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Torque reaction

Old 1st Jun 2021, 14:00
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Torque reaction

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!
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Old 1st Jun 2021, 14:51
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Torque is a force. Einstein's 'equal and opposite reaction' tells all. The torque from an aircraft propeller is substantial. An aeroplane however has substantial surfaces: main wings, tailplane and a fin all producing a reaction acting against the torque effect from a propeller and therefore it is not so noticeable. The torque is always there, as any tail wheel pilot knows. It is especially noticeable throughout the take-off phase when the pilot raises the tail. A helicopter has little surface area to oppose the torque and the tail rotor is very effective. The tail rotor also adds control to the pilot who often exploits the torque effect and the tail rotor to a benefit. You can change heading by simply rotating (yawing) in the hover.

A good experiment is to adapt a bicycle wheel (a handle on the axel) to use as a gyro. Spin it and attempt to change its plane of rotation, you will not find it easy but will learn a lot very simply.

Last edited by Fl1ingfrog; 1st Jun 2021 at 15:07.
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Old 1st Jun 2021, 15:17
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Pedantic Mode /ON/

Fl1ingfrog, I believe you meant to refer to Mr. Isaac Newton's Third Law of Motion. Einstein was busy trying to find a workable grand unification theory, E=mc˛, etc.

Pedantic Mode /OFF/

- Ed
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Old 1st Jun 2021, 16:05
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Yes Shumway, you're thinking about this correctly. Propeller and rotors are different in the way they convert the engine power into movement of air. Bear in mind that the 180 HP Lycoming in a Schweitzer 300 will easily power the helicopter into a climb. A 180 HP Lycoming in a Pitts really cannot even "hover" the plane for any time, let alone sustain a vertical climb indefinitely. the 180 HP is being transmitted into the air differently.

As for torque, yes, consider the aerodynamic center of drag being roughly halfway out the blade (be it propeller or rotor). That power creating drag at the center of drag much further out on the rotor blade than propeller blade creates much more torque to overcome - hence the tail rotor. The torque effect goes further in helicopters, as the side thrust of the tail rotor must also be overcome, so there are tweaks for that too. Depending upon American, or French (rotors turn opposite directions), the helicopter will land either left or right skid first.

For single propeller powered airplanes, there is still torque, but it's usually masked with small aerodynamic tweaks, not noticeable to the pilot in normal flying. But, even a 172 will exhibit the effects of engine torque when flying at very slow speeds. If you trim a 172 in slow flight, and gently increase or decrease engine power, you will roll the plane a little.
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Old 1st Jun 2021, 17:17
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I believe you meant to refer to Mr. Isaac Newton
Oops! thank you Cavuman1.

We must be careful not to confuse torque effect with the more easily identifiable asymmetric thrust and the resulting slipstream such as Yaw followed by roll. Helicopters, by the way, have an asymmetric down force causing roll. This is commonly offset by setting the rotor off centre utilising the weight of the helicopter as a balance. The lift/downdraft is different from the forward going blade (max) than the rearward one (less) which they refer to with a wonderful name; 'flapping to equality'. There are many more and I've always envied helicopter instructors who have so many expressive names in their vocabulary toolbox.

Last edited by Fl1ingfrog; 1st Jun 2021 at 17:48.
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Old 1st Jun 2021, 17:58
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Originally Posted by cavuman1 View Post
Pedantic Mode /ON/

Fl1ingfrog, I believe you meant to refer to Mr. Isaac Newton's Third Law of Motion. Einstein was busy trying to find a workable grand unification theory, E=mc˛, etc.

Pedantic Mode /OFF/

- Ed
pedant mode /on/

I’m pretty sure he was Knighted by Queen Anne, that he never got a DD or other doctorate and that he wasn’t a FRCS. Therefore, he’d properly be “Sir Isaac Newton” right?

pedant mode /off/
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Old 1st Jun 2021, 19:30
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Don't forget that power is proportional to the torque multiplied by the speed of rotation.

Imagine he same engine, fitted in either a fixed wing or helicopter. Using easy numbers, the rotor of the heli will be rotating (say, 200 rpm) at one tenth the propeller speed (say, 2000 rpm), so will produce 10 times the torque. Turbine engine fixed-wing operators can demonstrate this themselves on the ground; Set prop speed to max, and torque to around 80% of max. Now slowly reduce prop speed and watch the torque rise; watch your red lines!!! The multiple of torque and rpm will remain the same.

Edit (RTFQ!!) : So yes, its because the heli blades are longer, broader, etc. Nothing to do with weight; just has more drag than a typical prop.
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Old 1st Jun 2021, 20:40
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421dog - I bow to your superior knowledge! You are correct, of course. I owe you a cold beer....

- Col. Ed (of the Kentucky variety.)
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Old 1st Jun 2021, 20:50
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In the UK it is commonplace for knighted persons to not use their title other than in formal circumstances and when booking a table in a restaurant. Therefore 'Mr' would not be out of place nowadays.
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Old 1st Jun 2021, 20:51
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The torque of a helicopter main rotor is considerably greater than that of a propellor fitted to a similar sized flying machine!
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Old 4th Jun 2021, 20:49
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Tail Rotor

Don't assume the tail rotor is producing the same amount of thrust as an aircraft propeller. The blades are variable and might even 'go into reverse' if you need to yaw quickly in that direction. . . .
For an aircraft you can use the glide ratio, = L/D, to find out the Thrust of the propeller. Maybe there is a similar easy way to find the thrust from a tail-rotor.... Are there any helicopter engineers out there..?
.
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Old 5th Jun 2021, 02:53
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Helicopter tail rotors are very different to airplane propellers, as they have a quite different role. The blades have little if any twist, So much of the blade will always be at a non optimum AoA relative to the entering airflow. But this allows it to create thrust in either direction, and to have low drag when not called upon to create much thrust (cruise flight). They are also necessarily much lighter that propellers. The thrust produced by a tail rotor can be calculated by knowing dimensions, and the torque it is balancing.

Propellers, on the other hand, are much more optimized, so that in the normal operating conditions, they are highly efficient, and will transmit much more power effectively into the air. This is why airplane propellers have pretty poor efficiency when selected into reverse. Only the outboard portion of the blade is reversing the airflow, while the inboard is still making forward thrust (otherwise, air cooled piston engines would get really hot really fast when the prop is selected into reverse. So you'll get a horribly inefficient doughnut airflow with propeller reverse thrust, but reasonably similar, though less efficient thrust either way with a tailrotor. It's all a tradeoff.

Tail rotors can be lighter construction, as the blades are less constrained relative to their axis. Two bladed tail rotors can teeter, and multi blade tail rotors may have flapping and lead/lag freedom, which relieves immense forces when the axis changes, but would not work at all for an airplane propeller.

So apples to oranges....
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Old 8th Jun 2021, 11:31
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Thinking on from my last post. It is possible to use the same L/D estimation to work out the tail-rotor thrust...

If we assume the main rotor blades have a 10:1 L/D ratio, then they are producing a drag force at half the blade length of 400lbs for a 4000lb helicopter.
This is counterbalanced by the tail rotor, which is about 3 times further out from the main rotor shaft, of 400/3 which is 133lbs.
All these figures are very approximate, but should be somewhere near enough.
.
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Old 8th Jun 2021, 16:56
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Slightly off topic, but until reading this thread never thought of it before.
Why aren’t helicopter main rotors twisted like a propeller?
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Old 8th Jun 2021, 17:48
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Originally Posted by Fl1ingfrog View Post
The torque is always there, as any tail wheel pilot knows. It is especially noticeable throughout the take-off phase when the pilot raises the tail.

A good experiment is to adapt a bicycle wheel (a handle on the axel) to use as a gyro. Spin it and attempt to change its plane of rotation, you will not find it easy but will learn a lot very simply.
Are you, perhaps in both examples, confusing torque with gyroscopic precession?
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Old 8th Jun 2021, 18:42
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No, the term 'precession' describes the reaction; the resulting vectors of any change. 'Torque' is the measurement of the force that exist.
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Old 8th Jun 2021, 18:49
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Why aren’t helicopter main rotors twisted like a propeller?
Many are, though much less twist.
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Old 8th Jun 2021, 19:50
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Originally Posted by simmple View Post
Slightly off topic, but until reading this thread never thought of it before.
Why aren’t helicopter main rotors twisted like a propeller?
Because unlike most propellers they have to capable of providing thrust in both positive and negative directions.
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Old 8th Jun 2021, 20:25
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Originally Posted by Fl1ingfrog View Post
No, the term 'precession' describes the reaction; the resulting vectors of any change. 'Torque' is the measurement of the force that exist.
I don't understand that. The fact that my nose will (assuming I don't apply proper rudder input) swing left when I raise the tail is the reaction. The torque causing the propeller to turn and generate thrust is not involved. The left turning tendency would be just the same if a flywheel spinning freely on a stationary shaft replaced the propeller.

Are you referring to some torque other than the torque causing the propeller to rotate and generate thrust? There are certainly other torques involved.
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Old 9th Jun 2021, 00:53
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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.
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