Transient Torque Spikes in Helicopters

Welcome back to Helicopter Lessons in 10 Minutes or Less! I'm Jacob and this video topic is one that was requested quite a few times. The topic being Transient Torque Spikes. That said, be sure to leave your comments and video requests in the comments below. Hit like and subscribe as well.

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Transient torque spikes are aerodynamic phenomenon that occur when left or right (lateral) cyclic is applied. It is different than the operating limits labelled "torque transients" in operator's manuals that limit time at certain torque levels. These spikes, both up and down in torque value occur due to aerodynamic forces acting on the rotor disk because engine control units (ECU's) are trying to maintain a constant rotor RPM. Here's what it looks like: If I want to turn left, I need more lift on the right side of my rotor disk to do so. But, considering gyroscopic precession/ phase lag, the input has to be made 90 degrees prior. So to increase lift on the right, the increase in pitch is made over the tail. This simultaneously decreases lift on the left side of the rotor disk by making the input over the nose. The aircraft begins to turn to the left and torque increases.

This increase occurs because of the increase in lift and drag over the aft half of the disk where induced flow is greatest. This momentarily slows the rotor down. Aircraft equipped with ECU's, Full Authority Digital Electronic Units (FADEC's), Droop Compensators, or whatever brains in the engines are always trying to maintain constant rotor RPM. So when a decrease is sensed, they dump more fuel in the engine to bring the rotor back to say, 100%. The fuel is usually regulated by some sort of Hydromechanical Fuel Control (HMU). This surge in fuel to recover the rotor RPM is what causes a spike in torque. Turning to the left increases torque. Conversely, turning to the right decreases torque. Right cyclic increases pitch over the nose an decreases pitch over the tail. Gyroscopic precession/phase lag has its 90 degree delay effect and increases lift on the left side of the disk. The rotor momentarily increases in RPM. So the brains of the engine reduce fuel flow to try to get the rotor back down to 100%.

Consider the Running Man analogy. If he's maintaining a constant speed (constant rotor RPM) and you toss him a 20 lb weight and keep running. If he wants to maintain the same speed, his heart rate will go up (increase in torque/fuel flow) because more is demanded. Thats like turning to the left. I'm demanding more and I'll see a spike up in torque. Now the running man throws the weight down and maintains the same speed. That's like turning right. His heart rate will go down because the load demanded has been decreased.

All of this is important to know if you are aggressively maneuvering the helicopter and don't want to exceed a torque limit or "over torque" an aircraft. That said, there are factors that amplify these torque spikes.
1. Rate of Movement: how quickly are you moving the cyclic? Faster means bigger fluctuations.
2. Rate of Magnitude: a 1" cyclic displacement compared to a 4" displacement. More displacement means bigger spikes and dips in torque, especially full L to full Right or alternating Left Right Left.
3. Power Applied: Compare 60% to 95% at the start of maneuvering. The higher the initial power setting , the higher the torque spike will be.
4. Airspeed: Faster speeds generally mean more induced flow on the rear half which causes bigger fluctuations.
5. Weight: Heavier aircraft means more coning in the rotor disk which will make these spikes more pronounced.

So how do you compensate? think of string theory. Imagine a string is tied from the collective to the cyclic and your job is to keep it taut. If you turn left, lower the collective to keep the imaginary string tight. If you turn right, increase collective. These movements generally keep you compensating throughout the maneuvers.

That wraps up this subject. Thanks for watching. Be sure to hit like, subscribe, and leave a comment. As always I'm Jacob. Safe Flying.