Coriolis vs Conservation of Angular momentum
When a rotor flaps, the actual spin axis (rotor mast) doesn't move as the blades do
I don't understand. Why would the blades move at all? They're in a vacuum. There's no lift to cause displacement. Pitch/AOA would have no effect.
Completely untrue, consider the spacecraft in orbit. 90 degree phase lag.
Now you're presenting that "the shaft axis is the rotors spin axis" as a necessary feature of gyros that separates them from rotors. But this isn't a difference either, because it's false. See the cardboard on the pencil gyro video, where the spin axis was displaced from the shaft axis. Like a rotor.
I think this is what happens when you have knowledge as a collection of facts, rather than understanding as the inference from particular facts to a general principle, and then application of the general principle to some other particular thing in question.
To get to this topic, I click on "last page" but it never is the last page, there is always another para from Vessbot, who valiantly fights the fight of the Righteous. But sometimes you are the Wrongeous. But I no longer care, the glaze has descended over my eyeballs.
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To get to this topic, I click on "last page" but it never is the last page, there is always another para from Vessbot, who valiantly fights the fight of the Righteous. But sometimes you are the Wrongeous. But I no longer care, the glaze has descended over my eyeballs.
Violets are blue
How far ahead?
By π over 2
Errm. You may have glazed over, but I'm still interested.
The question was "Is phase lag not the same as gyroscopic precession". I can think of a couple of ways of making that question more accessible. First we can use the term torque induced precession (as Wikipedia does), in order to reduce the angst about comparisons between gyroscopes and rotor systems.
Second we could rephrase it as an exam question, maybe "Either (a) Describe in terms of its underlying physics any cause of control phase lag in a helicopter that is not wholly explained by torque induced precession; Or (b) choosing a suitable reference system estimate what proportion of helicopter control phase lag arises solely from torque induced precession".
I am genuinely interested in the answer. Many instructors have been told over the years that using torque induced precession as a first order explanation for helicopter control phase lag is either plain wrong, or a gross oversimplification. It would be nice to have that confirmed or squashed.
The question was "Is phase lag not the same as gyroscopic precession". I can think of a couple of ways of making that question more accessible. First we can use the term torque induced precession (as Wikipedia does), in order to reduce the angst about comparisons between gyroscopes and rotor systems.
Second we could rephrase it as an exam question, maybe "Either (a) Describe in terms of its underlying physics any cause of control phase lag in a helicopter that is not wholly explained by torque induced precession; Or (b) choosing a suitable reference system estimate what proportion of helicopter control phase lag arises solely from torque induced precession".
I am genuinely interested in the answer. Many instructors have been told over the years that using torque induced precession as a first order explanation for helicopter control phase lag is either plain wrong, or a gross oversimplification. It would be nice to have that confirmed or squashed.
Last edited by Wide Mouth Frog; 22nd Dec 2020 at 17:23.
To put the gyroscope notion to bed I'll offer up one example. A helicopter running on the deck of a wildly pitching and rolling ship, providing there is no SAS engaged, the rotor will maintain the same path relative to the helicopter and ship.
To put the gyroscope notion to bed I'll offer up one example. A helicopter running on the deck of a wildly pitching and rolling ship, providing there is no SAS engaged, the rotor will maintain the same path relative to the helicopter and ship.
that is an inconvenient truth for the pro-gyro clan, the lack of rigidity in space mentioned by AC a while ago and ignored by the playground bullies
With apologies to Casabianca:
The cab sat on the pitching deck,
Blades spinning round the head;
The gyro force that would try to wreck,
Was missing feared dead.
Yet steadfast were the critics stood,
‘Precession is the norm’
They spake as if ‘twas writ in blood,
A proud, though childlike form.
The deck it rolled, the disc it flapped,
Yet tip path plane it stayed in place;
The argument, now tightly wrapped,
A rotor’s not rigid in space.
The cab sat on the pitching deck,
Blades spinning round the head;
The gyro force that would try to wreck,
Was missing feared dead.
Yet steadfast were the critics stood,
‘Precession is the norm’
They spake as if ‘twas writ in blood,
A proud, though childlike form.
The deck it rolled, the disc it flapped,
Yet tip path plane it stayed in place;
The argument, now tightly wrapped,
A rotor’s not rigid in space.
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I am genuinely interested in the answer. Many instructors have been told over the years that using torque induced precession as a first order explanation for helicopter control phase lag is either plain wrong, or a gross oversimplification. It would be nice to have that confirmed or squashed.
But the gyroscopic effect is not some magic or extra phenomenon but is an emergent effect from the normal physical behavior of the masses rotating about one center.
When looking at a smaller scale for example at the movement of an individual rotor blade (or a narrow angular section of a gyroscope disk) then it becomes apparent that this delay is only caused by the time it takes to accelerate the blade upwards and downwards. The blade does not instantly reach its highest vertical displacement at the angular position with the highest vertical force. It has its highest vertical acceleration at the point of highest vertical force.
Now this could lead to a highest displacement at any delay angle, not necessarily at 90 degrees. Just looking at the aerodynamic forces one would expect a 180 degree phase lag, because the upwards movement needs to be slowed down first by downward forces.
But the blade is still attached to a center point so the centripetal force has a downward component relative to the original plane of rotation. This downward component leads to the blade decelerating towards its highest point much earlier than from the external forces, and then moving downward. This downward component of the centripetal force is proportional to the rotation speed, the flapping displacement and the mass of the blade.
This leads to a 0 degree phase shift between external accelerating force and vertical speed. Under ideal conditions (one unmoving center point) this leads to a 90 degree phase shift of the displacements.
Real helicopter rotors are not ideal gyroscopes and the rotor blade flapps about a hinge location that is not at the center of rotation. This leads to a phase lag somewhat smaller than 90 degrees.
But this imperfection is small and the phade lag is close to 90 degrees on real helicopters, especially with articulated rotor systems. Therefore I would call a helicopter rotor a gyro.
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This leads to a small tilt roughly 90 degrees relative to the ships tilt
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The primary reason the rotor disk follows the ship deck is because the swashplate stays aligned to the deck, and the rotor disk follows the swashplate. Disconnect the pitch links and the rotor doesn't follow the ship deck nearly as readily (hinge offset and other extraneous forces like elastomeric damping-friction-springiness are the cause for the alignment).
Just looking at the aerodynamic forces one would expect a 180 degree phase lag, because the upwards movement needs to be slowed down first by downward forces.
And you have completely forgotten the periodic drag changes as the blade spins round (the reason rotors have drag hinges or dampers)- something a gyro doesn't have - there are more differences than similarities between a rotor and a gyro but you can't seem to see past the spinning bit. Each blade speeds up and slows down in its journey because as CL increases due to increase in AoA, so does CD (coefficients of lift and drag respectively).
What would a gyro do if you sped it up on one side? It can't because it would speed up the other side too - unlike a rotor.
Can we please stop trying to answer the question "is a rotor a like a gyroscope", and try and answer the question posed which is "is torque induced precession the primary cause of helicopter control phase lag". The answer to the first question is a matter of opinion, the answer to the second (when we get to it) will be a matter of fact.
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Originally Posted by [email protected]
Or in fact by a reduction in the upward force due to the reduction in pitch angle and AoA as explained earlier and the reason it is around the 90 degree mark.
0 degrees - - >acceleration proportional to force
90 degrees - - > speed (integral of acceleration)
180 degrees - - > displacement (second integral of acceleration)
Originally Posted by [email protected]
And you have completely forgotten the periodic drag changes as the blade spins round (the reason rotors have drag hinges or dampers)- something a gyro doesn't have - there are more differences than similarities between a rotor and a gyro but you can't seem to see past the spinning bit. Each blade speeds up and slows down in its journey because as CL increases due to increase in AoA, so does CD (coefficients of lift and drag respectively).
What would a gyro do if you sped it up on one side? It can't because it would speed up the other side too - unlike a rotor.
What would a gyro do if you sped it up on one side? It can't because it would speed up the other side too - unlike a rotor.
After all a gyroscope is not ideally stiff. It is also subject to microscopic deformations. And forms a very minute cone from gravity. This cone also leads to coriolis effect when tilted. This will lead to tiny elastic deformations
Even sound waves lead to tiny displacements in a gyroscope.
I think we are not talking about absolute purity, are we?
A gyroscope is only a sample implementation. It does not define the gyroscopic effect.
Thats like saying "this is not true friction, if it is not exactly like a brake pad" or "this is not true bernoulli effect because it is not exactly like a venturi tube"
The primary reason the rotor disk follows the ship deck is because the swashplate stays aligned to the deck, and the rotor disk follows the swashplate. Disconnect the pitch links and the rotor doesn't follow the ship deck nearly as readily (hinge offset and other extraneous forces like elastomeric damping-friction-springiness are the cause for the alignment).
This does not lead to a sideways tilt of the rotor disk (or rotor cone), but this leads to a forward tilt. This means that the tilt hase 90 degree phase lag. This large scale phenomenon is called the gyroscopic effect.
But the blade is still attached to a center point so the centripetal force has a downward component relative to the original plane of rotation. This downward component leads to the blade decelerating towards its highest point much earlier than from the external forces, and then moving downward. This downward component of the centripetal force is proportional to the rotation speed, the flapping displacement and the mass of the blade.
an you explain in more detail, why you think that pure aerodynamic forces applied as a sinusoidal function of time would not lead to 180 degrees phase shift?
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Wiki also tells me that a 60mph wind over a long fetch will generate waves 15m tall with a period of about 15 seconds. I'm sure going up and down 15m every 15 seconds feels like wildly pitching, but it's not a huge angle change (about 5 degrees according to my trig). You'd need quite a beady eye to spot that amount of precession in that environment, especially if the hinge offset worked to reduce it.