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Flying Your Propeller: Converting fuel, noise and rotation into thrust.

A reader recently wrote to request an article on propellers: Why are "constant-speed" props called what they are, how can a fixed-pitch prop "lug" the engine and how does the number of propeller blades affect performance were some of the questions posed, and we promised to follow up. We'll talk mainly about how to fly common propeller systems on piston-powered small airplanes.

Remember that propeller blades are airfoils moving in a plane different from and usually perpendicular to the direction of flight. As an airfoil, the amount of lift the blade creates when moving through the air depends on its angle of attack, and its angle of attack--plus drag--can depend on a variety of factors, including the airplane's pitch attitude. Remember, too, that the outer portions of long prop blades move faster--they cover greater distance in the same amount of time--than shorter ones.

FIXED-PITCH/GROUND-ADJUST

The most basic and inexpensive propeller is a fixed-pitch version, possibly made from a single piece of alloyed aluminum. Other construction materials include laminated wood, probably with a metal leading edge, and composites, sometimes over a wood core and sometimes also with a metal leading edge. Regardless of the material used, a fixed-pitch prop is one whose blades cannot be pivoted in their hubs to change their angle of attack. What you see is what you get.

Fixed-pitch props can be manufactured to different blade angles, i.e., different pitches. Doing so can optimize a propeller for cruise flight--a coarser pitch that moves more air each rotation rather than one pitched to a shallower angle. Props pitched to a shallower angle can optimize takeoff and climb performance, at the expense of cruise speed for a given rpm. Props pitched for cruise allow higher airspeeds at the expense of takeoff and climb performance.

A variation on this theme is the ground-adjustable propeller. As its name implies, a qualified technician with the correct tools can change the blades' angle of attack on the ground, allowing per-flight optimization for cruise or climb, but not both. Ground-adjustable propellers are popular on FAR Part 103 ultralights, homebuilts and some LSAs. You fly a ground-adjustable prop the same way you do a fixed-pitch prop, since there's no airborne adjustment available.

A typical direct-drive airplane engine equipped with a fixed-pitch prop cannot achieve its full rated rpm--required for the engine to develop its rated power--even at full throttle while static or during takeoff. Once some airspeed is gained and/or level flight is attained, that same typical engine turning a fixed-pitch prop should be able to achieve near its rated rpm, and even exceed it in a dive. A fixed-pitch prop with a shallower blade angle--perhaps one configured as a climb prop--will allow the engine to turn faster at slow airspeeds, improving performance..

CONSTANT-SPEED PROPS

The takeoff-power limitations posed by fixed-pitch props led to engineers figuring out a way to pivot the blades in the propeller hub, thereby changing their angle of attack. The mechanisms to do so are beyond this article's scope, but they can be electrically powered or use engine oil as hydraulic fluid. Some designs will have pre-set positions: high rpm, fine pitch for takeoff and low rpm, coarse pitch for cruise. These are usually known as variable-speed propellers.

The constant-speed propellers on typical personal airplanes allow infinite adjustment between their high-rpm and a low-rpm stops. However, instead of measuring the blade's pitch angle, the pilot references the engine's rpm to set propeller pitch. In turn, throttle position is measured with manifold pressure. (A pilot's first introduction to a constant-speed prop often involves monitoring the tachometer while retarding the throttle and not understanding why rpm isn't decreasing. For more on manifold pressure, see the sidebar at the top of the opposite page.)

The genius of the constant-speed prop is that it does exactly what its name implies: turns at a constant rpm set by the pilot, who really is adjusting the propeller governor. The typical prop governor is a bolt-on device used to increase the engine oil's pressure and meter its flow into the propeller hub to hydraulically rotate the blades and change their angle of attack. A constant-speed prop will automatically change the propeller blades' angle of attack--perhaps minutely--to maintain the desired rpm as, for example, the throttle setting is changed, the airplane's angle of attack varies or it encounters a gust.

Constant-speed propellers are heavier and more expensive to acquire and maintain than fixed-pitch props. But as available power increases, it's unusual to see a fixed-pitch prop on an engine of more than, say, 180 hp because of its inability to keep up with the constant-speed variety when it comes to performance and efficiency.

FEATHERING/REVERSING

Once engineers figured out how to pivot the propeller blades in their hub, the logical next step was aligning them with the airplane's slipstream--feathering--or going past the feather position to reverse the propeller's thrust. A sidebar on the opposite page explores feathering and unfeathering systems.

Reversing is just what it sounds like: pivoting the blades in the hub past their feather position to push air ahead of the airplane, slowing it or even allowing it to move backward on the ground under its own power. Reversing thrust while airborne is a big no-no, and has resulted in crashes of both propeller-driven airplanes and jets. Few piston airplanes are equipped with reversible props, but they're common on turboprops.

For the most part, props don't need that much maintenance from year to year--dressing out nicks and touching up paint can be all the attention they need. But working on them generally requires special procedures and tooling your average mechanic doesn't have, so it has to go to a shop that does.

BY THE AVIATION SAFETY MAGAZINE STAFF

FEATHERING/UNFEATHERING

"Feathering" refers to changing a propeller blade's angle to minimize the drag produced, which is useful when the engine fails. They are rarely seen on a piston single but are almost universal in piston twins.

By pulling back the prop control on the typical piston twin after the engine has been shut down or fails, the prop enters the feather position, streamlining it and preventing it from windmilling in the slipstream, thereby reducing drag.

The details of propeller unfeathering systems vary from aircraft to aircraft, but each is designed to force the prop blades out of their feathered position so the engine can be restarted and used to produce thrust.

PROPELLER DESIGN 101

When a propeller with two or more blades turns, its rotation disturbs the air each blade encounters, reducing efficiency. But efficiency doesn't move an airplane. The "best" propeller is the one offering the greatest conversion of rotation into thrust, depending on the engine and airframe.

High-powered engines often mount multiple propeller blades to convert as much as possible of that power into thrust. Given the same engine/airframe combination, a two-blade prop generally will be slightly more efficient than a three-blade, resulting in faster cruising speeds on the same power. A three-blade prop in turn likely will offer better takeoff and climb performance. There are exceptions. Two-blade props can look wimpy compared to a three-blade, giving rise to the saying, "Two for the go, three for the show."

The longer the blade, the greater the prop's diameter and the faster its tips turn at a given rpm, often becoming supersonic and noisy. Meanwhile, the larger the prop's diameter, the greater the likelihood of a prop strike.

So-called "scimitar" blades, shown at right mounted on a Beech 58 Baron, are swept to reduce noise and may achieve greater efficiencies, depending at least in part on the engine/airframe combination.
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Title Annotation:AIRCRAFT SYSTEMS
Publication:Aviation Safety
Date:Jun 1, 2019
Words:1267
Previous Article:You're On Notice.
Next Article:Understanding Manifold Pressure.

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