vidual blades are free to flap, lag and change pitch but I will probably just focus on the two basic designs. I will discuss their features in more detail later in this article. Since I am going to be talking about forces in
this article, I want to throw out a couple of physics definitions so that we are on the same page.
FORCE: A push or pull acting on a body. Force usually causes some distortion of the body, a change in its velocity or both.
MASS: A measurement of the quantity of matter in an object. The measure of an objects resistance to acceleration or movement. Just walking up to a parked helicopter and looking at the head you can see the most obvious characteristic of the main rotor head — it is supporting two or more rotor blades that are attached, usually with just a couple of bolts. Just think of the weight of the stationary blades that are exposed to gravity and are being pulled toward the center of the earth. I’m sure you all remember the definition of torque but I will include it here anyway.
TORQUE: Simply put, torque is the turning effect of forces on the axis of rotation. In brief, it is a moment of force.
Using the formula for torque that we learned in A&P school: Torque = Force x Distance. If force is the weight of the blades and distance is the length of those blades, it doesn’t take a rocket scientist to know that there is a massive amount of torque being exerted at the blade bolt attachment points on the main rotor head. The bending forces of the blades are astronomical. In this static condition, gravity is the most predominant force on all of the components of this helicopter. They must be designed to handle the gravitational loads of landing and being parked or being towed. Just think of the amount of force being put on the main transmission with the total weight of the main rotor head, blades and mast resting on a set of tapered roller bearings in it. The transmission must support this weight when static. Now, let’s get this eggbeater moving.
THE MAIN ROTOR MAST POLE In regards to all of that mass that just talked about above, we need to get it turning. The engine or engines drive the main transmission through drive shafts and free-wheeling units so that it can reduce the speed of the drive shafts and increase the power (or torque) so that the mast can turn the main rotor head and blades. Not only is the mast pole required to sup- port the weight of the main rotor head and blades, it must also apply the torque required to spin the blades. I’m not going to dwell too long on the blades. They are a wonder in their own right, but I just want to point out their air foil design and Bernoulli’s Principle:
Constant pressure increases when fluid (or air) slows down. The pressure in a moving fluid (or air) decreases when speed increases and vice versa. We know that the air passing
over a wing must speed up which decreases the pressure compared to the air passing under the wing. This creates a low-pressure area above the blades. This creates lift. This lift is translated from the blades to the main rotor head to the
8
HelicopterMaintenanceMagazine.com December 2019 | January 2020 1-3 Page HeliMX
ad.indd 1 2/2/16 1:36 PM
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
