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Once again, I'm Jacob and this video's topic covers the Law of Conservation of Angular Momentum and how it applies to helicopters, more specifically maneuvering flight. I say maneuvering flight because you typically will not experience the effects of this unless you are aggressively maneuvering a helicopter. This isn't your typical traffic pattern stuff.
The Law of Conservation of Angular Momentum states that Angular Momentum of a rotating body will remain constant unless external forces are applied. In essence it's a variation of one of Newton's law that states that an object in motion will remain in motion unless acted upon by an outside force. Only here it is applied to a rotating body. Simply put, the rotor blades want tot maintain their momentum. Conservation of Angular Momentum is sometimes used synonymously with Coriolis Force/Effect which deals with the tendency of rotating bodies to increase in velocity as the center of mass (COM) or Center of Gravity (CG) shifts closer to the axis of rotation. In a nutshell, Conservation of Angular Momentum is used to explain the Coriolis Effect. But for the purposes of helicopter flying and this lesson, they are used interchangeably.
So how does all the physics stuff apply to flying a helicopter? I'm glad you asked. Imagine you're conducting flight maneuvers that increase and decrease the G-loading on your rotor system. As the helicopter G- loads, the rotor begins to cone. Because of this the blade diameter shrinks and the CG/COM shifts closer to the axis of rotation. Because of this the rotor RPM increases.
The easiest analogy to use to explain this is a figure skater spinning in a circle. As the skater's arms come closer to her body she speeds up. Her rotations per minute increases. Both the figure skater and your rotor blades have Angular Momentum while spinning. They want to maintain this momentum and when the center of gravity / center of mass changes, rotational velocity changes. Another way to think about it is a race car going around a track. If the car is traveling 60 mph and the trac is 1 mile, it takes 60 seconds to complete a lap. Now let's say the driver takes an inner loop while still maintaining 60 mph. The driver's momentum remains the same but now travels a shorter distance. So the momentum has maintained or been "conserved." But the RPM has increased. Now instead of 1 revolution in 60 seconds it may be something like 50 seconds. The time it takes to make 1 full revolutions has decreased.
But the engine control units of our helicopter want to maintain a specific rotor RPM. So when they see the rotor RPM increase, they reduce fuel flow to maintain the desired RPM. but when the rotor unloads the G forces and coning reduces, the rotor RPM decreases. So what do the engine control units do? They increase fuel flow to the engines to arrest this decreasing rotor RPM and speed the rotor back up. The engine control system that is supposed to maintain rotor RPM is always reacting to changes in rotor speed from aerodynamic forces.
Because of this, if you aren't aware if rapid rotor fluctuations resulting from Coriolis forces, you could have some negative effects while flying. These include:
1. Rotor overspeed: as the rotor cones the RPM can increase drastically
2. Rotor underspeed: as the rotor coning subsides the RPM can reduce drastically
3. Exceed Aircraft Torque Limit: in an effort to stop a rotor from underspeeding your engine control unit could cause your engines to produce more power than your drive system can handle and result in an overtorque.
Typically the quicker and more aggressive the maneuver, the greater the fluctuations in rotor RPM and torque.
That wraps up The Law of Conservation of Angular Momentum/ Coriolis Forces! The biggest takeaways are that the rotor always wants to maintain its momentum and that as the rotor canes due to G loading, it has a tendency if speed up and vice versa. These fluctuations can result in aircraft limitations being exceeded fairly rapidly. Thanks for watching! Don't forget to hit like and subscribe below. Be sure to like us on Facebook as well. As always, safe flying!
If you're just getting started and want more information, pictures, and more explanations, I'd recommend reading the Rotorcraft Flying Handbook - amzn.to/2ifPlnZ. If you've already got a basic understanding, and want to further your professional helicopter education with advanced helicopter concepts, I'd recommend reading Cyclic and Collective, by Shawn Coyle - amzn.to/2ifQGLx
15 сен 2024
