Unlike single-engine aircraft, the propellers on the multi-engine Piper Seminole are designed to fail in a feathered position. Here's how they work.

Review: Why Is Propeller Control So Important?

What's that blue lever next to the throttle? It's the propeller control, and when you fly a plane with a constant speed propeller, it gives you the ability to select the prop and engine speed you want for any situation. But what's the benefit, and how does it all work?

Constant speed propellers work by varying the pitch of the propeller blades. As the blade angle is increased, it produces more lift (thrust). At the same time, more force (torque) is required to spin the prop, slowing the engine down. The opposite is true when the blade angle is decreased: the torque required is decreased, and the engine speeds up.

In most cases, you take off and land with the prop control full forward, which means your propeller is in the flat, low pitch/high RPM setting. Having your prop in that position gives you a lot of takeoff power. But once you get off the ground and closer to your cruise altitude, you'll want to start pulling the prop lever back for a more efficient cruise power setting.

How The Prop It Works In A Piper Seminole

Like most constant speed propellers, blade pitch is controlled by oil pressure, aerodynamic twisting, counterweights, spring force, and a nitrogen charge.

HOWEVER, unlike single-engine constant speed props, in the PA-44 Seminole, oil pressure drives the propellers toward High RPM (unfeathered). The nitrogen charge and spring drive the propeller toward Low RPM (feathered). While our graphics and notes are specific to the Piper PA-44 Seminole, most multi-engine pistons work the exact same way.

Takeoff And Landing

• Low pitch/high RPM: Higher Oil Pressure, Aerodynamic Twisting
• High pitch/low RPM: Lower Oil Pressure, Spring Force, Nitrogen (N2) Charge, and Counter Weights

Cruise Flight

Adjusting your propeller for cruise flight means less drag and faster more efficient cruise legs, but what is actually happening when you pull the prop lever back?

When you pull the lever back mechanical linkages turn the threaded shaft, reducing the speeder spring's tension-- now that the spring is under less tension the flyweights are able to swing out from the rotational force of the engine. As the flyweights swing out this moves the pilot valve up, allowing oil to escape the hub. As the oil leaves the hub the piston in the hub moves forward allowing the blade's pitch to increase causing your RPMs to drop.

As your RPMs drop the centrifugal force on the flyweights reduces meaning the flyweights swing into equilibrium (straight up and down). When the flyweights reach equilibrium the pilot valve shuts. Here's an animation of what the system looks like in action:

Avoiding Asymmetric Drag In Multi-Engine Airplanes

If you're forced to fly single-engine in a multi-engine airplane, one of your primary goals is to preserve performance. While you lose 50% of your power during an engine failure in a twin, you lose roughly 80% of your performance. This is primarily because the failed engine, which was once producing thrust, is now contrbuting a large amount of drag.

Compared to a feathered propeller, a windmilling propeller creates more asymmetric drag. This is a problem in multi-engine airplanes, where Vmc is a concern during single-engine operations. If the propeller is windmilling (flat-facing the wind), the rudder must overcome two greater opponents: the operating engine's thrust and the windmilling propeller's drag. Due to the increased drag from a windmilling propeller, rudder effectiveness decreases, and your Vmc increases. When a propeller blade angle is moved towards feather, Vmc decreases and performance increases, that's why multi-engine constant speed propellers are designed differently.

Feathering: High Pitch / Low RPM

Each propeller is controlled by a blue prop lever in the cockpit. If you move the prop control fully aft through the low RPM detent into the feather position, oil leaves the propeller and is stored in an accumulator. The spring force and counterweights keep the propeller in the feathered position.

Less oil pressure = Lower RPM

Unfeathering: Low Pitch / High RPM

Unfeathering is accomplished by moving the prop control forward. This releases oil accumulated under pressure into the propeller hub and moves the propeller out of the feathered position. Oil pressure and aerodynamic twisting force keep the propeller in the unfeathered position in this case.

More oil pressure = Higher RPM

What Else Do You Want To Learn?

We hope this article helped simplify a relatively complex topic. What else do you want to learn about systems or multi-engine performance? Tell us in the comments below.