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5 Answers

Gyroscopic Precession

Asked by: 7372 views General Aviation

I came across 'Gyroscopic Precession' in the AFH "...The beginning pilot, not being familiar with the intricacies of flight by references to instruments, including such things as instrument lag and gyroscopic precession...."

 

Gyroscopic precession is then defined in the glossary as: "An inherent quality of rotating bodies, which causes an applied force to be manifested 90degrees in the direction of rotation from the point where the force is applied"

 

So what I understand from this is that with a Gyroscope, you're not going to immediately see changes in the instruments due to gyroscopic precession, as the change will show up 90 degrees downstream? (But I would think that as the gyroscopes are moving at such a rapid rate, something showing up 90 degrees downstream would be pretty fast to show up anyway?)

 

In summary, there's something about Gyroscopic precession which I don't quite dig here practically vs. theoretically.

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5 Answers

  1. Best Answer


    KDS on Aug 25, 2019

    Gyroscopic precession is just a principle of physics. How it plays into aviation depends on what is being considered.

    For example, with a taildragger, when the tail is lifted from the ground during the takeoff roll, there will be a left turning tendency. That is because the top part of the prop is being pushed forward and the bottom part is being pulled rearward. So, 90 degrees later, the blade which is now on the right side wants to go forward and the one on the left side wants to go rearward. This causes the airplane to want to turn left. The takeaway lesson for the pilot is when you lift the tail, you can wait for it to swerve a little and correct for it or you can know it’s going to happen and correct for it as you lift the tail.

    Another example is the main rotor on a helicopter. To get the helicopter to move forward, the rotating disc is tilted with the rear higher and the forward portion lower. This adds a horizontal component of lift to the rotor system and forward flight happens. To achieve this, the pilot moves the cyclic (the stick) forward. That results in the blade on the left side pitching up and the blade on the right side pitching down. Because of gyroscopic precession the force takes effect 90 degrees later. The rear part of the disc is up, and the forward part is down.

    An interesting story regarding that is one I’ve heard but cannot swear is true. I was told that when Igor Sikorsky made his first helicopter test flight that he pushed forward on the cyclic only to have the machine move sideways. Igor realized he had not allowed for gyroscopic precession. Hopefully Kris reads this and can throw some light on that story.

    However, for now, you’re focused on how it applies to instruments. I’d say the teaching point is to explain how it factors into making the gyro instruments work. It allows you to illustrate (with a picture or a cut away instrument) how the gyro rotates and then causes the movement that is seen on the face of the instrument.

    Here is a quote from AC 65-15A discussing the turn and bank indicator:

    TURN AND BANK INDICATOR

    The turn and bank indicator, figure 12-22, also referred to as the turn and slip or needle and ball indicator, shows the correct execution of a bank and turn and indicates the lateral attitude of the aircraft in level flight.

    The turn needle is operated by a gyro, driven either by a vacuum, air pressure, or electricity. The turn needle indicates the rate, in number of degrees per second, at which an aircraft is turning about its vertical axis. It also provides information on the amount of bank. The gyro axis is horizontally mounted so that the gyro rotates up and away from the pilot. The gimbal around the gyro is pivoted fore and aft.

    Gyroscopic precession causes the rotor to tilt when the aircraft is turned. Due to the direction of rotation, the gyro assembly tilts in the opposite direction from which the aircraft is turning. This prevents the rotor axis from becoming vertical to the earth’s surface. The linkage between the gyro assembly and the turn needle, called the reversing mechanism, causes the needle to indicate the proper direction of turn.

    I will also throw in that the gyro in the turn and slip indicator is mounted at 90 degrees where the gyro in the turn coordinator is mounted at 45 degrees. I have to confess that I instructed for quite a while without knowing the difference between those two instruments.

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  2. willmcc on Aug 26, 2019

    Thank you very much — that was very informative.

    I think my question is what does gyroscopic precession have to do with lag in instruments and about how much is that lag?

    I assume also that any change in plane attitude will show up in the actual physical rotating gyroscope 90 degrees from where it occurred? So there must be a compensational factor in this? I think somehow this is where I get confused / unclear.

    Now that you bring up the concept of a plane veering to the left, I had thought that this was because of the propeller slipstream hitting against the tail of the plane, moving the tail to the right and the front to the left. Is this wrong? I didn’t understand that it had to do with gyroscopic precession from the movement of the propeller

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  3. Kris Kortokrax on Aug 27, 2019

    There is no lag in the instrument indications. If there were, you could not control the aircraft by reference to the instruments. If there were a lag, the airplane would be moving in one or two axes (pitch and/or roll), while the AI would not indicate that movement until later. Or the AI would indicate a roll, if the airplane’s pitch changed. By the time you move the yoke to compensate, the plane could be moving in another direction. The gyroscopes inside the instruments are positioned to account for the difference between the position where the force is applied and the position where it becomes apparent. The movement of the gyro is instantaneous.

    The magnetic compass is one flight instrument which does exhibit a lag. The VSI is another, but the gyro instruments do not exhibit a lag. You can prove this to yourself in flight by moving the flight controls and observing the indication on the instruments.

    It is the same with rigging a helicopter. The pitch horn (which moves the blade) is positioned 90 degrees from the blade so that when the pilot moves the cyclic control forward, the blade pitch is increased on the left side of the aircraft and decreased on the right side.

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  4. KDS on Aug 28, 2019

    Hi willmcc,

    Let me address your reply of August 26th.

    The Instrument Flying Handbook (FAA-H-8083-15B) mentions “lag” in relation to three instruments. Those three are the airspeed indicator (ASI), the magnetic compass, and the vertical speed indicator (VSI). None of those uses a gyroscope.

    You are correct that there is a compensational factor in the gyroscopic instruments. I think the most fascinating one is the self-righting attitude indicator. When the instrument starts spinning, it doesn’t know which way is up or down. There are four vanes under the gyro that open or close based on where they sense gravity. If one is open, it emits air that pushes on the gyro and 90 degrees later, the instrument reacts and continues reacting until the vane is closed. It is difficult to describe in words alone, but you can go on youtube.com and search for “Pendulous Vanes” or “Inner Workings of an Attitude Indicator” to see some excellent demonstrations.

    One of the interesting aspects of that is it only senses where gravity is, not what is really up or down. So, if you fly for a period of time without keeping the ball in the center, your attitude indicator will show a bank despite the wings being level.

    You mentioned “propeller slipstream” as the cause of a left turning tendency. That is one of four.

    Back when I learned to fly, they were called the four left turning tendencies. Once “conventional gear” airplanes became unconventional, the phrase shifted to the four turning tendencies. The one I described is primarily a LEFT turning tendency when picking the tail off the ground on takeoff. In a tricycle geared airplane, rotating for takeoff is a right turning tendency.

    The four turning tendencies are described in Chapter 4 of the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25A). The handbook describes each one, but just to list them, they are:
    1. Torque reaction from engine and propeller,
    2. Corkscrewing effect of the slipstream,
    3. Gyroscopic action of the propeller, and
    4. Asymmetric loading of the propeller (P-factor).

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  5. ayavner on Sep 06, 2019

    Another thing to consider re precession and your question about “lag”, I think in context of the reaction to the force after 90 degrees of rotation – think about how fast your propeller is spinning (or the gyros for that matter) – that 90 degrees comes very quickly, so to your perception (and for practical purposes) there is no real lag in that sense.

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