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Torque reaction

Old 9th Jun 2021, 02:06
  #21 (permalink)  
 
Join Date: Apr 2009
Location: Phoenix, AZ
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Originally Posted by Fl1ingfrog View Post
It is much more simple: the term torque is the word we use to describe the rotational or twisting force. The term does not refer to the linear force, when 'Newtons' or 'horse power' is the measurement.

The yaw you describe is partly a gyroscopic effect. The rotating propeller and the helicopter rotor both can be considered as a disc; the bicycle wheel provides a good demonstrator. But the asymmetric thrust from the propeller (the down going blade produces more thrust than the upgoing blade) plus the resulting propeller slipstream acting on the fuselage and empennage, act together to cause the yaw to the left, but with engines rotating to the right. With Engine and propeller assemblies that rotate to the left the result is a yaw to the right.

The increased left yaw when the tail is raised is caused only by gyroscopic precession. The yaw due to asymmetric thrust reduces as the tail is raised, it does not increase.
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Old 9th Jun 2021, 15:57
  #22 (permalink)  
 
Join Date: Oct 2017
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The increased left yaw when the tail is raised is caused only by gyroscopic precession.
Yes, but I'm wary of using absolutes such as "only" which can come back to bite you.

The yaw due to asymmetric thrust reduces as the tail is raised, it does not increase.
Why would it change at all, the relationship of the propeller to the aeroplane remains the same? Of course, the propeller slipstream stretches and the yawing force from this will therefore reduce with the airspeed increasing but it will always be present. At the slow take-off speed the slipstream yawing force will still be substantial and require a sustained application of opposite rudder.
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Old 9th Jun 2021, 18:59
  #23 (permalink)  
 
Join Date: Apr 2009
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Originally Posted by Fl1ingfrog View Post
Why would it change at all, the relationship of the propeller to the aeroplane remains the same? Of course, the propeller slipstream stretches and the yawing force from this will therefore reduce with the airspeed increasing but it will always be present. At the slow take-off speed the slipstream yawing force will still be substantial and require a sustained application of opposite rudder.
On my airplane raising the tail causes a 12-13 degree change in pitch attitude and a corresponding change in the angle of attack of each propeller blade. With the tail down the downward moving blade has a higher angle of attack and produces more thrust than the upward moving blade. When the tail is up both blades have the same angle of attack and both produce the same thrust. It is the relationship of the propeller to the airflow that changes, not the relationship of the propeller to the airplane. (refer to explanations of P - factor)

This is probably over simplified but I am only talking about what changes when the tail is raised.
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Old 19th Jun 2021, 17:43
  #24 (permalink)  
 
Join Date: Mar 2011
Location: Canada
Age: 50
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Originally Posted by shumway76 View Post
Helicopters require tail rotor to counter the torque caused by the rotor blades.
Fixed wing single engine propeller aircraft also suffers from torque effect, but doesn't need something as dramatic as a "counter torque" mechanism.

Is the torque effect caused mainly by the length of the blade?
- helicopter has long blades, so more counter torque is required.
- fixed wing single engine propeller blades are short, so not such a great counter torque is required

Is my guess above correct?

Thank you!
Fixed wing aircraft do have anti-torque devices. They're called wings. If the aircraft starts to rotate in roll the different changes in angle of attack of the two wings (one will increase, the other decrease) applies an anti-roll force opposite to the rotation. Airplanes are inherently roll "stiff", in normal flight - they have very heavy roll damping. Ailerons are used to adjust this mechanism, and provide a small net angular momentum in either direction, at the pilot's discretion. The vertical stabilizer provides an effective mechanism for preventing the accumulation of angular momentum around the aircraft's normal axis. The rudder is used to trim this anti-rotational force.

The main difference between an airplane's anti-torque devices and those of a helicopter comes about because an aircraft is in forward motion: passive flight surfaces have a predictable airflow over them at all times and can be used. A helicopter's anti-torque mechanisms have to work while the aircraft is in hovering flight, moving only very slowly, or moving in a non-forward direction.

The torque effect from a helicopter comes from two contributions:

1. Spinning the rotor up to speed. As long as you do this while the helicopter is on the ground, the angular momentum imparted to the blades is balanced by twisting the planet the other way, through the wheels or skids. This effect stops when the rotor is at speed, as its angular momentum remains constant.

2. Spinning the air, as a by product of pushing it downward to generate lift while under power. This is a continous effect. To avoid building up angular momentum in the helicopter (which would cause the body to rotate around the rotor axis) the helicopter needs a source of angular momentum equal and opposite to what it's feeding into the air - which it gets from a tail rotor.

3. A rotor under autorotation doesn't apply any net angular momentum to the air. Gyrocopters don't have tail rotors.

4. What most people call "torque" effects in airplanes are precessional effects.

Other notes: torque is not a force. It has different physical units.

Last edited by photofly; 19th Jun 2021 at 19:03.
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