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 !