a quick question

[cforst513]

in reading my aviation literature i have come across the term "feathering." what does this mean?

 

[JeffSKDTW]

As in, "feather the prop," means to set the angle of a CS or VP propeller edge to the airflow to minimize drag and rotation.

 

[avbug]

Feathering is a process of rolling a student pilot in hot tar, and then coating his or her otherwise-naked body in young goose-down feathers, and then dunking the person in cold water to cool the tar. Seldom practiced today, it began as a jovial Irish solo tradition, but is usually now conducted by having the student sign his or her shirt tail, cutting it off, and nailing it to a wall.

Feathering is also a term used to describe turning a flying surface into the relative wind. You will see it most often used to describe an action involving a controllable pitch propeller. The blades of the propeller are rotated using hydraulic, electrical, or mechanical means, to align each blade with the wind. This is done to reduce drag.

You can see the effects of feathering for yourself by sticking your hand out the car window while driving down the highway (be sure there's no oncoming traffic, first). Place your palm facing forward, and feel the drag pushing your hand toward the rear of the car. Turn your hand so that your palm is facing down, and note the drag drastically decrease. Your hand has just "feathered," or been "feathered."

Fully articulating helicopter rotor systems also have a feathering hinge which serves to do the same thing, but for different reasons, and in a different way. An airplane propeller is feathered 90 degrees, or at a right angle, to the plane of rotation. That means that the chord line of the blade, the imaginary line that runs from the front or leading edge of the propeller blade to the trailing edge (rear edge) of the blade, is turned so that it's aligned with the wind...just like your hand in the car.

In a helicopter rotor, the opposite is done. The feathering action takes place in the plane of rotation. Feathering in the same way an airplane featheres wouldn't make much sense; the helicopter would drop out of the sky quicker than you can say aunt Jemima's toxic hot sauce. But adjusting the individual angle of attack of the blade, of in other words, helping align the chord line of the blade with the relative wind (in the direction the blade is moving) does have some merit. The hinge that helps do this, and which helps control collective pitch on the rotor blade, is called the feathering hinge.

Feathering is accomplished on an airplane to reduce the drag on the propeller. This is important in a multi engine airplane. A windmilling propeller, or one that isn't feathered, and is being driven by the wind (much like a child's pinwheel toy), incurs a bigger drag penalty than a giant plywood disc out there that is the same diameter as the prop arc. That's a lot of drag. By stopping the propeller from spinning, and by aligning the blades of the propeller with the relative wind (slipstream), the drag produced by that propeller is greatly reduced.

In a multi engine airplane, this serves to make the airplane much safer, more controllable, and to allow the airplane to use it's other engine(s) for performance instead of using the power to compensate for the bad engine. This may mean the difference between being able to stay in the air, or being forced to keep descending. It may also make the difference in weather the airplane can climb or not, a critical performance issue.

And possibly a life-saving one.