During my fixed-wing flight training, I witnessed what I thought to be a miracle – a Blackhawk helicopter landing at the same airport I was learning about the theory of airplane flight. Captivated by the powerful sound of the rotor blade, I instantly began to wonder. How did the rotor blades work to produce the lift necessary to keep the aircraft afloat? And were helicopter rotor systems susceptible to any of the same by-products of flight as the airplane?
I would soon discover the significance of words like flapping and feathering; that hunting was more than traipsing through the woods towards the nearest tree stand, and that coning and twist weren’t always referring to ice cream.
Helicopter flying has often been equated to rubbing your belly while patting your head and walking, all at the same time. There’s no doubt rotary wing flying involves a bit of manipulation unfamiliar to the fixed-wing crowd but proper manipulation of the wild yet fascinating components of the helicopter lead to the successful creation of lift, just like flying an airplane.
So how do helicopters fly? First, let’s decipher some helicopter vocabulary. Maybe in doing so, you will gain an appreciation (or at least a sense of awe, like I did) for helicopter flight.
Helicopter Rotor System Characteristics
Helicopters really come with two rotor blade systems – the main rotor system mounted above the cockpit and connected to the engine and the tail rotor, affixed to, well, the tail section (more on that in a future article). These two rotate simultaneously to produce and counteract lift, among other talents.
Like airplanes, helicopters must create enough lift to overcome weight to fly…it’s really all about balancing the forces. This vertical vector combined with centrifugal force produces a resultant force that’s not completely opposite the downward component of weight. So while your helicopter’s main rotor system is still creating lift, centrifugal force is stealing the thunder. If the goal is to take off vertically, the resultant vector needs some adjustment.
To make the resultant force more effective, the blades cone! Coning occurs to counteract our sneaky friend, centrifugal force. Ask and you shall receive…more lift that is. The blades flex upwards to more effectively concentrate the lift vertically. But beware – coning only augments lift to a certain point, after which it can actually degrade the amount of lift. Excessive coning can creep in at low RPMs, high gross weights, or high G maneuvers.
Helicopter rotor blades move fast! And they create a great deal of lift but the lift is not consistent along the blade so engineers design a twist into the blade. Twisting the blade distributes this lift more evenly along the length of the rotor blade.
Dissymmetry of Lift
A look at dissymmetry of lift is necessary to lay the groundwork before moving forward. Dissymmetry of lift is essentially the difference in the lift between the advancing half of the rotor disk and the retreating half. When the speed of the blade combines with the airspeed of the helicopter (wind affects both here), The advancing blade pulls ahead in the race as it moves much faster and acquires greater lift. Conversely, the retreating blade slows down and loses lift. And the closer you get to the tip of the blade, the faster the blade moves!
Although lift is a good thing, if half the helicopter has more than the other half, the aircraft may end up in a rolling situation (literally rolling over). To prevent the advancing blade from overpowering the retreating blade, we have to equalize lift. Several mechanisms exist to counteract this undesirable condition.
As the main rotor blades travel, they want to fight off the dissymmetry of lift while having some fun. So they climb (flap up) as they advance around the right half of the rotor’s path and dive (flap down) as they round out the left side. This is flapping. They can do this because they teeter on a hinge. You can see this when the helicopter is sitting on the ground, not running. The blades actually droop (and no, not because they’re sad).
The advancing blade flaps up, eventually decreasing its angle of attack. Conversely, the retreating blade flaps down, eventually creating an increase in the blade’s angle of attack and winning the battle against dissymmetry of lift.
Feathering, like blade flapping, has a role in countering dissymmetry of lift. Feathering is the rotation of the blade about its span wise axis, by collective or cyclic inputs, which causes a change in blade pitch angle.
Primary feathering occurs when you manipulate the cyclic, which in turn moves the thrust vector in the direction of movement (left, right, forward).
Leading and Lagging (also known as Hunting)
While the blade flaps up, the CG moves closer to the rotor mast. Why does this happen, you ask? Well, it’s all about Coriolis force. If you’ve ever watched ice skaters, you are familiar with Coriolis force (which simply states that as a mass moves closer to the center of rotation, it gains speed). So when the ice skater moves her arms closer to her body as she spins, her speed increases. The same thing occurs on a spinning rotor blade.
The faster blade also experiences a change in pitch and an increase in drag. If these stresses continue too long, the rotor blades risk excessive bending. Leading and lagging can give the blades some room to relax and unwind from their overstressed condition.
During leading and lagging, the rotor blade moves fore and aft (or hunts) in the plane of rotation. But this feature only frequents fully articulated rotor systems, so you may not encounter this when first learning to fly a helicopter.
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Dole, C. E. (1994). Flight Theory for Pilots. Redlands: Jeppesen Sanderson.
Headquarters, Department of the Army (2007). Fundamentals of Flight. Washington, D.C: U.S. Government Printing Office.