Sunday, January 22, 2012

Smoothing The Ride!

Working on an airplane is not so different from working on a car. Just like any other machine, there are a vast array of moving parts and systems which work together to perform a specified goal. In some ways, though, aircraft are much simpler than a comparable sized car. Thus an airplane can be easier for the aircraft mechanic to work on, when compared with the complex systems of the modern passenger vehicle that the car mechanic is required to deal with during maintenance. Much of this difference in complexity is due to differences in purpose. The purpose of a car is to provide transportation in a comfortable and economic way (though economy is not a factor with some types of cars). An aircraft on the other hand, needs to be, above all else, as light as possible. Thus the simple comfort and luxury in most passenger cars is found stripped out of an airplane due to weight needs. Though basic items, such as cushioned seats and insulation can be found in even the simplest of aircraft, the comfort of a flight mostly depends on how well the aircraft flies through the air. And for a helicopter, (which is the particular type of airplane I work on) this can become quite an involved task.
AS350 B2
I work on a smaller sized, French made Eurocopter AS350 B2 and B3 helicopter, which is used by rescue, police, military and many other commercial, government and civilian organizations, all over the world. For those of you who might be a little unfamiliar with helicopters, let me explain briefly, how this model of helicopter basically works. The AS350 uses a three bladed main rotor head system with a two bladed tail rotor. A small turbine engine turns a drive shaft which is connected to both the main rotor and the tail rotor through two separate transmission systems. As the engine turns it spins the main rotor head, with the three blades, which are like three small wings. As the rotor system spins faster, the blades create more lift and the helicopter can then raise off the ground. The pilot uses the control sticks in the cockpit to input small changes in the angles of the blades as they are turning (referred to as "angle of attack"). This allows the pilot to control forward, up and down and even backward and side to side movements...its almost like riding on a flying carpet! So what does the tail rotor do, you might ask? Well the the tail rotor blades are also like little wings that create lift. Except they push and pull on the tail boom of the helicopter in a lateral side to side motion. This enables pilots, through foot pedal inputs, to point the nose of the helicopter in any direction required. The tail rotor blade also keeps the helicopter from spinning in a circle when it leaves contact with the ground, it counteracts the twisting torque of the main rotor blades spinning overhead...remember Newton's third law.

A Balancing Act!

Add plate weights to balance the main rotor head
This spinning mass of metal and composites creates quite the wind storm. The rotor head resembles a giant wheel spinning on its side right on top of the helicopter and requires balancing, just like the wheels on a car. This balancing is accomplished by adding, or removing, small weighted plates to the inside bottom of each blade. But balancing is not the only adjustments that need to be made by the mechanic for the rotor head to operate efficiently. The main rotor blades also have to be "told" where to fly. Yes, I did say the pilot controls this from the cockpit, but it is a little more complicated than that. Remember that time your mini-van pulled hard to the right and seemed to have a mind of its own? The only way to fix the problem was to take it in for an alignment. Because the wheel alignment drifted due to hitting pot holes and and other types of wear, it made for an uncomfortable, and sometimes uncontrollable, ride. These small variations in wheel alignment caused the wheels to move out of the intended path which the driver inputted through the steering wheel.The same is true of the main rotor blades. Each one is like a small wing that has to be set a certain way for it to fly correctly. Even the smallest variation in blade flight path can cause a very uncomfortable (and damaging) vertical vibration. Very similar to the way in which an out of balance wheel can make for a rough ride. This vibration can be corrected by making small adjustments to the angle of the blades as they sit on the rotor head. While the helicopter is in the hanger before it makes its first flight, the initial blade angle is set at 7 degrees using a standard rigging tool. This is adjust by the pitch control links, which are metal rods that connect the swashplate to each individual blade. No a swashplate is not some kind of weird dinner plate, it is a simple system which connects the spinning rotor head with the stationary controls that are inside the helicopter, which allow the pilot to control the aircraft. Once this initial blade angle set up is done, then its flying time!

Pitch Control Links and side view of the swashplate
So Whats The Angle?

Now that the helicopter is flying, this is where things get a little touchy! Even though the blades have an angle set on the ground, before even leaving the hanger, the blades can take on a mind of their own when the helicopter starts flying forward. Each of these blades are hand made out of composite materials, fiberglass, resin, metal, and graphite. Because each blade is hand made and completely original, they each fly in their own unique ways. This individuality reveals itself pretty quickly after reaching cruising speed (around 100-120 knots), this is when it starts to get bumpy. The only way to get these unruly blades to fly right is to adjust their individual tabs on the trailing edge of each blade. These tabs are like tiny ailerons which control the particular angle ("angle of attack") that each blade flies. Using a special tool the mechanic can make small bends to these tabs. If the tab is bent up, the blade flies higher, if the tab is bent down the blade flies lower. The closer the blades fly to each other, the smoother the ride. The best way to explain this phenomenon is to imagine having a three legged race with someone who has a shorter leg. This would cause for a bumpy race each time the shorter leg is stepped on and used to propel the racers forward. The same is true of a blade that is not flying in the same circular path as the other blades. Thus, the closer the blades fly to each other, the smoother the ride. Simple right! Not so fast. There is one more problem the mechanic has to deal with, turbulence. Remember when you last went fishing on the lake, and those pesky motor boats kept speeding by and made such a large wake that you left in disgust? As the boat raced through the water, it created waves that reached out behind the boat as it traveled forward in the water. Well the same is true for each of these blades. As the blade passes through the air (remember it is a fluid) it creates its own wake in the air that disturbs the smooth ride of the blade right behind it in the rotor path. So you have to be careful not let the blades fly too close or the ride will get bumpy again. This final act of adjustment can be quite the feat and is a combination of art and science. After you master this, you got it! The helicopter is flying smooth and efficient!
Blade tab adjustment tool
The small aileron like blade tabs. But only bend the two inboard ones, the other four are set at the factory.
Bend the tab up, the blade flies higher. Bend the tab down, the blade flies lower.

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