UNDERSTANDING GYROSCOPIC PRECESSION

You will likely notice something very strange when you move your cyclic RC helicopter controls . The pitch change on the rotor doesn’t occur where it should. It seems to happen 90 degrees before it should… “I must have built something wrong or programmed the wrong CCPM mode into my radio” you say to your self.

Nope! What you are witnessing are the cyclic helicopter controls compensating for the forces of Gyroscopic Precession.

Here we go again - another example.

Say we are looking down at our helicopter from above. The nose is pointed forward (12 o-clock position) and the tail is pointed back at our feet (6 o-clock position). In this example we will assume the rotor rotation is clockwise (the most common direction for most helicopters).

Now we give a forward cyclic command. The natural assumption is the rotor blade will have more positive pitch at 6 o-clock than at 12 o-clock so the rotor lifts at the back more than the front and pitches the helicopter forward.

However because of gyroscopic forces acting on our spinning rotor, the lift forces of the rotor actually occur 90 degrees later in the rotation. This phenomenon is called Gyroscopic Precession.

In the above example with a clockwise rotating rotor and where the rear blade at 6 o-clock has more lift than the front blade at 12 o-clock - our helicopter would actually roll to the right, not pitch forward. Why?

Remember the gyroscopic precession will cause the force applied to the rotor to occur approximately 90 degrees later in the revolution. So the rotor force actually lifts more a 9 o-clock than at 3 o-clock.

The rotor tilts to the right and our helicopter follows along and rolls right. In essence, we have just given a right cyclic helicopter control command.

OK - just to make sure you have got this gyroscopic precession idea all figured out, here is a skill testing question:

Where in the rotor cycle should the blade have applied more lift and less lift to pitch our helicopter forward with a clockwise rotating rotor?

Yes - more lift at 3 o-clock and less at 9 o-clock respectively. What about if the rotor is spinning counter clockwise? Yup, more lift at 9 and less at 3 o-clock.

Most helicopter instructions don’t include this very useful tidbit of information and believe me, it has had more than a few of us newbies scratching our heads.

I told you this was fun... or maybe just funny.