“P” Factor

“Grasshopper, maybe it was the “P” Factor?” Gone are the good old days when something went wrong, terribly wrong when you were just learning to fly and you did a poor job with your instructor as you were trying to control your airplane. With a straight face you could easily and without hesitation blame it on, you guessed it, “P” Factor! Sadly, or maybe happily, the dynamics and the causes of “P” Factor have almost been entirely swallowed up and neutralized in the redesigning of today’s modern student pilot trainer airplanes.

Some attempts of redesign have, in my opinion, have actually gone way too far past the continuum by producing an opposite desired effect. I will give an example of this a little bit later without embarrassing the airplane manufacturer. Back to “P” Factor. Can you remember what this actually stands for? At first I thought this was some sort of mysteriously mystical word that my early and little experienced flight instructors would use when I thought perhaps the demons were taking over as I added power for takeoff in my little 65 horsepower Luscombe airplane and we started to wonder and twist and turn as if the plane had a mind of its own when we started down the 8,000’ foot and 500’ foot wide runway at Quonset Point (OQU), Rhode Island. This was scary. Finally I was lucky to switch to an old salty and long time experienced flight instructor whose name was Gene.

Someone told me that Gene had actually taught one of the Wright brothers, though this may not be true. I asked Gene why this was happening to me, and was it just me? He gently turned toward me from the right seat, grinned, and said softly, “P” Factor. I first I thought he was joking and making fun of me. Then he said it was time for some serious ground instruction. We parked the plane, and tied it down. As I started walking back to the classroom he called out, “Wo, this is now your classroom and your little Luscombe is our training aid.” Gene began by saying if there ever were a plane designed to show all the aspects of “P” Factor, the little Luscombe 8A had to be it. To begin to unravel the mystery Gene began by explaining that “P” Factor stood for all the different forces caused by the propeller. Different forces? I was asking myself, What different forces? This to me was not obvious nor apparent.

I asked Gene what other force could the propeller produce besides make the airplane go forward? Gene laughed and said if we were to count that force also, that would make six forces, one force to make the airplane go forward, and five other forces of which we had to be aware and possibly have to counteract and control. I am sure you already know these five forces, but I was really amazed to hear this and quite anxious to learn what and why all this was happening to my airplane, and me! First Gene explained that the Luscombe has a very, very narrow stance since the main wheels are not very far apart. Putting it in terms of the many pilots who have flown a Luscombe, the plane on the ground is very squirrely. It did not particularly want to taxi in a straight line even if you did everything right. And this had nothing to do with “P” Factor. Next issue, the Luscombe is a taildragger. There is no nose wheel, the tail sits low to the ground. The whole aspect of the airplane certainly is not level like a plane with a nose wheel, but is stationed, sitting on the ground with its nose already pointing in the air. Gene said this is where we start to see where the “P” Factors are originating.

"P" Factor #1#

We all learned as taildragger students that in order for our little plane to gain sufficient airspeed for takeoff, we had to raise the tail. This would put the plane in a level aspect to the ground thereby reducing the drag. So what? Let’s look at this propeller turning clockwise from the cockpit. You are looking at a giant gyroscope. To demonstrate what is happening when you lift the tail of the airplane, get an old- fashioned, large vinyl phonograph record from an antique shop or from your great uncle’s attic, (be sure you have not hit the jackpot getting an original Beatles’ album), stick a pencil through the whole, hold the record in front of you with your left hand and in a tilted up aspect to imitate the position of the propeller. Now holding the record with the pencil, spin the record clockwise and then lift quickly the pencil parallel to the ground. Whoa!!!! Hey! What just happened? The entire record disk swung in the direction of 90 degrees to the left. You have just experienced gyroscopic precession. So what is gyroscopic precession? The simplest explanation I can give you is what my physics teacher, Mr O’Connell, told me in tenth grade. If you apply a force (lifting the pencil) perpendicular to the angular momentum (rotating record to the right) it gets defected by 90° (to the left). If you want a more detailed explanation of gyroscopic precession, I suggest you take a two semester course in physics at either Harvard or MIT. So now you know as you start your take off roll and add full power in your little taildragger, you better start thinking about adding some right rudder pressure to counteract the plane from swinging to the left as you are lifting the tail.

"P" Factor #2:

Asymmetrical Thrust #1 The plane is now safely tied down and the magneto switch has been verified to be in the office position. Gene says to first imagine the plane is horizontally level and you would see that the pitch of the left side of the propeller is equal and opposite to the pitch in right side of the propeller. This means that each side of the properly is taking an equal bite of air. Yes, but that is when the airplane is level. Once we get airborne we pitch the nose up to start our climb. This would put the airplane in about the same attitude as it is now sitting on the ground. Now let’s look at the bite of the pitch of the blade on the left and right side of the propeller. Hmmmmm. Looks like the pitch on the left side is almost flat. But wow, the pitch on the right side is really very steep. The conclusion is that the right side of the propeller is much more efficient than the left side and is pulling the airplane to the left since the right side is overpowering the left side. Looks like I better add a little right rudder pressure to prevent the plane from turning to the left.

"P" Factor #3:

Asymmetrical Thrust#2 So now our little plane is climbing. As we are climbing through the air we have created an airstream equal and opposite to our upward direction of flight. You can use the same analogy as a helicopter blade is moving through the air. The right side of the blade is more efficient because the relative motion of the air is greater than the left side of the left retreating blade. In helicopter language, this is while regular helicopters have a limited forward airspeed due to the onset of retreating blade stall. As our plane is climbing, the left side of our propeller is less efficient due to the fact that side of the propeller has less relative wind across its “retreating” blade. The result is what as known asymmetrical thrust. We better add a little right rudder pressure to make up for the unbalanced “weaker” left side of the propeller.

"P" Factor #4:

We are still happily climbing. The little propeller is whirling around causing a wash of circular air to swirl around the entire body of the plane in a clockwise direction. That is okay. However, as the moving air mass hits the rear of the plane, it pushes the tail to right, forcing the nose of the plane to the left. Looks like we should add a little right rudder pedal to offset this rotational air force, too.

"P" Factor # 5:

I saved the earliest one for last. I am sure you already guessed this one from Newton’s third law of motion. As Mr. O’Connell would have so aptly reminded us, “A force is a push or a pull that acts upon an object as a results of its interaction with another object. … These two forces are called action and reaction forces and are the subject of Newton’s third law of motion. Formally stated, Newton’s third law is: “For every action, there is an equal and opposite reaction.” That is right. The propeller force is turning to the right, causing the body of the plane now wanting to turn in the opposite direction to the left. I guess I better add a little right rudder pressure to prevent the plane from turning to the left.

So what did the aircraft industry do to fight these gremlins? First, they gave us a nose wheel airplane.

No need to raise the tail anymore. Goodbye gyroscopic precession

Engine slightly canted to the right under the cowling. – Goodbye asymmetrical thrust.

Reshaping the dyhedral of the wing. – Goodbye airplane pulling to the left.

Little fixed trim tab on the rudder. -Goodbye propellor wash.

They never did anything about Newton’s Third Law of Motion…….yet.

And what about the little high tail elevator; add full power and instead of the nose pitching up, the nose pitches down. I cannot seem to remember who made that plane.

Wishing you clear skies and following winds. Keep the blue side up. Check wheels down, you are cleared to land.

By Carl Dworman.