a mechanic. This situation is a great example of why it can be beneficial to know who does maintenance on the airplane. If it’s someone you’re comfortable talking to, just head into the shop and ask some questions, or have the technician take a look. As a general rule, it’s always better to ask for help than to assume everything is OK.

Wooden wonders

wood prop
For true romanticism, only a wooden propeller will do. For that reason, and many others, they remain popular more than 100 years after their introduction. Wooden propellers are usually made of laminated wood (layer upon layer glued together) that’s at least partially hand carved. Many also feature a metal leading edge to help protect them from grass, dirt, and other hazards where they are known to operate.

The biggest consideration when flying an airplane with a wooden propeller is that rain can strip the varnish, potentially damaging the wood. Cracks also have been known to start at the fastening location of the metal leading-edge strip, so careful preflight inspection is required in that area.

Depending on the severity of the issue, the propeller will either need to be filed down and properly dressed locally by a mechanic, or sent away for some specialized work. Believe it or not, even multiple dings, corrosion, and problems with tips can be addressed by a mechanic, although the depth, placement, and number of blemishes matter. The FAA publishes a graph that enables mechanics to determine what can be repaired. To the untrained eye this makes performance charts look like preschool work.

The one thing a local mechanic can’t adjust is the blade twist. This is spelled out clearly in all the FAA’s guidance. Apparently it’s a common problem that after the blade strikes a hard object and bends, the mechanic will attempt to straighten it for shipping to a specialized shop. Because metal weakens after it is bent, only very minor straightening is allowed—and then only by those specifically authorized to do so. In other words, if you hit a large rock, collapse the gear, and strike the propeller, or run into anything else hard, it’s likely the propeller is a goner.

One of the best things an aircraft owner can do to keep a metal propeller in top shape is to have it painted. Repainting usually is done at inspection time, which is when the hub and blades are examined by the mechanic in detail, often with a magnifying glass. Ignoring this guidance can lead to some less than desirable consequences. In June 2008, a Piper Seminole lost a blade in flight soon after some work was done to file out a leading-edge blemish. There have been other accidents in which too little propeller was left after the work, and they too lost blades. The good news is that these accidents are quite rare. In the past decade only 75 accidents can be traced to propeller issues, and many of those were problems with homebuilt airplanes.

Regular inspections also will include a close examination of the propeller hub, which is fixed to the blades as one seamless piece in a fixed-pitch propeller or a separate piece in a constant-speed model. With the common fixed-pitch propeller, this inspection looks for wear on the mounting hardware and mounting holes in the aluminum. Although basic, this is important because the spinner completely covers the hub on most aircraft.

Even if everything is checking out properly, propeller manufacturers recommend regular overhaul intervals, just like the aircraft engine. These typically occur between 1,500 and 2,000 hours, or five years, whichever comes first. The aircraft’s propeller logbook should reveal the details of such work, assuming it’s been done.

When the propeller comes back from overhaul, it will look brand-new, having been properly cleaned, dressed, and repainted. It can then be reinstalled to once again pull us into the air with style and grace as it gets twisted, turned, and pummeled by thousands of pounds of force.

Ian J. Twombly is editor of Flight Training magazine.

fixed pitch prop

As the propller turns

The unique characteristics of the propeller bring with them unique flight considerations. Each of the major left-turning tendencies can be attributed to the propeller. They are:

Torque—As seen from the cockpit, most propellers turn clockwise. That turning force has an equal and opposite reaction, which causes the airplane to want to roll left.

Precession—The fast-spinning propeller acts like a gyroscope (imagine holding on to a spinning bicycle wheel). When the airplane pitches up, the force is applied 90 degrees ahead, causing the left-yawing force.

Slipstream—The turning propeller causes a spiraling airflow around the fuselage that eventually contacts the vertical stabilizer on the left side, pushing the tail to the right and nose to the left.

P-factor—The easiest way to think of P-factor is to look at the profile of the propeller. As the arc comes down on the right, the angle of attack is higher than when it comes up on the left. That higher angle of attack means more lift, or pulling force, which results in a tendency to turn left.

Fixed-pitch versus constant-speed propeller

General aviation aircraft commonly employ one of two types of propeller: the fixed pitch or constant speed. Fixed pitch is pretty self-explanatory. The blade angle is fixed to a central hub, which is bolted directly to the engine. Power to the propeller is controlled by the throttle, which simply makes the engine—and the blades—turn faster or slower. The wind’s friction force also will affect this type of propeller, making it slow down or speed up, depending on the speed of the airplane.

In a constant-speed propeller, the pitch of the blades continually changes to maintain a constant rpm, which the pilot sets through the propeller control. The individual blades of a constant-speed propeller attach to a central hub that also houses a governor, a somewhat complex device that automatically adjusts blade angle through oil pressure, springs, and counterweights. Oil found leaking from a constant-speed propeller hub during preflight indicates a potential problem.

Although more expensive, complex, and with a risk of failure, constant-speed propellers are superior because they allow the pilot to set the optimal blade pitch for takeoff, climb, cruise, and descent. A fixed-pitch propeller, on the other hand, is optimized for one flight segment only. What is good for climb isn’t necessary good for cruise.