Torque reaction
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 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.
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.
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.
This is probably over simplified but I am only talking about what changes when the tail is raised.
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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 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!
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.
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.
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.
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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
Last edited by Fl1ingfrog; 22nd Jun 2021 at 12:39.
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
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.
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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.
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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.
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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 !
