Helicopter torque
 

Why does the helicopter yaw left when throttle is rollled off in a Counter clockwise rotor? When throttle is applied we have torque effect from the engine I get that but what's the explanation for a rapid left yaw when torque effect is removed? Thanks guys

Because you've taken away the torque but you've still got a boot load of left pedal...

There it is in a nutshell. You've set the yaw trim (pedal displacement) to compensate for the torque, when you remove the torque you have to retrim the pedals or you will get a considerable left yaw. In a power off configuration the aircraft will actually require some right trim (from neutral) as the fuselage will have a tendency to rotate in the direction of rotation due to the friction reaction by the main gearbox.

That makes sense but what about in the case of a stuck right pedal procedure?Assuming a shallow low approach, before touchdown we roll on to turn to the right (because of torque reaction) but when we roll off our nose will not only straighten but turn to the left. I understand that in an auto at altitude we have our vertical and horizontal stabilizers and transmission drag to cause a need for some right pedal but in the mini flare or a stuck pedal with only 5 or ten knots we still get a left nose yaw when coming off the power? What would cause that left yaw?

There's a lot to go over in a conversation on this topic, it would benefit you to get with your instructor and have a face to face discussion. That being said there are some basics that you need to consider.
Stuck pedal/anti-torque training maneuvers are a demonstration of balancing the torque reaction of power applied to a "fixed" anti-torque pedal position in order to land the aircraft. A stuck right pedal maneuver is not a demonstration of a total loss of tail rotor thrust. A stuck left pedal is not a demonstration of landing with maximum tail rotor thrust. The left/right reference is in relation to the amount of tail rotor thrust required to hover the aircraft at its present GW.
At a stabilized hover note the pedal position, that is your reference point. Less left pedal than that would be a stuck right, more left displacement would be a stuck left.
You are not trying to land the aircraft with full left, or full right pedal. To do that would require a lot of aircraft to train with and a full time crash crew.
What you are practicing is how to balance the power applied with a fixed anti-torque setting (and also vary airspeed to take advantage of aerodynamic factors) in order to land the aircraft.

That makes sense but even the demonstration of slight stuck right pedal with power reduced the nose will still yaw back to the left? (At a low airspeed)I'm working on my CFI and just trying not to memorize everything but fully know and comprehend it as well. The insight on the stuck pedals was very helpful though!

In a standard rotor, hold the collective and extend your finger straight down. Then point your finger where you want the nose to go.

That's because a "stuck right" is simply pedal position with centered pedals as a reference. Centered pedals are not the zero thrust position of the tail rotor. If you want to know the zero thrust position it will roughly be the right pedal applied to keep the nose straight when you close the throttle to initiate a hovering auto (not scientific, but close).

A student should be the only one who doesn't understand this concept. A commercial pilot has been tested and is expected to understand it.

An instructor needs an even deeper understanding of it to be able to teach it.

Why did airspeed have a say in it? Because the vertical fin, canted off to one side, creates thrust to unload the tail rotor - remember how much left pedal you need in the hover, but when you are pulling the same power in forward flight, the pedals are much closer to neutral and might even be slightly right pedal forward.

As you slow down for the fixed thrust/stuck pedal landing, the fin becomes less effective, and so does the main rotor, requiring more collective to stay off the ground.


Here is one of many techniques, which I have found works for any machine I have flown, from an R22 to an S-76:

If you can line yourself up on a runway or landing area with the wind from the left, in a gentle descent around 30 kt, look for the speed where the nose is about 30 degrees left of straight ahead. Hold that speed until you are about 2' off the ground, then verrrrry slowly start to reduce the groundspeed.

As you slow down, it will want to sink, so you smoothly add lever to stay off the ground - the nose will come a bit to the right when you do so. Slow a bit, hold it off, see if the nose is straight yet. The important bit is :
DON'T LET THE NOSE COME UP!! You will slow down too fast, pull too much lever, and snap to the right.

Keep the gentle slow-down going until the nose is straight, then lower the nose and run on at that speed.

There are lots of other techniques. Ask your instructor, (if he/she has 5000 hrs or more) which one works the best for them. If your instructor only has 100 more hours than you, ask for a change. In cases like this, lots of experience in lots of situations is needed.

That does make sense to me I just had a hard time getting it in my head that we spin as quickly as we do but like you said the pedals are not at zero pitch when they are centered so there is the extra force that spins us faster than what I would have imagined

I appreciate the belittling comment! I won't let that affect me wanting to learn from people who know a lot more than me. I appreciate the info on how to do a stuck pedal though

I should probably leave AC to add the final word since he made the argument so clearly, but I just wanted to check you'd got his point.

The 'extra force' that made you spin sharply to the left as you slowed down too much is nothing to do with the position of the pedals, that has been constant throughout. What made you spin left so sharply is the sudden disappearance of some forces that were keeping you more or less pointing in the right direction when you were moving faster. Those forces include (but are not limited to) the aerodynamic thrust from the fixed tail fin, and the airflow over the shape of the airframe (slipstream).

So the take home message for a student is as AC says, don't let the nose come up/slow down too much because you will enter an unrecoverable spin. Obviously as a trainee instructor you have responsibilities way beyond that.

One of the most important aspects of performing stuck pedal maneuvers as an instructor pilot is to never lose sight of the fact that the pedals are not really fixed. Don't forget that it's only a simulated malfunction. If it isn't working out as planned don't try to salvage the maneuver. Resume normal flight, figure out what went wrong, try it again.
Just like simulated engine failures are simulated. If it isn't working out end the simulation.

Ascend Charlie says:

Just a nit - not all fins are canted (I don't think the R22 is, the R44 & 66 are). You can kind of tell by how an R22 flies at cruise - still needs a fair bit of left pedal compared to a 206.

Also, some (like Enstrom) don't really have much fin at all.

The original question asked about why the left yaw, and I agree that what people have been mentioning are the biggest factors, but nobody mentioned transmission drag, which is typically what's yawing your nose left during the flare of an autorotation. The friction inside the gearbox tends to want to drag the nose, but is typically being countered by torque, fin, and pedal. But during unpowered flight, and especially when you wind the RPM up, the drag is sufficient to be noticed, although not a huge effect (if there's any crosswind it'll typically hide any transmission drag effects).

One of the big differences between a touchdown auto and an actual engine failure is the nose swing as you cushion. When performing a touchdown auto (training maneuver) the engine is at idle and tends to counter that left yaw as you cushion the landing. With an actual engine failure you notice that you need some right pedal as the rpm comes down when you cushion. (my experience with turbine engines, recips...been too long ago to remember)

Don't forget that as you decay the Nr during the cushion phase of an EOL, the TR loses thrust very quickly so lots more pedal required whether at idle or shutdown.