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Saturday, February 4, 2012

How to Not Look Stupid

In this series of posts, I will occasionally try to perpetuate common pet peeves of the pilot community.

There is a right way and a wrong way to set up counter-rotating twin engines.  The right way is for the motors to spin top blade in, so both top blades turn toward the fuselage.

Here is an example of incorrect counter-rotation (as evidenced by prop pitch):

Wrong direction of prop counter-rotation
The reasons are P-Factor, torque and slipstream.

P-Factor:  When an airplane is airborne, there is usually some positive Angle of Attack (AOA) between the airflow and the wing chord to create lift, and there is always a very large AOA when you are landing.

When there is positive AOA , each prop disk slices through the airstream at an angle, instead of the usual, mostly perpendicular orientation.  That means there is one side of the prop disk with forward traveling blades while the other side has rearward traveling blades.  The forward traveling side adds the planes airspeed to the prop speed, increasing thrust on that half of the arc.  Vice versa for the rearward traveling side.  This causes an increasing yawing moment from each propeller as aircraft AOA increases.
Correct direction of prop

Proper landings require touch down at a high AOA to minimize ground speed, so P-Factor is close to maximum.   A lot of single engine pilots mistake left-yaw at slow speeds and high AOAs for torque, but if you feel left-yaw increase without a change in power setting, especially as you slow down, it is more likely P-Factor that is steering the nose left. 

If twin motors are spinning their top blades away from the fuselage, then P-Factor becomes additive, instead of subtractive, to the natural thrust asymmetry when flying on only one engine.

Torque:  Similar to P-factor reasoning, when flying single engine you want torque to help counter the natural thrust asymmetry, not compound it.

Slipstream:  P-Factor creates more backward air flow on one side of the disk than the other side, due to the blades on that side taking a bigger bite of air.  If the engines are in conventional tractor configuration (i.e. not a pusher) then wing lift is usually aided by the prop wash.  Any prop wash speed differential generates a lift differential on parts of the wing.  This creates a rolling moment when running on a single engine.  To minimize any rolling moment into the bad engine, the props need to spin the proper direction so the fastest air is closest to the fuselage, not out toward the wing tip.

Additionally, the presence of the fuselage usually creates a "funnel" that naturally speeds the slipstream behind the inside half of the prop disk, due to the Bernoulli effect.  When combined with P-Factor, the proper spin direction results in the most possible thrust and wing lift with one, or both engines operating.

When you combine these three effects, wrong direction counter-rotation can become unlandable in a single engine situation.

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