This one has been doing the rounds for many years.
One is supposed to park up the aircraft before allowing mechanical gyros to spin down, otherwise they can get damaged (by being moved around at a low gyro RPM).
What is the exact physical reason for it (if true) ?
Newton’s 2nd law of motion states that the deflection of a moving body is directly proportional to the force applied and inversely proportional to the weight and speed of the body.
This is the reason for the design of the gyro-rotor: relatively great weight and high speed.
A heavy, fast-moving body will not deviate from its path as much as a light, slow-moving object when the same deflective force is applied.
I think when the gyro isn’t at top speed after shut-down it will get tossed around quite a bit (including its gimbals) when you move the aircraft. This again may lead to light damage to the gyro bearings causing a higher friction and lateron in normal operation – a certain amount of unwanted precession.
There must be a certain speed during shut-down the bearings don’t like at all when being moved – in comparison to moving the aircraft without any rotational activity of the gyros.
We had a number of gyro failures in the Caravan. The solution was to leave the aircraft parked for at least 15 minutes after shutdown before moving it around. This reduced the failure rate. As mentioned, the logic presumably is that the gyro has lots of stability when spinning above some rpm, but below that rpm it gets tossed around and damages the bearings.
The solution is solid state gyros…
What I’ve read is that if you move the aircraft while the gyro is spinning down, it can bang hard against internal stops.
I have to admit that I don’t understand that… I can’t visualise it. On a normal AI, there is no “stop” in roll i.e. the roll gimbal goes all the way round through 360 degrees. Connections are made with brushes and slip rings. On the pitch gimbal there are normally stops, I think. I will ask somebody who makes gyros…
bq.This one has been doing the rounds for many years Quote
Yeah, ’cause many many years ago, it mattered. The old old gyros which could be caged for aerobatics, because they did not go all the way around might be caged on the ground. Thus if you moved the plane with the gyros winding down, they would be strained against the bearings – but much less so than when you were tumbling through the sky doing aerobatics with them caged! Those gyros are only now found in aircraft so old, that I doubt many people here are flying them.
Modern gyros have more freedom of motion. Certainly no motion of a plane on the apron is going to upset one of these gyros. Consider the gyros: The AH has a gyro rotating on a vertical axis, so moving a plane on the ground, around that axis, is hardly going to introduce a force to be reconed with, The DG has a gyro on a horizontal axis, but it is free to rotate with the compass card on the vertical axis, so, again no resistance there at all to ground handling. And, the T&B has a gyro on a horizontal axis, but it’s business IS measuring turns, so I hardly expect that ground handling would have the aircraft turning as quickly as the T&B can withstand during maneuvering flight.
So it’s great to consider gentle handling of the aircraft, fo the sake of preservation, but this is not, to me, a concerning issue….
On a normal AI, there is no “stop” in roll
It depends on what you think is normal I suppose. There are quite a number of older vacuum and electrical mechanical AI’s which have stops for both pitch and roll, roll limit is typically > 100 degrees.
Newer gyro’s tend to have more freedom.
Those gyros are only now found in aircraft so old, that I doubt many people here are flying them.
I’m one of them (our aircraft was built in 1945) – we have World War II era DG (of the compass style, not the vertical card style), AI and turn and bank. The AI can’t be caged and it often does impressive somersaults when winding down. The DG can be caged (and if you don’t cage it, it’ll start spinning wildly when it gets slow). Despite their great age, on a recent 2.5 hour flight I think I only adjusted the DG perhaps 3 times, it seems to stay in sync with the compass as well as a new one. The AI is very slow to erect but otherwise works fine.
I have a couple of spare gyros of this style at home (in full working order). The ancient sticker on the back of the instrument says “Handle like eggs”.
