Gyroscopic Precession

January 29, 2012 by Tim McAdams

When reading about helicopter aerodynamics you will see the term gyroscopic precession. It refers to a principle of gyroscopes that states when a force is applied to a spinning object, the maximum reaction occurs approximately 90 degrees later in the direction of rotation. Since a helicopter’s main rotor acts like a gyroscope, this principle applies to the rotor disc.

 

To understand what this means for a helicopter we need to know the basics of how a rotor disc tilts in the direction commanded by the pilot. Fundamentally, the pitch of each blade is controlled by a swash plate, and each blade is connected to the upper ring (also called the rotating part) of the swash plate via a pitch link. The cyclic flight control is connected to the lower ring (also called the non-rotating part). When control inputs are made it tilts the swash plate changing the pitch of each blade independently, according to the blade’s position, to tilt the rotor disk in the desired direction. 

This can be observed during a pre-flight. If you move the cyclic control forward you will see the swash plate tilt forward. While the swash plate actually tilts in the direction that the cyclic control moves, each blade’s pitch must change approximately 90 degrees prior to get the rotor disk to also tilt in that direction. This is accomplished by the blades’ pitch horns which are offset approximately 90 degrees. Therefore, to tilt the rotor disc forward in-flight (in a rotor system that spins counter-clock-wise) the pitch of the blade on the right decreases and the one on the left increases. Since maximum deflection takes place approximately 90 degrees later, the disc tilts forward.

12 Responses to “Gyroscopic Precession”

  1. Christian Tillmanns Says:

    Funny that you show the Robinson rotor head. The offset on Robinson Helicopters isn’t 90 degrees, it’s 72 (as Frank about the Wa-we effect). It‘s 78 in a H300 and never really 90 degrees.
    Unfortunately the whole thing is much more complicated and understanding it, wouldn‘t make us better pilots but overqualified in high school grade physics.
    In the world of helicopter engineering, it‘s called „phase lag“. Each helicopter has it‘s own phase lag. Since in a gyro, the 90 degrees are a fact, why isn‘t it like that in a helicopter.
    It has lot‘s to do with the flapping and even more with aerodynamical forces, which are much stronger, than the gyro effect.
    And why do we learn it in the basic helicopter handbook? See above. It‘s extremely complicated, therefore all basic helicopter handbooks revert to a kind of „lie to children“.

  2. Jim Borger Says:

    I’m perfectly fine with the explanation, “It’s PFM”.

  3. Alan Barnes Says:

    To further what Christian said… a rotor system is *like* a gyro and has some gyro-like properties. But it’s not a fixed system like a gyro. The blades are allowed to flap or flex and, in a fully articulated system, lead and lag. This moves where the reaction takes place from 90 degrees to a spot based off that specific rotor system – it’s different for every helicopter.

    One of the best explanations I’ve found for it is in the Navy’s Introduction to Helicopter Aerodynamics workbook (P-401) available here https://www.cnatra.navy.mil/pubs/folder5/TH57/P-401.pdf It starts on page 103 of the pdf, section 405 and 406. This is also a valuable resource for a lot of other rotor aero subjects.

  4. Paul Goldasich Says:

    I enjoy learning about technical things especially having to do with helicopters. I often watch the mechanics and ask questions to learn more about the machines I love to fly. During training, there are so many things to learn, and the principles of flight is one of the biggest. Hitting the highlights is about all you get and if you want to learn more, it’s on you. To demonstrate the precession principle, my school had a bicycle wheel (with the tire mounted) and a handle bolted to the axle. You would hold the handle and get the wheel spinning. You could then feel the gyroscopic force applied by the spinning wheel. Great learniing tool to explain this concept. I cannot do a better job explaining it than Christian did, and I agree with Jim, that as pilots, the explanation is just fine.

  5. Gary Says:

    Well, I thought I had learned something. Then, from the comments above I’ve reverted to my prior understanding. I knew it couldn’t be that simple so yes, helicopter flight IS a fig-newton of our imagination, collection of parts, etc. I even tried it. Once. Rub stomach and pat head, or was it rub head and pat stomach? Repeat.
    My hat is off to all that do it!

  6. Rich Says:

    Tail dragger pilots also become familiar with gyroscopic precession. When raising the tail during take off at high rpm the effect is noticeable by creating a sudden left turning tendancy as the propeller rotates forward and down.

    And that need to reset the DG once in a while is caused by gyroscopic precession.
    Holy moly Batman it is all around us..

  7. Bob Kuriger Says:

    Gyroscopic precession is not as mysterious as most college physics professors and their texts would have you believe. Helicopter rotor blades, for instance, climb and dive under the influence of increased or decreased cyclic pitch. Since maximum climb rate occurs at a point 180 degrees from maximum dive rate, it is only natural that halfway between, i.e 90 degrees in the direction of rotation, the blade will be neutral and at its highest point of the revolution. Also good to know, gyroscopic precession only occurs when the forcing function is relatively mild compared to the inertia of the rotating disk.

  8. Bob Kuriger Says:

    With regards to resetting your DG every so often, be advised the DG wants to stay aligned with some point in space, while the world rotates 15 degrees per hour. That is not exactly gyroscopic precession, but close.

  9. Alan Barnes Says:

    That’s not gyroscopic precession but, rather, another important property of gyros called rigidity in space. Essentially the gyro instrument wants to stay in the same location while the aircraft rotates around it. Having to reset the DG occasionally is more a function of drag inside the instrument. It can be due to age (it’s just worn out :D), a manufacturing defect, or even a vacuum failure.

  10. Dale Long Says:

    Enjoyed reading all the comments. Thanks for creating a platform to share HELI info.

  11. Philip Casey Says:

    Good read! Thanks for the info. I’m very interested in how helicopters operate and I hope to learn more by reading your blog.

  12. JPH Says:

    Comparison of the fundamental equations describing the behaviour of a
    gyroscope and that of a helicopter rotor (specifically the blades)
    clearly indicate they are not same physical phenomenon.

    The control advance angle of the Robinson R-22 helicopter is 78 degrees,
    that of the Air & Space 18A gyroplane 52 degrees, and the Lockheed L-286 helicopter about 45 degrees. This is not gyroscopic precession which is always 90 degrees.

    The unstabilized rotor also does not hold its position in space but left
    to its own devices wanders all over the place. Try flying an R-22 hands-off.

    A rotor is more accurately described as a system of whirling pendulums
    subject to – and readily explained by – the laws of vibration.

    Unfortunately, at some time early in the helicopter era the analogy of a
    rotor as a gyroscope took hold. This analogy is not only incorrect but
    results in confusion as to what is really going on.

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