|
|
| On occasion, at the field you will hear someone say "Why did it do that?" Sometimes an airplane will do things that at first seem very strange. Why does a left turn seem easier to make than a right turn? Why did it roll out at the top of that loop? Why does the left wing drop when I flare out to land? Why does it always go left when I am taking off? These are just a few things an airplane will do that seem strange. Not knowing why these strange things happen will let them become a problem, but if you know what is going on, what causes it, and when it will happen the problem is no longer there. |
| If your airplane is trimmed out properly it will always want to go to the left on takeoff. That doesn't sound logical, but never less, it is true. There are three forces acting upon an airplane during takeoff to make this happen. They are p-factor, Torque, and Prop wash. The hardest one to visualize is P-Factor so it will be discussed first. |
|
P-factor- The P in P-Factor stands for pitch. In figure 1 is an airplane that is just starting its takeoff roll. The tail wheel is still on the ground. The horizontal arrow is the direction the plane is traveling while the angled arrow is at a right angle to the prop. Since the prop is angled into the air stream, this makes the prop pitch at the top more than it is at the bottom. This is hard to see, but if you pick up a prop and look at it a little while from different angles you will eventually see it. Because of this difference in pitch angle, the prop is producing more lift at the top of its arc than at the bottom. Since the prop is producing more lift at the top of the arc, it is also producing more drag. The prop is turning counter-clockwise so every time one of the blades swings into the top of the arc it is pushing the nose of the plane to the left. P-Factor will continue to affect the airplane until the tail comes up and starts flying and then its effect will be minimal. P-Factor will come back into effect if the angle of attack is increased. This is most noticeable just after takeoff when the planes flying speed is still slow and the nose is horsed up. Torque- Engine Torque is pretty easy to visualize. Newtons Laws state that every action has an equal and opposite reaction. Since the prop is turning counter-clockwise it is always trying to turn the airplane clockwise. This is more pronounced during takeoff or any other time the plane is in slow speed high power situations. |
|
Prop wash- The rotating prop of an airplane creates rotating air behind it. In figure 2 the plane is being viewed from the top. The rotating air from the prop or prop wash is striking the vertical stabilizer and rudder on the left side which is pushing the tail of the plane to the right and the nose to the left. The prop wash is also striking under the left wing and on top of the right wing which, to a lesser degree is trying to roll the plane to the right. The right rolling motion is negligible because the wings are large enough to offer more stability. There are a few 'tricks of the trade' to minimize the effect of P-factor, Torque, and Prop wash. The most common is to use a small amount of right side thrust in the engine. A little down thrust will help counter p-factor in a tail dragger plane. A trike gear eliminates p-factor during ground roll on take off by putting the prop at a right angle to the direction the plane is going but torque and prop wash are still there. In any case, these three forces cannot be eliminated. Too much right side thrust and the plane will turn right under power and left under glide. Too much down thrust will desensitize elevator control. |