Why Planes Veer Left on Take-Off

 

            Have you ever noticed that you need a little extra right rudder on take-off?  Would you like your plane to track a straighter line?  Do you support Stockwell Day changing his first name to Doris?  But I digress.  Why do planes veer to the left during the take off, climb out or during a go-around?  The answer lies in the propeller.

            The four main reasons that a plane tends to the left are: torque from the engine, slipstream from the propeller, greater thrust from the down going blade and gyroscopic effect.  Lets look at each individually.


            Sir Isaac Newton’s third law (made famous by Garfield) states that “for every action there is an equal and opposite reaction”.  The torque from the engine causes the propeller and aircraft to rotate in opposite directions.  When the crankshaft and propeller rotate clockwise (when viewed from the tail of the plane), the reaction is for the plane to rotate counter-clockwise.  The left wing drops and the plane veers to the left.   

 


Plane turns counter-clockwise
 

Propeller turns clockwise
 
            The next reason is slipstream.  The airflow comes off the propeller in a corkscrewing pattern.  The air rotates clockwise around the fuselage and strikes the rudder on the left side.  The rudder is pushed to the right and the plane goes to the left. 


 


            The next cause of left veering aircraft is just a wee bit more complicated.  First, lets remember that one of the components for the lift formula is velocity squared (the faster you go, the more lift you get).  Next, this formula also applies to thrust since a propeller is nothing more than a rotating wing.  Ok, here goes.  Essentially, because the plane has a slightly positive (nose up) angle, the downward going (right hand) blade of the propeller moves slightly forward and has a slightly greater relative velocity than the upward going (left hand) blade which moves slightly backward.  The result is slightly more thrust on the right side of the propeller and a tendency for the plane to, you guessed it, veer left.  Let’s extrapolate this to the extreme.  Lets assume that the plane has 90o nose up attitude and that the propeller is now rotating parallel to the relative airflow.  The right blade is moving upwind, creating all sorts of lift and left blade is moving downwind, creating very little lift.  You are now into a situation where you must increase Angle of Attack of the downwind (left) blade in order to generate an amount lift equal to the upwind (right) blade.  This is exactly the problem that our helicopter friends face and it is the price that they must pay for insisting on flying an airplane at 900.

            The last cause of left veering airplanes is gyroscopic effect.  Have you ever held a spinning bike tire and noticed that it is very hard to control.  This is because a force applied to a gyro (spinning disk) will act at 900.  Let’s say you held a clockwise spinning bike tire vertically and applied a force to the top.  The force would be transposed 900 and the right side would move.  When a propeller is turning, it becomes a disk with all of the properties of a gyro.  So there you are barreling down the runway in your brand spanking new Piper Cub, you raise the tail (which applies a force to the top of the propeller, which is transposed 900 to the right side of the propeller) and your plane veers left.


           

 


            So now that we know the causes, what are the solutions.  The simplest way to stop the plane veering left is to turn the propeller in the opposite direction, and the plane would then veer right.  Another way would be to have one propeller turning clockwise and another one turning counter-clockwise.  The two forces would cancel each other out and your plane would track nice and straight.  Problem is, it is very mechanically complicated to build a counter-rotating engine.  On the bright side, you would steal the show at the Scale Masters if you showed up with a counter-rotating RC engine.  We could get rid of the propeller all together and install a jet in out Piper Cup (another good way to steal the show).  You could set in some rudder trim, but this would only be good at a particular speed or you could build some offset into your firewall.  By angling the engine towards the right (30 seems to be a good starting point) you counteract the plane wanting to go left.  Lastly, and this is my goal for the coming year, you could become more proficient with the rudder.

 

Questions or comments

Craig Skinner

kc.skinner@sympatico.ca