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!

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.

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=mc², etc.

Pedantic Mode /OFF/

- Ed :ok:

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.

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.

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/

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.

421dog - I bow to your superior knowledge! You are correct, of course. I owe you a cold beer....

- Col. Ed (of the Kentucky variety.)

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.

The torque of a helicopter main rotor is considerably greater than that of a propellor fitted to a similar sized flying machine!

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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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....

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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Slightly off topic, but until reading this thread never thought of it before.
Why aren’t helicopter main rotors twisted like a propeller?

Are you, perhaps in both examples, confusing torque with gyroscopic precession?

No, the term 'precession' describes the reaction; the resulting vectors of any change. 'Torque' is the measurement of the force that exist.

Many are, though much less twist.

Because unlike most propellers they have to capable of providing thrust in both positive and negative directions.

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.

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.

Yes, but I'm wary of using absolutes such as "only" which can come back to bite you.

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.

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.

Using round numbers, the Cessna 152 and Robinson R22 are powered by Lycoming piston engines of 110/120 hp. Whilst the Cessna's prop is ungeared, the Robinson's transmission is geared to turn the rotor at 1/5 of engine speed.

Rotational power is a function of torque multiplied by speed of rotation. To absorb the engine's power at 1/5 of the speed, the rotor has size and aerodynamic properties in order to load the transmission at 5 times the torque, to absorb the same power from the engine.

So although the fixed wing and rotary wing employ similar power, because of its speed of rotation the fixed wing airframe (in this example) requires only 1/5 of the anti-torque.

Duncan :ok:

torque (ft.-lbs.) = 5255 * brake horse power / output shaft speed (RPM)

That's very true, but it doesn't help us understand why a 120HP R22 needs an active anti-torque device that requires a gearbox, driveshaft and blades with pitch control, but a high performance aircraft like a 1700HP Spitfire, with way more engine torque than the R22, doesn't.

There is a great deal of inertia resulting from the flying surfaces: wings, tailplane and fin of an aeroplane plus the air resistance produced by these large surface areas. However, do not shove the throttle to the fire wall to recover from the stall on very high powered types - the Spitfire might be one of them - because in that condition the aeroplane will very quickly flip inverted. A helicopter has little resistance around the normal axis (the rotor shaft) and so needs the tail rotor for control in yaw

The Spitfire pilot notes of the very high power models have such a warning for stall recovery, also full power couldn't be used on take off as it suffered undue tyre scrubbing and could actually roll the tyres off the rims if overly enthusiastic. Going to the contra rotating prop solved both issues.It's the direction in which the torque is applied, in the case of helicopters the torque is about a vertical shaft, hence all single rotor helicopters require a tail rotor to counteract main rotor torque, the tail rotor also provides a means of directional control for the pilot by altering the thrust (pitch change controlled by pilots pedals). In a single engine fixed wing the torque is applied about the fore/aft axis ie wanting to roll the aircraft. The rolling moment is generally controlled by ailerons, though some old aircraft used different angles of incidence on each wing.

Right. Airplanes have forward motion, and can use passive surfaces to react the torque. But it’s also interesting to note that airplanes are inherently roll stiff in a way helicopters aren’t (yaw stiff): even absent aileron deflection a rolling motion changes the angles of attack of the two wings: the down-going wing has a higher angle of attack and generates more lift than the up-going wing (unless stalled), inherently providing a torque contrary to the roll. Ailerons are the pilots mechanism to adjust this couple, to effect desired roll.

I did some flight testing a number of years ago in a Bell 206B and Bell LongRanger, where tail rotor effect had to be evaluated before and after an external camera installation. I set up a test along an abandoned dirt road I could fly down, and flew progressively more slowly, without touching the tail rotor pedals. In both cases, I was able to fly down to 22 knots with only the vertical stabilizer holding against the torque. Below 22 knots, it suddenly swung, and a prompt application of pedal was required. Happily, pre and post mod were the same, so test element passed. Most helicopters I have flown will cruise quite happily, with only a touch, if any pedal applied.

photofly - that is an impressively simple and lucid explanation. I'm just in the middle of studying principles of flight for ATPL, I wish my study notes were that good !