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Gyro & Precession

Hi all, I have found the above rather confusing and spending 3-4 hours looking at it confused myself totally!!

When they talk about precession it says here that if an external force or torque (what does that mean!) is applied to change the direction of the rotor axis, the gyro resists angular movement in the plain of the torque applied, instead moved in a plain of right angles to the torque.

For some reason I simply cannot picture this, I have watched various youtube video's and none have helped me out!!

The whole 90 degree part of it or right angle part. The Gyro will precess in a direction 90 to the applied force? Can anyone put this in simple terms!?

Why on a turn indicator does the gyro rotor move in the opposite direction??

AS I understand it if you have a bike wheel held on both sides and spin in a vertical plain, you only need it supported on one side, is this precession? Or is precession when the bike wheel will start moving in a clockwise or anti clockwise motion because of this 90 degree thing?

Any explanations I have found on the internet just make it sound really complicated.

I do also under stand how a gyro tries to stay upright in space when it is turning.

Many thanks

Mark

Mark,

a gyro wants to maintain its spin axis, if you apply a force and try to move it, it will react with a counterforce to keep its rigidity in space.

A turn coordinator is on gimbals in two directions. The gimbals allow it to move in two dimensions and if you turn the airplane (i.e. move the airplane away from its spin axis) it will precess. The erection mechanism will force it to the new position over time, the mechanism is calibrated and this is how you get the turn rate.

I am not going to be able to give an "aviation exam correct" answer but I have seen "precession" used in two contexts:

1) The unavoidable 360 degrees per 24hrs rotation in the heading, caused by the earth's rotating underneath you as you are flying along, which means that even a perfectly frictionless and infinitely stiff DI needs adjusting by 15 degrees every hour

2) The random-direction heading errors caused by a knackered DI

IME, 2) is dominant in the training fleet, with perhaps 10 degree errors every say 10 minutes

The above is what leads to a slaved compass system, whereby a magnetometer (a remotely mounted compass) is usually mounted in the wingtip and this feeds a signal to the DI to slowly correct its drift.

It's easy to intuitively understand gyroscopic behaviour, if you see a gyro instrument opened up and powered-up. I would recommend that; most schools have some knackered old DI which they use for that.

But I have always found it hard to get the terminology right to answer the exam questions.

Administrator
Shoreham EGKA, United Kingdom

"The whole 90 degree part of it or right angle part. The Gyro will precess in a direction 90 to the applied force? Can anyone put this in simple terms!?"

Try this demo: Get a bicycle wheel, hold it by its axle ends and spin the wheel.

Imagine a line running end to end through the axle spindle - this is the horizontal axis. Imagine another line running up from the floor through the middle of the axle and to the ceiling - this is the vertical axis. A third line runs from you to the straight ahead - the longitudinal axis. These same axes apply to an aircraft relative to the pilot, though to be more precise, they run through the CofG of the a/c for the purposes of instruments detecting movement.

Back to the rotating cycle wheel. Whilst it is spinning, turn the axle of the wheel to the left about the vertical axis - as you might do if you wanted to turn left when it is installed in your bike.

As you turn the axle left, the vertical axis of the wheel tries to 'tip' some degrees to the right. That tipping effect is precession. Change the direction of rotation of the wheel and repeat the experiment, and it will 'tip' in the opposite direction when you 'turn left'. This precession effect is used in a/c instruments to indicate change in direction of the a/c with respect to a spinning wheel or cylinder. In vacuum instruments the wheel is caused to spin by a jet of air on a cylinder. As the a/c changes direction in the axis the instrument is designed to detect, the precession effect is used to deflect a needle.

I hope this helps and sorry my explanation is a bit wordy.
Nothing like an experiment to prove something to yourself . . .

EuropaBoy
EGBW

Try this link

Thanks very much to all!

Europaboy that was really helpful.

Rgds

MArk

Europaboy,

When you say: As you turn the axle left, the vertical axis of the wheel tries to 'tip' some degrees to the right. That tipping effect is precession

Ok so if the wheel is not supported by both my hands just one on the axel, then wheel then starts rotating as I understand it, what is that then??

So where does this 90 degree movement come in and what moves in 90 degrees to what??

Thanks

Mark

Hi Mark,

Firstly we need to agree some terminology. A turning force is called a torque - which is a force multiplied by a distance. The other name for this is a 'moment'. it is also the basis for calculating CofG - which is simply the resultant of a combination of masses multiplied by a combination of moment arms. When you turn a nut on a bolt using a spanner you are applying a torque (hence 'torque-wrench').

The wheel axle held with both hands has its spin axis horizontal (as previously). Gravity is acting through the CofG of the wheel (more or less in a line aligned to the vertical axis - the imaginary line coming up from the floor), and at a position about halfway between your two holding points on the axle. Your two hands apply forces at the two ends of the axle that are equal and opposite to the force of gravity acting down the vertical axis as a result of the wheels' mass - so it stays in position.

Now the situation that you hold just one end of the axle whilst the wheel is spinning:-

achimba is right when he says "a gyro (in this case the spinning cycle wheel) wants to maintain its spin axis . . .".

By holding the axle at just one end, the mass of the wheel is now reacted by the force applied by your hand, but this time, you re applying that force slightly offset to one side compared to where gravity is acting on the centre of the wheel - along the vertical axis. So in this case you are inadvertently applying a torque by holding just one end of the axle- which also results in precession.

The 90 degree thing:

There is a good graphic here: http://en.wikipedia.org/wiki/File:Gyroscopicprecession256x256.png

One way of thinking about this is to visualise the three axes. If the wheel [gyro] spins about the horizontal axis, and you try to turn this spinning wheel about the vertical axis, then the precession effect will result in a movement around the only axis left - the fore-aft axis, i.e. it will tip to the right or left depending on the direction of rotation of spin.

I realise in trying to explain this, how difficult it is to do/understand using just words . . .

I hope helpful

EuropaBoy
EGBW

I agree it is hard to explain in words!

I thought about gyroscopes a lot while preparing for a helicopter technical exam, and finally pictured it for myself as you see below. I am no particulat expert on gryos, but this picture worked for me.

The picture shows a grey tyre with pink sides, turning as shown by the orange arrows. You could imagine it as the front wheel of a bicycle moving in the "Forward" direction. Continuing with the bicycle idea for a moment, imagine twisting the handlebars to turn right.

The yoke is typically cambered, so a rightwards force is applied through the frame to those parts of the wheel in the top-forward position, and a leftwards force is applied to those parts in the bottom-rear position. No forces act on the bottom front and top-rear. The biggest of these forces are shown in red.

Now forget all about bicycles and just look at what the turning forces do to our spinning gyroscope. Assume for the moment that neighbouring bits of rim exert no forces on each other, so that each individual part of the rim is subjected to a force, and so an acceleration, which depends on its angular position.

As a rim element passes the point of maximum rightwards acceleration, its speed will increase very rapidly, and then more slowly, until it reaches a non-accelerating zone in the bottom-forward position. This will be a maximum rightward speed, and it is 90 degrees after the maximum rightward force position.

After that it will go though a leftwards acceleration zone, culminating in a maximum leftward speed 90 degrees after the maximum leftward force.

Finally it returns to our starting point.

The rightwards speeds are shown in green, and one (maximum) leftward speed is also shown. Clearly the gyroscope will rotate in accordance with the green arrows, at right angles to the axis of the turning force.

A couple of loose ends:

The wheel as whole is not moving left or right, and so its average sideways speed is zero, and so from symmetry the left/right speeds must be zero at the maximum acceleration stations, as shown in the diagram.

I assumed that the rim elements exert no forces on each other. I won't try to prove that here, but if you imagine a gryoscope made only of balls on the ends of spokes, then the balls will form a disk in the same shape as the actual gryroscope disc, and so there is no reason to expect forces between the rim elements.

Ironically, although my diagram did help me to understand what happens to helicopter rotor blades, it turns out they do not behave like simple gyroscopes, because of "blade flapping".

In effect, the swash-plate controls the vertical speed of the rotor blades, and the min/max vertical blade positions are 90 degrees after the min/max vertical speeds.

White Waltham EGLM, United Kingdom
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