I have a question of the book "The foundations of helicopter flight" by Simon Newman.
The question is from page 186:
1) How can we feel gyroscopic effect of tail rotor with hover spot turns?
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1$\begingroup$ I suggest you split this question into four questions. $\endgroup$– Steve V.Aug 2, 2014 at 21:58
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$\begingroup$ @SteveV. OK I edited it. Thanks for your comment $\endgroup$– YadiAug 3, 2014 at 8:44
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1$\begingroup$ I'm sure this will kick off a war of words. It is a myth that a helicopter rotor is a gyro. It is not. The blades fly to their position, they do not precess. The phase angle changes with wind direction and for each blade. The precession angle for a gyro is always, and must be, 90 degrees. When the angle of attack of the blades is altered, the rotor thrust alters accordingly and acts through the plain of rotation. If a rotor was a gyro, it would probably tear itself off when tilted in response to pitch changes. $\endgroup$– SimonAug 4, 2014 at 18:44
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$\begingroup$ Just thought of a good demonstration of why a rotor is not a gyro. Watch a pilot doing a rotor disc flying check. The rotor can tilt very quickly. Now imagine a gyro of that mass and angular momentum and the forces involved in trying to move it from it's plane of rotation. Wow! I can't find a video anywhere but you can get quite "frisky" with the cyclic so long as the collective is firmly planted. $\endgroup$– SimonAug 4, 2014 at 18:54
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$\begingroup$ @Simon can you give a reference to where it is documented that it is not like a gyro as even experimental helicopter test-pilots confirm that it acts like a gyro. Even though the force doesn't act exactly 90° later, but perhaps 75-85° $\endgroup$– Chris VMar 5, 2016 at 22:26
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2 Answers
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The tilt and the gyroscopic effect cause the helicopter to hover with a slight bank in a no-wind condition, a compromise that is acceptable. It should also be noted that a crosswind can exaggerate the bank, eliminate it, or even cause the helicopter to bank in the opposite direction.
In helicopters, the controls are rigged is such a way that when forward cyclic is applied, the helicopter moves forward, likewise for aft, etc. To accomplish this, the pitch horn is offset 90º to the rotor blade. The controls still tilt the swash plate in the same direction as the control input is made, but due to the pitch horn placement, the input to the blade occurs 90º earlier in the plane of rotation. The tail rotor as a gyroscope is rather insignificant
Also, speed decreases all the way from the start of the approach to the hover point, and the power, controlled by the collective-mounted throttle, decreases during the first half of the approach, and then increases during the latter half - as the helicopter nears the hover spot and loses translational lift.
In a stable, no-wind hover everything is in equilibrium. The engine is providing just enough power to keep the main rotor turning at just the right rpm to hold the height over the ground and to keep the tail rotor spinning at just the right angle to counteract the torque of the main rotor. When you push one or the other of the tail rotor pedals forward, you upset this balance and must do something to compensate in order to hold the same hover height.
In a hover, the torque of the main rotor tries to turn the fuselage to the right and therefore pressure on the left pedal is required to keep the nose straight. Forward left pedal means the tail rotor blades are biting the air at a greater angle of attack and therefore producing more lift. If you add even more forward left pedal to initiate a left hovering turn, you increase the tail rotor blade pitch angle even more. This requires more power from the main gearbox and, if engine power remains constant, the only way the gearbox can satisfy this increased demand for power from the tail rotor is to allow main rotor rpm to decrease.
The tail rotor provides horizontal thrust but only to the right (nose left). The tail rotor counters the torque on the fuselage caused by the engine driving the main rotor
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Contrary to some misguided views, all helicopter rotors, both main and tail, are gyroscopic and precess at exactly 90 degrees to the sum of ALL applied moments. The control mechanisms of all helicopters are offset in phase to allow for that.
Because they are gyroscopic, rotors simply do not roll easily. The aerodynamic forces needed to roll the rotor are typically four or five times the forces needed to roll the hull. A violent roll may reflect in the helicopter's torque meter reading because of the extra power needed.
In the case of a tail rotor, when a hovering yaw is carried out, the tail rotor axis is being turned about a vertical axis and so it will precess about the roll axis of the helicopter.
This is accommodated using mechanical coupling in the tail rotor hub, typically by turning the axis of the teetering bearing by something like 45 degrees. When the tail rotor disc rolls, the coupling causes cyclic pitch changes on the tail rotor blades that oppose the roll. As a result the tail rotor follows the hull in a yaw instead of precessing and munching bits off the fin.
