Welcome back to helicopter lessons in 10 minutes or less. I'm Jacob and this video covers one topic that's been requested quite a few times. The topic being transient torque spikes and helicopters. That said be sure to leave your comments and video requests below if you have topics that you'd like to see in the future because that's heavily in which videos I start pushing through production.
Also be sure to hit like and subscribe. Let's get started. Transient torque spikes are an aerodynamic phenomena that occur with left or right cyclic applied.
That is lateral cyclic. So short quick definition short or it's a torque spike as a result of lateral cyclic. Now it's generally different, or it is different, than what you see in operating limits of operators'manuals labeled torque transients, and that's just limiting the time at certain torque values to keep from over-torquing.
Transient torque spikes are both increases and decreases in torque value that occur due to aerodynamic forces acting on the rotor disc because of an engine control unit trying to maintain a constant rotor RPM. That is, some sort of ECU... FADAC, a fuel governing system, whatever it is that brands in the engine trying to maintain 100% rotor RPM or constant rotor RPM. So let's see what that looks like. If I want to turn to the left, let's say I have my rotor disc here, there's the fuselage with tail rotor counterclockwise turning rotor system and I want to turn to the left so a lateral cyclic input in this case if I want to turn to the left I need more lift on the right side of the disc less on the left side of the disc but considering during gyroscopic procession or phase lag, the input is actually made 90 degrees prior.
So that looks like something like this. This increase in pitch is actually happening over the tail. This decrease in pitch is happening over the nose. If I want to increase the lift on the right, these are where the inputs have to be made to make that increase and decrease respectively. In this situation, the torque is going to increase with a left turn.
So if I turn left, it will be an increase in torque percentage. you're asking why is this? Well this increase occurs because of the increase in lift and drag over the aft half of the disc where the induced flow is at its greatest. It's not as severe on the front half but as you fly forward in flight there's more and more induced flow and drag on the aft half of the disc and now I'm increasing pitch in this region which is demanding more. So this momentarily slows down the rotor.
Aircraft equipped with an electronic control unit or a full-authority digital electronic control unit or droop compensators or whatever the brands of the engines are, they're always trying to maintain that constant 100% rotor RPM. So when a decrease is sensed, say I'm increasing pitch here, it slows down the rotor, decreases its rotor, these control units sense that and they add more fuel to try to bring that rotor back up to the 100%. The fuel is usually regulated by some sort of hydro mechanical fuel control, but they surge fuel into the engine to recover the decreasing rotor RPM Which causes the spike in torque so turning to the left is going to be an increase in torque Conversely turning to the right is going to be a decrease in torque and that's gonna look something like this So if this is my rotor, there's fuselage counterclockwise rotating rotor. If I want to now turn to the right, it's going to be an increase in pitch on the left, decrease in pitch on the right. This, because of gyroscopic precession and phase lag, has the 90 degrees, or the input made 90 degrees prior.
I am now decreasing the pitch over the tail which is already the most induced flow so that's lightening the load on the engines increasing the pitch over the front half which is already in the clean air so if I turn to the right this is going to be a momentary decrease in torque percentage. So one way to think about this is consider a man just out for a jog running. If he's maintaining a constant speed, a constant rotor rpm, if you were to throw him a 20 pound weight and tell him to hold it while he's running, if he wants to maintain the same speed his heart rate is going to go up because I'm now demanding more of them.
That's like turning to the left. I'm demanding a little bit more of the rotor. It's going to take a little bit more fuel.
I'm going to see a spike in torque. Now let's say the running man throws the weight down his speed. Let's say he needs to maintain the same speed.
Well now his heart rate is going to go down because now the load demanded of that runner has been decreased. So that's kind of what it's doing. It's the management of that constant rotor speed. Once again, left cyclic increases torque. right cyclic decreases torque.
All this is for helicopters that have some sort of electronic control unit or governor or fuel regulator, something that maintains the constant rotor speed. Otherwise, if you didn't have this piece, then a left cyclic would just cause a drop in rotor RPM. A right cyclic would cause an increase in rotor RPM. But all this is important to know is in case you're aggressively maneuvering the helicopter and you don't want to exceed a torque limit or over-torque the aircraft.
So that said, there's some factors that amplify these torque spikes. Now that is the first one being the rate of movement. Rate of movement getting into how quickly are you moving the cyclic. Faster cyclic inputs mean higher fluctuations in torque.
The next one is going to be magnitude of movement. This is all in the cyclic here. So say a one inch disc. cyclic displacement versus a 4 inch cyclic displacement.
More displacement means bigger torque spikes, especially if you're going from one extreme to another. Say from a full right cyclic to a full left cyclic or a quick alternating left right left right in the cyclic. This type of magnitude, the amount of displacement is going to increase the torque spike.
Third is going to be power applied. So how much power is in at the beginning of the maneuver? If you're say 60% power applied versus 95% power at the beginning of the maneuver, the higher the initial power setting, the higher the spikes are going to be.
Next up is going to be airspeed. Faster forward airspeeds means more induced flow on the aft half of the disc, which causes increases in the torque fluctuations when I'm doing these maneuvers because of more induced flow, induced drag on the aft half. Next is going to be weight. The heavier I am, the more the road rotors are coning which is going to just make all maneuvers a little bit more pronounced especially accelerations and decelerations in the rotor. So if I don't want to have these huge torque spikes or I want to mitigate them how can I compensate for them?
Easiest way to compensate for them is just think of string theory and I'm not going into extreme physics here this is just a simple imagine a string is tied from the collective to the cyclic and you must keep the string tight for the entire flight. If you turn hard to the left, you need to be lower in that collective to keep the string tight. If you turn to the right, you're increasing collective to keep the string tight.
This is generally just a brain tool or a tool in the aircraft to keep the torque, the collective, everything where it should be when you're making these maneuvers. This is just a way to keep you compensating throughout the maneuvers. But that wraps up transient torque spikes. They are increases or decreases in torque as a result of lateral cyclic inputs.
If not taken into consideration, you could possibly over torque your aircraft doing aggressive maneuvering. But thanks for watching. Be sure to hit like, subscribe below.
As always, this is Helicopter Lessons in 10 minutes or less. I'm Jacob.